Stellar corona

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Stellar_corona 

During a total solar eclipse, the Sun's corona and prominences are visible to the naked eye.

A corona (pl.: coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma.

The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. Spectroscopic measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvins, much hotter than the surface of the Sun, known as the photosphere.

Corona (Latin for 'crown') is, in turn, derived from Ancient Greek κορώνη (korṓnē) 'garland, wreath'.

History

Corona sketched by José Joaquín de Ferrer during the solar eclipse of June 16, 1806 in Kinderhook, New York.

In 1724, French-Italian astronomer Giacomo F. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun, not to the Moon. In 1809, Spanish astronomer José Joaquín de Ferrer coined the term 'corona'. Based in his own observations of the 1806 solar eclipse at Kinderhook (New York), de Ferrer also proposed that the corona was part of the Sun and not of the Moon. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, which was called helium (from Greek helios 'sun'). French astronomer Jules Jenssen noted, after comparing his readings between the 1871 and 1878 eclipses, that the size and shape of the corona changes with the sunspot cycle. In 1930, Bernard Lyot invented the coronograph, which allows viewing the corona without a total eclipse. In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny 'nanoflares', miniature brightenings resembling solar flares that would occur all over the surface of the Sun.

Historical theories

The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV, or Fe¹³⁺). Bengt Edlén, following the work of Walter Grotrian in 1939, first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line from Fe¹³⁺ at 5303Å, but also the red Fe-X line from Fe⁹⁺ at 6374Å).

Physical features

Configuration of solar magnetic flux during the solar cycle

The Sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the corona's temperature is 1 to 3 million kelvin compared to the photosphere's average temperature – around 5800kelvin. The corona is far less dense than the photosphere, and produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include episodic energy releases from the pervasive magnetic field and magnetohydrodynamic waves from below. The outer edges of the Sun's corona are constantly being transported away, creating the "open" magnetic flux entrained in the solar wind.

The corona is not always evenly distributed across the surface of the Sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans approximately 11 years, from solar minimum to the following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun's equator (differential rotation), sunspot activity will be more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the relatively dark sun spots.

Since the corona has been photographed at high resolution in the X-ray range of the spectrum by the satellite Skylab in 1973, and then later by Yohkoh and the other following space instruments, it has been seen that the structure of the corona is quite varied and complex: different zones have been immediately classified on the coronal disc. The astronomers usually distinguish several regions, as described below.

Active regions

Main article: Active region

Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million kelvin, while the density goes from 10⁹ to 10¹⁰ particles per cubic centimetre.

Solar prominences and sunspots

Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights above the Sun's surface: sunspots and faculae occur in the photosphere; spicules, Hα filaments and plages in the chromosphere; prominences in the chromosphere and transition region; and flares and coronal mass ejections (CME) happen in the corona and chromosphere. If flares are very violent, they can also perturb the photosphere and generate a Moreton wave. On the contrary, quiescent prominences are large, cool, dense structures which are observed as dark, "snake-like" Hα ribbons (appearing like filaments) on the solar disc. Their temperature is about 5000–8000K, and so they are usually considered as chromospheric features.

In 2013, images from the High Resolution Coronal Imager revealed never-before-seen "magnetic braids" of plasma within the outer layers of these active regions.

Coronal loops

Main article: Coronal loop

Image from TRACE at 171Å wavelength (extreme ultraviolet) showing coronal loops

Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of the open-magnetic flux that can be found in coronal holes and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma. Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and CMEs.

The solar plasma that feeds these structures is heated from under 6000K to well over 10⁶ K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points (siphon flow due to a pressure difference, or asymmetric flow due to some other driver).

When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), its dark filaments obvious against the solar disk or prominences off the Sun's limb.

Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops (example).

Coronal loops are very important to our understanding of the current coronal heating problem. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE. An explanation of the coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present (i.e., radiation contributions along the line-of-sight propagation). In-situ measurements are required before a definitive answer can be determined, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. The next mission of the NASA Parker Solar Probe will approach the Sun very closely, allowing more direct observations.

Large-scale structures

Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions.

They were first detected in the June 8, 1968, flare observation during a rocket flight.

The large-scale structure of the corona changes over the 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component).

Interconnections of active regions

The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare.

Some other features of this kind are helmet streamers – large, cap-like coronal structures with long, pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered to be sources of the slow solar wind.

Filament cavities

Image taken by the Solar Dynamics Observatory on October 16, 2010. A very long filament cavity is visible across the Sun's southern hemisphere.

Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes.

Filament cavities are cooler clouds of plasma suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure ${\displaystyle p}$ can be calculated by the state equation of a perfect gas: ${\displaystyle p=n{k}_{B}T}$, where ${\displaystyle n}$ is the particle number density, ${\displaystyle k_B}$ the Boltzmann constant and ${\displaystyle T}$ the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.

Bright points

Bright points are small active regions found on the solar disk. X-ray bright points were first detected on April 8, 1969, during a rocket flight.

The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in the X-ray emission.

Coronal holes

Main article: Coronal hole

Coronal holes are unipolar regions which look dark in the X-rays since they do not emit much radiation. These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions.

In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind. These are the coronal plumes. More precisely, they are long thin streamers that project outward from the Sun's north and south poles.

The quiet Sun

The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun.

The equatorial region has a faster rotation speed than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore, the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle. Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are coronal holes.

Alfvén surface

Main article: Alfvén surface

The Alfvén surface is the boundary separating the corona from the solar wind defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal.

Researchers were unsure exactly where the Alfvén critical surface of the Sun lay. Based on remote images of the corona, estimates had put it somewhere between 10 and 20 solar radii from the surface of the Sun. On April 28, 2021, during its eighth flyby of the Sun, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface.

Variability of the corona

A portrait, as diversified as the one already pointed out for the coronal features, is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve at differential times. Studying coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table.

Coronal event	Typical time-scale	Typical length-scale (Mm)
Active region flare	10 to 10000seconds	10–100
X-ray bright point	minutes	1–10
Transient in large-scale structures	from minutes to hours	~100
Transient in interconnecting arcs	from minutes to hours	~100
Quiet Sun	from hours to months	100–1000
Coronal hole	several rotations	100–1000

Flares

Main article: Solar flares

On August 31, 2012, a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted at 4:36 p.m. EDT

Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.

