CNO cycle

From Wikipedia, the free encyclopedia

 

Overview of the CNO-I Cycle

The CNO cycle (for carbon–nitrogen–oxygen) is one of the two (known) sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction. Unlike the latter, the CNO cycle is a catalytic cycle. Theoretical models^[which?] show that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun.^[1] The proton–proton chain is more important in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around 4×10⁶ K^[2] (4 megakelvins), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts occurring at approximately 15 MK, but its energy output rises much more rapidly with increasing temperatures.^[1] At approximately 17 MK, the CNO cycle starts becoming the dominant source of energy.^[3] The Sun has a core temperature of around 15.7 MK, and only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. The CNO-I process was independently proposed by Carl von Weizsäcker^[4] and Hans Bethe^[5] in 1938 and 1939, respectively.

In the CNO cycle, four protons fuse, using carbon, nitrogen and oxygen isotopes as a catalyst, to produce one alpha particle, two positrons and two electron neutrinos. Although there are various paths and catalysts involved in the CNO cycles, simply speaking all these cycles have the same net result:

4 1
1H  +  2 e−  →  4
2He  +  2 e+  +  2 e−  +  2 ν
e  +  3 γ  +  24.7 MeV  →  4
2He  +  2 ν
e  +  3 γ  +  26.7 MeV

The positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. One nucleus goes to become carbon, nitrogen, and oxygen isotopes through a number of transformations in an endless loop.

Cold CNO cycles

Under typical conditions found in stellar plasmas, catalytic hydrogen burning by the CNO cycles is limited by proton captures. Specifically, the timescale for beta decay of radioactive nuclei produced is faster than the timescale for fusion. Because of the long timescales involved, the cold CNO cycles convert hydrogen to helium slowly, allowing them to power stars in quiescent equilibrium for many years.

CNO-I

The first proposed catalytic cycle for the conversion of hydrogen into helium was at first simply called the carbon–nitrogen cycle (CN cycle), also honorarily referred to as the Bethe–Weizsäcker cycle, because it does not involve a stable isotope of oxygen. Bethe's original calculations suggested the CN-cycle was the Sun's primary source of energy, owing to the belief at the time that the Sun's composition was 10% nitrogen;^[5] the solar abundance of nitrogen is now known to be less than half a percent. This cycle is now recognized as the first part of the larger CNO nuclear burning network.

The main reactions of the CNO-I cycle are 12
6C→13
7N→13
6C→14
7N→15
8O→15
7N→12
6C:^[6]

12 6C	+	1 1H	→	13 7N	+	γ			+	1.95 MeV
13 7N			→	13 6C	+	e+	+	ν e	+	1.20 MeV	(half-life of 9.965 minutes[7])
13 6C	+	1 1H	→	14 7N	+	γ			+	7.54 MeV
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	1.73 MeV	(half-life of 122.24 seconds[7])
15 7N	+	1 1H	→	12 6C	+	4 2He			+	4.96 MeV

where the Carbon-12 nucleus used in the first reaction is regenerated in the last reaction. After the two positrons emitted annihilate with two ambient electrons producing an additional 2.04 MeV, the total energy released in one cycle is 26.73 MeV; it should be noted that in some texts, authors are erroneously including the positron annihilation energy in with the beta-decay Q-value and then neglecting the equal amount of energy released by annihilation, leading to possible confusion. All values are calculated with reference to the Atomic Mass Evaluation 2003.^[8]

The limiting (slowest) reaction in the CNO-I cycle is the proton capture on 14
7N; it was recently experimentally measured down to stellar energies, revising the calculated age of globular clusters by around 1 billion years.^[9]

The neutrinos emitted in beta decay will have a spectrum of energy ranges, because although momentum is conserved, the momentum can be shared in any way between the positron and neutrino, with either being emitted at rest and the other taking away the full energy, or anything in between, so long as all the energy from the Q-value is used. All momentum which get the electron and the neutrino together is not great enough to cause a significant recoil of the much heavier daughter nucleus and hence, its contribution to kinetic energy of the products, for the precision of values given here, can be neglected. Thus the neutrino emitted during the decay of nitrogen-13 can have an energy from zero up to 1.20 MeV, and the neutrino emitted during the decay of oxygen-15 can have an energy from zero up to 1.73 MeV. On average, about 1.7 MeV of the total energy output is taken away by neutrinos for each loop of the cycle, leaving about 25 MeV available for producing luminosity.^[10]

CNO-II

In a minor branch of the reaction, occurring in the Sun's inner part, the core, just 0.04% of the time, the final reaction shown above does not produce carbon-12 and an alpha particle, but instead produces oxygen-16 and a photon and continues 15
7N→16
8O→17
9F→17
8O→14
7N→15
8O→15
7N:

15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)
17 8O	+	1 1H	→	14 7N	+	4 2He			+	1.19 MeV
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)

Like the carbon, nitrogen, and oxygen involved in the main branch, the fluorine produced in the minor branch is merely an intermediate product and at steady state, does not accumulate in the star.

