Gyrokinetic electron acceleration in the force-free corona with anomalous resistivity

¹ Paul Scherrer Institut, 5232 Villigen, Switzerland e-mail: arzner@astro.phys.ethz.ch
² Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece e-mail: vlahos@astro.auth.gr

Received: 2 February 2006
Accepted: 27 March 2006

Abstract

Aims.We numerically explore electron acceleration and coronal heating by dissipative electric fields.

Methods.Electrons are traced in linear force-free magnetic fields extrapolated from SOHO/MDI magnetograms, endowed with anomalous resistivity (η) in localized dissipation regions where the magnetic twist exceeds a given threshold. Associated with is a parallel electric field that can accelerate runaway electrons. In order to gain observational predictions, we inject electrons inside the dissipation regions and follow them for several seconds in real time.

Results.Precipitating electrons that leave the simulation system at height are associated with hard X rays, and electrons that escape at height  km are associated with normal-drifting type IIIs at the local plasma frequency. A third, trapped population is related to gyrosynchrotron emission. Time profiles and spectra of all three emissions are calculated, and their dependence on the geometric model parameters and on η is explored. It is found that precipitation generally precedes escape by fractions of a second and that the electrons perform many visits to the dissipation regions before leaving the simulation system. The electrons impacting reach higher energies than the escaping ones, and non-Maxwellian tails are observed at energies above the largest potential drop across a single dissipation region. Impact maps at z = 0 show the tendency of the electrons to arrive at the borders of sunspots of one polarity.

Conclusions.Although the magnetograms used here belong to non-flaring times, so that the simulations refer to nanoflares and “quiescent” coronal heating, it is conjectured that the same process, on a larger scale, is responsible for solar flares.