The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration - large-scale Alfven wave pulses - transport the energy and magnetic-field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. Firstly, in a coronal plasma with beta < m_e/m_p, the waves propagate as inertial Alfven waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wavefronts. Secondly, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently-observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare "number problem" of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.
Multi-wavelength observations of an unusual impulsive flare associated with CME
对2001年3月10日发生在NOAA AR 9368（N27 W 42）上的一次强脉冲太阳耀斑1B/M6.7的多波长观测结果进行了详细的分析。观测结果表明，耀斑是一个非常强的脉冲，在HXR中有非常硬的光谱，显示非热辐射占主导地位。另一方面，该耀斑还产生了II型射电爆发和日冕物质抛射（CME），这不是脉冲耀斑的一般特征。在H $α$中，我们观察到亮质量喷出物（BME），然后是暗质量喷出物（DME）。基于BME和CME的爆发时间和方向的一致性，我们认为这两种现象是密切相关的。据推测，能量积累发生由于光球重连接之间出现的正寄生极性和占主导地位的负极性，这导致作为一个后果的通量抵消。由于正寄生极性的进一步出现，导致通量的强烈增强的抵消，剪切增加到$>80^o$。看来，这种增强的磁通量抵消在一个强烈的剪切区触发了脉冲耀斑。
Spectroscopic observations of a solar limb flare recorded by SUMER on SOHO reveal, for the first time, hot fast magnetic reconnection outflows in the corona. As the reconnection site rises across the SUMER spectrometer slit, significant blue- and red-shift signatures are observed in sequence in the Fe XIX line, reflecting upflows and downflows of hot plasma jets, respectively. With the projection effect corrected, the measured outflow speed is between 900-3500 km/s, consistent with theoretical predictions of the Alfvenic outflows in magnetic reconnection region in solar impulsive events. Based…