CNO-III

This subdominant branch is significant only for massive stars. The reactions are started when one of the reactions in CNO-II results in fluorine-18 and gamma instead of nitrogen-14 and alpha, and continues 17
8O→18
9F→18
8O→15
7N→16
8O→17
9F→17
8O:

17 8O	+	1 1H	→	18 9F	+	γ			+	5.61 MeV
18 9F			→	18 8O	+	e+	+	ν e	+	1.656 MeV	(half-life of 109.771 minutes)
18 8O	+	1 1H	→	15 7N	+	4 2He			+	3.98 MeV
15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)

CNO-IV

A proton reacts with a nucleus causing release of an alpha particle.

Like the CNO-III, this branch is also only significant in massive stars. The reactions are started when one of the reactions in CNO-III results in fluorine-19 and gamma instead of nitrogen-15 and alpha, and continues 19
9F→16
8O→17
9F→17
8O→18
9F→18
8O→19
9F:

19 9F	+	1 1H	→	16 8O	+	4 2He			+	8.114 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)
17 8O	+	1 1H	→	18 9F	+	γ			+	5.61 MeV
18 9F			→	18 8O	+	e+	+	ν e	+	1.656 MeV	(half-life of 109.771 minutes)
18 8O	+	1 1H	→	19 9F	+	γ			+	7.994 MeV

Hot CNO cycles

Under conditions of higher temperature and pressure, such as those found in novae and x-ray bursts, the rate of proton captures exceeds the rate of beta-decay, pushing the burning to the proton drip line. The essential idea is that a radioactive species will capture a proton more quickly than it can beta decay, opening new nuclear burning pathways that are otherwise inaccessible. Because of the higher temperatures involved, these catalytic cycles are typically referred to as the hot CNO cycles; because the timescales are limited by beta decays instead of proton captures, they are also called the beta-limited CNO cycles.

HCNO-I

The difference between the CNO-I cycle and the HCNO-I cycle is that 13
7N captures a proton instead of decaying, leading to the total sequence 12
6C→13
7N→14
8O→14
7N→15
8O→15
7N→12
6C:

12 6C	+	1 1H	→	13 7N	+	γ			+	1.95 MeV
13 7N	+	1 1H	→	14 8O	+	γ			+	4.63 MeV
14 8O			→	14 7N	+	e+	+	ν e	+	5.14 MeV	(half-life of 70.641 seconds)
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)
15 7N	+	1 1H	→	12 6C	+	4 2He			+	4.96 MeV

HCNO-II

The notable difference between the CNO-II cycle and the HCNO-II cycle is that 17
9F captures a proton instead of decaying, and helium is produced in a subsequent reaction on 18
9F, leading to the total sequence 15
7N→16
8O→17
9F→18
10Ne→18
9F→15
8O→15
7N:

15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F	+	1 1H	→	18 10Ne	+	γ			+	3.92 MeV
18 10Ne			→	18 9F	+	e+	+	ν e	+	4.44 MeV	(half-life of 1.672 seconds)
18 9F	+	1 1H	→	15 8O	+	4 2He			+	2.88 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)

HCNO-III

An alternative to the HCNO-II cycle is that 18
9F captures a proton moving towards higher mass and using the same helium production mechanism as the CNO-IV cycle as 18
9F→19
10Ne→19
9F→16
8O→17
9F→18
10Ne→18
9F:

18 9F	+	1 1H	→	19 10Ne	+	γ			+	6.41 MeV
19 10Ne			→	19 9F	+	e+	+	ν e	+	3.32 MeV	(half-life of 17.22 seconds)
19 9F	+	1 1H	→	16 8O	+	4 2He			+	8.11 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F	+	1 1H	→	18 10Ne	+	γ			+	3.92 MeV
18 10Ne			→	18 9F	+	e+	+	ν e	+	4.44 MeV	(half-life of 1.672 seconds)

Use in astronomy

While the total number of "catalytic" CNO nuclei are conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than main sequence stars, which is considered to be convincing evidence for the operation of the CNO cycle.^{[citation needed]}

The presence of the heavier elements carbon, nitrogen and oxygen places an upper bound of approximately 150 solar masses on the maximum size of massive stars.^{[citation needed]} It is thought^{[by whom?]} that the "metal-poor" early universe could have had stars, called Population III stars, up to 250 solar masses without interference from the CNO cycle at the beginning of their lifetime.