Flares are impulsive phenomena, of average duration of 15 minutes, and the most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature.

An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.

In the corona, the morphology of flares is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and is very complex. However, two kinds of basic structures can be distinguished: 

• Compact flares, when each of the two arches where the event is happening maintains its morphology: only an increase of the emission is observed without significant structural variations. The emitted energy is of the order of 10²² – 10²³ J.
• Flares of long duration, associated with eruptions of prominences, transients in white light and two-ribbon flares: in this case the magnetic loops change their configuration during the event. The energies emitted during these flares are of such great proportion they can reach 10²⁵ J.

Filament erupting during a solar flare, seen at EUV wavelengths (TRACE)

As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event:

• An initial impulsive phase, whose duration is on the order of minutes, strong emissions of energy are often observed even in the microwaves, EUV wavelengths and in the hard X-ray frequencies.
• A maximum phase
• A decay phase, which can last several hours.

Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.

Coronal mass ejections

Main article: Coronal mass ejection

Often accompanying large solar flares and prominences are coronal mass ejections (CME). These are enormous emissions of coronal material and magnetic field that travel outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger CMEs can propel hundreds of millions of tons of material into interplanetary space at roughly 1.5 million kilometers an hour.

Stellar coronae

Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung–Russell diagram. These coronae can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. For example, FK Comae Berenices is the prototype for the FK Com class of variable star. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (L_x ≥ 10³² erg·s⁻¹ or 10²⁵W) and the hottest known with dominant temperatures up to 40 MK.

The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group showed that F-, G-, K- and M-stars have chromospheres and often coronae much like the Sun. The O-B stars, which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.

Physics of the corona

This image, taken by Hinode on 12 January 2007, reveals the filamentary nature of the corona.

The matter in the external part of the solar atmosphere is in the state of plasma, at very high temperature (a few million kelvin) and at very low density (of the order of 10¹⁵ particles/m³). According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.

The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization of its heavier elements than that found in the photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere, but by collisional equilibrium in the optically thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.

The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately a million times higher than on the solar surface. However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average (for the equipartition theorem), electrons have a mass roughly 1800 times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from the photospheric radiative processes), or on thermal conduction. Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields. Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma, even though it is still not clear how they can be transmitted or generated in the corona.

Radiation

See also: Coronal radiative losses

Coronal plasma is optically thin and therefore transparent to the electromagnetic radiation that it emits and to that coming from lower layers. The plasma is very rarefied and the photon mean free path overcomes by far all the other length-scales, including the typical sizes of common coronal features.

Electromagnetic radiation from the corona has been identified coming from three main sources, located in the same volume of space:

• The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight Thomson scattering off free electrons; doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines.
• The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light.
• The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

Thermal conduction

A mosaic of the extreme ultraviolet images taken from STEREO on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195Å—green), 60000–80000°C (304 Å—red), and 2.5 million °C (286 Å—yellow).

STEREO – First images as a slow animation

In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.

When there is a magnetic field the thermal conductivity of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction. A charged particle moving in the direction perpendicular to the magnetic field line is subject to the Lorentz force which is normal to the plane individuated by the velocity and the magnetic field. This force bends the path of the particle. In general, since particles also have a velocity component along the magnetic field line, the Lorentz force constrains them to bend and move along spirals around the field lines at the cyclotron frequency.

If collisions between the particles are very frequent, they are scattered in every direction. This happens in the photosphere, where the plasma carries the magnetic field in its motion. In the corona, on the contrary, the mean free-path of the electrons is of the order of kilometres and even more, so each electron can do a helicoidal motion long before being scattered after a collision. Therefore, the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction.

In the direction longitudinal to the magnetic field, the thermal conductivity of the corona is

$${\displaystyle k=20{\left(\frac{2}{\pi }\right)}^{3/2}\frac{{\left(k_{\text{B}}T\right)}^{5/2}{k}_{\text{B}}}{m_e^{1/2}{e}^4\ln \mathrm{\Lambda }}\approx \frac{T^{5/2}}{\ln \mathrm{\Lambda }}\times 1.8\times {10}^{-10}\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}}$$

where ${\displaystyle k_{\text{B}}}$ is the Boltzmann constant, ${\displaystyle T}$ is the temperature in kelvin, ${\displaystyle m_e}$ is the electron mass, ${\displaystyle e}$ is the electric charge of the electron,

$${\displaystyle \ln \mathrm{\Lambda }=\ln \left(12\pi n{\lambda }_D^3\right)}$$

is the Coulomb logarithm, and

$${\displaystyle {\lambda }_D=\sqrt{\frac{k_{\text{B}}T}{4\pi n{e}^2}}}$$

is the Debye length of the plasma with particle density ${\displaystyle n}$. The Coulomb logarithm ${\displaystyle \ln \mathrm{\Lambda }}$ is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10¹⁵ particles/m³, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 10¹⁸ particles/m³, and in practice it can be assumed constant.

Thence, if we indicate with ${\displaystyle q}$ the heat for a volume unit, expressed in J m⁻³, the Fourier equation of heat transfer, to be computed only along the direction ${\displaystyle x}$ of the field line, becomes

$${\displaystyle \frac{\mathrm{\partial }q}{\mathrm{\partial }t}=0.9\times {10}^{-11}\frac{{\mathrm{\partial }}^2{T}^{7/2}}{\mathrm{\partial }{x}^2}.}$$

Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.

Coronal seismology

Main article: Coronal seismology

Coronal seismology is a method of studying the plasma of the solar corona with the use of magnetohydrodynamic (MHD) waves. MHD studies the dynamics of electrically conducting fluids – in this case, the fluid is the coronal plasma. Philosophically, coronal seismology is similar to the Earth's seismology, the Sun's helioseismology, and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kinds are used to probe a medium. The potential of coronal seismology in the estimation of the coronal magnetic field, density scale height, fine structure and heating has been demonstrated by different research groups.

