Particle Acceleration at 5 au Associated with Turbulence and Small-scale Magnetic Flux Ropes
Particle Acceleration at 5 au Associated with Turbulence and Small-scale Magnetic Flux Ropes
复制标题
DOI：
10.3847/1538-4357/aafcb2
复制
发表时间：
2019-02
期刊：
影响因子：
--
通讯作者：
L.-L. Zhao-L.;G. Zank;Y. Chen;Q. Hu;J. L. le Roux;S. Du;L. Adhikari
中科院分区：
文献类型：
--
作者：
L.-L. Zhao-L.;G. Zank;Y. Chen;Q. Hu;J. L. le Roux;S. Du;L. Adhikari
本文对Ulysses航天器在5Au附近测量到的一个非典型高能粒子事件进行了观测分析。先前的一项研究将高能质子通量的异常增强归因于小规模动态磁岛或磁通绳的存在。在这里，我们发现这些磁岛的产生可能与流相互作用区(SIR)和日球层电流片(HCS)的相互作用有关。粒子被加速并被困在SIR结构中，其特征是正向激波-反向波对。高能粒子强度谱分析表明，63keV-4.4 MeV质子在激波附近形成了一个幂定律(PL)谱，其斜率比扩散激波加速(DSA)理论预测的要陡峭得多。在粒子通量异常增强的区域发现了双荧光光谱，并且光谱的低能部分在远离激波的地方变得更硬。作为对比，我们讨论了嵌入大尺度磁云(MC)的后向波-反波对。与小尺度磁岛相比，磁岛对应的是高能质子通量的减少。功率谱密度分析表明，由于航天器的速度几乎垂直于行星际磁场，压缩区域的湍流水平增加，观测到的湍流功率大部分存在于二维分量中。这与SIR中磁通绳的增强产生是一致的，这是准2D MHD湍流的内在特征。利用一种自动的Grad-Shafrov重建技术来识别感兴趣周期内的磁绳结构，并给出了它们的详细参数。观测证据表明，激波/压缩波与HCS的相互作用可能是产生小尺度动态磁岛的关键因素，这些磁岛随后加速带电粒子，补充了经典的DSA机制。
An observational analysis of an atypical energetic particle event near 5 au measured by the Ulysses spacecraft is presented. A previous study has attributed the unusual enhancement of energetic proton flux to the presence of small-scale dynamic magnetic islands or flux ropes. Here, we find that the generation of these magnetic islands may be related to the interaction of a stream interaction region (SIR) and the heliospheric current sheet (HCS). Particles are accelerated and trapped within the SIR structure characterized by a forward shock–reverse wave pair. Analysis of the energetic particle intensity spectra shows that the 63 keV–4.4 MeV protons form a power-law (PL) spectrum near the shock, with a slope much steeper than predicted by the diffusive shock acceleration (DSA) theory. Double PL spectra are found in the region of unusual particle flux enhancement, and the lower energy part of the spectrum gets harder farther away from the shock. In comparison, we discuss a later forward wave–reverse wave pair with an embedded large-scale magnetic cloud (MC). In contrast to small-scale magnetic islands, the MC corresponds to a decrease in energetic proton fluxes. A power spectral density analysis suggests that the turbulence level increases in the compression regions, and the majority of the observed turbulence power resides in the two-dimensional (2D) component because the spacecraft velocity is almost perpendicular to the interplanetary magnetic field. This is consistent with the enhanced generation of magnetic flux ropes, which are instrinsic to quasi-2D MHD turbulence, in the SIR. An automatic Grad–Shafranov reconstruction technique is used to identify flux rope structures within the period of interest, and their detailed parameters are included in the paper. The observational evidence suggests that the interaction of shock/compressional waves with the HCS may be a key element in generating small-scale dynamic magnetic islands, which subsequently accelerate charged particles and complement the classical DSA mechanism.