Shapes of Magnetically Controlled Electron Density Structures in the Dayside Martian Ionosphere
Shapes of Magnetically Controlled Electron Density Structures in the Dayside Martian Ionosphere
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DOI：
10.1002/2017ja025140
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发表时间：
2018-05
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影响因子：
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通讯作者：
C. Dieval;A. Kopf;J. Wild
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文献类型：
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作者：
C. Dieval;A. Kopf;J. Wild
与近径向地壳磁场区域相关的非水平局域电子密度结构通常是通过火星白天侧的雷达斜回波来探测的，火星快车上搭载的火星亚表面和电离层探测高级雷达(MARSIS)的电离层探测模式。以前的研究大多是在固定的等离子体频率下研究这些结构，并假设与周围正常电离层相比，这些结构的表观高度较大，这意味着它们是凸起。然而，当信号通过等离子体传播时，它会受到色散的影响，因此基于视高度的解释应该谨慎对待。我们进一步研究了48个密度结构事件形状的频率依赖关系(即高度依赖关系)，使用经过信号色散校正的MARSIS电子密度剖面时间序列。在这些时间序列中发现了四种可能的最简单的形状，它们可以产生倾斜的回声：凸起、倾斜、下坡和上坡。密度结构与边缘的高度差绝对值在低频(高海拔)大于高频(低海拔)，随着频率的增加从几十公里到几公里。在大多数频率范围内，凸起在数量上占主导地位。最后，密度结构的地理扩展覆盖了广泛的地壳磁场取向，如预期的那样，近垂直场朝向其中心，近水平场朝向其边缘。运输过程被认为是这些密度结构的关键驱动因素。
Nonhorizontal localized electron density structures associated with regions of near‐radial crustal magnetic fields are routinely detected via radar oblique echoes on the dayside of Mars with the ionospheric sounding mode of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar onboard Mars Express. Previous studies mostly investigated these structures at a fixed plasma frequency and assumed that the larger apparent altitude of the structures compared to the normal surrounding ionosphere implied that they are bulges. However, the signal is subjected to dispersion when it propagates through the plasma, so interpretations based on the apparent altitude should be treated with caution. We go further by investigating the frequency dependence (i.e., the altitude dependence) of the shape of 48 density structure events, using time series of MARSIS electron density profiles corrected for signal dispersion. Four possible simplest shapes are detected in these time series, which can give oblique echoes: bulges, dips, downhill slopes, and uphill slopes. The altitude differences between the density structures and their edges are, in absolute value, larger at low frequency (high altitude) than at high frequency (low altitude), going from a few tens of kilometers to a few kilometers as frequency increases. Bulges dominate in numbers in most of the frequency range. Finally, the geographical extension of the density structures covers a wide range of crustal magnetic fields orientations, with near‐vertical fields toward their center and near‐horizontal fields toward their edges, as expected. Transport processes are suggested to be a key driver for these density structures.