G. Vekstein

PARTICLE ACCELERATION IN FRAGMENTING PERIODIC RECONNECTING CURRENT SHEETS IN SOLAR FLARES

Proton and electron acceleration in a fragmenting periodic current sheet (CS) is investigated, based on the forced magnetic reconnection scenario. The aim is to understand the role of CS fragmentation in high-energy beam generation in solar flares. We combine magnetohydrodynamics and test-particle models to consider particle trajectories consistent with a time-dependent reconnection model. It is shown that accelerated particles in such a model form two distinct populations. Protons and electrons moving in open magnetic field have energy spectra that are a combination of the initial Maxwellian distribution and a power-law high-energy (E > 20 keV) part. The second population contains particles moving in a closed magnetic field around O-points. These particles move predominantly along the guiding field and their energies fall within quite a narrow range between similar to 1 MeV and similar to 10 MeV. It is also found that particles moving in an open magnetic field have a considerably wider pitch-angle distribution.

Energy release and plasma heating by forced magnetic reconnection

A simple model of forced magnetic reconnection in a force-free magnetic field is considered, which allows the calculation of the magnetic energy release during the current sheet reconnection. The dependence of this energy on characteristics of the magnetic configuration has been studied, and it was found that the released energy becomes very large when the field is near the marginal tearing stability. A persistent plasma heating provided by ongoing external driving and internal reconnection is most efficient when the time-scales of these processes are comparable. Possible implications of the obtained results for the problem of solar coronal heating are briefly discussed.

Particle acceleration at an X‐type reconnection site with a parallel magnetic field

The acceleration of charged particles at a two-dimensional magnetic reconnection site is investigated. The magnetic field has an X-type neutral point, while reconnection is driven by a uniform transverse electric field; the effect of including a uniform magnetic field component parallel to the driving electric field and transverse to the plane of the X point is studied. We focus on the adiabatic motion of strongly magnetized particles, a valid assumption everywhere for sufficiently strong parallel magnetic fields but one which excludes a region around the neutral point for weaker fields. The regime of interest is fast driven reconnection, in which the electric drift is strong. The trajectories of particles and their dependence on the magnitude of the parallel magnetic field component are investigated. Particles can be accelerated along the magnetic field lines both because of the coupling of the perpendicular electric drift with the parallel motion, which occurs in an inhomogeneous magnetic field, and the direct acceleration by the electric field. The energy spectra of particles leaving the reconnection site are also calculated.

Electric-drift generated trajectories and particle acceleration in collisionless magnetic reconnection

Adiabatic acceleration of charged particles along magnetic field lines originates from the coupling between the electric drift and longitudinal motion in a nonunidirectional magnetic field. As a result, initially slow particles entering the reconnection site of an X-type magnetic geometry can leave the latter as substantially accelerated jets directed along the magnetic separatrices. The corresponding energy spectrum has a power-law form, with the spectral index depending on the angle between the separatrices.

On plasma heating by reconnective magnetic relaxation

A self-consistent derivation of magnetic energy dissipation provided by ongoing external driving and internal current sheet reconnection in a force-free magnetic field is reported. The resulting plasma heating rate shows characteristic relaxation-type dependence on the driving frequency, being most efficient when the time scales of the driving and reconnection are comparable.

Nonlinear magnetic reconnection with collisionless dissipation

An analytical model of two‐dimensional collisionless reconnection in an X‐type magnetic geometry is presented. The conversion of magnetic energy to the kinetic energy of accelerated ions takes place in the vicinity of the neutral line. The structure of this dissipation region and the magnetic energy release rate have been investigated both for linear and nonlinear regimes of collisionless reconnection. A simple model of global reconnection flow has been constructed, assuming an incompressible ideal magnetohydrodynamics approximation outside the dissipation region. The corresponding scaling for the external Mach number (Me) is found, which predicts a maximum reconnection rate Me=1/2Rm −1/2, where Rm≊(Le/λi)2≫1 is the effective magnetic Reynolds number for collisionless reconnection (Le is the global size of the system and λi is the ion inertial skin depth).

GENERATION OF ELECTRIC CURRENTS IN THE CHROMOSPHERE VIA NEUTRAL-ION DRAG

We consider the generation of electric currents in the solar chromosphere where the ionization level is typically low. We show that ambient electrons become magnetized even for weak magnetic fields (30 G); that is, their gyrofrequency becomes larger than the collision frequency while ion motions continue to be dominated by ion-neutral collisions. Under such conditions, ions are dragged by neutrals, and the magnetic field acts as if it is frozen-in to the dynamics of the neutral gas. However, magnetized electrons drift under the action of the electric and magnetic fields induced in the reference frame of ions moving with the neutral gas. We find that this relative motion of electrons and ions results in the generation of quite intense electric currents. The dissipation of these currents leads to resistive electron heating and efficient gas ionization. Ionization by electron-neutral impact does not alter the dynamics of the heavy particles; thus, the gas turbulent motions continue even when the plasma becomes fully ionized, and resistive dissipation continues to heat electrons and ions. This heating process is so efficient that it can result in typical temperature increases with altitude as large as 0.1-0.3 eV km–1. We conclude that this process can play a major role in the heating of the chromosphere and corona.

Observational appearance of nanoflares with SXT and TRACE

We quantitatively investigate intensity fluctuations observed with the Yohkoh SXT, which is sensitive to hot (>2 MK) plasma, and TRACE, which is sensitive to cool (~1 MK) plasma. We find that the TRACE light curves contain fluctuations that are significantly larger than the photon noise and that TRACE is more sensitive to the emission from nanoflare heating than is the SXT. We discover that the standard deviation of the fluctuation (the photon noise is removed) is proportional to the mean intensity for both the SXT and TRACE loops. We also analyze the autocorrelation functions in order to obtain the duration of the intensity fluctuations. While the duration of the intensity fluctuations for the SXT loops is relatively short because of the significant photon noise, that for the TRACE loops agrees well with the characteristic cooling timescale. This is evidence that coronal loops are continuously heated by impulsive nanoflares. We estimate the energy of nanoflares to be 1025 ergs for SXT loops and 1023 ergs for TRACE loops. The occurrence rate of nanoflares is about 0.4 and 30 nanoflares s−1 in a typical SXT loop and a typical TRACE loop, respectively.

Hall assisted forced magnetic reconnection

The role of the Hall effect in forced magnetic reconnection is investigated analytically for the so-called Taylor problem. In the latter, a tearing stable slab plasma equilibrium, which is chosen here to be a simple magnetic field reversal, is subjected to a small-amplitude boundary deformation that drives magnetic reconnection (hence the adjective “forced” ) at the neutral surface within the plasma. It is shown that such reconnection becomes substantially accelerated by the Hall effect when the nondimensional parameter di=(c∕ωpi)∕a exceeds S−1∕5. Here, c∕ωpi is the ion inertial skin depth, a is the width of the plasma slab, and S≫1 is the Lundquist number of a highly conducting plasma. Two different types of external perturbation are considered. In the case of continuous quasistatic driving, with a frequency ω such that ωτA≪1, τA being the Alfven transit time, various reconnection regimes are identified. The corresponding heating rates, which are determined by the parameters di, S, and ωτA, are derived. ...

On the correspondence between forced magnetic reconnection and Alfvén resonances

This note aims to clarify the correspondence between forced magnetic reconnection and Alfven resonances in a plasma with a sheared magnetic field subjected to continuous external driving. It is shown how a transition from one regime to another occurs, and what implications this has on the magnetic energy dissipation rate.