On the Possibility of Heating the Solar Corona by Heat Fluxes from Coronal Magnetic Structures
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On the Possibility of Heating the Solar Corona by Heat Fluxes from Coronal Magnetic Structures V.V. Zaitsev1 · A.V. Stepanov2
· P.V. Kronshtadtov1
Received: 21 July 2020 / Accepted: 31 October 2020 © Springer Nature B.V. 2020
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 104 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 1010 – 1012 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. Keywords Coronal heating · Convection · Electric current · Magnetic loops · Magnetic-flux tubes
B A.V. Stepanov
[email protected]
1
Institute of Applied Physics, Ulianova str. 46, Nizhny Novgorod, 603950, Russia
2
Pulkovo Observatory, Pulkovo chaussee 65, Saint Petersburg, 196140, Russia
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V.V. Zaitsev et al.
1. Introduction The problem of the heating of stellar coronas remains one of the important unsolved problems in astrophysics. The optical radiation of the Sun’s photosphere cannot heat the corona to one million Kelvin and higher, as the effective temperature of the photosphere is approximately 6 × 103 K. At the same time, the solar corona loses a significant amount of energy due to radiation and thermal conductivity. Therefore, in order to maintain the coronal temperature, sufficiently powerful heating sources are needed to compensate for these losses. Observations obtained with the Interface Region Imaging Spectrograph (IRIS: De Pontieu et al., 2014) and the High-resolution Coronal Imager (Hi-C: Kobayashi et al., 2014) indicate that
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