Solar prominence

A prominence, sometimes referred to as a filament,[lower-alpha 1] is a large plasma and magnetic field structure extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the solar corona. While the corona consists of extremely hot plasma, prominences contain much cooler plasma, similar in composition to that of the chromosphere.

False-color image of a solar prominence captured in 304 Å (30.4 nm) radiation emitted by He II

Prominences form over timescales of about a day and may persist in the corona for several weeks or months, looping hundreds of thousands of kilometers into space. Some prominences may give rise to coronal mass ejections. Scientists are currently researching how and why prominences are formed.

A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 km (500,000 mi) long,[2] roughly a solar radius.

History
Solar prominences (in red) visible around the edge of the Sun during a solar eclipse.

The first detailed description of a solar prominence was in 14th-century Laurentian Codex, describing the Solar eclipse of 1 May 1185. They were described as "flame-like tongues of live embers."[3][4][5]

Prominences were first photographed during the solar eclipse of July 18, 1860 by Angelo Secchi. From these photographs, altitude, emissivity, and many other important parameters were able to be derived for the first time.[6]

During the solar eclipse of August 18, 1868, spectroscopes were for the first time able to detect the presence of emission lines from prominences. The detection of a hydrogen line confirmed that prominences were gaseous in nature. Pierre Janssen was also able to detect an emission line corresponding to an at the time unknown element now known as helium. The following day, Janssen confirmed his measurements by recording the emission lines from the now unobstructed Sun, a task that had never been done before. Using his new techniques, astronomers were able to study prominences daily.[7]

Classification
H-alpha image of the solar disk showing quiescent filaments (QF), intermediate filaments (IF), and active region filaments (ARF).

There are a number of different prominence classification schemes in use today. One of the most widely used and basic schemes divides prominences into three classes based on the magnetic environment in which they had formed. These three classes are known as the active region prominences, quiescent prominences, and intermediate prominences.[8] Active region prominences are defined as those having formed within the relatively strong magnetic field at the centers of active regions, whereas, in contrast, quiescent prominences are defined as those having formed in the weak background field far from any active regions. In between these two lies the intermediate prominences defined as having formed between weak unipolar plage regions and active regions.

Active region prominences and quiescent prominences differ in fundamental ways. The former, as a consequence of only being located within active regions, are usually found in the lower heliographic latitudes whereas the latter are typically found in the higher latitudes around the polar crown.[9][10] Additionally, active region prominences, having lifetimes of only a few hours to days, are more eruptive than quiescent prominences which have lifetimes ranging from weeks to months.[11] Quiescent prominences generally reach much greater heights than active region prominences.

Active region and quiescent prominences can also be differentiated by their emitted spectra. The spectra of active region prominences is identical to that of the upper chromosphere having strong He II lines but very weak ionized metal lines. On the other hand, the spectra of quiescent prominences is identical to the spectra measured at 1,500 km (930 mi) in the chromosphere with strong H, He I, and ionized metal lines, but weak He II lines.[12]

Morphology

Filament channels

Prominences are thought to form in magnetic structures known as filament channels. These channels are found in the lower corona above divisions between regions of opposite photospheric magnetic polarity known variously as polarity inversion lines (PIL), polarity reversal boundaries (PRB), or neutral lines.[13][14] Filament channels are able to thermally shield prominences from the surrounding corona and support them against gravity.[7]

Spines and barbs

Typical prominences posses a narrow structure oriented along the filament channel known as a spine. The spine defines the upper main body of a prominence and is generally in the form of a vertical sheet. Many prominences also posses smaller structures that diverge from the spine towards the chromosphere referred to as barbs. Spines and barbs are both composed of thin threads that trace the magnetic field similar to chromospheric fibrils.[7][14]

H-alpha image of an active region filament showing a spine and two barbs.[13]

Overlying structures
Main article: Helmet streamer

Above filament channels lie overarching magnetic arcades which can extend from 50,000 to 70,000 km (31,000 to 43,000 mi) into the corona. Above these arcades, the closed coronal magnetic field may extend radially outward forming what is known as a helmet streamer.[15] These streamers may reach a solar radius or more above the Sun's surface.[7]

Eruption
Main article: Coronal mass ejection

Some prominences are so powerful that they throw out matter from the Sun into space at speeds ranging from 600 km/s to more than 1000 km/s.[1]

See also

Explanatory notes
  1. When viewed against the background of space (off-limb), they are referred to as prominences; when viewed against the Sun's surface (on-disk), they are referred to as filaments.[1]

References
  1. "About Filaments and Prominences". solar.physics.montana.edu. Retrieved 2 January 2010.
  2. Atkinson, Nancy (6 August 2012). "Huge Solar Filament Stretches Across the Sun". Universe Today. Retrieved 11 August 2012.
  3. "1185: The first description of solar prominences". SOLAR PHYSICS HISTORICAL TIMELINE (0–1599). High Altitude Observatory. 2008.
  4. "1185: The first description of solar prominences" (PDF). Great Moments in the History of Solar Physics. Université de Montréal. 2008. Archived from the original on 21 August 2015. Retrieved 30 March 2015.
  5. Poitevin, Patrick; Edmonds, Joanne (2003). "Solar Eclipse Newsletter" (PDF). Retrieved 30 March 2015.
  6. Secchi, Angelo (1870). Le Soleil, Part 1. Paris. p. 378.
  7. Vial, Jean-Claude; Engvold, Oddbjørn (2015). Solar Prominences. Springer. ISBN 978-3-319-10415-7.
  8. Engvold, Oddbjørn (1998). "Observations of Filament Structure and Dynamics". International Astronomical Union Colloquium. 167: 22–31. doi:10.1017/S0252921100047229.
  9. Menzel, Donald H.; Jones, F. Shirley (December 1962). Solar Prominence Activity, 1944-1954.
  10. Minarovjech, M.; Rybanský, M.; Rušin, V. (1998). "Time-Latitude Prominence and the Green Corona Distribution Over the Solar Activity Cycle". International Astronomical Union Colloquium. 167: 484–487. doi:10.1017/S0252921100048132.
  11. Mackay, D. H.; Karpen, J. T.; Ballester, J. L.; Schmieder, B.; Aulanier, G. (April 2010). "Physics of Solar Prominences: II—Magnetic Structure and Dynamics". Space Science Reviews. 151 (4): 333–399. arXiv:1001.1635. doi:10.1007/s11214-010-9628-0.
  12. Zirin, Harold; Tandberg-Hanssen, Einar (1960). "Physical Conditions in Limb Flares and Active Prominences. IV. Comparison of Active and Quiescent Prominences". The Astrophysical Journal. 131: 717–724.
  13. Parenti, Susanna (2014). "Solar Prominences: Observations" (PDF). Living Reviews in Solar Physics. 11. doi:10.12942/lrsp-2014-1. Retrieved 29 January 2022.
  14. Gibson, Sarah E. (December 2018). "Solar prominences: theory and models: Fleshing out the magnetic skeleton" (PDF). Living Reviews in Solar Physics. 15 (1): 7. doi:10.1007/s41116-018-0016-2. Retrieved 29 January 2022.
  15. Guo, W. P.; Wu, S. T. (10 February 1998). "A Magnetohydrodynamic Description of Coronal Helmet Streamers Containing a Cavity". The Astrophysical Journal. 494 (1): 419–429. doi:10.1086/305196. Retrieved 19 April 2022.

Further reading
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