Solar Corona

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During a total solar eclipse, the solar corona can be seen by the naked eye.

A corona (Latin, 'crown') is an aura of plasma that surrounds the Sun and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word "corona" is a Latin word meaning "crown", from the Ancient Greek κορώνη (korōnē, “garland, wreath”).

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". These spectral features have since been traced to highly ionized iron (Fe-XIV). Bengt Edlén, following the work of Grotrian (1939), first identified the coronal lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green FeXIV line at 5303 Å, but also the red line FeX at 6374 Å). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvin.^[1]

Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures 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.^[2]

Physical features

A drawing demonstrating the 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 photosphere's average temperature is 5800 kelvin compared to the corona's one to three million kelvin. The corona is 10⁻¹² times as dense as the photosphere, and so 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 induction by the Sun's magnetic field and MHD waves from below. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating 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 a differential rotation at the solar equator (the equator rotates quicker than the poles), 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 dark (when compared to the solar disk) spots.

Since the corona has been photographed at high resolution in the X-rays 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 very various and complex: different zones have been immediately classified on the coronal disc.^[3][4][5] The astronomers usually distinguish several regions,^[6] as described below.

Active regions

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 particle per cm³.

Illustration depicting solar prominences and sunspots

Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights on the Sun's surface:^[6] sunspots and faculae, happening in the photosphere, spicules, Hα filaments and plages in the chromosphere, prominences in the chromosphere and transition region, and flares and coronal mass ejections happening in the corona and chromosphere, but if flares are very violent can perturb also the photosphere and generate a Moreton wave, as described by Uchida. On the contrary, quiescent prominences are large, cool dense structures which are observed as dark, "snake-like" Hα ribbons (filaments) on the solar disc. Their temperature is about 5000–8000 K, 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.^[7]

Coronal loops

TRACE 171Å 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 hole (polar) regions and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma.^[8] Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and coronal mass ejections (CMEs).

Solar plasma feeding these structures is heated from under 6000 K to well over 1×10⁶ K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one foot point and drain from the other (siphon flow due to a pressure difference,^[9] or asymmetric flow due to some other driver).

When the plasma goes upward from the foot points towards the loop top, as it always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools falling down towards the photosphere, we have the 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), creating dark filaments in the solar disk or prominences off the limb.

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

Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE; they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the LOS). In-situ measurements are required before a definitive answer can be arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time being). The next mission of the NASA Solar Probe Plus will approach the Sun very closely allowing more direct observations.

Coronal arches connecting regions of opposite magnetic polarity (A) and the unipolar magnetic field in the coronal hole (B)

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.^[10]

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, in 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 as sources of the slow solar wind.^[11]

Filament cavities

Image taken by the Solar Dynamics Observatory on Oct 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.^[10]

Filament cavities are cooler clouds of gases (plasma) suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in the 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 can be calculated by the state equation of a perfect gas , where is the particle number density, the Boltzmann constant and the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect makes sunspots dark in the photosphere.

Bright points

Bright points are small active regions spread over the whole solar disk. X-ray bright points were first detected in April 8, 1969 during a rocket flight.^[10]

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.^[12]

Coronal holes

Coronal holes are the Polar Regions which look dark in the X-rays since they do not emit much radiation.^[13] 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 exactly, they are long thin streamers that project outward from the Sun's north and south poles.^[14]

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 velocity rotation 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 lower 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 the coronal holes.

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 in times very different among them. Studying the 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 10,000 seconds	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	100–1,000
Coronal hole	several rotations	100–1,000

Flares

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

Flares take place in active regions and provoke a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they interest several zones of the solar atmosphere and involve 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, even if the most energetic events can last several hours. Flares involve 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 EUV wavelengths and in the X-rays, typical of the chromospheric and coronal emission.

In the corona the morphology of flares, which can be grasped from the observations in the soft and hard X-rays, at the UV wavelengths and in Hα, is very complex. However, two kinds of basic structures can be distinguished: ^[15]

• 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 to eruptions of prominences, transients in white light and two-ribbon flares:^[16] in this case the magnetic loops change their configuration during the event. The energies emitted during these flares of such large proportions 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. These times, moreover, can depend on the range of wavelengths used to observe the event even considerably:

• An initial impulsive phase, whose duration is of the order of minutes, strong emissions of energy are often observed even in the microwaves, at EUV wavelengths and in the hard X-rays.
• 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.

