arXiv 1406.08022014-06-03astro-ph.SR

太阳耀斑中快磁声波对电子的随机加速：速度andWavenumber空间各向异性的影响

Stochastic Acceleration of Electrons by Fast Magnetosonic Waves in Solar Flares: the Effects of Anisotropy in Velocity andWavenumber Space

Peera Pongkitiwanichakul, Benjamin D. G. Chandran

DOI：: 10.1088/0004-637X/796/1/45

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摘要：

我们发展了一个太阳耀斑中电子随机加速的模型。与之前的几个模型一样，电子通过传输时间阻尼（TTD）相互作用被湍流快磁声波（“快波”）加速。(In在TTD相互作用中，快波就像移动的磁镜，推动电子平行或反平行于磁场）。我们还包括库仑碰撞和波的平行电场的影响。与以前的模型不同，我们的模型是二维的动量空间和波数空间，并考虑到各向异性的波功率谱$F_k$和电子分布函数$f_{\rm e}$。我们利用弱湍流理论和准线性理论得到一组描述$F_k$和$f_{\rm e}$耦合演化的方程。我们数值求解这些方程，发现电子分布函数在有限的能量范围内发展出一个类似幂律的非热尾$E\in (E_{\rm nt}, E_{\rm max})$。我们获得近似的解析表达式$E_{\rm nt}$和$E_{\rm max}$，它描述了这些最小和最大能量如何依赖于参数，如电子数密度和速度，快波能量注入到加速区域在大尺度。我们对比我们的结果与以前的研究，假设$F_k$和$f_{\rm e}$是各向同性的，我们比较我们的数值计算与时间相关的硬X射线光谱观察期间，1980年6月27日耀斑。在我们的数值计算中，电子能谱比各向同性$F_k$和$f_{\rm e}$模型中的电子能谱更软（更陡），更接近于从太阳耀斑观测中推断出的值。

英文摘要：

We develop a model for stochastic acceleration of electrons in solar flares. As in several previous models, the electrons are accelerated by turbulent fast magnetosonic waves ("fast waves") via transit-time-damping (TTD) interactions. (In TTD interactions, fast waves act like moving magnetic mirrors that push the electrons parallel or anti-parallel to the magnetic field). We also include the effects of Coulomb collisions and the waves' parallel electric fields. Unlike previous models, our model is two-dimensional in both momentum space and wavenumber space and takes into account the anisotropy of the wave power spectrum $F_k$ and electron distribution function $f_{\rm e}$. We use weak turbulence theory and quasilinear theory to obtain a set of equations that describes the coupled evolution of $F_k$ and $f_{\rm e}$. We solve these equations numerically and find that the electron distribution function develops a power-law-like non-thermal tail within a restricted range of energies $E\in (E_{\rm nt}, E_{\rm max})$. We obtain approximate analytic expressions for $E_{\rm nt}$ and $E_{\rm max}$, which describe how these minimum and maximum energies depend upon parameters such as the electron number density and the rate at which fast-wave energy is injected into the acceleration region at large scales. We contrast our results with previous studies that assume that $F_k$ and $f_{\rm e}$ are isotropic, and we compare one of our numerical calculations with the time-dependent hard-x-ray spectrum observed during the June 27, 1980 flare. In our numerical calculations, the electron energy spectra are softer (steeper) than in models with isotropic $F_k$ and $f_{\rm e}$ and closer to the values inferred from observations of solar flares.

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