Solar wind

From Wikipedia, the free encyclopedia

The heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the solar wind.

The solar wind is a stream of plasma released from the upper atmosphere of the Sun. It consists of mostly electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in density, temperature, and speed over time and over solar longitude. These particles can escape the Sun's gravity because of their high energy, from the high temperature of the corona and magnetic, electrical and electromagnetic phenomena in it.

The solar wind flows outward supersonically to great distances, filling a region known as the heliosphere, an enormous bubble-like volume surrounded by the interstellar medium. Other related phenomena include the aurora (northern and southern lights), the plasma tails of comets that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines and create strong currents in power grids on Earth.

History

The existence of a continuous stream of particles flowing outward from the Sun was first suggested by British astronomer Richard C. Carrington. In 1859, Carrington and Richard Hodgson independently made the first observation of what would later be called a solar flare. This is a sudden outburst of energy from the Sun's atmosphere. On the following day, a geomagnetic storm was observed, and Carrington suspected that there might be a connection. George FitzGerald later suggested that matter was being regularly accelerated away from the Sun and was reaching the Earth after several days.^[1]

Laboratory simulation of the magnetosphere's influence on the Solar Wind; these auroral-like Birkeland currents were created in a terrella, a magnetised anode globe in an evacuated chamber.

In 1910 British astrophysicist Arthur Eddington essentially suggested the existence of the solar wind, without naming it, in a footnote to an article on Comet Morehouse.^[2] The idea never fully caught on even though Eddington had also made a similar suggestion at a Royal Institution address the previous year. In the latter case, he postulated that the ejected material consisted of electrons while in his study of Comet Morehouse he supposed them to be ions.^[2] The first person to suggest that they were both was Norwegian physicist Kristian Birkeland. His geomagnetic surveys showed that auroral activity was nearly uninterrupted. As these displays and other geomagnetic activity were being produced by particles from the Sun, he concluded that the Earth was being continually bombarded by "rays of electric corpuscles emitted by the Sun".^[1] In 1916, Birkeland proposed that, "From a physical point of view it is most probable that solar rays are neither exclusively negative nor positive rays, but of both kinds". In other words, the solar wind consists of both negative electrons and positive ions.^[3] Three years later in 1919, Frederick Lindemann also suggested that particles of both polarities, protons as well as electrons, come from the Sun.^[4]

Around the 1930s, scientists had determined that the temperature of the solar corona must be a million degrees Celsius because of the way it stood out into space (as seen during total eclipses). Later spectroscopic work confirmed this extraordinary temperature. In the mid-1950s the British mathematician Sydney Chapman calculated the properties of a gas at such a temperature and determined it was such a superb conductor of heat that it must extend way out into space, beyond the orbit of Earth. Also in the 1950s, a German scientist named Ludwig Biermann became interested in the fact that no matter whether a comet is headed towards or away from the Sun, its tail always points away from the Sun. Biermann postulated that this happens because the Sun emits a steady stream of particles that pushes the comet's tail away.^[5] Wilfried Schröder claims in his book, Who First Discovered the Solar Wind?, that the German astronomer Paul Ahnert was the first to relate solar wind to comet tail direction based on observations of the comet Whipple-Fedke (1942g).^[6]

Eugene Parker realised that the heat flowing from the Sun in Chapman's model and the comet tail blowing away from the Sun in Biermann's hypothesis had to be the result of the same phenomenon, which he termed the "solar wind".^[7][8] Parker showed in 1958 that even though the Sun's corona is strongly attracted by solar gravity, it is such a good conductor of heat that it is still very hot at large distances. Since gravity weakens as distance from the Sun increases, the outer coronal atmosphere escapes supersonically into interstellar space. Furthermore, Parker was the first person to notice that the weakening effect of the gravity has the same effect on hydrodynamic flow as a de Laval nozzle: it incites a transition from subsonic to supersonic flow.^[9]

Opposition to Parker's hypothesis on the solar wind was strong. The paper he submitted to the Astrophysical Journal in 1958 was rejected by two reviewers. It was saved by the editor Subrahmanyan Chandrasekhar (who later received the 1983 Nobel Prize in physics).