Coronal heating problem

Unsolved problem in physics:

Why is the Sun's corona so much hotter than the Sun's surface?

The coronal heating problem in solar physics relates to the question of why the temperature of the Sun's corona is millions of kelvins versus the thousands of kelvins of the surface. Several theories have been proposed to explain this phenomenon, but it is still challenging to determine which is correct. The problem first emerged after the identification of unknown spectral lines in the solar spectrum with highly ionized iron and calcium atoms. The comparison of the coronal and the photospheric temperatures of 6000K, leads to the question of how the 200-times-hotter coronal temperature can be maintained. The problem is primarily concerned with how the energy is transported up into the corona and then converted into heat within a few solar radii.

The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the second law of thermodynamics prevents heat from flowing directly from the solar photosphere (surface), which is at about 5800K, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10MK).

Between the photosphere and the corona, the thin region through which the temperature increases is known as the transition region. It ranges from only tens to hundreds of kilometers thick. Energy cannot be transferred from the cooler photosphere to the corona by conventional heat transfer as this would violate the second law of thermodynamics. An analogy of this would be a light bulb raising the temperature of the air surrounding it to something greater than its glass surface. Hence, some other manner of energy transfer must be involved in the heating of the corona.

The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region. It is about 1 kilowatt for every square meter of surface area on the Sun's chromosphere, or 1/40000 of the amount of light energy that escapes the Sun.

Many coronal heating theories have been proposed, but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares). Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.

In 2012, high resolution (<0.2″) soft X-ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) is assumed to originate from MHD waves.

NASA's Parker Solar Probe is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind. It was successfully launched on August 12, 2018 and as of fall 2022 has completed the first 13 of more than 20 planned close approaches to the Sun.

Competing theories of heating mechanisms

Heating models
Hydrodynamic	Magnetic
No magnetic field Slow rotating stars	DC (reconnection)	AC (waves)
	B-field stresses Reconnection events Flares-nanoflares Uniform heating rates	Photospheric foot point shuffling MHD wave propagation High Alfvén wave flux Non-uniform heating rates

Wave heating theory

The wave heating theory, proposed in 1949 by Évry Schatzman, proposes that waves carry energy from the solar interior to the solar chromosphere and corona. The Sun is made of plasma rather than ordinary gas, so it supports several types of waves analogous to sound waves in air. The most important types of wave are magneto-acoustic waves and Alfvén waves. Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to ultra low frequency radio waves that have been modified by interaction with matter in the plasma. Both types of waves can be launched by the turbulence of granulation and super granulation at the solar photosphere, and both types of waves can carry energy for some distance through the solar atmosphere before turning into shock waves that dissipate their energy as heat.

One problem with wave heating is delivery of the heat to the appropriate place. Magneto-acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona, both because of the low pressure present in the chromosphere and because they tend to be reflected back to the photosphere. Alfvén waves can carry enough energy, but do not dissipate that energy rapidly enough once they enter the corona. Waves in plasmas are notoriously difficult to understand and describe analytically, but computer simulations, carried out by Thomas Bogdan and colleagues in 2003, seem to show that Alfvén waves can transmute into other wave modes at the base of the corona, providing a pathway that can carry large amounts of energy from the photosphere through the chromosphere and transition region and finally into the corona where it dissipates it as heat.

Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the Solar and Heliospheric Observatory space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet (EUV) for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a 1000second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.

It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as 100mHz (10 second period). Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as 200Hz, well into the range of human hearing. These waves are very difficult to detect under normal circumstances, but evidence collected during solar eclipses by teams from Williams College suggest the presences of such waves in the 1–10Hz range.

Recently, Alfvénic motions have been found in the lower solar atmosphere and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory. These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft.

Solar wind observations with the Wind spacecraft have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.

Magnetic reconnection theory

Main article: Magnetic reconnection

Arcing active region by Solar Dynamics Observatory

The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona. The currents then collapse suddenly, releasing energy as heat and wave energy in the corona. This process is called "reconnection" because of the peculiar way that magnetic fields behave in plasma (or any electrically conductive fluid such as mercury or seawater). In a plasma, magnetic field lines are normally tied to individual pieces of matter, so that the topology of the magnetic field remains the same: if a particular north and south magnetic pole are connected by a single field line, then even if the plasma is stirred or if the magnets are moved around, that field line will continue to connect those particular poles. The connection is maintained by electric currents that are induced in the plasma. Under certain conditions, the electric currents can collapse, allowing the magnetic field to "reconnect" to other magnetic poles and release heat and wave energy in the process.

Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in the Solar System. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–1000km across. These small magnetic poles are buffeted and churned by the constant granulation. The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this "magnetic carpet", so the energy released by the reconnection is a natural candidate for the coronal heat, perhaps as a series of "microflares" that individually provide very little energy but together account for the required energy.

The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial. In particular, ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light, but there seem to be too few of these small events to account for the energy released into the corona. The additional energy not accounted for could be made up by wave energy, or by gradual magnetic reconnection that releases energy more smoothly than micro-flares and therefore does not appear well in the TRACE data. Variations on the micro-flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.

Spicules (type II)

For decades, researchers believed spicules could send heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so the theory was discounted.

As per studies performed in 2010 at the National Center for Atmospheric Research in Colorado, in collaboration with the Lockheed Martin's Solar and Astrophysics Laboratory (LMSAL) and the Institute of Theoretical Astrophysics of the University of Oslo, a new class of spicules (TYPE II) discovered in 2007, which travel faster (up to 100 km/s) and have shorter lifespans, can account for the problem. These jets insert heated plasma into the Sun's outer atmosphere.

Thus, a much greater understanding of the corona and improvement in the knowledge of the Sun's subtle influence on the Earth's upper atmosphere can be expected henceforth. The Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite which was used to test this hypothesis. The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply.

These observations reveal a one-to-one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona.

Corona discharge

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Corona_discharge 

Long exposure photograph of corona discharge on an insulator string of a 500 kV overhead power line. Corona discharges represent a significant power loss for electric utilities.

