Editing Stellar corona (section)

====Filament cavities====
[[File:Crackling with Solar Flares.jpg|thumb|left|Image taken by the [[Solar Dynamics Observatory]] on October 16, 2010. A very long filament cavity is visible across the Sun's southern hemisphere.]]

Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected ''coronal holes''.<ref name=Giacconi />

Filament cavities are cooler clouds of plasma suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the [[magnetic pressure]] and plasma pressure must be constant everywhere on the [[heliosphere]] in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure <math>p</math> can be calculated by the [[state equation]] of a perfect gas: <math> p = n k_B T</math>, where <math>n</math> is the [[particle number density]], <math>k_B</math> the [[Boltzmann constant]] and <math>T</math> the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.{{Citation needed|date=February 2022}}