On the Possibility of Heating the Solar Corona by Heat Fluxes from Coronal Magnetic Structures

Abstract

We estimate the role of heat flux from hot magnetic loops and open flux tubes into the surrounding corona as a possible source of heating of the corona. We show that hot magnetic-flux tubes (Type-II spicules) provide a more efficient source of coronal heating than hot magnetic loops, as the closed magnetic structure of a loop substantially restricts the heat flux into the corona. In order to compensate for radiation and thermal-conduction losses, approximately $10^{4}$ Type-II spicules with a temperature of several million Kelvin are required, which is about 1% of the number of spicules simultaneously observed on the solar disk. Our analysis shows that the principal source of energy that heats the coronal plasma is photospheric convection, which generates electric currents of about $10^{10}$ – $10^{12}$ A in magnetic loops and spicules. Dissipation of the currents increases significantly in the partially ionized plasma, i.e. when ion–atom collisions and the associated Cowling conductivity occur. This results in two important effects: heating of plasma in magnetic structures up to several million Kelvin, and ejection of hot plasma from open magnetic-flux tubes to the corona, replenishing the corona with hot plasma. The ejection of hot plasma results from the heating of the spicule foot-point by electric-current dissipation, which grows with a sporadic increase in the velocity of photospheric convection, for example, due to five-minute oscillations or the Rayleigh–Taylor instability. As a result, the rate of heating the photospheric foot-points of the spicules by ring currents exceeds radiation losses, which leads to a jump in the pressure gradient and the ejection of hot plasma into the corona from the open tips of the magnetic-flux tubes.