Transients

Accompanying solar flares or large solar prominences, "coronal transients" (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material traveling 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 ejections can propel hundreds of millions of tons of material into space at roughly 1.5 million kilometers an hour.

A solar storm

These movies have been taken by the satellite SOHO during two weeks in October and November 2003. The images have been taken at the same time by the different instruments on board SOHO: the MDI, producing magnetograms, the Extreme ultraviolet Imaging Telescope (EIT), which photographs the corona in the ultraviolets, and the Large Angle and Spectrometric Coronagraph (LASCO).

The first video at the top on the left (in grey) shows the magnetograms as they vary in time. At the top on the right (in yellow) the photosphere can be seen in white light as taken by the MDI.

Furthermore the EIT filmed the event in its four filters which are sensitive to different wavelengths, selecting plasma at different temperatures. The images in orange (on the left) refers to chromospheric plasma, while that one in green (on the right) to the corona.

In the last movie at the centre the Sun's images taken in the ultraviolet filter by the EIT have been combined with those taken by the coronograph LASCO blue and white in this movie.

All the instruments registered the storm which is considered as one of the largest solar activity events observed by SOHO and maybe since the advent of space-based solar observations. The storm involved all the plasma of the solar atmosphere from the chromosphere to the corona, as can be seen from the movies, which are ordered from left to right, from top to bottom, in the outward direction of the increasing temperature on the Sun: photosphere (yellow), chromosphere-transition region (orange), low corona (green) and extended corona (blue).

The corona is visible to the SOHO/LASCO coronagraph instruments, which block the bright disk of the Sun so the significantly fainter corona can be seen. In this movie, the inner coronagraph (designated C2) is combined with the outer coronagraph (C3).

As the movie plays, we can observe a number of features of the active Sun. Long streamers radiate outward from the Sun and wave gently due to their interaction with the solar wind. The bright white regions are visible due to their high density of free electrons which scatter the light from the photosphere towards the observer. Protons and other ionized atoms are there as well, but are not as visible since they do not interact with photons as strongly as electrons. Coronal Mass Ejections (CMEs) are occasionally observed launching from the Sun. Some of these launch particle events can saturate the cameras with snow-like artifacts.

Also visible in the coronagraphs are stars and planets. Stars are seen to drift slowly to the right, carried by the relative motion of the Sun and the Earth. The planet Mercury is visible as the bright point moving left of the Sun.

The horizontal "extension" in the image is called blooming and is due to charge leakage during readout of saturated pixels in the camera's CCD imager.

Stellar coronae

Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung–Russell diagram.^[17] 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.^[17]

The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group^[18] showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun. The O-B stars, which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronas, 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

Taken by Hinode on Jan 12 2007 this image 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 Kelvins) and at very low density (of the order of 10¹⁵ particle/m³). According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.

The composition is the same as the one in the Sun's interior, mainly hydrogen, but completely ionized, thence protons and electrons, and a small fraction of the other atoms in the same percentages as they are present in the photosphere. Even 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. 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 photosphere is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximatively 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,^[19] even if it is not still clear how they can be transmitted or generated in the corona.

Radiation

The corona emits radiation mainly in the X-rays, observable only from space.
The plasma is transparent to its own radiation and to that one coming from below, therefore we say that it is optically-thin. The gas, in fact, is very rarefied and the photon mean free-path overcomes by far all the other length-scales, including the typical sizes of the coronal features.

Different processes of radiation take place in the emission, due to binary collisions between plasma particles, while the interactions with the photons, coming from below; are very rare. Because the emission is due to collisions between ions and electrons, the energy emitted from a unit volume in the time unit is proportional to the squared number of particles in a unit volume, or more exactly, to the product of the electron density and proton density.^[20]

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 (195 Å—green), 60,000–80,000 °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.^[21] 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^[21]

where is the Boltzmann constant, is the temperature in Kelvin, the electron mass, the electric charge of the electron,

the Coulomb logarithm, and

the Debye length of the plasma with particle density . The Coulomb logarithm 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 the heat for a volume unit, expressed in J m⁻³, the Fourier equation of heat transfer, to be computed only along the direction of the field line, becomes
.
Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.

