# Stars/Sun/X-ray sources

The Sun as an X-ray source is a curiosity.
At right is a visual image of the Sun, the star around which the Earth orbits. This image shows the ball that is the photosphere of the Sun, the surface of the Sun. The effective temperature of the photosphere is too low to emit X-rays.

## Notations

Notation: let the symbol $R_{\odot }$ indicate the solar radius.

## Prominences

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."

## Radiation

Def. the point of origin of a ray, beam, or stream of small cross section traveling in a line is called a radiation source.
The image at right is the first X-ray light image of the Sun by the satellite GOES-15 Solar X-ray Imager (SXI) on June 2, 2010. The surface of the Sun, beneath the coronal cloud layer is dark. The coronal cloud is the actual source of the X-rays.
"X-rays span 3 decades in wavelength, frequency and energy. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays."
"Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air."

## Solar sciences

The GOES 14 spacecraft carries a Solar X-ray Imager that took this image [at right] of the Sun during the most recent quiet period. The Sun appears dark because of the wavelength band of observation and the lack of X-rays.
Except for X-ray emission that suggests a circular disc with some isolated X-ray sources at specific locations, the Sun is almost invisible. X-rays are primarily emitted from plasmas near 10$^6$ K.

## Colors

At right is 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.

## Theoretical solar astronomy

Def. "[t]he star at the center of our solar system" is called the Sun.
Def. "[a] luminous celestial body, made up of plasma (particularly hydrogen and helium) and having a spherical shape" is called a star.
"Depending on context the sun may or may not be included."
Def. "[a]ny small luminous dot appearing in the cloudless portion of the night sky, especially with a fixed location relative to other such dots" is called a star.
Def. "[t]he natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible" is called light.
Def. "emitting light" is called luminous.

## Dynamos

"[M]otions resulting from [a linear magnetohydrodynamic] instability act as a dynamo to sustain the magnetic field." "Supersonic flows are initially generated by the Balbus-Hawley magnetic shear instability."
"A plasma with local magnetohydrodynamic instabilities creates mechanical turbulence, motion, or shear (a dynamo) which in turn generates or sustains the local magnetic field."
"The solar dynamo is the physical process that generates the Sun's magnetic field. The Sun is permeated by an overall dipole magnetic field, as are many other celestial bodies such as the Earth. The dipole field is produced by a circular electric current flowing deep within the star, following Ampère's law. The current is produced by shear (stretching of material) between different parts of the Sun that rotate at different rates, and the fact that the Sun itself is a very good electrical conductor (and therefore governed by the laws of magnetohydrodynamics)."

## Magnetic reconnections

"Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnection occurs on timescales intermediate between slow resistive diffusion of the magnetic field and fast Alfvénic timescales. The qualitative description of the reconnection process is such that magnetic field lines from different magnetic domains (defined by the field line connectivity) are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical or magnetic energy in both space and time."

## Sources

"Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus, the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere."

## Strong forces

"Due to the very low energy of the colliding protons in the Sun, only states with no angular momentum (s-waves) contribute significantly. One can consider it as a head-on collision, so that angular momentum plays no role. Consequently, the total angular momentum is the sum of the spins, and the spins alone control the reaction. Because of Pauli's exclusion principle, the incoming protons must have opposite spins. On the other hand, in the only bound state of deuterium, the spins of the neutron and proton are aligned. Hence a spin flip must take place [...] The strength of the nuclear force which holds the neutron and the proton together depends on the spin of the particles. The force between an aligned proton and neutron is sufficient to give a bound state, but the interaction between two protons does not yield a bound state under any circumstances. Deuterium has only one bound state."
The "force acting between the protons and the neutrons [is] the strong force".
"A potential of 36 MeV is needed to get just one energy state."
The width of a bound proton and neutron is "2.02 x 10$^{-13}$ cm".