In January 1959, the Soviet satellite Luna 1 first directly observed the solar wind and measured its strength.^[10][11][12] They were detected by hemispherical ion traps. The discovery, made by Konstantin Gringauz, was verified by Luna 2, Luna 3 and by the more distant measurements of Venera 1. Three years later its measurement was performed by Americans (Neugebauer and collaborators) using the Mariner 2 spacecraft.^[13]

In the late 1990s the Ultraviolet Coronal Spectrometer (UVCS) instrument on board the SOHO spacecraft observed the acceleration region of the fast solar wind emanating from the poles of the Sun, and found that the wind accelerates much faster than can be accounted for by thermodynamic expansion alone. Parker's model predicted that the wind should make the transition to supersonic flow at an altitude of about 4 solar radii from the photosphere; but the transition (or "sonic point") now appears to be much lower, perhaps only 1 solar radius above the photosphere, suggesting that some additional mechanism accelerates the solar wind away from the Sun. The acceleration of the fast wind is still not understood and cannot be fully explained by Parker's theory. The gravitational and electromagnetic explanation for this acceleration is, however, detailed in an earlier paper by 1970 Nobel laureate for Physics, Hannes Alfvén.^[14][15]

The first numerical simulation of the solar wind in the solar corona including closed and open field lines was performed by Pneuman and Kopp in 1971. The magnetohydrodynamics equations in steady state were solved iteratively starting with an initial dipolar configuration.^[16]

In 1990, the Ulysses probe was launched to study the solar wind from high solar latitudes. All prior observations had been made at or near the Solar System's ecliptic plane.^[17]

Emission

While early models of the solar wind used primarily thermal energy to accelerate the material, by the 1960s it was clear that thermal acceleration alone cannot account for the high speed of solar wind. An additional unknown acceleration mechanism is required, and likely relates to magnetic fields in the solar atmosphere.

The Sun's corona, or extended outer layer, is a region of plasma that is heated to over a million degrees Celsius. As a result of thermal collisions, the particles within the inner corona have a range and distribution of speeds described by a Maxwellian distribution. The mean velocity of these particles is about 145 km/s, which is well below the solar escape velocity of 618 km/s. However, a few of the particles achieve energies sufficient to reach the terminal velocity of 400 km/s, which allows them to feed the solar wind. At the same temperature, electrons, due to their much smaller mass, reach escape velocity and build up an electric field that further accelerates ions - charged atoms - away from the Sun.^[18]

The total number of particles carried away from the Sun by the solar wind is about 1.3×10³⁶ per second.^[19] Thus, the total mass loss each year is about (2–3)×10⁻¹⁴ solar masses,^[20] or about one billion kilograms per second. This is equivalent to losing a mass equal to the Earth every 150 million years.^[21] However, only about 0.01% of the Sun's total mass has been lost through the solar wind.^[22] Other stars have much stronger stellar winds that result in significantly higher mass loss rates.

Components and speed

The solar wind is divided into two components, respectively termed the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 km/s, a temperature of 1.4–1.6×10⁶ K and a composition that is a close match to the corona. By contrast, the fast solar wind has a typical velocity of 750 km/s, a temperature of 8×10⁵ K and it nearly matches the composition of the Sun's photosphere.^[23] The slow solar wind is twice as dense and more variable in intensity than the fast solar wind. The slow wind also has a more complex structure, with turbulent regions and large-scale structures.^[19][24]

The slow solar wind appears to originate from a region around the Sun's equatorial belt that is known as the "streamer belt". Coronal streamers extend outward from this region, carrying plasma from the interior along closed magnetic loops.^[25][26] Observations of the Sun between 1996 and 2001 showed that emission of the slow solar wind occurred between latitudes of 30–35° around the equator during the solar minimum (the period of lowest solar activity), then expanded toward the poles as the minimum waned. By the time of the solar maximum, the poles were also emitting a slow solar wind.^[27]

The fast solar wind is thought to originate from coronal holes, which are funnel-like regions of open field lines in the Sun's magnetic field.^[28] Such open lines are particularly prevalent around the Sun's magnetic poles. The plasma source is small magnetic fields created by convection cells in the solar atmosphere. These fields confine the plasma and transport it into the narrow necks of the coronal funnels, which are located only 20,000 kilometers above the photosphere. The plasma is released into the funnel when these magnetic field lines reconnect.^[29]

Solar wind pressure

The wind exerts a pressure at 1 AU typically in the range of 1–6 nPa (1–6×10⁻⁹ N/m²), although it can readily vary outside that range.

