Particle Acceleration by Slow Modes in Strong Compressible Magnetohydrodynamic Turbulence, with Application to Solar Flares
Particle Acceleration by Slow Modes in Strong Compressible Magnetohydrodynamic Turbulence, with Application to Solar Flares
复制标题
强可压缩磁流体动力湍流中慢模式的粒子加速及其在太阳耀斑中的应用
DOI：
10.1086/379317
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
B. Chandran
中科院分区：
文献类型：
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作者：
B. Chandran
高能粒子经过强烈的俯仰角散射并在含有强可压缩MHD湍流的等离子体中扩散时，其在动量空间中的扩散系数为Dp。当rms湍流速度为alfv<s:1>速度vA阶时，慢模涡流对Dp的贡献为≃(2p2vA/9l)[ln(lvA/D||) + 2γ - 3]，其中l为湍流的外标度，γ≃0.577为欧拉常数，D||为高能粒子的空间扩散系数，假设该系数满足D||≪lvA。考虑到库仑损失和粒子从加速区域逃逸，逃逸时间与能量无关，由此导出了该Dp值下加速粒子的能谱。D||《lvA极限》中的慢模不太可能解释太阳耀斑中的电子加速达到10-100 keV的能量，因为对于太阳耀斑的情况，预测的加速时间太长，而且预测的能谱也太困难。本文讨论的加速机制原则上可以解释100-5000 keV范围内回旋同步加速器发射电子的相对硬谱，但前提是D||对这类粒子≪lvA。
Energetic particles that undergo strong pitch-angle scattering and diffuse through a plasma containing strong compressible MHD turbulence undergo diffusion in momentum space with diffusion coefficient Dp. If the rms turbulent velocity is of the order of the Alfvén speed vA, the contribution to Dp from slow-mode eddies is ≃(2p2vA/9l)[ln(lvA/D||) + 2γ - 3], where l is the outer scale of the turbulence, γ ≃ 0.577 is Euler's constant, and D|| is the spatial diffusion coefficient of energetic particles, which is assumed to satisfy D|| ≪ lvA. The energy spectrum of accelerated particles is derived for this value of Dp, taking into account Coulomb losses and particle escape from the acceleration region with an energy-independent escape time. Slow modes in the D|| ≪ lvA limit are an unlikely explanation for electron acceleration in solar flares to energies of 10-100 keV, because for solar flare conditions, the predicted acceleration times are too long, and the predicted energy spectra are too hard. The acceleration mechanism discussed in this paper could in principle explain the relatively hard spectra of gyrosynchrotron-emitting electrons in the 100-5000 keV range, but only if D|| ≪ lvA for such particles.