## Electromagnetics

A fossil stellar magnetic field is a relic "of the primordial field that [threads] the interstellar gas out of which stars [form]. As "[o]hmic decay times in stable radiative envelopes ... are very long, [a] primordial field sufficiently concentrated by the star formation process [may survive] through most or all of such a star’s main-sequence [lifetime]. "[T]he slow rotation of most magnetic ... stars relative to [non-magnetics of the same spectral type] is ... [the] result of magnetic braking by the field threading the stars." "Invoking fossil origins for the observed surface magnetism, with predominate dipole structure surviving, is a favored explanation ... since strong fields could more easily be obtained."
The Sun consists of hot plasma interwoven with magnetic fields."
The Sun is a magnetically active star. It [has] a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum. The Sun's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity [may] have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth's outer atmosphere.
"The first systematic attempt to base a theory of the origin of the solar system on electromagnetic or hydromagnetic effects was made in Alfvén (1942). The reason for doing so was that a basic difficulty with the old Laplacian hypothesis: how can a central body (Sun or planet) transfer angular momentum to the secondary bodies (planets or satellites) orbiting around it? It was demonstrated that this could be done by electromagnetic effects. No other acceptable mechanism has yet been worked out. [...] the electromagnetic transfer mechanism has been confirmed by observations, as described in the monograph Cosmic Plasma (Alfvén, 1981, pp. 28, 52, 53 0."
"If charged particles (electrons, ions or charged grains) move in a magnetic dipole field - strong enough to dominate their motion - under the action of gravitation and the centrifugal force, they will find an equilibrium in a circular orbit if their centrifugal force is 2/3 of the gravitational force [...] The consequence of this is that if they become neutralized, so that electromagnetic forces disappear, the centrifugal force is too small to balance the gravitation. Their circular orbit changes to an elliptical orbit with the semi-major axis a = 3/4a$_0$ and e = 1/3 (where a$_0$ is the central distance where the neutralization takes place [...] Collisional (viscous) interaction between the condensed particles will eventually change the orbit into a new circular orbit with a = 2/3a$_0$ and e = 0."
"If [...] there is plasma in the region [collisional interaction results in] matter in the 2/3-[region]. [...] matter in the region [...] will produce a [cosmogonic] shadow in the region".
"[A] variety of geophysical and astrophysical phenomena can be explained by a net charge on the Sun of -1.5 x 10$^{28}$ e.s.u." This figure was later reduced by a factor of five.
    $Q_{\odot }=-0.3\times 10^{28}\,{\hbox{esu}}.$

## Bands

"The hydromagnetic approach led to the discovery of two important observational regularities in the solar system: (1) the band structure [such as in the rings of Saturn and in the asteroid belt], and (2) the cosmogonic shadow effect (the two-thirds fall down effect)."

## Backgrounds

During more than a decade of observations of X-ray emission from the Sun, evidence of the existence of an isotropic X-ray background flux was obtained in 1956.

## Solar radii

The "[s]olar radius is a unit of distance used to express the size of stars in astronomy equal to the current radius of the Sun:
    $R_{\odot }=6.955\times 10^{8}\,{\hbox{m}}=0.0046491\,{\hbox{AU}}=2.254\times 10^{-8}\,{\hbox{pc}}$
The solar radius is approximately 695,500 kilometres (432,450 miles) or about 110 times the radius of the Earth ($R_{\oplus }$), or 10 times the average radius of Jupiter. It varies slightly from pole to equator due to its rotation, which induces an oblateness of order 10 parts per million."

## Photospheres

"When we speak of the surface of the Sun, we normally mean the photosphere." "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."
Illumination of the Sun's photosphere is in part by gamma rays. Each gamma ray that interacts with the photosphere is converted into several million photons of visible light. At the visible surface of the Sun, the temperature has dropped to 5,700 K and the density to only 0.2 g/m$^3$ (about 1/6,000th the density of air at sea level).
"The tremendous power output of the Sun is not due to its high power per volume, but instead due to its large size. ... Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H$^−$ ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H$^−$ ions. ... The photosphere has a particle density of ~10$^{23}$ m$^{−3}$ (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy)."

## Temperatures

"The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere." The effective temperature of the surface of the Sun's photosphere is 5,778 K. The "[t]emperature at [the] bottom of [the Sun's] photosphere [is] 6600 K", while the "[t]emperature at [the] top of [the] photosphere [is] 4400 K". The photosphere is "~400 km" in thickness.
The peak emittance wavelength of 501.5 nm (~0.5 eV) makes the photosphere a primarily green radiation source. The figure at the right shows the extraterrestrial solar spectral irradiance as compared with a blackbody spectrum. The sharper than black-body cutoff at the shorter wavelength end indicates an even lower likelihood that X-rays are emitted from the photosphere.

## Polar temperatures

From model calculations based on data from Ulysses and Skylab, “[i]nside 2 R$_ʘ$ the [interplanetary medium] temperature is a minimum over the poles, with values T$_{eff}$ ~10$^6$ K, while farther from the Sun the temperature is a maximum over the poles with T$_{eff}$ ~3 x 10$^6$ K at its maximum value [at about 5 R$_ʘ$ out to 7 R$_ʘ$].” T$_{eff}$ is estimated to be ~2 x 10$^5$ K at 1 AU over the poles and ~1.5 x 10$^5$ in the equatorial region out at 1 AU, which compare well with spacecraft observations.

## Meteors

"[A] medium-strength flare erupted from the sun on July 19, 2012. The blast also generated the enormous, shimmering plasma loops, which are an example of a phenomenon known as "coronal rain," agency officials said."
"Hot plasma in the corona cooled and condensed along strong magnetic fields in the region" slowly falling back to the solar surface as plasma "rain".
"Many CMEs have also been observed to be unassociated with any obvious solar surface activity".
In the images at right, a CME, or "arcs rise above an active region on the surface of the Sun in this series of images taken by the STEREO (Behind) spacecraft on January 27, 2010. The arcs are plasma, superheated matter made up of moving charged particles (electrons and ions). Just as iron filings arc from one end of a magnet to another, the plasma is sliding in an arc along magnetic field lines. In a movie of STEREO observations made between January 26 and January 29, the dynamic streams were initially just over the Sun’s edge and readily spotted as the Sun rotated them more into view."

