质子各向同性边界的统计特征
Statistical Characteristics of the Proton Isotropy Boundary
AI总结：
该研究基于ELFIN对284个夜侧质子各向同性边界事件进行统计，揭示其地方时、L壳、能量和沉降功率特征，并表明磁场线曲率散射可造成显著高能质子损失。
摘要：
利用来自ELFIN卫星的粒子数据，我们对夜侧磁层中的284个质子各向同性边界事件进行了统计研究，刻画了它们随地磁活动变化的发生情况及其在地方时、纬度（L壳）、能量和沉降能通量方面的分布。对于给定的带电粒子种类和能量，其各向同性边界（IB）是这样一个磁纬度：在其向极一侧首次观测到持续各向同性的投掷角分布（$J_{prec}/J_{perp}\sim 1$）。这种各向同性化被解释为赤道磁层中磁场线曲率（FLC）散射所致。我们发现，在所有观测到的活动水平下都能观测到质子IB，其跨越16至05 MLT，在19至03 MLT之间发生率约为100%，在黎明/黄昏趋向于60%。这些结果还与使用ELFIN观测到的电子IB特性进行了比较，我们发现二者在地方时和活动水平上具有相似趋势，其中能量$\geq$50 keV质子IB的起始位置平均低2个L壳，并且提供3至10倍的沉降功率。质子IB通常跨越$64^\circ$-$66^\circ$磁纬度（L壳为5-6），对应环电流外缘，并随地磁活动增强趋向更低的IGRF纬度。研究发现，IB通常出现在等离子体层顶之外0.3-2.1 Re处。质子IB的能量通常跨越$<$50 keV至$\sim$1 MeV，在22 MLT附近达到最大，并向黎明和黄昏降低至300-400 keV的典型上限；在活跃时段，观测到的峰值各向同性能量增加约$\sim$500 keV。这些结果表明，IB附近的FLC可为高能质子提供显著的耗散机制，并且由FLC散射产生的夜侧总沉降功率约为100 MW量级，有时$\geq$10 GW。
英文摘要：
Using particle data from the ELFIN satellites, we present a statistical study of 284 proton isotropy boundary events on the nightside magnetosphere, characterizing their occurrence and distribution in local time, latitude (L-shell), energy, and precipitating energy flux, as a function of geomagnetic activity. For a given charged particle species and energy, its isotropy boundary (IB) is the magnetic latitude poleward of which persistently isotropic pitch-angle distributions ($J_{prec}/J_{perp}\sim 1$) are first observed to occur. This isotropization is interpreted as resulting from magnetic field-line curvature (FLC) scattering in the equatorial magnetosphere. We find that proton IBs are observed under all observed activity levels, spanning 16 to 05 MLT with $\sim$100% occurrence between 19 and 03 MLT, trending toward 60% at dawn/dusk. These results are also compared with electron IB properties observed using ELFIN, where we find similar trends across local time and activity, with the onset in $\geq$50 keV proton IB occurring on average 2 L-shells lower, and providing between 3 and 10 times as much precipitating power. Proton IBs typically span $64^\circ$-$66^\circ$ in magnetic latitude (5-6 in L-shell), corresponding to the outer edge of the ring current, tending toward lower IGRF latitudes as geomagnetic activity increases. The IBs were found to commonly occur 0.3-2.1 Re beyond the plasmapause. Proton IBs typically span $<$50 keV to $\sim$1 MeV in energy, maximizing near 22 MLT, and decreasing to a typical upper limit of 300-400 keV toward dawn and dusk, with peak observed isotropic energy increasing by $\sim$500 keV during active intervals. These results suggest that FLC in the vicinity of IBs can provide a substantial depletion mechanism for energetic protons, with the total nightside precipitating power from FLC-scattering found to be on the order of 100 MW, at times $\geq$10 GW.
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