The dynamic pressure is a function of wind speed and density. The formula is

P = 1.6726×10⁻⁶ * n * V²

where pressure P is in nPa (nano Pascals), n is the density in particles/cm³ and V is the speed in km/s of the solar wind.^[30]

Coronal mass ejection

Both the fast and slow solar wind can be interrupted by large, fast-moving bursts of plasma called interplanetary coronal mass ejections, or ICMEs. ICMEs are the interplanetary manifestation of solar coronal mass ejections, which are caused by release of magnetic energy at the Sun. CMEs are often called "solar storms" or "space storms" in the popular media. They are sometimes, but not always, associated with solar flares, which are another manifestation of magnetic energy release at the Sun. ICMEs cause shock waves in the thin plasma of the heliosphere, launching electromagnetic waves and accelerating particles (mostly protons and electrons) to form showers of ionizing radiation that precede the CME.
When a CME impacts the Earth's magnetosphere, it temporarily deforms the Earth's magnetic field, changing the direction of compass needles and inducing large electrical ground currents in Earth itself; this is called a geomagnetic storm and it is a global phenomenon. CME impacts can induce magnetic reconnection in Earth's magnetotail (the midnight side of the magnetosphere); this launches protons and electrons downward toward Earth's atmosphere, where they form the aurora.

ICMEs are not the only cause of space weather. Different patches on the Sun are known to give rise to slightly different speeds and densities of wind depending on local conditions. In isolation, each of these different wind streams would form a spiral with a slightly different angle, with fast-moving streams moving out more directly and slow-moving streams wrapping more around the Sun. Fast moving streams tend to overtake slower streams that originate westward of them on the Sun, forming turbulent co-rotating interaction regions that give rise to wave motions and accelerated particles, and that affect Earth's magnetosphere in the same way as, but more gently than, CMEs.

Effect on the Solar System

Over the lifetime of the Sun, the interaction of the Sun's surface layers with the escaping solar wind has significantly decreased its surface rotation rate.^[31] The wind is considered responsible for the tails of comets, along with the Sun's radiation.^[32] The solar wind contributes to fluctuations in celestial radio waves observed on the Earth, through an effect called interplanetary scintillation.^[33]

Magnetospheres

Schematic of Earth's magnetosphere. The solar wind flows from left to right.

As the solar wind approaches a planet that has a well-developed magnetic field (such as Earth, Jupiter and Saturn), the particles are deflected by the Lorentz force. This region, known as the magnetosphere, causes the particles to travel around the planet rather than bombarding the atmosphere or surface. The magnetosphere is roughly shaped like a hemisphere on the side facing the Sun, then is drawn out in a long wake on the opposite side. The boundary of this region is called the magnetopause, and some of the particles are able to penetrate the magnetosphere through this region by partial reconnection of the magnetic field lines.^[18]

Noon meridian section of magnetosphere.

The solar wind is responsible for the overall shape of Earth's magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earth's local space environment. For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and the shape and location of the magnetopause and bow shock wave upstream of it can change by several Earth radii, exposing geosynchronous satellites to the direct solar wind. These phenomena are collectively called space weather.

From the European Space Agency’s Cluster mission, a new study has taken place that proposes it easier for the solar wind to infiltrate the magnetosphere than it had been previously believed. A group of scientists directly observed the existence of certain waves in the solar wind that were not expected. A recent publishing in the Journal of Geophysical Research shows that these waves enable incoming charged particles of solar wind to breach the magnetopause. This suggests that the magnetic bubble forms more as a filter than a continuous barrier. This latest discovery occurred through the distinctive arrangement of the four identical Cluster spacecraft, which fly in a strictly controlled configuration through near-Earth space. As they sweep from the magnetosphere into interplanetary space and back again, the fleet provides exceptional three-dimensional insights on the processes that connect the sun to Earth.