## Neutrons

A "new detector to observe solar neutrons [has been in operation] since 1990 October 17 [...] at the Mount Norikura Cosmic Ray Laboratory (CRL) of [the] Institute for cosmic Ray Research, the University of Tokyo."
"On 1991 June 1, an active sunspot appeared at N25 E90 on the Sun (NOAA region 6659). The commencement of an enormous bright flare was observed at 03:37 UT on 1991 June 4 [...] The flare was classified as 3 B and the location was at N31 E70 of the solar surface."
"The solar neutron telescope [image at right] consists of 10 blocks of scintillator [...] and several lead plates which are used to place kinetic energies T$_n$ of incoming particles into three bands (50-360 MeV, 280-500 MeV, and ≥ 390 MeV)." The telescope is inclined to the direction of the Sun by 15°. The plane area of the detector is 1.0 m$^2$ and protected by lead plates (Pb) to eliminate gamma-ray and muon background from the side of the detector. The anti-coincident counter (A) is used to reject the muons and gamma rays, coming from the side of the detector and the top scintillators. (P) and (G) are used to identify the proton events and gamma rays. The central scintillator blocks are optically separated into 10 units.
"The horizontal scintillator just above the 10 vertical scintillators distinguishes neutral particles (neutrons) from the charged particles (mainly muons, protons and electrons)."
"Mount Norikura Cosmic-Ray Laboratory has an elevation of 2770 m above sea level. The geographical latitude is 36.10° N and the longitude is 137.55° E. The zenith angle of the Sun at 03:37 UT on June 4 is 18.9° and the solar neutron telescope was set at a zenith angle of 15° on this day."

## Electrons

Electrons "provide remote-sensing observations of distant targets in the heliosphere - the Sun, the Moon, Jupiter, and various heliospheric structures."

## Neutrinos

"A star is considered to be at zero age (protostellar) when it is assumed to have a homogeneous composition and to be just beginning to derive most of its luminosity from nuclear reactions (so neglecting the period of contraction from a cloud of gas and dust). To obtain the SSM, a one solar mass stellar model at zero age is evolved numerically to the age of the Sun. The abundance of elements in the zero age solar model is estimated from primordial meteorites. Along with this abundance information, a reasonable guess at the zero-age luminosity (such as the present-day Sun's luminosity) is then converted by an iterative procedure into the correct value for the model, and the temperature, pressure and density throughout the model calculated by solving the equations of stellar structure numerically assuming the star to be in a steady state. The model is then evolved numerically up to the age of the Sun. Any discrepancy from the measured values of the Sun's luminosity, surface abundances, etc. can then be used to refine the model. For example, since the Sun formed, the helium and heavy elements have settled out of the photosphere by diffusion. As a result, the Solar photosphere now contains about 87% as much helium and heavy elements as the protostellar photosphere had; the protostellar Solar photosphere was 71.1% hydrogen, 27.4% helium, and 1.5% metals. A measure of heavy-element settling by diffusion is required for a more accurate model."
"Nuclear reactions in the core of the Sun change its composition, by converting hydrogen nuclei into helium nuclei by the proton-proton chain and (to a lesser extent in the Sun than in more massive stars) the CNO cycle. This decreases the mean molecular weight in the core of the Sun, which should lead to a decrease in pressure. This does not happen as instead the core contracts. By the Virial Theorem half of the gravitational potential energy released by this contraction goes towards raising the temperature of the core, and the other half is radiated away. By the ideal gas law this increase in temperature also increases the pressure and restores the balance of hydrostatic equilibrium. The luminosity of the Sun is increased by the temperature rise, increasing the rate of nuclear reactions. The outer layers expand to compensate for the increased temperature and pressure gradients, so the radius also increases."
"Most of the neutrinos produced in the sun come from the first step of the pp chain but their energy is so low (<0.425 MeV) they are very difficult to detect. A rare side branch of the pp chain produces the "boron-8" neutrinos with a maximum energy of roughly 15 MeV, and these are the easiest neutrinos to detect. A very rare interaction in the pp chain produces the "hep" neutrinos, the highest energy neutrinos predicted to be produced by our sun. They are predicted to have a maximum energy of about 18 MeV."
"All of the interactions described above produce neutrinos with a spectrum of energies. The electron capture of $^7$Be produces neutrinos at either roughly 0.862 MeV (~90%) or 0.384 MeV (~10%)."

## Gamma rays

Illumination of the Sun's photosphere is in part by gamma rays. Each gamma ray that interacts with the photosphere is converted into several million photons of visible light. At the visible surface of the Sun, the temperature has dropped to 5,700 K and the density to only 0.2 g/m$^3$ (about 1/6,000th the density of air at sea level).