The corona discharge around a high-voltage coil

Corona discharge from a spoon attached to the high voltage terminal of a Tesla coil.

Large corona discharges (white) around conductors energized by a 1.05 million volt transformer in a U.S. NIST laboratory in 1941

A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them (St. Elmo's fire).

In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O₃) and nitric oxide (NO), and in turn, nitrogen dioxide (NO₂), and thus nitric acid (HNO₃) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.

Corona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.

Introduction

A variety of forms of corona discharge, from various metal objects. Notice, especially in the last two pictures, how the discharge is concentrated at the points on the objects.

A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.

When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.

Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge"—an electric arc. A corona forms only when the conductor is widely enough separated from conductors at the opposite potential that an arc cannot jump between them. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.

Corona discharge forms only when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at sea level pressure of 101 kPa, the critical value is roughly 30 kV/cm, but this decreases with pressure, therefore, corona discharge is more of a problem at high altitudes. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in the air, corona can start at potentials of 2–6 kV. In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses. Corona rings are often added to insulators of high voltage transmission lines.

Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly curved electrode. If the curved electrode is positive with respect to the flat electrode, it has a positive corona; if it is negative, it has a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.

An important reason for considering coronas is the production of ozone around conductors undergoing corona processes in air. A negative corona generates much more ozone than the corresponding positive corona.

Applications

Corona discharge has a number of commercial and industrial applications:

• Removal of unwanted electric charges from the surface of aircraft in flight and thus avoiding the detrimental effect of uncontrolled electrical discharge pulses on the performance of avionic systems
• Manufacture of ozone
• Sanitization of pool water
• In an electrostatic precipitator, removal of solid pollutants from a waste gas stream, or scrubbing particles from the air in air-conditioning systems
• Photocopying
• Air ionisers
• Production of photons for Kirlian photography to expose photographic film
• EHD thrusters, lifters, and other ionic wind devices
• Nitrogen laser
• Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion mobility spectrometer
• Static charge neutralization, as applied through antistatic devices like ionizing bars
• Refrigeration of electronic devices by forced convection

Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles onto oppositely charged plates.

The free radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone.

Problems

Corona discharges on the 380kV overhead power line over the Albula Pass (Switzerland) in foggy weather conditions (long time exposure 30 seconds).

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. Therefore, power transmission equipment is designed to minimize the formation of corona discharge.

Corona discharge is generally undesirable in:

• Electric power transmission, where it causes:
  • Power loss
  • Audible noise
  • Electromagnetic interference
  • Purple glow
  • Ozone production
  • Insulation damage
  • Possible distress in animals that are sensitive to ultraviolet light
• Electrical components such as transformers, capacitors, electric motors, and generators:
  • Corona can progressively damage the insulation inside these devices, leading to equipment failure
  • Elastomer items such as O-rings can suffer ozone cracking
  • Plastic film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization

In many cases, coronas can be suppressed by corona rings, toroidal devices that serve to spread the electric field over a larger areas and decrease the field gradient below the corona threshold.

Mechanism

Corona discharge occurs when the electric field is strong enough to create a chain reaction; electrons in the air collide with atoms hard enough to ionize them, creating more free electrons that ionize more atoms. The diagrams below illustrate at a microscopic scale the process which creates a corona in the air next to a pointed electrode carrying a high negative voltage with respect to ground. The process is:

1. A neutral atom or molecule, in a region of the strong electric field (such as the high potential gradient near the curved electrode), is ionized by a natural environmental event (for example, being struck by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.
   
2. The electric field accelerates these oppositely charged particles in opposite directions, separating them, preventing their recombination, and imparting kinetic energy to each of them.
3. The electron has a much higher charge/mass ratio and so is accelerated to a higher velocity than the positive ion. It gains enough energy from the field that when it strikes another atom it ionizes it, knocking out another electron, and creating another positive ion. These electrons are accelerated and collide with other atoms, creating further electron/positive-ion pairs, and these electrons collide with more atoms, in a chain reaction process called an electron avalanche. Both positive and negative coronas rely on electron avalanches. In a positive corona, all the electrons are attracted inward toward the nearby positive electrode and the ions are repelled outwards. In a negative corona, the ions are attracted inward and the electrons are repelled outwards.
   
4. The glow of the corona is caused by electrons recombining with positive ions to form neutral atoms. When the electron falls back to its original energy level, it releases a photon of light. The photons serve to ionize other atoms, maintaining the creation of electron avalanches.
   
5. At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this, the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.

Thermodynamically, a corona is a very nonequilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1–3 ev, the rest of the surrounding gas is close to ambient temperature.

The onset voltage of corona or corona inception voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas.

Positive coronas

Properties

A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though many of the emissions are in the ultraviolet. The uniformity of the plasma is caused by the homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions.

A positive corona has a much lower density of free electrons compared to a negative corona; perhaps a thousandth of the electron density, and a hundredth of the total number of electrons. However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of the high potential gradient (and therefore the electrons have high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires high activation energy, positive coronas may support a greater reaction constant than corresponding negative coronas; though the total number of electrons may be lower, the number of very high energy electrons may be higher.

Coronas are efficient producers of ozone in the air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to increased production.

Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.

Mechanism

As with a negative corona, a positive corona is initiated by an exogenous ionization event in a region of a high potential gradient. The electrons resulting from the ionization are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.

In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionization of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begins the process of creating further avalanches inside the plasma.

Negative coronas

Properties

A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of the corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it. The total number of electrons and electron density is much greater than in the corresponding positive corona. However, they are of predominantly lower energy, owing to being in a region of lower potential gradient. Therefore, whilst for many reactions, the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require higher electron energy may take place at a lower rate.

Mechanism

Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionization event generating a primary electron, followed by an electron avalanche.

Electrons ionized from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionization energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionized neutral gas as a source of ionization is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.

Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation.

The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward.

A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as oxygen and water vapor), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'.

