Emission of Joule heating events in simulations of the solar corona

¹ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
e-mail: charalambos.kanella@astro.uio.no

Received: 13 June 2018
Accepted: 14 November 2018

Abstract

Context. Nanoscale events in cooperation with steady heating from a slow heating mechanism, such as slow-burning current-sheets, could be able to heat the corona; however, their observational traces are hard to detect via current instrumentation. After we locate heating events in magnetohydrodynamic (MHD) simulations and synthesise observational data, we extract observational signatures of small-scale events.

Aims. Our mission is threefold. The first goal is to observe the manifestation of small-scale events via three observational tools: intensity maps of three extreme ultraviolet (EUV) filters in the Atmospheric Imaging Assembly (AIA) instrument with resolution better than that in AIA images, emission measure (EM) analysis, and time-lag maps. The second goal is to identify the reason why we cannot quantify the energy release from observed events. The third goal is to study the differences between the radiation from isolated heating events and that from the whole corona.

Methods. We employed a three-dimensional magnetohydrodynamic (3D-MHD) simulation using the Bifrost code. We simulated the atmosphere of a network embedded in the quiet Sun (QS), and we identified 3D heating events in the corona in several time-steps. Then we synthesised the three observational tools for two cases. First, we considered information from the total column mass in the corona, and then we considered only regions that exhibit heating events.

Results. We report on the differences between the two regions of investigation, which also consist of the evidence to justify why observers cannot identify small-scale heating events in observations. We found that the combination of multiple heating events at different cooling phases along the line of sight gives the impression of thin elongated threads of events. For this reason, the EM as a function of temperature has a multi-thermal distribution. Both the radiation and the emission measure of the isolated heating events have values at least ten times lower than the signal calculated from the total corona. We also found that heating events move together with diffuse emission from the slow heating mechanism, and for this reason we cannot differentiate between the two. In addition, we find that the frequency of heating events and their intensity affect the EM distribution as a function of temperature. We also find that the filter’s intensity, EM, and time-lag maps of heating events are different to those incorporating information from the total column mass of the corona. However, the two regions have, on average, comparable values, which are slightly smaller than the analytical cooling timescales calculated for an optically thin and radiation-dominated atmosphere.