On the generation of Alfvén waves by solar energetic particles
On the generation of Alfvén waves by solar energetic particles
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太阳高能粒子产生阿尔文波
DOI：
10.1051/0004-6361:20030822
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发表时间：
2003
影响因子：
6.5
通讯作者：
R. Vainio
中科院分区：
文献类型：
--
作者：
R. Vainio
研究了流太阳高能粒子放大阿尔芬波的一种简单解析理论。指出高能质子的有限时间积分净通量必须通过空间中的每一点，才能期望阿尔芬波被流不稳定性显著地改变。计算了时积分波增长速率（简称波增长）超过单位所需的时积分净质子通量。假设质子流比波快得多，我们对特定质子注入能谱$\mathrm{d}N/\mathrm{d}E$计算波的增长作为位置和波数的函数。发现波的增长与$vp, $ mathm {d}N/ $ mathm {d}E$成正比，其中v和p是粒子的速度和动量，并与局部阿尔芬速度$V_{\ mathm {A}}$成正比。因此，最大的波增长是在最大$V_{\ mathm {A}}$的位置（在几个太阳半径处）实现的，而波增长超过单位所需的$\ mathm {d}N/\ mathm {d}E$的最小值是在太阳表面每单位立体角（在坐标空间中）有几倍10$^{32}/vp$质子。如果$\ mathm {d}N/\ mathm {d}E$低于此值，则测试粒子理论是粒子输运和加速的有效描述。在小的渐进SEP事件中，在1 AU时，峰值1-MeV质子强度低于~ 10个质子(cm$^{2}\, $sr $ MeV) -1，则不超过该值（高于1 MeV能量）。波增长的空间和动量依赖性也可用于估计行星际介质中运动质子源的最大发射强度。对于以恒定的超阿尔芬速度在太阳风中运动的强源，单位时间内逃逸粒子的数量和通量管截面在时间上近似恒定，预测了源前观测到的高原型时间强度剖面。该模型再现了10兆电子伏特左右的流受限强度观测结果，并解释了在大型SEP事件中观测到的双峰注入剖面。
A simple analytical theory of Alfven waves amplified by streaming solar energetic particles (SEPs) is studied. It is pointed out that a finite time-integrated net flux of energetic protons has to pass each point in space before we can expect Alfven waves to be significantly modified by the streaming instability. The time-integrated net proton flux needed for the time-integrated wave growth rate (or wave growth, for short) to exceed unity is evaluated. Assuming that protons stream much faster than the waves, we evaluate the wave growth as a function of position and wavenumber for a specified proton injection energy spectrum, $\mathrm{d}N/\mathrm{d}E$. The wave growth is found to be proportional to $vp\, \mathrm{d}N/\mathrm{d}E$, where v and p are the particle speed and momentum, and to the local Alfven speed $V_{\mathrm{A}}$. Thus, maximum wave growth is achieved at the location of maximum $V_{\mathrm{A}}$ (at a few solar radii), and the minimum value of $\mathrm{d}N/\mathrm{d}E$ required for the wave growth to exceed unity there is a few times 10$^{32}/vp$ protons per unit solid angle (in coordinate space) at the solar surface. If $\mathrm{d}N/\mathrm{d}E$ is below this value, test-particle theory is a valid description of particle transport and acceleration. The value is not exceeded (above 1 MeV energies) in small gradual SEP events having peak 1-MeV proton intensities below ∼ 10 protons (cm$^{2}\, $sr s MeV) -1 at 1 AU. The spatial and momentum dependence of the wave growth can also be used to estimate the maximum emission strength of a moving proton source in the interplanetary medium. For a strong source moving through the solar wind at constant super-Alfvenic speed, the number of escaping particles per unit time and flux-tube cross section is approximately constant in time, predicting a plateau-type time–intensity profile observed ahead of the source. The model reproduces observations of streaming-limited intensities at energies around 10 MeV and explains the double peaked injection profiles observed in large SEP events.