Electrical wind

Corona discharge on a Wartenberg wheel

Ionized gases produced in a corona discharge are accelerated by the electric field, producing a movement of gas or electrical wind. The air movement associated with a discharge current of a few hundred microamperes can blow out a small candle flame within about 1 cm of a discharge point. A pinwheel, with radial metal spokes and pointed tips bent to point along the circumference of a circle, can be made to rotate if energized by a corona discharge; the rotation is due to the differential electric attraction between the metal spokes and the space charge shield region that surrounds the tips.

St. Elmo's fire

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/St._Elmo%27s_fire

Illustration of St. Elmo's fire on a ship at sea

Electrostatic discharge flashes across the windscreen of a KC-10 cockpit

St. Elmo's fire (also called witchfire or witch's fire) is a weather phenomenon in which luminous plasma is created by a corona discharge from a rod-like object such as a mast, spire, chimney, or animal horn in an atmospheric electric field. It has also been observed on the leading edges of airplanes, as in the case of British Airways Flight 009, and by U.S. Air Force pilots.

The intensity of the effect, a blue or violet glow around the object, often accompanied by a hissing or buzzing sound, is proportional to the strength of the electric field and therefore noticeable primarily during thunderstorms or volcanic eruptions.

St. Elmo's fire is named after St. Erasmus of Formia (also known as St. Elmo), the patron saint of sailors. The phenomenon, which can warn of an imminent lightning strike, was regarded by sailors with awe and sometimes considered to be a good omen.

Cause

St. Elmo's fire is a reproducible and demonstrable form of plasma. The electric field around the affected object causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Conditions that can generate St. Elmo's fire are present during thunderstorms, when high-voltage differentials are present between clouds and the ground underneath. A local electric field of about 100 kV/m is required to begin a discharge in moist air. The magnitude of the electric field depends greatly on the geometry (shape and size) of the object. Sharp points lower the necessary voltage because electric fields are more concentrated in areas of high curvature, so discharges preferentially occur and are more intense at the ends of pointed objects.

The nitrogen and oxygen in the Earth's atmosphere cause St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow, albeit at a different colour due to the different gas involved.

In 1751, Benjamin Franklin hypothesized that a pointed iron rod would light up at the tip during a lightning storm, similar in appearance to St. Elmo's fire.

In an August 2020 paper, researchers in MIT's Department of Aeronautics and Astronautics demonstrated that St. Elmo's fire behaves differently in airborne objects versus grounded structures. They show that electrically isolated structures accumulating charge more effectively in high wind, in contrast to the corona discharge observed in grounded structures.

In history and culture

• In ancient Greece, the appearance of a single instance of St. Elmo's fire was called Helene (Ancient Greek: Ἑλένη), literally meaning "torch", with two instances referred to as Castor and Pollux, names of the mythological twin brothers of Helen.
• After the medieval period, St. Elmo's fire was sometimes associated with the Greek element of fire, such as with one of Paracelsus's elementals, specifically the salamander, or, alternatively, with a similar creature referred to as an acthnici.
• Welsh mariners referred to St. Elmo's fire as canwyll yr ysbryd or canwyll yr ysbryd glân ("candles of the Holy Ghost" or the "candles of St. David").
• Russian sailors also historically documented instances of St. Elmo's fire, known as "Saint Nicholas" or "Saint Peter's lights", also sometimes called St. Helen's or St. Hermes' fire, perhaps through linguistic confusion.
• St. Elmo's fire is reported to have been seen during the Siege of Constantinople by the Ottoman Empire in 1453. It was reportedly seen emitting from the top of the Hippodrome. The Byzantines attributed it to a sign that the Christian God would soon come and destroy the conquering Muslim army. According to George Sphrantzes, it disappeared just days before Constantinople fell, ending the Byzantine Empire.
• Accounts of Magellan's first circumnavigation of the globe refer to St. Elmo's fire (calling it the body of St. Anselm) being seen around the fleet's ships multiple times off the coast of South America. The sailors saw these as favorable omens.
• En route to Nagasaki with the Fat Man atom bomb on 9 August 1945, the B-29 Bockscar experienced an uncanny luminous blue plasma forming around the spinning propellers, "as though we were riding the whirlwind through space on a chariot of blue fire."
• St Elmo's fire was seen during the 1955 Great Plains tornado outbreak in Kansas and Oklahoma.
• Among the phenomena experienced on British Airways Flight 9 on 24 June 1982, were glowing light flashes along the leading edges of the aircraft, including the wings and cockpit windscreen, which were seen by both passengers and crew. While the bright flashes of light shared similarities with St Elmo's fire, the glow experienced was from the impact of ash particles on the leading edges of the aircraft, similar to that seen by operators of sandblasting equipment.
• St. Elmo's fire was observed and its optical spectrum recorded during a University of Alaska research flight over the Amazon in 1995 to study sprites.
• Ill-fated Air France Flight 447 from Rio de Janeiro–Galeão International Airport to Paris Charles de Gaulle Airport in 2009 is understood to have experienced St. Elmo's fire 23 minutes prior to crashing into the Atlantic Ocean; however, the phenomenon was not a factor in the disaster.
• Apoy ni San Elmo – commonly shortened to santelmo – is a bad omen or a flying spirit in Filipino folklore, although the description for santelmo is more similar to ball lightning than St. Elmo's fire. There are various indigenous names for santelmo which has existed before the term santelmo was coined. The term santelmo originated from Spanish colonial rule in the Philippines.

Notable observations

Classical texts

St. Elmo's fire is referenced in the works of Julius Caesar (De Bello Africo, 47) and Pliny the Elder (Naturalis Historia, book 2, par. 101), Alcaeus frag. 34. Earlier, Xenophanes of Colophon had alluded to the phenomenon.

Zheng He

In 15th-century Ming China, Admiral Zheng He and his associates composed the Liujiagang and Changle inscriptions, the two epitaphs of the treasure voyages, where they made a reference to St. Elmo's fire as a divine omen of Tianfei (天妃), the goddess of sailors and seafarers.

The power of the goddess, having indeed been manifested in previous times, has been abundantly revealed in the present generation. In the midst of the rushing waters it happened that, when there was a hurricane, suddenly a divine lantern was seen shining at the masthead, and as soon as that miraculous light appeared the danger was appeased, so that even in the peril of capsizing one felt reassured and that there was no cause for fear.

