The surface of the Sun, the photosphere, has a temperature of ~ 6000 Kelvin and emits mainly in the visible light. Above the photosphere, the Sun has an extended corona of plasma with temperatures in excess of a few million Kelvin, much hotter than the photosphere. The corona emits in the X-ray band and, contrary to the photosphere, it is highly inhomogeneous, energetic and dynamic. The model shows the X-ray emission arising from the solar corona. Its building blocks are arch-like magnetic flux tubes which confine the hot plasma. These coronal loops are mostly concentrated in bright active regions. Occasionally, sudden increases of the radiative flux occur in the most energetic and bright portion of active regions, the cores. These bright and energetic flares are transient phenomena that heat the plasma up to temperatures of tens of millions Kelvin. The solar magnetic field is responsible for the heating, but the details of the mechanism are sill debated.
Credit: INAF-Osservatorio Astronomico di Palermo.
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