Coronal cloud

A coronal cloud is a cloud composed of plasma and usually associated with a star or other celestial or astronomical body, extending sometimes millions of kilometers into space, or thousands of light-years, depending on the associated body. The high temperature of the coronal cloud gives it unusual spectral features. These features have been traced to highly ionized atoms of elements such as iron which indicate a plasma's temperature in excess of 10⁶ K (MK).

Although a coronal cloud is usually filled with high-temperature plasma at temperatures of 1–2 (MK), hot active regions and postflare loops have plasma temperatures of 2–40 MK.

In a coronal cloud are magnetic field lines filled with plasma-like fluxtubes, which are governed by magnetohydrodynamics (MHD) and can exhibit propagating and standing waves.

Resonance lines of hydrogen-like and helium-like ions from carbon and all heavier elements occur below 6.0 nm. 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).

Coronal star

'Coronal stars' are ubiquitous among the stars in the cool half of the Hertzsprung-Russell diagram. Generally, a star is a massive, luminous ball of plasma held together by gravity.

The plasma density of the solar corona is on the order of 5 x 10⁻¹⁷ g cm⁻³. Near the surface of the chromosphere this increases to 10⁻¹⁴ g cm⁻³. With the mass of a hydrogen atom at ~1.67 x 10⁻²⁴ g, if the solar corona is only monatomic hydrogen then the atom density is on the order of 3 x 10⁷ atoms/cm³. Near to a chromosphere or chromospheric type plasma cloud, this density increases to ~6 x 10⁹ atoms/cm³.

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.

Free-flowing coronal wind

The high temperature of a coronal cloud requires magnetic confinement unless it is a free-flowing coronal wind.

The total number of particles carried away from the Sun by the solar wind is about 1.3 per second. or 6.7 billion tons per hour. This is equivalent to losing a mass equal to the Earth every 150 million years. However, only about 0.01% of the Sun's total mass has been lost through the solar wind. Other stars have much stronger stellar winds that result in significantly higher mass loss rates.

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 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 K and it nearly matches the composition of the Sun's photosphere. 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.

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. 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.

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. 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.

Coronal mass ejection

The ejected material is a plasma consisting primarily of electrons and protons, but may contain small quantities of heavier elements such as helium, oxygen, and even iron.

A typical coronal mass ejection (CME) may have any or all of three distinctive features:

# a cavity of low electron density, # a dense core (the prominence, which appears as a bright region on coronagraph images embedded in this cavity), and # a bright leading edge.

Coronal mass ejections reach velocities between 20 km/s to 3200 km/s with an average speed of 489 km/s, based on SOHO/LASCO measurements between 1996 and 2003. The average mass is 1.6 x 10¹² kg. The values are only lower limits.

CMEs are daily occurrences averaged over a solar cycle and involve significant masses, typically 10¹⁵ to 10¹⁶ g.

Local hot bubble

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. The most likely candidate for the remains of this supernova is Geminga ("Gemini gamma-ray source"), a pulsar in the constellation Gemini. It is supposed that Geminga is a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago. This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System.

Interstellar medium

(known by astronomers as H II from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere as observed with the Wisconsin Halpha Mapper .]] The 'interstellar medium' is the gas and dust that pervade interstellar space: the matter that exists between the stars within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy, in the form of electromagnetic radiation, that occupies the same volume is the 'interstellar radiation field'.

The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99 % gas and 1 % dust by mass. Densities range from a few thousand to a few hundred million particles per cubic meter with an average value in the Milky Way of a million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.

The Galactic interstellar medium has a hot (T ~ 3 x 10⁵ – 10⁶ K component. The presence of this gas is indicated by observations of

# soft X-rays and # O VI absorption.

Some 30–70 % fractional volume of the interstellar medium is 10⁶ – 10⁷ K in temperature and is referred to as the 'hot interstellar medium' (HIM). Its density ranges from 10⁻⁴ to 10⁻² atoms/cm³ and its hydrogen is ionized as are its metals (highly ionized). These regions emit X-rays and exhibit absorption lines of the highly ionized metals, primarily in the ultraviolet.

Galactic corona

The existence of hot coronal gas within the Milky Way halo is one of the major discoveries of the ROSAT mission.

