region (PCTR) where there is a steep temperature gradient. The PCTR is responsible for most of the optical emission of prominences. Overlying structures Above filament channels lie overarching magnetic s which can extend from into the corona. Above these arcades, the closed coronal magnetic field may extend radially outward forming what is known as a helmet streamer. These streamers may reach a solar radius or more above the Sun's surface. Chirality Filament channels and their prominence, if present, exhibit chirality. When observed from the side of the filament channel with positive magnetic polarity, the channel is said to be dextral if the horizontal magnetic field is oriented rightward and sinistral if it is oriented leftward. Dextral channels have been found more frequently in the Sun's northern hemisphere and sinistral channels more frequently in the southern hemisphere. The horizontally oriented magnetic field causes chromospheric fibrils along the filament channel to lie nearly parallel to the PIL and anti-parallel to one another on opposite sides of the PIL. The directions that these fibrils are oriented depend on the chirality of the channel. On the side of the PIL with positive magnetic polarity, dextral channels have fibrils which stream to the right and barbs which bear to the right, whereas sinistral channels have fibrils which stream to the left and barbs which bear to the left. Additionally, the overlying magnetic arcades of dextral channels are left-skewed, and those of sinistral channels are right-skewed. Formation The exact mechanism which leads to the formation of solar prominences is not currently known. Models must be able to explain the formation of the filament channel and its hemisphere-dependent chirality, as well as the origin of the dense plasma that makes up the prominence core. Eruption Some prominences are ejected from the Sun in what is known as a prominence eruption. These eruptions can have speeds ranging from 600 km/s to more than 1000 km/s. At least 70% of prominence eruptions are associated with an ejection of coronal material into the solar wind known as a coronal mass ejection. See also Active region Hyder flare Solar flare Explanatory notes References Further reading External links Articles containing video clips Solar phenomena