— Admiral Zheng He and his associates (Changle inscription) 

Accounts associated with Magellan and da Gama

Mention of St. Elmo's fire can be found in Antonio Pigafetta's journal of his voyage with Ferdinand Magellan. St. Elmo's fire, also known as "corposants" or "corpusants" from the Portuguese corpo santo ("holy body"), is also described in The Lusiads, the epic account of Vasco da Gama's voyages of discovery.

Robert Burton

Robert Burton wrote of St. Elmo's fire in his Anatomy of Melancholy (1621): "Radzivilius, the Lithuanian duke, calls this apparition Sancti Germani sidus; and saith moreover that he saw the same after in a storm, as he was sailing, 1582, from Alexandria to Rhodes". This refers to the voyage made by Mikołaj Krzysztof "the Orphan" Radziwiłł in 1582–1584.

John Davis

On 9 May 1605, while on the second voyage of John Davis commanded by Sir Edward Michelborne to the East Indies, an unknown writer aboard the Tiger describes the phenomenon: "In the extremity of our storm appeared to us in the night, upon our maine Top-mast head, a flame about the bigness of a great Candle, which the Portugals call Corpo Sancto, holding it a most divine token that when it appeareth the worst is past. As, thanked be God, we had better weather after it".

Pierre Testu-Brissy

Pierre Testu-Brissy was a pioneering French balloonist. On 18 June 1786, he flew for 11 hours and made the first electrical observations as he ascended into thunderclouds. He stated that he drew remarkable discharges from the clouds by means of an iron rod carried in the basket. He also experienced Saint Elmo's fire.

William Bligh

William Bligh recorded in his log on Sunday 4 May 1788, on board HMS Bounty of 'Mutiny On The Bounty' fame: 'Corpo-Sant. Some electrical Vapour seen about the Iron at the Yard Arms about the Size of the blaze of a Candle.' The location of this event was in the South Atlantic sailing from Cape Horn, (having failed to round the cape in the winter months), en route to Cape of Good Hope and west of Tristan da Cunha. The log records the ship's location as: Latd. 42°:34'S, Longd (by the time keeper K2) as 34°:38'W. Reference: Log of the Proceedings of His Majestys Ship Bounty in a Voyage to the South Seas, (to take the Breadfruit plant from the Society Islands to the West Indies,) under the Command of Lieutenant William Bligh, 1 December 1787 – 22 October 1788 Safe 1/46, Mitchell Library, State Library of NSW

William Noah

William Noah, a silversmith convicted in London of stealing 2,000 pounds of lead, while en route to Sydney, New South Wales on the convict transport ship Hillsborough, recorded two such observations in his detailed daily journal. The first was in the Southern Ocean midway between Cape Town and Sydney and the second was in the Tasman Sea, a day out of Port Jackson:

26 June 1799: At 4 Began to Blow very Hard with Heavy Shower of Rain & Hail and Extraordinary Heavy Clap of Thunder & Lightning when fell a Cormesant [corposant] a Body of Fire which collect from the Lightning & Lodge itself in the Foretopmast Head where it was first seen by our Captain when followed a Heavy Clap of Thunder & Lightning which occasioned it to fall & Burst on the Main Deck the Electrific of the Bursting of this Ball of Fire had such power as to shake several of their Leg not only On the Main Deck as the fire Hung much round the smith Forge being Iron but had the same Effect on the Gun Deck & Orlop [deck] on several of the Convicts.
25 July 1799: We were now sourounded with Heavy Thunder & Lightning and the Dismal Element foaming all round us Shocking to see with a Cormesant Hanging at the Maintop mast Head the Seamen was here Shock'd when a flash of Lightning came Burst the Cormesant & Struck two of the Seamen for several Hours Stone Blind & several much hurt in their Eyes.

While the exact nature of these weather phenomena cannot be certain, they appear to be mostly about two observations of St. Elmo's fire with perhaps some ball lightning and even a direct lightning strike to the ship thrown into the mix.

James Braid

On 20 February 1817, during a severe electrical storm, James Braid, surgeon at Lord Hopetoun's mines at Leadhills, Lanarkshire, had an extraordinary experience whilst on horseback:

On Thursday 20th, I was gratified for a few minutes with the luminous appearance described above [viz., "such flashes of lightning from the west, repeated every two or three minutes, sometimes at shorter intervals, as appeared to illumine the whole heavens"]. It was about nine o'clock, P.M. I had no sooner got on horseback than I observed the tips of both the horse's ears to be quite luminous: the edges of my hat had the same appearance. I was soon deprived of these luminaries by a shower of moist snow which immediately began to fall. The horse's ears soon became wet and lost their luminous appearance; but the edges of my hat, being longer of getting wet, continued to give the luminous appearance somewhat longer.

I could observe an immense number of minute sparks darting towards the horse's ears and the margin of my hat, which produced a very beautiful appearance, and I was sorry to be so soon deprived of it.

The atmosphere in this neighbourhood appeared to be very highly electrified for eight or ten days about this time. Thunder was heard occasionally from 15th to 23rd, during which time the weather was very unsteady: frequent showers of hail, snow, rain, &c.

I can find no person in this quarter who remembers to have ever seen the luminous appearance mentioned above, before this season, – or such a quantity of lightning darting across the heavens, – nor who have heard so much thunder at that season of the year.

This country being all stocked with sheep, and the herds having frequent occasion to pay attention to the state of the weather, it is not to be thought that such an appearance can have been at all frequent, and none of them to have observed it.

— James Braid, 1817

Weeks earlier, reportedly on 17 January 1817, a luminous snowstorm occurred in Vermont and New Hampshire. Saint Elmo's fire appeared as static discharges on roof peaks, fence posts, and the hats and fingers of people. Thunderstorms prevailed over central New England.