## X-radiation

Def. the point of origin of a ray, beam, or stream of small cross section traveling in a line is called a radiation source.
The image at right is the first X-ray light image of the Sun by the satellite GOES-15 Solar X-ray Imager (SXI) on June 2, 2010. The surface of the Sun, beneath the coronal cloud layer is dark. The coronal cloud is the actual source of the X-rays.
"X-rays span 3 decades in wavelength, frequency and energy. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays."
"Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air."

## Ultraviolets

Ultraviolet emission from the Sun actually comes from the chromospheres.

## Visuals

The visible light we see is produced as electrons react with hydrogen atoms to produce H$^−$ ions. The photosphere has a particle density of ~10$^{23}$ m$^{−3}$ (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy).

## Reds

"The flare was observed by [the commencement of an enormous bright flare was observed at 03:37 UT on 1991 June 4 (K. Yamaguchi, M. Ire, & M. Miyashita 1991, private communication$^5$; Sakurai et al. 1992) with] a 14 cm aperture Hα monochromatic heliograph of the National Astronomical Observatory [Mitaka, Tokyo 181, Japan]."

## Submillimeters

The "solar submillimeter telescope [(SST) is] at the El Leoncito Observatory located at 2550 m altitude in the Argentina Andes. The SST has a 1.5 m reflector with four 212 GHz and two 405 GHz radiometers operating simultaneously with 5 ms time resolution. The main-beam cluster consists of three 212 GHz beams (about 4 half-power beamwidth) partially overlapping each other and one 405 GHz beam (about 2) in the center of the three".

## Plasma objects

Def. the "luminous plasma atmosphere of the Sun or other star, extending millions of kilometres into space, most easily seen during a total solar eclipse" is called a corona, or stellar corona.
"Beginning with the daguerreotype of the corona of 1851, the Reverend Lecturer had thrown on the screen pictures illustrating the form of the corona in different years. The drawings of those of 1867, 1878, and 1900 all showed long equatorial extensions with openings at the solar poles filled with beautiful rays." "The intermediate years, as, for example, 1883, 1886, and 1896 showed the four groups of synclinals which mainly constitute the corona gradually descending towards the equator of the sun, with a corresponding opening of the polar regions."
"Some of the theories of the solar corona were then illustrated and discussed."
* "The corona is not of the nature of an atmosphere round the sun, for the pressure at the sun's limb would be enormous, while the thinness of the chromospheric lines show that it is not."
* "comets, such as that of 1843, have approached the sun with enormous velocities within the region of the prominences without suffering disruption or retardation."
* "If not an atmosphere of particles of gas, still less is it an atmosphere of solid stones or meteorites."
* "Meteor streams do circle round the sun, but there is no reason why the positions of the orbits, or the intrinsic brightness of such streams should vary with the sun-spot period."
* "the appearance of the corona does not seem to be such as the projection of meteor streams upon the celestial vault would give."
* "Prof. Schaeberle has proposed a mechanical origin of the solar corona, due to the forces of ejection of particles from the solar limb, as evidenced by the prominences, and the force of gravity under the particular conditions of the solar rotation and the inclination of its axis to the earth's orbit."
* "The electrical theory of the corona does not negative the mechanical theory, but supplements it. In addition to the forces of gravity and ejection, it takes account of the repulsive force which the sun exerts on matter which has the same electrical sign as itself, and which has been ejected from it."
* "it would seem that the solar corona is of the nature of an electrical aurora round the sun."
* "the coronoidal discharges in poor vacua obtained by Prof. Pupin about an insulated metal ball are exceedingly like the rays and streamers of the solar corona."
The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.
"[T]he sun's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. Inconsistencies in this wind and larger events on the surface of the star, such as coronal mass ejections, form a system that has features analogous to conventional weather systems (such as pressure and wind) and is generally known as space weather. Coronal mass ejections have been tracked as far out in the solar system as Saturn. The activity of this system can affect planetary atmospheres and occasionally surfaces. The interaction of the solar wind with the terrestrial atmosphere can produce spectacular aurorae, and can play havoc with electrically sensitive systems such as electricity grids and radio signals."

## Gaseous objects

The solar photosphere is a "weakly ionized [n$_i$/(n$_i$ + n$_a$)] ~ 10$^{-4}$, relatively cold and dense plasma".

## Heliums

Helium "was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer during the solar eclipse of 1868, and Lockyer was the first to propose that the line was due to a new element, which he named."

## Berylliums

"Stars with an initial mass less than the solar mass have a large deficiency of light elements — lithium, beryllium, and boron — which burn up almost completely both in the interior and in the convective envelope."

## Oxygens

"X-radiation from the Sun impacts oxygen atoms, knock electrons out of the inner parts of their electron clouds, and excite the atoms to a higher energy level in the process. The atoms almost immediately return to their lower energy state and may emit a fluorescent X-ray in this process with an energy characteristic of the atom involved - oxygen in this case. A similar process involving ultraviolet light produces the visible light from fluorescent lamps."