Coronal seismology

Coronal seismology is a new way of studying the plasma of the solar corona with the use of magnetohydrodynamic (MHD) waves. Magnetohydrodynamics 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

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A new visualisation technique can provide clues to the coronal heating problem.

The coronal heating problem in solar physics relates to the question of why the temperature of the Sun's corona is millions of kelvin higher than that of the surface. 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, or surface, at about 5800 K, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10 MK).

The thin region of temperature increase from the chromosphere to the corona is known as the transition region and can range from tens to hundreds of kilometers thick. An analogy of this would be a light bulb heating the air surrounding it hotter than its glass surface. The second law of thermodynamics would be broken.

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, or 1/40000 of the amount of light energy that escapes the Sun.

Many coronal heating theories have been proposed,^[22] but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares).^[23] Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.

The NASA mission 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.

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. The authors 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) in this case is supposed to be MHD waves.^[24]

Axions may hold the key to the Solar Corona heating problem.^[25]

Competing 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
	Competing theories

Wave heating theory

The wave heating theory, proposed in 1949 by Evry 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.^[26] Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to ULF 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 into the corona and then dissipate 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 SOHO 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 1,000 second 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 100 mHz (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 200 Hz, 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 1–10 Hz range.

Recently, Alfvénic motions have been found in the lower solar atmosphere ^{[27] [28]} and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory.^[29] These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft .^[30]

Solar wind observations with the WIND (spacecraft) have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.^[31]

Magnetic reconnection theory

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.^[32] 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 our solar system. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–1,000 km 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,^[33] 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 doesn't 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.^[34] 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.^[35]

Solar cycle

From Wikipedia, the free encyclopedia

400 year sunspot history

"The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013 so the official maximum will be at least this high. The smoothed sunspot number has been rising again towards this second peak over the last five months and has now surpassed the level of the first peak (66.9 in February 2012). Many cycles are double peaked but this is the first in which the second peak in sunspot number was larger than the first. We are currently over five years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906."^[1] The monthly sunspot number was still rising as of March 2014.^[2]

The solar cycle (or solar magnetic activity cycle) is the periodic change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (visible in changes in the number of sunspots, flares, and other visible manifestations). Solar cycles have an average duration of about 11 years. They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for hundreds of years.

Solar variation causes changes in space weather, weather, and climate on Earth. It causes a periodic change in the amount of irradiation from the Sun that is experienced on Earth.

It is one component of solar variation, the other being aperiodic fluctuations.

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Evolution of magnetism on the Sun.

Powered by a hydromagnetic dynamo process, driven by the inductive action of internal solar flows, the solar cycle:

• Structures the Sun's atmosphere, its corona and the wind;
• Modulates the solar irradiance;
• Modulates the flux of short-wavelength solar radiation, from ultraviolet to X-ray;
• Modulates the occurrence frequency of solar flares, coronal mass ejections, and other geoeffective solar eruptive phenomena;
• Indirectly modulates the flux of high-energy galactic cosmic rays entering the solar system.

History

Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots

Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century

The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk.^[3] Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today.

Until recently it was thought that there were 28 cycles in the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784–1799) seems actually to have been two cycles,^[4][5] so that the average length is only around 10.66 years. Cycles as short as 9 years and as long as 14 years have been observed, and in the double cycle of 1784-1799 one of the two component cycles had to be less than 8 years in length. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.

Following the numbering scheme established by Wolf, the 1755–1766 cycle is traditionally numbered "1". The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing data.^[6] This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer. In the second half of the nineteenth century it was also noted (independently) by Richard Carrington and by Spörer that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by the husband-wife team of E. Walter and Annie Maunder in the early twentieth century (see graph below). Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-latitude diagram.