Charles Darwin

Charles Darwin noted the effect while aboard the Beagle. He wrote of the episode in a letter to J. S. Henslow that one night when the Beagle was anchored in the estuary of the Río de la Plata:

Everything is in flames – the sky with lightning, the water with luminous particles, and even the very masts are pointed with a blue flame.

— Charles Darwin, 1832

He also describes the above night in his book The Voyage of the Beagle:

On a second night we witnessed a splendid scene of natural fireworks; the mast-head and yard-arm-ends shone with St.Elmo's light; and the form of the vane could almost be traced, as if it had been rubbed with phosphorous. The sea was so highly luminous, that the tracks of the penguins were marked by a fiery wake, and the darkness of the sky was momentarily illuminated by the most vivid lightning.

— Charles Darwin, 1832

Richard Henry Dana

In Two Years Before the Mast, Richard Henry Dana Jr., (1815–1882) describes seeing a corposant in the horse latitudes of the northern Atlantic Ocean. However, he may have been talking about ball lightning; as mentioned earlier, it is often erroneously identified as St. Elmo's fire:

The observation by R. H. Dana of this phenomenon in Two Years Before the Mast is a straightforward description of an extraordinary experience apparently only known to mariners and airline pilots.

There, directly over where we had been standing, upon the main top-gallant mast-head, was a ball of light, which the sailors name a corposant (corpus sancti), and which the mate had called out to us to look at. They were all watching it carefully, for sailors have a notion that if the corposant rises in the rigging it is a sign of fair weather, but if it comes lower down, there will be a storm. Unfortunately, as an omen, it came down, and showed itself on the topgallant yardarm. We were off the yard in good season, for it is held as a fatal sign to have the pale light of the corposant thrown upon one's face.

— Richard Henry Dana, 1840

Nikola Tesla

Nikola Tesla created St. Elmo's fire in 1899 while testing a Tesla coil at his laboratory in Colorado Springs, Colorado, United States. St. Elmo's fire was seen around the coil and was said to have lit up the wings of butterflies with blue halos as they flew around.

Mark Heald

A minute before the crash of the Luftschiffbau Zeppelin's LZ 129 Hindenburg on 6 May 1937, Professor Mark Heald (1892–1971) of Princeton saw St. Elmo's Fire flickering along the airship's back. Standing outside the main gate to the Naval Air Station, he watched, together with his wife and son, as the airship approached the mast and dropped her bow lines. A minute thereafter, by Heald's estimation, he first noticed a dim "blue flame" flickering along the backbone girder about one-quarter the length abaft the bow to the tail. There was time for him to remark to his wife, "Oh, heavens, the thing is afire," for her to reply, "Where?" and for him to answer, "Up along the top ridge" – before there was a big burst of flaming hydrogen from a point he estimated to be about one-third the ship's length from the stern.

William L. Laurence

St. Elmo's fire was reported by The New York Times reporter William L. Laurence on 9 August 1945, as he was aboard Bockscar on the way to Nagasaki.

I noticed a strange, eerie light coming through the window high above in the Navigator's cabin and as I peered through the dark all around us I saw a startling phenomenon. The whirling giant propellers had somehow become great luminous discs of blue flame. The same luminous blue flame appeared on the plexiglass windows in the nose of the ship, and on the tips of the giant wings it looked as though we were riding the whirlwind through space on a chariot of blue fire. It was, I surmised, a surcharge of static electricity that had accumulated on the tips of the propellers and on the dielectric material in the plastic windows. One's thoughts dwelt anxiously on the precious cargo in the invisible ship ahead of us. Was there any likelihood of danger that this heavy electric tension in the atmosphere all about us may set it off? I express my fears to Captain Bock, who seems nonchalant and imperturbed at the controls. He quickly reassures me: "It is a familiar phenomenon seen often on ships. I have seen it many times on bombing missions. It is known as St. Elmo's Fire."

In popular culture

In literature

One of the earliest references to the phenomenon appears in Alcaeus's Fragment 34a about the Dioscuri, or Castor and Pollux. It is also referenced in Homeric Hymn 33 to the Dioscuri who were from Homeric times associated with it. Whether the Homeric Hymn antedates the Alcaeus fragment is unknown.

The phenomenon appears to be described first in the Gesta Herwardi, written around 1100 and concerning an event of the 1070s. However, one of the earliest direct references to St. Elmo's fire made in fiction can be found in Ludovico Ariosto's epic poem Orlando Furioso (1516). It is located in the 17th canto (19th in the revised edition of 1532) after a storm has punished the ship of Marfisa, Astolfo, Aquilant, Grifon, and others, for three straight days, and is positively associated with hope:

But now St. Elmo's fire appeared, which they had so longed for, it settled at the bows of a fore stay, the masts and yards all being gone, and gave them hope of calmer airs.

— Ludovico Ariosto, 1516

In William Shakespeare's The Tempest (c. 1623), Act I, Scene II, St. Elmo's fire acquires a more negative association, appearing as evidence of the tempest inflicted by Ariel according to the command of Prospero:

PROSPERO

Hast thou, spirit,

Perform'd to point the tempest that I bade thee?

ARIEL

To every article.

I boarded the king's ship; now on the beak,

Now in the waist, the deck, in every cabin,

I flamed amazement: sometime I'd divide,

And burn in many places; on the topmast,

The yards and bowsprit, would I flame distinctly,

Then meet and join.

— Act I, Scene II, The Tempest

The fires are also mentioned as "death fires" in Samuel Taylor Coleridge's The Rime of the Ancient Mariner:

About, about, in reel and rout,

The death fires danced at night;

The water, like a witch's oils,

Burnt green and blue and white.

— l. 127–130

Later in the 18th and 19th centuries, literature associated St. Elmo's fire with a bad omen or divine judgment, coinciding with the growing conventions of Romanticism and the Gothic novel. For example, in Ann Radcliffe's The Mysteries of Udolpho (1794), during a thunderstorm above the ramparts of the castle:

"And what is that tapering of light you bear?" said Emily, "see how it darts upwards,—and now it vanishes!"

"This light, lady," said the soldier, "has appeared to-night as you see it, on the point of my lance, ever since I have been on watch; but what it means I cannot tell."

"This is very strange!" said Emily.