## Sun

"On April 23, 1978 a system of bright loop prominences was observed at Oslo Solar Observatory, Harestua, on the east limb of the Sun."
"X-ray photons can be effectively backscattered by photosphere atoms and electrons (Tomblin 1972; Bai & Ramaty 1978). ... [A]t energies not dominated by absorption the backscattered albedo flux must be seen virtually in every solar flare spectrum, the degree of the albedo contribution depending on the directivity of the primary X-ray flux (Kontar et al. 2006). The solar flare photons backscattered by the solar photosphere can contribute significantly (the reflected flux is 50-90 % of the primary in the 30 - 50 keV range for isotropic sources) to the total observed photon spectrum. for the simple case of a power-law-like primary solar flare spectrum (without albedo), the photons reflected by the photosphere produce a broad 'hump' component. Photospheric albedo makes the observed spectrum flatter below ~ 35 keV and slightly steeper above, in comparison with the primary spectrum."

## Coronal clouds

Def. a cloud, or cloud-like, natural astronomical entity, composed of plasma is called a coronal cloud.
From coronal cloud: "Initially the concept of a coronal cloud developed with observations of clouds or cloud-like structures forming above the photospheric surface of the Sun."
"Coronal clouds, type IIIg, form in space above a spot area and rain streamers upon it." Type IIIg is a subdivision of sunspot prominences (class III). "Occasionally dots form in space above a sunspot group. Other dots then appear, and all coalesce into a suspended cloud from which streamers pour into the spot area."
Although a coronal cloud (as part or all of a stellar or galactic corona) is usually "filled with high-temperature plasma at temperatures of T ≈ 1–2 (MK), ... [h]ot active regions and postflare loops have plasma temperatures of T ≈ 2–40 MK."
The preflare solar material is observed "to be an elevated cloud of prominence-like material which is suddenly lit up by the onslaught of hard electrons accelerated in the flare; the acceleration may be inside or outside the cloud, and brightening is seen in other areas of the solar surface on the same magnetic field lines.", per coronal cloud.
"A hot coronal cloud at T ~ 10$^7$ K is left behind, presumably evaporated from the original material." "[O]nce ionized, the gas is rapidly heated by Coulomb collisions to the coronal cloud temperature, but as this material peels off, a cooler hydrogen-emitting region is left."
At right is an image showing the Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite. The bright, loop-like structures are hot plasma (MK) confined by magnetic fields apparently rooted in the solar interior. An image of the Sun in visible light would show sunspots at the feet of many of these loops. The halo of gas extends well beyond the Sun and almost appears like a shell around the Sun. The darker regions at the North and South poles of the Sun are coronal holes, where the magnetic field lines are open to space and allow particles to escape. These holes are the source for the solar wind.

## Coronal heating

The photosphere of the Sun has an effective temperature of 5,570 K yet its corona has an average temperature of 1–2 x 10$^6$ K. However, the hottest regions are 8–20 x 10$^6$ K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.
Although the total energy output of the photosphere is larger than that of the coronal cloud there appears to be no mechanism to focus it into the corona.
The image at right shows the first detection of high temperature nanoflares. The false-color temperature map of solar active region AR10923, observed close to center of the sun's disk, contains nanoflare regions (blue, indicating plasma near 10 million degrees K).
"Nanoflares are small, sudden bursts of heat and energy. "They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop," says Klimchuk. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun's corona. ... Observations from the NASA-funded X-Ray Telescope (XRT) and Extreme-ultraviolet Imaging Spectrometer (EIS) instruments aboard Hinode reveal that ultra-hot plasma is widespread in solar active regions. The XRT measured plasma at 10 million degrees K, and the EIS measured plasma at 5 million degrees K. "These temperatures can only be produced by impulsive energy bursts,"says Klimchuk ... "Coronal loops are bundles of unresolved strands that are heated by storms of nanoflares." ... when a nanoflare suddenly releases its energy, the plasma in the low-temperature, low-density strands becomes very hot—around 10 million degrees K—very quickly. The density remains low, however, so the emission, or brightness, remains faint. Heat flows from up in the strand, where it's hot, down to the base of the coronal loop, where it's not as hot. This heats up the dense plasma at the loop’s base. Because it is so dense at the base, the temperature only reaches about 1 million degrees K. This dense plasma expands up into the strand. Thus, a coronal loop is a collection of 5-10 million degree K faint strands and 1 million degree K bright strands. "What we see is 1 million degree K plasma that has received its energy from the heat flowing down from the superhot plasma," says Klimchuk. "For the first time, we have detected this 10 million degree plasma, which can only be produced by the impulsive energy bursts of nanoflares.""
As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."
The source of heat that brings the coronal cloud near the Sun hot enough to emit X-rays may be an electron beam heating effect due to "high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside".
"Electron beams can be generated by thermionic emission, field emission or the anodic arc method. The generated electron beam is accelerated to a high kinetic energy and focused towards the [target]. When the accelerating voltage is between 20 kV – 25 kV and the beam current is a few amperes, 85% of the kinetic energy of the electrons is converted into thermal energy as the beam bombards the surface of the [target]. The surface temperature of the [target] increases resulting in the formation of a liquid melt. Although some of incident electron energy is lost in the excitation of X-rays and secondary emission, the [target] material evaporates under vacuum."

