A prominence, sometimes referred to as a filament, 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.
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 long, roughly a solar radius.
History
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."
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.
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.
Classification
There are a number of different prominence classification schemes in use today. One of the most widely used and basic schemes classifies prominences based on the magnetic environment in which they had formed. There are three classes: Active region prominences, or active region filaments, form within the relatively strong magnetic fields at the centers of active regions. Active region prominences have lifetimes ranging from hours to days and erupt more often than prominences belonging to the other classes. As a consequence of being located within active regions, active region prominences are usually found in low latitudes. Intermediate prominences, or intermediate filaments, form between a weak unipolar plage regions and active regions. Quiescent prominences, or quiescent filaments, form in the weak background magnetic field far from any active regions. Unlike active region prominences, quiescent prominences are relatively stable and can have lifetimes ranging from weeks to months, hence the name . Quiescent prominences are typically located at high latitudes around what is referred to as the polar crown. Additionally, quiescent prominences generally reach much greater heights in the corona 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 in the chromosphere with strong H, He I, and ionized metal lines, but weak He II lines.
Morphology
Filament channels
Prominences form in magnetic structures known as filament channels where they are thermally shielded from the surrounding corona and supported against gravity. These channels are found in the chromosphere and lower corona above divisions between regions of opposite photospheric magnetic polarity known as polarity inversion lines (PIL). The presence of a filament channel is a necessary condition for the formation of a prominence, but a filament channel can exist without containing a prominence. Multiple prominences may form and erupt from within one filament channel over the channel's lifetime. The magnetic field making up the filament channel is predominantly horizontal, pointing in the same direction on both sides of the PIL (see ).
Prominence material does not occupy the entire width of the filament channel; a tunnel-like region less dense than the corona, known as a coronal cavity, occupies the volume between the prominence and the overlying magnetic arcade.