The sunspot butterfly diagram. This modern version is constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

The physical basis of the solar cycle was elucidated in the early twentieth century by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (this was the first detection of magnetic fields outside the Earth), and in 1919 went on to show that the magnetic polarity of sunspot pairs:

• Is always the same in a given solar hemisphere throughout a given sunspot cycle;
• Is opposite across hemispheres throughout a cycle;
• Reverses itself in both hemispheres from one sunspot cycle to the next.

Hale's observations revealed that the solar cycle is a magnetic cycle with an average duration of 22 years. However, because very nearly all manifestations of the solar cycle are insensitive to magnetic polarity, it remains common usage to speak of the "11-year solar cycle".

Half a century later, the father-and-son team of Harold Babcock and Horace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order a dipole; and that this dipole also undergoes polarity reversals with the same period as the sunspot cycle (see graph below). These various observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.

Time vs. solar latitude diagram of the radial component of the solar magnetic field, averaged over successive solar rotation. The "butterfly" signature of sunspots is clearly visible at low latitudes. Diagram constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

Phenomena, measurement, and causes

Spots from two adjacent cycles can co-exist for some time, and since it was discovered that the Sun reverses magnetic polarity from one solar cycle to the next, spots from different cycles can be distinguished by direction of their magnetic field. However, it takes some months before a definite decision can be made as to the true date of solar minimum. One of the principal authorities that determine the date of the solar minimum is SIDC (the Solar Influences Data Analysis Center), which is located in Belgium and works with agencies such as NASA and ESA.
The most important information today comes from SOHO (a project of international cooperation between ESA and NASA), such as the MDI magnetogram, where the solar "surface" magnetic field can be seen.

The basic causes of the solar variability and its cycles are still under debate, with some researchers suggesting a link with the tidal forces due to the gas giants Jupiter and Saturn,^[7][8] or due to the solar inertial motion.^[9][10] Another cause of Sun spots can be solar jet stream "torsional oscillation".

Patterns have been noted in solar cycles. For example, the Waldmeier effect is the phenomenon that cycles with larger maximum amplitudes tend to take less time to reach their maxima than cycles with smaller amplitudes;^[11] there is also a negative correlation between maximum amplitudes and the lengths of earlier cycles, which allows a degree of prediction.^[12]

Effects of the solar cycle

Activity cycles 21, 22 and 23 seen in sunspot number index, TSI, 10.7cm radio flux, and flare index. The vertical scales for each quantity have been adjusted to permit overplotting on the same vertical axis as TSI. Temporal variations of all quantities are tightly locked in phase, but the degree of correlation in amplitudes is variable to some degree.

The Sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to a host of phenomena collectively known as solar activity. All of solar activity is strongly modulated by the solar magnetic cycle, since the latter serves as the energy source and dynamical engine for the former.

Surface magnetism

Sunspots may exist anywhere from a few days to a few months, but they eventually decay, and this releases magnetic flux in the solar photosphere. This magnetic field is dispersed and churned by turbulent convection, and solar large-scale flows. These transport mechanisms lead to the accumulation of the magnetized decay products at high solar latitudes, eventually reversing the polarity of the polar fields (notice how the blue and yellow fields reverse in the graph above).

The dipolar component of the solar magnetic field is observed to reverse polarity around the time of solar maximum, and reaches peak strength at the solar minimum. Sunspots, on the other hand, are produced from a strong toroidal (longitudinally directed) magnetic field within the solar interior. Physically, the solar cycle can be thought of as a regenerative loop where the toroidal component produces a poloidal field, which later produces a new toroidal component of sign such as to reverse the polarity of the original toroidal field, which then produces a new poloidal component of reversed polarity, and so on.

