Diagnostics of Ellerman Bombs with High-resolution Spectral Data

Ellerman bombs (EBs) are tiny brightenings often observed near sunspots. The most impressive characteristic of the EB spectra is the two emission bumps in both wings of the H$alpha$ and ion{Ca}{II} 8542 {AA} lines. High-resolution spectral data of three small EBs were obtained on 2013 June 6 with the largest solar telescope, the 1.6 meter New Solar Telescope (NST), at the Big Bear Solar Observatory. The characteristics of these EBs are analyzed. The sizes of the EBs are in the range of 0.3arcsec--0.8arcsec and their durations are only 3--5 minutes. Our semi-empirical atmospheric models indicate that the heating occurs around the temperature minimum region with a temperature increase of 2700--3000 K, which is surprisingly higher than previously thought. The radiative and kinetic energies are estimated to be as high as 5$times$10$^{25}$--3.0$times$10$^{26}$ ergs despite the small size of these EBs. Observations of the magnetic field show that the EBs appeared just in a parasitic region with mixed polarities and accompanied by mass motions. Nonlinear force-free field extrapolation reveals that the three EBs are connected with a series of magnetic field lines associated with bald patches, which strongly implies that these EBs should be produced by magnetic reconnection in the solar lower atmosphere. According to the lightcurves and the estimated magnetic reconnection rate, we propose that there is a three phase process in EBs: pre-heating, flaring and cooling phases.

Energy Spectrum of the Electrons Accelerated by a Reconnection Electric Field: Exponential or Power Law?

The direct current (DC) electric field near the reconnection region has been proposed as an effective mechanism to accelerate protons and electrons in solar flares. A power-law energy spectrum was generally claimed in the simulations of electron acceleration by the reconnection electric field. However in most of the literature, the electric and magnetic fields were chosen independently. In this paper, we perform test-particle simulations of electron acceleration in a reconnecting magnetic field, where both the electric and magnetic fields are adopted from numerical simulations of the MHD equations. It is found that the accelerated electrons present a truncated power-law energy spectrum with an exponential tail at high energies, which is analogous to the case of diffusive shock acceleration. The influences of reconnection parameters on the spectral feature are also investigated, such as the longitudinal and transverse components of the magnetic field and the size of the current sheet. It is suggested that the DC electric field alone might not be able to reproduce the observed single or double power-law distributions.