Rapid Magnetic Energy Release, Its Possible Role in Coronal Heating and Solar Wind Acceleration

摘要

Studies of the EUV emission from coronal bright points, active regions and apicules suggest that stochastic mechanisms may plan an important role in heating the solar atmosphere at chromospheric and coronal levels. In small magnetic bipolar regions (spatial extent < 1') the observed EUV variations suggest that impulsive heating at chromospheric and coronal levels appears to be very important, possibly the dominant form of heating. The mechanism most likely involves rapid release of magnetic energy, possibly associated with the emergence of magnetic flux from lower levels into the chromosphere and corona. In larger scale (> 1') magnetic bipolar regions, there is evidence for both quasi-steady and impulsive heating, with quasi-steady heating dominating. This heating could be caused by either a mechanism such as steady-state current dissipation, or by a stochastic process whose integrated effect (resulting from the smoothing caused by finite radiative and conductive cooling times) yields a nearly constant radiative output. The widespread variability of the emission in spectral lines formed at transition region temperatures (100000 < T < 1000000 K) provides eveidence that impulsive energy releases are a common, nearly continuous phenomenon in bipolar magnetic regions. However, at the present time we do not know what fraction of the total energy deposition in the atmosphere (chromosphere and low corona) originates in impulsive phenomena.