Total solar irradiance

The total solar irradiance (TSI) is the amount of solar radiative energy incident on the Earth's upper atmosphere. TSI variations were undetectable until satellite observations began in late 1978. A series of radiometers carried on satellites from the 1970s to the 2000s.^[13] TSI differed from 1360 to 1370 W/m2 across ten satellites. The controversial 1989-1991 “ACRIM gap” between non-overlapping satellites has been interpolated by an ACRIM composite showing +0.037%/decade rise by the ACRIM group, and a PMOD composite with a -0.008%/decade downward trend by the PMOD group.^[14] This 0.045%/decade difference strongly impacts climate models.
Satellite measurements show that solar irradiance varies systematically over the 11-year sunspot cycle,^[15] both in total irradiance and in the relative components of the irradiance (UV Light ratios to Visible Light Ratios). The solar luminosity is about 0.07 percent brighter during solar maximum than during solar minimum. Photospheric magnetism appears to be the primary cause (96%) of 1996-2013 TSI variation.^[16] Observations from spacecraft in the 2000s showed that the ratio of ultraviolet to visible light is much more variable than previously thought.^[17]

The major finding of satellite observations is that TSI varies in phase with the solar magnetic activity cycle^[18] with an amplitude of about 0.1% and an average value of about 1361.5 W/m^2[19] (the "solar constant"). Variations about the average up to −0.3% are caused by large sunspot groups and of +0.05% by large faculae and bright network on a week to 10-day timescale^[20] (see TSI variation graphics).^[21] TSI variations over the several decades of continuous satellite observation show small but detectable trends.^[22][23]

TSI is higher at solar maximum, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the "bright" network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler but less numerous sunspots. The primary driver of TSI changes on solar rotational and sunspot cycle timescales is the varying photospheric coverage of these radiatively active solar magnetic structures.

Short-wavelength radiation

A solar cycle: a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001. Credit: the Yohkoh mission of ISAS (Japan) and NASA (US).

With a temperature of 5870 kelvins, the photosphere of the Sun emits a very small proportion of radiation in the extreme ultraviolet (EUV) and above. However, hotter upper layers of the Sun's atmosphere (chromosphere and corona) emit more short-wavelength radiation. Since the upper atmosphere is not homogeneous and contains significant magnetic structure, the solar ultraviolet (UV), EUV and X-ray flux varies markedly in the course of the solar cycle.

The photo montage to the left illustrates this variation for soft X-ray, as observed by the Japanese satellite Yohkoh from after August 30, 1991, at the peak of cycle 22, to September 6, 2001, at the peak of cycle 23. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the SOHO or TRACE satellites.

Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.

Solar radio flux

Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions.^[24] The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7 cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or solar flux units (1 SFU = 10⁻²² W m⁻² Hz⁻¹). It represents a measure of diffuse, nonradiative heating of the coronal plasma trapped by magnetic fields over active regions. It is an excellent indicator of overall solar activity levels and correlates well with solar UV emissions.

The solar F10.7 index is measured daily at local noon in a bandwidth of 100 MHz centered on 2800 MHz at the Penticton site of the Dominion Radio Astrophysical Observatory (DRAO), Canada. The solar F10.7 cm record extends back to 1947, and is the longest direct record of solar activity available, other than sunspot-related quantities.^[25][26]

Sunspot activity has a major effect on long distance radio communications particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere.

It has been suggested that 10.7 cm solar flux could interfere with point-to-point terrestrial communications.^[27]

Geoeffective eruptive phenomena

An overview of three solar cycles shows the relationship between the sunspot cycle, galactic cosmic rays, and the state of our near-space environment.^[28]

The solar magnetic field structures the corona, giving it its characteristic shape visible at times of solar eclipses. Complex coronal magnetic field structures evolve in response to fluid motions at the solar surface, and emergence of magnetic flux produced by dynamo action in the solar interior. For reasons not yet understood in detail, sometimes these structures lose stability, leading to coronal mass ejections into interplanetary space, or flares, caused by sudden localized release of magnetic energy driving copious emission of ultraviolet and X-ray radiation as well as energetic particles. These eruptive phenomena can have a significant impact on Earth's upper atmosphere and space environment, and are the primary drivers of what is now called space weather.

