Abstracts submitted by Lyndsay Fletcher
Energy transport by large-scale Alfvenic wave pulses in the flare impulsive phase, and the electron acceleration problem
Lyndsay Fletcher and Hugh S. Hudson
Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
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 occurs as the coronal magnetic field reconfigures and relaxes following reconnection. Therefore, we investigate a scenario in which the electron acceleration is caused by the agents of this reconfiguration -- large-scale Alfven wave pulses which propagate the field changes throughout the atmosphere. In a coronal plasma with beta less than the electron to proton mass ratio, these waves propagate as inertial Alfven waves which, in the presence of strong spatial gradients, generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above (bearing in mind the strong magnetic fields and high Alfven speeds that pertain in the low corona in the core of an active region). Once they arrive at the chromosphere, the generation of a turbulent fast-mode spectrum and consequent wave damping offers an additional acceleration channel, as well as a source of lower-atmosphere heating. This novel view also provides an interpretation of the recently-observed rapid variations of the line-of-sight component of the photospheric magnetic field during the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare ``number problem'' of finding sufficient and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission. |
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