Precipitation of ring current protons caused by wave-particle interactions with satellite conjugated observation
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摘要:
波粒相互作用是环电流损失的重要机制之一，但波粒相互作用导致的环电流离子沉降而损失迄今为止缺乏直接的观测证据.基于磁层及电离层卫星的协同观测，本文报道了发生在2015年9月7日，由电磁离子回旋波（EMIC波）导致环电流质子沉降的共轭观测事件.在等离子体层的内边界，Van Allen Probe B卫星观测到，存在EMIC波的区域和不存在EMIC波的区域相比，离子通量的投掷角分布的各向异性变弱.我们将Van Allen Probe B卫星沿着磁力线投影到电离层高度，同时在该投影区域内DMSP 16卫星在亚极光区域观测到环电流质子沉降.而且，通过从理论上计算质子弹跳平均扩散系数，我们进一步证实观测的EMIC波确实能将环电流质子散射到损失锥中.本文的研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据，揭示了环电流衰减的重要物理机制：EMIC波将环电流质子散射到损失锥中，从而沉降到低高度大气层中而损失.
Abstract:Wave-particle interaction is considered as an important mechanism for the loss of ring currents. However, the direct observational proof of the loss of ring currents caused by wave-particle interaction has not been reported so far. Based on the combination of observations from magnetosphere to ionosphere, this paper presents conjugated observations of ring current (RC) proton precipitation caused by electromagnetic ion cyclotron (EMIC) waves on September 7, 2015. In the inner boundary of the plasmasphere, the Van Allen Probe B satellite observed that the ion flux anisotropy of pitch angle distributions was weaker in the region where the EMIC wave existed than the region where the EMIC wave did not appear. While at ionospheric altitudes, the DMSP 16 satellite at the footprint of the Van Allen probe B simultaneously observed the RC proton precipitation. In addition, we demonstrate that observed EMIC waves can truly scatter RC ions into the loss cone through theoretically calculating the pitch angle diffusion coefficients for protons. Our result provides a direct proof of precipitation of ring current protons caused by EMIC waves so as to reveal the important mechanism for the loss of ring currents, i.e., EMIC waves can scatter ring current ions into the loss cone and lead to the precipitation and loss of ring current ions in the low-altitude atmosphere.
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图 1
DMSP 16卫星和Van Allen Probe B卫星共轭观测位置图
Figure 1.
Conjugated observation map of DMSP 16 and Van Allen Probe B satellites
图 4
(a) EMIC波的高斯拟合曲线(蓝色实线); (b)质子对应于赤道损失锥角(3.2°)的弹跳平均扩散系数〈Dαα〉(黑线)，其中红线表示强投掷角扩散系数DSD; (c)投掷角为0°~90°的质子弹跳平均扩散系数〈Dαα〉，其中黄色虚线对应子图(b)中的〈Dαα〉
Figure 4.
(a) Gaussian fitting (blue solid line); (b) Bounce-averaged pitch angle diffusion rates for protons 〈Dαα〉 (black line) near the equatorial loss cone (3.2°), and the red line represents the rate of strong pitch diffusion (DSD); (c) Bounce-averaged pitch angle diffusion rates for protons 〈Dαα〉 with pitch angles of 0°~90° and the yellow dashed line corresponds to 〈Dαα〉 in subgraph (b)
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