"My fellow-guard," continued the man, "has the same flame on his arms; he says he has sometimes seen it before...he says it is an omen, lady, and bodes no good."

"And what harm can it bode?" rejoined Emily.

"He knows not so much as that, lady."

— Vol. III, Ch. IV, The Mysteries of Udolpho

In the 1864 novel Journey to the Centre of the Earth by Jules Verne, the author describes the fire occurring while sailing during a subterranean electrical storm (chapter 35, page 191):

On the mast already I see the light play of a lambent St. Elmo's fire; the outstretched sail catches not a breath of wind, and hangs like a sheet of lead.

In Herman Melville's novel Moby-Dick, Starbuck points out "corpusants" during a thunder storm in the Japanese sea in chapter 119, "The Candles".

St. Elmo's fire makes an appearance in The Adventures of Tintin comic, Tintin in Tibet, by Hergé. Tintin recognizes the phenomenon on Captain Haddock's ice-axe.

The phenomenon appears in the first stanza of Robert Hayden's poem "The Ballad of Nat Turner"; it is also referred to with the term "corposant" in the first section of his long poem "Middle Passage".

In Kurt Vonnegut's Slaughterhouse-Five, Billy Pilgrim sees the phenomenon on soldiers' helmets and on rooftops. Vonnegut's The Sirens of Titan also notes the phenomenon affecting Winston Niles Rumfoord's dog, Kazak, the Hound of Space, in conjunction with solar disturbances of the chrono-synclastic infundibulum.

In Robert Aickman's story "Niemandswasser" (1975), the protagonist, Prince Albrecht von Allendorf, is "known as Elmo to his associates, because of the fire which to them emanated from him". "There was an inspirational force in Elmo of which the sensitive soon became aware, and which had led to his Spottname or nickname."

In On the Banks of Plum Creek by Laura Ingalls Wilder, St. Elmo's fire is seen by the girls and Ma during one of the blizzards. It was described as coming down the stove pipe and rolling across the floor following Ma's knitting needles; it did not burn the floor (pages 309–310). The phenomenon as described, however, is more similar to ball lightning.

In Voyager, the third major novel in Diana Gabaldon's popular Outlander series, the primary characters experience St. Elmo's fire while lost at sea in a thunderstorm between Hispaniola and coastal Georgia.

St. Elmo's fire is also mentioned in the novel, Castaways of the Flying Dutchman by Brian Jacques.

It is referenced multiple times in the novel Pet Sematary by Stephen King.

In television

On the children's television series The Mysterious Cities of Gold (1982), episode four shows St. Elmo's fire affecting the ship as it sailed past the Strait of Magellan. The real-life footage at the end of the episode has snippets of an interview with Japanese sailor Fukunari Imada, whose comments were translated to: "Although I've never seen St. Elmo's fire, I'd certainly like to. It was often considered a bad omen, as it played havoc with compasses and equipment". The TV series also referred to St. Elmo's fire as being a bad omen during the cartoon. The footage was captured as part of his winning solo yacht race in 1981.

On the American television series Rawhide, in a 1959 episode titled "Incident of the Blue Fire", cattle drovers on a stormy night see St. Elmo's fire glowing on the horns of their steers, which the men regard as a deadly omen. St. Elmo's fire is also referenced in a 1965 episode of Bonanza in which religious pilgrims staying on the Cartwright property believe an experience with St. Elmo's fire is the work of Satan.

On the Netflix original animated series Trese (2021), the Santelmo (St. Elmo's Fire) is one of the protagonist's, Alexandra Trese's, allies whom she contacts using her old Nokia phone, dialing the date of the Great Binondo fire, 0003231870.

In film

• In Moby Dick (1956), St. Elmo's fire stops Captain Ahab from killing Starbuck.
• In The Last Sunset (1961), outlaw/cowhand Brendan "Bren" O'Malley (Kirk Douglas) rides in from the herd and leads the recently widowed Belle Breckenridge (Dorothy Malone) to an overview of the cattle. As he takes the rifle from her, he proclaims, "Something out there, you could live five lifetimes, and never see again," the audience is then shown a shot of the cattle with a blue or violet glow coming from their horns. "Look. St. Elmo's fire. Never seen it except on ships," O'Malley says as Belle says, "I've never seen it anywhere. What is it?" Trying to win her back, he says, "Well, a star fell and smashed and scattered its glow all over the place."
• In St. Elmo's Fire (1985), Rob Lowe's character Billy Hicks erroneously claims that the phenomenon is "not even a real thing."
• In the Western miniseries Lonesome Dove (1989–1990), lightning strikes a herd of cattle during a storm, causing their horns to glow blue.
• In Lars von Trier's 2011 film Melancholia, the phenomenon features in the opening sequence and later in the film as the rogue planet Melancholia approaches Earth for an impact event.
• In Robert Eggers's 2019 horror film The Lighthouse, it appears in reference to the mysterious salvation that lighthouse keeper Thomas Wake (Willem Dafoe) is hiding from Ephraim Winslow (Robert Pattinson) inside the Fresnel lens of the lantern.

In music

• Brian Eno's third studio album Another Green World (1975) contains a song titled "St. Elmo's Fire" in which guesting King Crimson guitarist Robert Fripp (credited with playing "Wimshurst guitar" in the liner notes) improvises a lightning-fast solo that would imitate an electrical charge between two poles on a Wimshurst high-voltage generator.
• "St. Elmo's Fire (Man in Motion)" is a song recorded by John Parr. It hit number one on the Billboard Hot 100 on 7 September 1985, remaining there for two weeks. It was the main theme for Joel Schumacher's 1985 film St. Elmo's Fire.
• "St. Elmo's Fire" by Michael Franks.
• The Sammarinese entry for the 2017 Eurovision Song Contest in Kyiv "Spirit of the Night" contains references to St. Elmo's Fire.
• "St. Elmo's Fire" is mentioned in Bowling For Soup's 2004 song "1985". The lyrics contain, "She's seen all the classics, she knows every line / Breakfast Club, Pretty In Pink, even St. Elmo's Fire."