## Holes

Def. "[a] hollow spot in a surface" is called a hole.
Def. "[a]n opening in a solid[, liquid, gas, or plasma]" is called a hole.
Def. "[a] round or irregular patch on [or apparently on] the surface of [an entity] having a different color, texture etc. and generally round in shape" is called a spot.
Def. "[a]n opening through which [entities such as] gases ... can pass" is called a vent.
"Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles. Although they are at temperatures of roughly 3,000–4,500 K (2,727–4,227 °C), the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (49,710 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere."
"For the greater part of the sun-spot period there is practically but one zone of spots in each hemisphere. The departure from this condition of things near or at the time of minimum, when the spots of the dying cycle are approaching the equator, and the forerunners of the new cycle are beginning to appear in high latitudes, is the only case in which the solar spots are distinctly separated into more than a single zone in each hemisphere." "Spöerer's law ... involves that in a minimum year the zone about 15° should be entirely barren".
"First of all there were only fifteen groups seen during the entire year [1901], north and south put together. Of these, seven were in the north, and the mean latitude for the north was 8.6°, exactly the latitude of one spot of the seven, and this very naturally, seeing that it was by far the greatest group of the year, the celebrated "eclipse group."" Bold added. "Greatest group" and "eclipse group" are both relative synonyms for "dominant group".
"[T]he spot-groups have been carefully examined for cases of return, and where it appeared clear that the same group has returned a second time or more frequently, without any temporary disappearance or subsidence, such a long-continued group has been treated as an entity throughout." Bold added. "It has been forgotten that, whatever the cause which produces this variation of rotation rate with latitude, the causes producing difference of rate within any given latitude are more effective still."
"[T]here is a slight retardation of the rotation period from the first cycle to the second, shown by both northern and southern hemispheres."
"Studies of stratigraphic data have suggested that the solar cycles have been active for hundreds of millions of years, if not longer; measuring varves in precambrian sedimentary rock has revealed repeating peaks in layer thickness, with a pattern repeating approximately every eleven years. It is possible that the early atmosphere on Earth was more sensitive to changes in solar radiation than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity.
This would presume annual layering; however, alternate explanations (diurnal) have also been proposed."
"Analysis of tree rings has revealed a detailed picture of past solar cycles: Dendrochronologically dated radiocarbon concentrations have allowed for a reconstruction of sunspot activity dating back 11,400 years, far beyond the four centuries of available, reliable records from direct solar observation."
"The earliest surviving record of sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records."
The second drawing at right is of a group of sun spots and veiled spots. This group was observed on June 17, 1875, at 07:30 a.m. The interior of these spots appears to have a larger granule size than the outer surface of the photosphere itself.
The spiral sunspot in the third drawing at right is a vortex seen on May 5, 1857, at Rome. "[T]he substance of the photosphere is rushing with an eddying motion [into the spot]."
"Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun."
"The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun."
"Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars."
"Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loops (prominences) and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured."
"The sunspot itself can be divided into two parts:
* The central umbra, which is the darkest part, where the magnetic field is approximately vertical (normal to the Sun's surface).
* The surrounding penumbra, which is lighter, where the magnetic field is more inclined."
"[T]he rate of helicity change dH/dt due to horizontal motions is
    ${\frac {dH}{dt}}=-2\oint (\mathbf {v} \cdot \mathbf {A_{p}} )\mathbf {B_{n}} \mathrm {d} \mathbf {S} ,$
where B$_n$ is the vertical component of the magnetic field on the photosphere and v the photospheric horizontal velocity."
"Helicity [(H)] is a quantitative measure of the chiral properties of the structures observed in the solar atmosphere. ... [H]elicity [is] injected to the corona by photospheric horizontal shearing motions (other than differential rotation) or by [vertical] magnetic flux. ... [D]ifferential rotation cannot provide the required helicity to the [CME] field ejected to interplanetary space."
Solar active region AR 10030 contained a group of sunspots including the largest one partially included in the image at the right. It is a planet-sized sunspot showing for the first time the dark cores of the filaments extending into the sunspot. These filaments are thousands of km long by about 100 km wide. The image is recorded on July 15, 2002, using the Swedish Solar Telescope (SST).
"The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.
HD 154345b has "a 9.2 year, circular orbit with radius 4.2 AU. ... We also detect a ~ 9 year activity cycle in this star [HD 154345] photometrically and in chromospheric emission. ... We note that the Sun's 11 year activity cycle has a period similar to that of Jupiter's orbit, and that the Mount Wilson survey demonstrated that decadal activity cycles are a common feature of old G stars (Baliunas et al. 1995)."
"Naked sunspots are spots seen in Hα to be devoid of associated plage. In magnetograms and K-line little if any opposite polarity field is found, and in soft X-ray images a blank appears in the region of the spot. In almost all cases ... in which naked spots resulted the spot groups had emerged in unipolar regions of the same polarity as the naked spot. At least half of the naked spots are associated with coronal holes."
"[N]aked spots are long-lived and show rotation rates close to the Newton-Nunn curve. Most of the naked spots had bright rims in Hα".
"Though sunspots themselves are darker, they form when there are particularly magnetically active regions, which is when larger, brighter, longer-duration Faculae are more common as well".

