On the Importance of Gradients in the Low-Energy Electron Phase Space Density for Relativistic Electron Acceleration
On the Importance of Gradients in the Low-Energy Electron Phase Space Density for Relativistic Electron Acceleration
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DOI：
10.1029/2019ja026516
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发表时间：
2019-04-01
影响因子：
2.8
通讯作者：
Del Zanna, Giulio
中科院分区：
文献类型：
--
作者：
Allison, Hayley J.;Horne, Richard B.;Del Zanna, Giulio
对电子辐射带的观测表明，低能种子数量的增加与>1 MeV通量的增强之间存在联系。在活动期间，低能电子被引入辐射带区域，然后通过一系列机制被加速到更高的能量。种子种群的变化对1兆电子伏通量水平的影响进行了探讨，使用英国南极调查辐射带模型。我们发现，在2013年4月21日至5月9日的一段时间内，低能电子通量的增加对于重现5月1日观察到的1 MeV通量增强至关重要，但对于2013年4月27日的1 MeV增强不太重要。为了更好地了解不同能量群之间的关系，进行了一系列理想化的实验，2-D英国南极调查辐射带模型，突出了损失和合唱波加速之间的仔细平衡。种子种群的增强通过增加相空间密度梯度来改变这种平衡，并因此改变能量扩散的速率，从而允许加速超过损失。此外，我们表明，即使具有相同的合唱团扩散系数和相同的低能量边界条件，类似于500 keV至1 MeV的电子的通量增加时，开始与硬谱，但软初始谱下降。这表明相空间密度中的初始能量梯度对于确定是否由于合唱波加速而增强>500 keV的电子是重要的。
Observations of the electron radiation belts have shown links between increases in the low-energy seed population and enhancements in the >1-MeV flux. During active times, low-energy electrons are introduced to the radiation belt region before being accelerated to higher energies via a range of mechanisms. The impact of variations in the seed population on the 1-MeV flux level were explored using the British Antarctic Survey Radiation Belt Model. We find that, for a period from the 21 April to 9 May 2013, the increase in the low-energy electron flux was vital to recreate the observed 1-MeV flux enhancement on the 1 May but was less important for the 1-MeV enhancement on the 27 April 2013. To better understand the relationships between the different energy populations, a series of idealized experiments with the 2-D British Antarctic Survey Radiation Belt Model were performed, which highlight a careful balance between losses and acceleration from chorus waves. Seed population enhancements alter this balance by increasing the phase space density gradient, and consequently, the rate of energy diffusion, allowing acceleration to surpass loss. Additionally, we demonstrate that even with the same chorus diffusion coefficients and the same low-energy boundary condition, the flux of similar to 500-keV to 1-MeV electrons increased when starting with a hard spectrum but decreased for a soft initial spectrum. This suggests that initial energy gradients in the phase space density were important to determine whether >500-keV electrons were enhanced due to chorus wave acceleration.