Entropy mode at a magnetic null point as a possible tool for indirect observation of nanoflares in the solar corona

¹ Group of Astrophysics, Institute of Physics, UMCS, ul. Radziszewskiego 10, 20-031 Lublin, Poland
e-mail: kmur@kft.umcs.lublin.pl
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: teimuraz.zaqarashvili@oeaw.ac.at
³ Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
e-mail: V.Nakariakov@warwick.ac.uk
⁴ Abastumani Astrophysical Observatory, Ilia State University, University St. 2, Tbilisi, Georgia
⁵ Central Astronomical Observatory, Russian Academy of Sciences, Pulkovo, 196140 St Petersburg, Russia

Received: 23 March 2011
Accepted: 12 July 2011

Abstract

Aims. We aim to explore the dynamics of the entropy mode perturbation excited by an energy release in the vicinity of a magnetic null point that is embedded in a gravitationally stratified solar corona.

Methods. We solve two-dimensional, time-dependent magnetohydrodynamic equations numerically to find spatial and temporal signatures of the entropy mode that is triggered impulsively by a spatially localized pulse of the gas pressure.

Results. We find that the properties of the entropy mode are determined by the sign of the initial pressure pulse. The initial increase in the gas pressure creates, together with the magnetoacoustic-gravity waves, a stationary void of the rarefied plasma at the launching place, associated with the entropy mode. In contrast, an initial decrease in the gas pressure, which corresponds to a rapid (or catastrophic) cooling, forms a blob of the dense plasma at the launching place.

Conclusions. The cool, dense blobs at magnetic null points may be observed in transition region and chromospheric spectral lines at coronal heights off the solar limb and may be associated with the places of nanoflare occurrence. Therefore, extensions of entropy mode studies may produce a diagnostic tool for indirect observations of nanoflares. The dense cool blobs may be affected by the gravity or carried by downflows, hence may initiate a coronal rain.