Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS
Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS
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DOI：
10.1029/2021ja030029
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发表时间：
2022-01
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影响因子：
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通讯作者：
A. Vu;T. Liu;Hui Zhang;C. Pollock
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作者：
A. Vu;T. Liu;Hui Zhang;C. Pollock
前震瞬变，如前震气泡（FB），热流异常（HFA），和自发热流异常（SHFA）显示加热，纤细的核心和压缩边界的大流量偏转。THEMIS和Cluster的观测结果表明，一些核包含局部密度增强，可以通过研究这些增强来更好地理解前震瞬变的演化过程。然而，在磁层多尺度使命获得更高分辨率的数据之前，对这些子结构进行更仔细的检查是不可行的。我们确定了164个FB-样，HFA-样，和SHFA事件从两个MMS日侧阶段的统计研究，调查他们的太阳风条件，属性和子结构属性。这三种事件类型的发生率在磁场强度较低、太阳风速度和马赫数较高以及准平行弓形激波的情况下较高。事件通常沿弓形激波面沿着跨越3 RE，并从弓形激波向上游延伸6 RE。虽然有和没有子结构的事件表现出类似的太阳风条件，但有子结构的事件更有可能具有更长的核心持续时间和更大的规模。子结构密度与体积流量呈正相关，与温度呈负相关。子结构大小在4和24离子惯性长度之间变化，表明多种形成机制。子结构可以是两个前震瞬态事件之间的边界，它们合并成一个事件，快模式变化，由慢模式或镜像模式不稳定性产生，或由压缩边界或冲击的参数梯度引起的不稳定性产生。
Foreshock transients such as foreshock bubbles (FBs), hot flow anomalies (HFAs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that some cores contain local density enhancements which can be studied to better understand the evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the higher resolution data from the Magnetospheric Multiscale mission (MMS). We identify 164 FB‐like, HFA‐like, and SHFA events from two MMS dayside phases for a statistical study to investigate their solar wind conditions, properties, and substructure properties. Occurrence rates of the three event types are higher for lower magnetic field strengths, higher solar wind speeds and Mach numbers, and quasi‐parallel bow shocks. Events usually span up to 3 RE along the bow shock surface and extend up to 6 RE upstream from the bow shock. Though events with and without substructures exhibit similar solar wind conditions, events with substructures are more likely to have longer core durations and larger sizes. Substructure densities display a positive correlation with bulk flows and a negative correlation with temperatures. Substructure sizes vary between 4 and 24 ion inertial lengths, indicating multiple formation mechanisms. Substructures could be the boundary between two foreshock transient events that have merged into a single event, fast‐mode variations, generated by slow or mirror mode instabilities, or produced from instabilities due to parameter gradients at the compressional boundaries or shocks.