## Solar cycles

"The solar cycle has a great influence on space weather, and a significant influence on the Earth's climate since the Sun's luminosity has a direct relationship with magnetic activity. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during this period. During this era, known as the Maunder minimum or Little Ice Age, Europe experienced unusually cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures."
"MOST current literature on solar activity assumes that the planets do not affect it, and that conditions internal to the Sun are primarily responsible for the solar cycle. Bigg$^1$, however, has shown that the period of Mercury's orbit appears in the sunspot data, and that the influence of Mercury depends on the phases of Venus, Earth, and Jupiter."
"It is shown that starting with the alignment of Venus with Jupiter at perihelion position, these two planets will perfectly align at Jupiter's perihelion after every 23.7 years".
"The tidal forces hypothesis for solar cycles has been proposed by Wood (1972) and others. Table 2 below shows the relative tidal forces of the planets on the sun. Jupiter, Venus, Earth and Mercury are called the "tidal planets" because they are the most significant. According to Wood, the especially good alignments of J-V-E with the sun which occur about every 11 years are the cause of the sunspot cycle. He has shown that the sunspot cycle is synchronous with the alignments, and J. Schove's data for 1500 year of sunspot maxima supports the 11.07 year J-V-E period average."
"Both the 11.86 year Jupiter tropical period (time between perihelion's or closest approaches to the sun and the 9.93 year J-S alignment periods are found in sunspot spectral analysis. Unfortunately direct calculations of the tidal forces of all planets does not support the occurrence of the dominant 11.07 year cycle. Instead, the 11.86 year period of Jupiter's perihelion dominates the results. This has caused problems for several researchers in this field."
"[B]y assuming a harmonic variation having a period of 11.13 years ... the phases of such a variation [in polar diameter minus equatorial diameter of the Sun] coincide to within one-fifth of a year with the phases of the sun-spot fluctuations; that, at times corresponding to minimum of sun-spottedness, the polar diameter is relatively larger; that, at times of maximum sun-spottedness, the equatorial diameter is relatively larger. The amplitude of the variation is extremely small, but its reality would seem to be established. [This] at least renders the existence of such periodic fluctuations in the shape of the sun more probable than their non-existence."
"Solar oblateness, the difference between the equatorial and polar diameters, reflects certain fundamental properties of the Sun. ... the oblateness reflects properties of the Sun's interior, ... [There is] a time varying, excess equatorial brightness [producing] a difference between the equatorial and polar limb darkening functions ... at times when the excess brightness is reduced, the intrinsic visual oblateness can be obtained from the observations without detailed knowledge of the excess brightness. A period of reduced excess brightness occurred in 1973 September." The period of reduced excess equatorial brightness occurred between solar cycle maximum around 1970 and minimum around 1975. Considering excess equatorial brightness and seeking to perform measurements at opportunities of reduced excess equatorial brightness has the effect of reducing solar oblateness from some 86.6 ± 6.6 milli-arcsec to 18.4 ± 12.5 milli-arcsec.
"The Babcock Model describes a mechanism which can explain magnetic and sunspot patterns observed on the Sun."
* "The start of the 22-year cycle begins with a well-established dipole field component aligned along the solar rotational axis. The field lines tend to be held by the highly conductive solar plasma of the solar surface."
* "The solar surface plasma rotation rate is different at different latitudes, and the rotation rate is 20 percent faster at the equator than at the poles (one rotation every 27 days). Consequently, the magnetic field lines are wrapped by 20 percent every 27 days."
* "After many rotations, the field lines become highly twisted and bundled, increasing their intensity, and the resulting buoyancy lifts the bundle to the solar surface, forming a bipolar field that appears as two spots, being kinks in the field lines."
* "The sunspots result from the strong local magnetic fields in the solar surface that exclude the light-emitting solar plasma and appear as darkened spots on the solar surface."
* "The leading spot of the bipolar field has the same polarity as the solar hemisphere, and the trailing spot is of opposite polarity. The leading spot of the bipolar field tends to migrate towards the equator, while the trailing spot of opposite polarity migrates towards the solar pole of the respective hemisphere with a resultant reduction of the solar dipole moment. This process of sunspot formation and migration continues until the solar dipole field reverses (after about 11 years)."
* "The solar dipole field, through similar processes, reverses again at the end of the 22-year cycle."
* "The magnetic field of the spot at the equator sometimes weakens, allowing an influx of coronal plasma that increases the internal pressure and forms a magnetic bubble which may burst and produce an ejection of coronal mass, leaving a coronal hole with open field lines. Such a coronal mass ejections are a source of the high-speed solar wind."
* "The fluctuations in the bundled fields convert magnetic field energy into plasma heating, producing emission of electromagnetic radiation as intense ultraviolet (UV) and X-rays."

