A Novel Determination of the Foreshock ULF Boundary: Statistical Approach

IF 2.6 2区 地球科学 Q2 ASTRONOMY & ASTROPHYSICS
A. Salohub, J. Šafránková, Z. Němeček, G. Pi
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Abstract

The location and spatial extent of the region populated by the foreshock waves depend on the IMF orientation. We performed a systematic statistical study of wave activity in the frequency range of 0.03 0.15 $0.03-0.15$  Hz observed during an initial phase of the THEMIS mission. Wave activity is quantified by standard deviations of the IMF magnitude and its components over 10-min intervals. We apply the foreshock coordinate system defined as the angle between the bow shock normal and upstream magnetic field vectors and the distance from the spacecraft to bow shock along the magnetic field line. We have found that the Ultra-low Frequency (ULF) foreshock boundary (a) is well defined in these coordinates, (b) it tends to shift outward with an increasing solar wind bulk speed, and (c) with an increasing Mach number. However, the change of the fluctuation level in the foreshock is not uniform because the increasing solar wind bulk speed enhances the fluctuation level mainly in a close proximity of the bow shock whereas the increasing Mach number leads to an intensification of fluctuation levels at the foreshock boundary.

Abstract Image

一种确定前震极低频边界的新方法:统计方法
前冲击波聚集区域的位置和空间范围取决于IMF方向。我们对THEMIS任务初始阶段观测到的0.03-0.15$ 0.03-0.15$ Hz频率范围内的波活动进行了系统的统计研究。波浪活动是通过IMF震级及其分量在10分钟间隔内的标准偏差来量化的。我们将前激波坐标系定义为弓形激波法向和上游磁场矢量之间的夹角以及航天器沿磁力线到弓形激波的距离。我们发现超低频率(ULF)前震边界(a)在这些坐标中定义得很好,(b)随着太阳风体积速度的增加,它倾向于向外移动,(c)随着马赫数的增加。然而,前激波波动水平的变化并不均匀,增大的太阳风体速度主要增强了艏激波附近的波动水平,而增大的马赫数导致前激波边界的波动水平加剧。
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来源期刊
Journal of Geophysical Research: Space Physics
Journal of Geophysical Research: Space Physics Earth and Planetary Sciences-Geophysics
CiteScore
5.30
自引率
35.70%
发文量
570
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