The team of scientists was able to characterize how variances in IMF formation largely influenced by Kelvin-Helmholtz waves (which occur upon the interface of two fluids) as a result of differences in thickness and numerous other characteristics of the boundary layer. Experts believe that this was the first occasion that the appearance "of Kelvin-Helmholtz waves at the magnetopause has be displayed at high latitude dawnward orientation of the IMF. These waves of being seen in unforeseeable places under solar wind conditions that were formerly believed to be undesired for their generation.The discoveries found through this mission are of great importance by ESA project scientists because it shows how Earth’s magnetosphere can be penetrated by solar particles under specific interplanetary magnetic field circumstances. The findings are also relevant to studies of magnetospheric progressions around other planetary bodies in the solar system. This study suggests that Kelvin-Helmholtz waves can be a somewhat common, and possibly constant, instrument for the entrance of solar wind into terrestrial magnetospheres under various IMF orientations.^[34]

Atmospheres

The solar wind affects the other incoming cosmic rays interacting with the atmosphere of planets. Moreover, planets with a weak or non-existent magnetosphere are subject to atmospheric stripping by the solar wind.

Venus, the nearest and most similar planet to Earth in the Solar System, has an atmosphere 100 times denser than our own, with little or no geo-magnetic field. Modern space probes have discovered a comet-like tail that extends to the orbit of the Earth.^[35]

Earth itself is largely protected from the solar wind by its magnetic field, which deflects most of the charged particles; however some of the charged particles are trapped in the Van Allen radiation belt. A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth's upper atmosphere and ionosphere in the auroral zones. The only time the solar wind is observable on the Earth is when it is strong enough to produce phenomena such as the aurora and geomagnetic storms. Bright auroras strongly heat the ionosphere, causing its plasma to expand into the magnetosphere, increasing the size of the plasma geosphere, and causing escape of atmospheric matter into the solar wind. Geomagnetic storms result when the pressure of plasmas contained inside the magnetosphere is sufficiently large to inflate and thereby distort the geomagnetic field.

Mars is larger than Mercury and four times farther from the Sun, and yet even here it is thought that the solar wind has stripped away up to a third of its original atmosphere, leaving a layer 1/100th as dense as the Earth's. It is believed the mechanism for this atmospheric stripping is gas being caught in bubbles of magnetic field, which are ripped off by solar winds.^[36]

Planetary surfaces

Mercury, the nearest planet to the Sun, bears the full brunt of the solar wind, and its atmosphere is vestigial and transient, its surface bathed in radiation.

Mercury has an intrinsic magnetic field, so under normal solar wind conditions, the solar wind cannot penetrate the magnetosphere created around the planet, and particles only reach the surface in the cusp regions. During coronal mass ejections, however, the magnetopause may get pressed into the surface of the planet, and under these conditions, the solar wind may interact freely with the planetary surface.

The Earth's Moon has no atmosphere or intrinsic magnetic field, and consequently its surface is bombarded with the full solar wind. The Project Apollo missions deployed passive aluminum collectors in an attempt to sample the solar wind, and lunar soil returned for study confirmed that the lunar regolith is enriched in atomic nuclei deposited from the solar wind. There has been speculation that these elements may prove to be useful resources for future lunar colonies.^[37]

Outer limits

The solar wind "blows a bubble" in the interstellar medium (the rarefied hydrogen and helium gas that permeates the galaxy). The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the heliopause, and is often considered to be the outer border of the Solar System. The distance to the heliopause is not precisely known, and probably varies widely depending on the current velocity of the solar wind and the local density of the interstellar medium, but it is known to lie far outside the orbit of Pluto. Scientists hope to gain more perspective on the heliopause from data acquired through the Interstellar Boundary Explorer (IBEX) mission, launched in October 2008.

Notable events

• From May 10 to May 12, 1999, NASA's Advanced Composition Explorer (ACE) and WIND spacecraft observed a 98% decrease of solar wind density. This allowed energetic electrons from the Sun to flow to Earth in narrow beams known as "strahl", which caused a highly unusual "polar rain" event, in which a visible aurora appeared over the North Pole. In addition, Earth's magnetosphere increased to between 5 and 6 times its normal size.^[38]
• See also the solar variation entry.
• On 13 December 2010, Voyager 1 determined that the velocity of the solar wind, at its location 10.8 billion miles from Earth has now slowed to zero. "We have gotten to the point where the wind from the Sun, which until now has always had an outward motion, is no longer moving outward; it is only moving sideways so that it can end up going down the tail of the heliosphere, which is a comet-shaped-like object," said Dr. Edward Stone, the Voyager project scientist.^[39][40]