Solar prominence and filament

Solar prominences consist of relatively cool, dense plasma that is suspended in the solar corona at heights up to 10⁵ km above the chromosphere. When solar prominences locate at the solar limb, they appear as bright features against the dark background; and they are called filaments when solar prominences are observed on the solar disk, where they are seen in absorption and appear to be dark structure. Solar prominences and solar filaments are different manifestations of the same object. Figure 1 shows a full disk image of the Sun observed in the Hα(656.3 nm) line by the Big Bear Solar Observatory (BBSO), on which filaments and prominences are clear seen.

Solar prominences are very dynamic and appear in thousands of postures. They often appear in the forms of arches, hedges, bushes, clouds, jets, and tongues of fire (Figure 2).
The feet of a solar prominence are believed to locate below the photosphere and its main body extends to the corona. The density of the corona is just about 0.1 grams per cubic meter and the temperature exceeds millions of Kelvin. However, density of the solar prominence is about hundreds times higher than the corona while the temperature is lower than one-percent of the corona. Solar prominences may exist weeks and months in the corona. It is absolutely amazing that two such different objects coexist simultaneously for so long a time, just like ice does not dissolve in a furnace.

Solar prominences are supported by non-potential magnetic structures, which can store enough magnetic free energy to power solar eruptions. Solar prominences may last a few hours to several months. Under certain conditions, they may become unstable and eventually erupt. Solar prominence eruptions are often accompanied by solar flares and/or coronal mass ejections (CMEs), which further impact on the Earth and human activities via the so-called space weather effect.
The outstanding questions in solar prominence studies include: How solar prominences are formed and maintained? How are solar prominences supported? What is the trigger mechanism of prominence eruptions? How are prominence eruptions related to solar flares and CMEs?