Stochastic acceleration of electrons in solar flares

The generation of lower-hybrid waves by cross-field currents is applied to reconnection processes proposed for solar flares. Recent observations on fragmentation of energy release and acceleration, and on hard X-ray (HXR) spectra are taken into account to develop a model for electron acceleration by resonant stochastic interactions with lower-hybrid turbulence. The continuity of the velocity distribution is solved including collisions and escape from the turbulence region. It describes acceleration as a diffusion process in velocity space. The result indicates two regimes that are determined by the energy of the accelerating electrons which may explain the double power-law often observed in HXR spectra. The model further predicts an anticorrelation between HXR flux and spectral index in agreement with observations.     

On a common derivation of the averaging method and the two-timescale method

In this paper it is shown that the well-known averaging method (of Krylov, Bogoliubov-Mitropolski) and the two-timescale method, applied to periodic first-order ordinary differential equations, can be derived from one common principle, as two more or less complementary special cases. The uniformity of this treatment includes the proof of asymptotic convergence of both methods, since a single proof can be given under certain hypotheses, which are verifieda posteriori in both cases.     

Plastic flow and state-dilatancy for geomaterials

In this paper, the stress-dilatancy hypothesis is revisited and a stress-dilatancy framework is established such that it can be compatibly used for different yield functions. The influence of density–pressure and other state variables are systematically accommodated. The framework is further evaluated by obtaining and extending some flow rules for the Mohr–Coulomb criterion and novel stress-dilatancy formalism for the Hoek–Brown criterion.     

An Interpretation of the Coronal Holes’ Visibility in the Millimeter Wavelength Range

Various observations indicate that coronal holes generally appear as low brightness temperature regions (LTRs) in the centimeter and millimeter wavelength ranges. However, within their borders local enhancements of radiation, that is, high brightness temperature regions (HTRs), often occur. The theory behind the described behavior is not fully understood and therefore we analyze full-disk solar images obtained at a wavelength of 8 mm at Metsähovi Radio Observatory and compare them with data simultaneously taken in other wavelength ranges. The observational finding that the average brightness temperature of coronal holes is not much different from the quiet-Sun level (with localized deviations toward higher and lower intensities on the order of a few percent) is compared with theoretical models of the thermal bremsstrahlung radiation originating in the solar chromosphere, transition region, and corona. Special attention is devoted to the interpretation of the localized enhancements of radiation observed inside coronal holes at millimeter wavelengths. The main conclusion is that the most important contribution to the brightness temperature comes from an increased density in the transition region and low corona (i.e., at the heights where the temperature is below 10⁶ K). This can explain both the LTRs and HTRs associated with coronal holes.