The occurrence frequency of coronal mass ejections and flares is strongly modulated by the solar activity cycle. Flares of any given size are some 50 times more frequent at solar maximum than at minimum. Large coronal mass ejections occur on average a few times a day at solar maximum, down to one every few days at solar minimum. The size of these events themselves does not depend sensitively on the phase of the solar cycle. A good recent case in point are the three large X-class flares having occurred in December 2006, very near solar minimum; one of these (an X9.0 flare on Dec 5) stands as one of the brightest on record.^[29]

Cosmic ray flux

The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy cosmic rays entering the solar system from elsewhere in the galaxy. Since the frequency of solar eruptive events is strongly modulated by the solar cycle, the degree of cosmic ray scattering in the outer solar system varies in step. As a consequence, the cosmic ray flux in the inner solar system is anticorrelated with the overall level of solar activity. This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface.

A drawing of a sunspot in the Chronicles of John of Worcester.

Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents to cause occasionally nuclear spallation reactions. Some of the fission products include radionuclides such as ¹⁴C and ¹⁰Be, which settle down on Earth's surface. Their concentration can be measured in ice cores, allowing a reconstruction of solar activity levels into the distant past.^[30] Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that Maunder minimum-like epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.

Effects on Earth

Terrestrial organisms

The impact of the solar cycle on living organisms has been investigated (see chronobiology). Some researchers claim to have found connections with human health.^[31][32]
The amount of ultraviolet UVB light at 300 nm reaching the Earth varies by as much as 400% over the solar cycle due to variations in the protective ozone layer. In the stratosphere, ozone is continuously regenerated by the splitting of O₂ molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a decrease in the concentration of ozone, allowing increased UVB to penetrate to the Earth's surface.^[33]

Radio communication

Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) through the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays. This affects the path (propagation) of the radio wave in complex ways which can either facilitate or hinder local and long distance communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators, and shortwave broadcasters. 

These users utilize frequencies within the High Frequency or 'HF' radio spectrum which are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications.

Terrestrial climate

Both long-term and short-term variations in solar activity are hypothesized to affect global climate, but it has proven extremely challenging to directly quantify the link between solar variation and the earth’s climate.^[34] The topic continues to be a subject of active study.
Early research attempted to find a correlation between weather and sunspot activity, mostly without notable success.^[35] Later research has concentrated more on correlating solar activity with global temperature. Most recently, research suggests that there may also be regional climate impacts due to the solar cycle. Measurements from the Spectral Irradiance Monitor on NASA’s Solar Radiation and Climate Experiment show that solar UV output is more variable over the course of the solar cycle than scientists had previously thought, resulting in, for example, colder winters in the US and southern Europe and warmer winters in Canada and northern Europe during solar minima.^[36]

There are three suggested mechanisms by which solar variations are hypothesized to have an effect on climate:

• Solar irradiance changes directly affecting the climate ("Radiative forcing").
• Variations in the ultraviolet component. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might cause an effect on climate.
• Effects mediated by changes in cosmic rays (which are affected by the solar wind) such as changes in cloud cover.

The sunspot cycle variation of 0.1% has small but detectable effects on the Earth’s climate.^[37][38][39] Work by Camp and Tung suggests that changes in solar irradiance correlate with a variation of 0.18 K ±0.08 K (0.32 °F ±0.14 °F) in measured average global temperature between the peak and minimum of the 11-year solar cycle.^[40]

The effect of solar variation at time scales longer than a solar cycle is also of interest to climate science. The current scientific consensus is that solar variations do not play a major role in determining present-day global warming,^[34] since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases,^[41] but the level of understanding of solar impacts is low.^[42]

Effects on spacecraft

The coronal mass ejections ("CME") associated with solar flares produces a radiation flux of high-energy protons, sometimes known as solar cosmic rays. These can cause radiation damage to electronics and solar cells in satellites. The solar proton events also can cause single-event upset (SEU) events on electronics; at the same, the reduced flux of galactic cosmic radiation during solar maximum (see section "Cosmic ray flux" above) will decrease the high-energy component of particle flux.

If astronauts on a space mission are above the shielding effect produced by the Earth's magnetic field, the radiation from a CME would also be dangerous to humans; many future mission designs (e.g., for a Mars Mission) therefore incorporate a radiation-shielded "storm shelter" for astronauts to retreat to during such a radiation event.

In view of the problems in space flight occurring during high solar activity, prediction of the latter becomes more and more important. A particular method that relies on several consecutive cycles was established by Wolfgang Gleißberg.^[43]