Solar Impulsive Hard X-Ray Emission and Two-Stage Electron Acceleration

Heating and acceleration of electrons in solar impulsive hard X-ray （HXR） flares are studied according to the two-stage acceleration model developed by Zhang for solar ^3Herich events. It is shown that electrostatic H-cyclotron waves can be excited at a parallel phase velocity less than about the electron thermal velocity and thus can significantly heat the electrons （up to 40 MK） through landau resonance. The preheated electrons with velocities above a threshold are further accelerated to high energies in the flare-acceleration process. The flareproduced electron spectrum is obtained and shown to be thermal at low energies and power law at high energies. In the non-thermal energy range, the spectrum can be double power law if the spectral power index is energy dependent or related. The electron energy spectrum obtained by this study agrees quantitatively with the result derived from the Reuven Ramaty High Energy Solar Spectroscopic Imager （RHESSI） HXR observations in the flare of 2002 July 23. The total flux and energy flux of electrons accelerated in the solar flare also agree with the measurements.

Comparative analyses of current three-dimensional numerical solar wind models

We present a comparative study of the most advanced three-dimensional time-dependent numerical simulation models of solar wind. These models can be classified into two categories： （I） theoretical, empirical and numerically based models and （Ⅱ） self-consistent multi-dimensional numerical magnetohydrodynamic （MHD） models. The models of Category I are used to sep- arately describe the solar wind solution in two plasma flows regions： transonic/trans-Alfvrnic and supersonic/super-Alfvenic, respectively. Models of Category II construct a complete, single, numerical solar wind solution through subsonic/sub-Alfvrnic region into supersonic/super-Alfvrnic region. The Wang-Sheeley-Arge （WSA）/ENLIL in CISM is the most successful space weather model that belongs to Category I, and the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme （BATS-R-US） code in SWMF （Space Weather Modeling Framework） and the solar-interplanetary conservative element solution element MHD （SIP-CESE MHD） model in SWIM （Space Weather Integrated Model） are the most commonly-used models that belong to Category II. We review the structures of their frameworks, the main results for solar wind background studies that are essential for solar transient event studies, and discuss the common features and differences between these two categories of solar wind models. Finally, we conclude that the transition of these two categories of models to operational use depends on the availability of computational resources at reasonable cost and point out that the models＇ prediction capabilities may be improved by employing finer computational grids, incorporating more observational data and by adding more physical constraints to the models.