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arXiv 1003.3263astro-ph.SR
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自相互作用太阳耀斑快电子的热化和硬X射线韧致辐射效率

Thermalisation and hard X-ray bremsstrahlung efficiency of self-interacting solar flare fast electrons

R. K. Galloway, P. Helander, A. L. MacKinnon, J. C. Brown

备注：
9 pages, 7 figures, accepted for Astronomy & Astrophysics; this version clarifies arguments around Eqs. (11) and (20)
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摘要：

大多数关于太阳耀斑韧致辐射产生的理论描述都假设是被加速的稀粒子与冷而致密的靶等离子体的碰撞，忽略了快粒子之间的相互作用。这是不够的情况下，与这种背景等离子体的碰撞是不完全占主导地位的，可能是这样的，例如，在低密度的日冕源。我们的目标是制定一个模型的自相互作用，完全快速的电子人口在没有一个致密的背景等离子体，调查其影响所观察到的韧致辐射光谱和耀斑能量预算。我们使用福克-普朗克方法推导出快电子随时间变化的分布函数的近似表达式。我们使用这些表达式来产生合成韧致辐射X射线光谱，将看到从相应的日冕源。我们发现，我们的模型定性地再现了观察到的一些耀斑的行为。随着耀斑的发展，模型的初始幂律谱被一个较低能量的热分量加入。幂律分量减少，和不断增长的热分量的收益占主导地位的总排放量与耀斑观测一致的时间尺度。幂律表现出渐进的光谱硬化，正如在一些耀斑日冕源中所看到的那样。我们还发现，我们的模型需要一个7 - 10倍的加速电子比冷，厚靶模型产生一个等效的硬X射线通量。这个模型的基础上形成的耀斑快电子，一个过程，提供了一个更有效的手段来产生韧致辐射光子，从而可以降低效率的要求放在粒子加速机制之间的自相互作用的治疗。它还提供了一个有用的描述热化的快电子在日冕源。

英文摘要：

Most theoretical descriptions of the production of solar flare bremsstrahlung radiation assume the collision of dilute accelerated particles with a cold, dense target plasma, neglecting interactions of the fast particles with each other. This is inadequate for situations where collisions with this background plasma are not completely dominant, as may be the case in, for example, low-density coronal sources. We aim to formulate a model of a self-interacting, entirely fast electron population in the absence of a dense background plasma, to investigate its implications for observed bremsstrahlung spectra and the flare energy budget. We derive approximate expressions for the time-dependent distribution function of the fast electrons using a Fokker-Planck approach. We use these expressions to generate synthetic bremsstrahlung X-ray spectra as would be seen from a corresponding coronal source. We find that our model qualitatively reproduces the observed behaviour of some flares. As the flare progresses, the model's initial power-law spectrum is joined by a lower energy, thermal component. The power-law component diminishes, and the growing thermal component proceeds to dominate the total emission over timescales consistent with flare observations. The power-law exhibits progressive spectral hardening, as is seen in some flare coronal sources. We also find that our model requires a factor of 7 - 10 fewer accelerated electrons than the cold, thick target model to generate an equivalent hard X-ray flux. This model forms the basis of a treatment of self-interactions among flare fast electrons, a process which affords a more efficient means to produce bremsstrahlung photons and so may reduce the efficiency requirements placed on the particle acceleration mechanism. It also provides a useful description of the thermalisation of fast electrons in coronal sources.

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