For no galactic corona, the gas pressure above some height (H) above the galactic disk is small compared to that in the disk. The galactic halo region is that region at a distance greater than H. When the only source of energy and mass for the halo is the galactic disk, gas streams into the halo until the pressure gradient is such that the pressure in the halo approaches the disk pressure, on a timescale of about 5 x 10⁷ yr.

Intergalactic medium

Surrounding and stretching between galaxies, there is a rarefied plasma that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the universe in rich clusters of galaxies.

The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of 10⁵ K to 10⁷ K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). Computer simulations indicate that on the order of half the atomic matter in the universe might exist in this warm–hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10⁸ K and above.

Warm–hot intergalactic medium

The 'warm–hot intergalactic medium' (WHIM) refers to a sparse, hot gas which is believed by cosmologists to exist in the spaces between galaxies. In May 2010 a giant reservoir of WHIM was detected by the Chandra X-ray Observatory lying along the wall shaped structure of galaxies (Sculptor Wall) some 400 million light-years from Earth.

Hot intergalactic medium

The 'hot intergalactic medium' is the hot intragroup gas within galaxy clusters and groups such as the Local Group of galaxies.

Observed coronal cloud

From the SIMBAD data base is a collection of objects which are both astronomical X-ray sources and coronal clouds.

{| class="wikitable sortable" |+ Observed coronal clouds |- ! Source !! Abbreviation !! Number !! Known X-ray sources !! Known gamma-ray sources (gam) |- | Active galactic nucleus || AGN || 34,232 || 6265|| 386 |- | Astronomical gamma-ray source || gam + gB || 1872 + 7638 || 986 + 23 || 1872 |- | Astronomical infrared source || IR || 1,888,729 || 28,497 || 484 |- | Astronomical blue source || blu || 19,323 || 383 || 31 |- | Astronomical radio source || Rad || 487,066 || 5211 || 409 |- | Astronomical red source || red || 269 || 17 || 0 |- | Astronomical ultraviolet source || UV || 87,216 || 2767 || 119 |- | Astronomical X-ray source || X || 203,637 || 203,637 || 986 |- | Cloud || Cld || 8379 || 4 || 0 |- | Dark nebula || DNe || 20,972 || 6 || 1 |- | Dwarf nova || DN* || 478 || 82 || 2 |- | Emission object || EmO || 11,837 || 181 || 9 |- | Galaxy || G || 1,287,663 || 5984 || 397 |- | HI region || HI || 5724 || 36 || 7 |- | HII region || HII || 25,654 || 198 || 6 |- | Interstellar medium || ISM || 4530 || 66 || 1 |- | Molecular cloud || MoC || 5679 || 7 || 3 |- | Nova || No* || 1360 || 74 || 13 |- | Nova-like star || NL* || 103 || 27 || 0 |- | Open galactic cluster || OpC || 2161 || 21 || 0 |- | Planetary nebula || PN || 10,737 || 48 || 2 |- | Quasar || QSO || 148,801 || 5796 || 323 |- | Reflection nebula || RNe || 1799 || 7 || 0 |- | Star || * || 1,792,160 || 18,291 || 227 |- | Supernova || SN || 6391 || 21 || 7 |- | Supernova remnant || SNR || 1166 || 246 || 29 |- | Young stellar object || Y*O || 8151 || 1694 || 0 |- |}

From inspection of the above table, a large number of X-ray sources are only known as X-ray sources.

See also

Astronomical cosmic-ray source

Astronomical X-ray sources

Dark matter

Dark nebula

Extragalactic cosmic ray

Galactic cosmic ray

Index table for X-ray and gamma-ray sources

Ionization

Large-scale structure of the cosmos

Nebula

Orion-Eridanus Superbubble

Rotating magnetic field

Soft X-ray transient

Solar cosmic ray

Solar energetic particle

Solar flare

Solar surface fusion

Star-forming region

Super soft X-ray source

Ultra-high-energy cosmic ray

Visibly dark X-ray source

X-ray astronomy

X-ray generation

X-ray transient

References

Further reading

General overview.

External links

How Many Known X-Ray (and Other) Sources Are There?

NASA/IPAC Extragalactic Database – NED

SIMBAD Astronomical Database

X-ray all-sky survey on WIKISKY

Category:Astronomical X-ray sources Category:H II regions Category:Intergalactic media Category:Interstellar media Category:Large-scale structure of the cosmos Category:Milky Way Galaxy Category:Plasma physics Category:X-ray astronomy