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

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 ∇ ×b exceeds a given threshold. Associated with η > 0 is a parallel electric field E = ηj which 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 which leave the simulation system at height z = 0 are associated with hard X rays, and electrons which escape at height z ∼ 3·10 4 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 preceeds 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 z = 0 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 a 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.

Magnetic Field Extrapolations into the Corona: Success and Future Improvements

“…Radio The radio emission can be observed during eruptive events such as flares, CMEs or filament eruptions, and it has been often suggested that the radio emission in the high corona occurs at the interface between closed and open magnetic field regions (Klassen et al, 1999;Bentley et al, 2000;Paesold et al, 2001;Aurass et al, 2003Aurass et al, , 2005Aurass et al, , 2011Grechnev et al, 2006;Arzner and Vlahos, 2006;Hofmann and Ruždjak, 2007;Wen, Wang, and Zhang, 2007;Klein et al, 2008;Chen et al, 2011;Tun, Gary, and Georgoulis, 2011;Del Zanna et al, 2011;Iwai et al, 2012;Hao et al, 2012). The gyroemission is also used to determine the strength of the magnetic field in the corona (Pallavicini, Sakurai, and Vaiana, 1981;Schmahl et al, 1982;Hildebrandt, Seehafer, and Krueger, 1984;Brosius et al, 1997Brosius et al, , 2002Lee et al, 1998Lee et al, , 1999Grebinskij et al, 2000;Ryabov et al, 2005;Bogod, Stupishin, and Yasnov, 2012).…”

3D Solar Null Point Reconnection MHD Simulations

“…The diffusive part of the electric field (ηj) included in the induction equation is responsible for both changing the magnetic field topology and for transforming magnetic energy into Joule dissipation in the MHD picture. In the Sun the diffusive electric field component parallel to the magnetic field is responsible for particle acceleration (Arzner and Vlahos, 2006). It is therefore of particular interest to see how this field evolves with the boundary displacement, and to find out where it concentrates and how large values it reaches.…”

Astrophysics in 2006

“…Simulations of this kind revealed how particles are accelerated in 3D magnetic nullpoints (Dalla and Browning 2006), in 2D X-points with a single X-line (Hannah and Fletcher 2006), in force-free fields with anomalous resistivity (Arzner and Vlahos 2006), in strong turbulence regions , in particular for 3 He and 4 He ions (Liu et al 2006e), in spatially intermittent turbulence regions (Decamp and Malara 2006), in stochastic current sheets of stressed fields (Turkmani et al 2006), in 3D Harris reconnecting current sheets where particles follow chaotic, regular, and mirror-type regular orbits (Gontikakis et al 2006), or in fan 3D reconnection geometries (Litvinenko 2006). The highest energies of accelerated, gamma-ray producing particles reported this year went up to >200 MeV, produced by neutral pion decay (Kuznetsov et al 2006).…”