## Solar winds

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."
"An alternative theory is based on electromagnetic heating during an episode of strong solar wind from the early proto-Sun when our star experienced a T Tauri phase, as predicted by modern stellar astrophysics."

## Venus

"During a rare period of very low density solar outflow, the ionosphere of Venus was observed to become elongated downstream, rather like a long-tailed comet. ... Under normal conditions, the solar wind has a density of 5 - 10 particles per cubic cm at Earth's orbit, but occasionally the solar wind almost disappears, as happened in May 1999. ... A rare opportunity to examine what happens when a tenuous solar wind arrives at Venus came 3 - 4 August 2010, following a series of large coronal mass ejections on the Sun. NASA's STEREO-B spacecraft, orbiting downstream from Venus, observed that the solar wind density at Earth's orbit dropped to the remarkably low figure of 0.1 particles per cubic cm and persisted at this value for an entire day."
"The observations show that the night side ionosphere moved outward to at least 15 000 km from Venus' centre over a period of only a few hours," said Markus Fraenz, also from the Max Planck Institute for Solar System Research, who was the team leader and a co-author of the paper. "It may possibly have extended for millions of kilometres, like an enormous tail."
"Although we cannot determine the full length of the night-side ionosphere, since the orbit of Venus Express provides limited coverage, our results suggest that Venus' ionosphere resembled the teardrop-shaped ionosphere found around comets, rather than the more symmetrical, spherical shape which usually exists."
"The side of Venus' ionosphere that faces away from the sun can billow outward like the tail of a comet, while the side facing the star remains tightly compacted, researchers said. ... "As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions," ESA officials said in a statement today (Jan. 29). It only takes 30 to 60 minutes for the planet's comet-like tail to form after the solar wind dies down. Researchers observed the ionosphere stretch to at least 7,521 miles (12,104 kilometers) from the planet, said Yong Wei, a scientist at the Max Planck Institute in Katlenburg, Germany who worked on this research."
Venus has been detected as a gaseous object using X-ray through red astronomy.

## Earth

Energetic charged particles from the Sun energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the ionosphere, causing the X-ray emission.
"Although auroras might first appear to be moonlit clouds, they only add light to the sky and do not block background stars from view. Called "Northern Lights" in the Northern Hemisphere, auroras are caused by collisions between charged particles from the magnetosphere and air molecules high in the Earth's atmosphere. If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well. Predictable auroras might occur a few days after a powerful magnetic event has been seen on the sun."
"Ionization [of the ionosphere] depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. Thus there is a diurnal (time of day) effect and a seasonal effect. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the Sun is associated with the sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions)."

## Interplanetary medium

"[I]nterplanetary space ... is a stormy and sometimes very violent environment permeated by energetic particles and radation constantly emanating from the Sun."

## Heliospheres

Def. the "region of space where interstellar medium is blown away by solar wind; the boundary, heliopause, is often considered the edge of the Solar System" is called the heliosphere.
"The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself."
"The point where the solar wind slows down is the termination shock". Further out "is the heliosheath area ... As of June 2011, the heliosheath area is thought to be filled with magnetic bubbles (each about 1 AU wide), creating a "foamy zone".

## Electron winds

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."

## Heliopauses

"[T]he point where the interstellar medium and solar wind pressures balance is called the heliopause".
"[T]he point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock".

## Local hot bubbles

The 'local hot bubble' is a "hot X-ray emitting plasma within the local environment of the Sun." "This coronal gas fills the irregularly shaped local void of matter (McCammon & Sanders 1990) - frequently called the Local Hot Bubble (LHB)."

## Recent history

The recent history period dates from around 1,000 b2k to present. The beginning of the search for X-ray sources from above the Earth's atmosphere is on August 5, 1948, 12:07 GMT. A US Army (formerly German) V-2 rocket as part of Project Hermes is launched from White Sands Proving Grounds. The first solar X-rays are recorded by T. Burnight. After detecting X-ray photons from the Sun in the course of the rocket flight, T. Burnight wrote, “The sun is assumed to be the source of this radiation although radiation of wave-length shorter than 4 angstroms would not be expected from theoretical estimates of black body radiation from the solar corona.”

## Hypotheses

* The source of electrons to bombard the Sun is external to the Sun.