Group-standing of whistler-mode waves near the Moon
Group-standing of whistler-mode waves near the Moon
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月球附近哨声模波的群体站立
DOI：
10.1002/2013ja019585
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
and M. Matsushima
中科院分区：
文献类型：
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作者：
Y. Tsugawa;Y. Katoh;N. Terada;T. Ono;H. Tsunakawa;F. Takahashi;H. Shibuya;H. Shimizu;and M. Matsushima
在月球附近观测到频率接近1hz的窄带哨声模式波。研究表明，在群速度矢量几乎被航天器框架内的太阳风速度矢量抵消的条件下，可以解释航天器的窄带光谱、1 Hz附近的频率集中以及波矢量、磁场矢量和向阳方向之间的关系。此后，我们将这种情况称为群体站立条件。由于太阳风等离子体框架与航天器框架的频率宽度不同，对谱密度进行了修正，在满足群驻条件的频率处有一个峰值。此外，如果波被减速为群驻波，能量通量的守恒会导致该频率处的波幅增强。我们还推导了随群速度变化的修正量的解析表达式。这些效应可以解释1 Hz附近的窄带光谱，并支持波矢量、磁场矢量和向阳方向之间的关系。在估计的误差条内，满足群站立条件的估计频率与观测频率吻合良好。考虑到群站立的条件，我们认为在航天器框架中观测到的窄带波起源于低混合频率附近频率的斜哨声波，这可能是由月球磁异常反射离子产生的。
Narrowband whistler mode waves with frequencies near 1 Hz have been observed near the Moon. We reveal that the narrowband spectra, the frequency concentration near 1 Hz, and the relations between the wave vector, magnetic field vector, and sunward directions can be explained by a condition in which the group velocity vector is almost canceled by the solar wind velocity vector in the spacecraft frame. Hereafter, we refer to this condition as the group‐standing condition. The spectral density is modified and has a peak at the frequency satisfying the group‐standing condition because of the difference of the frequency width between the solar wind plasma frame and the spacecraft frame. In addition, if the waves were decelerated to be group‐standing, the conservation of the energy flux results in the intensification of the wave amplitude at that frequency. We also derive the analytical expression of the amount of the modifications, which depend on the group velocity. These effects can explain the narrowband spectra near 1 Hz and support the relations between the wave vector, magnetic field vector, and sunward directions. The estimated frequency which satisfies the group‐standing condition is in good agreement with the observed frequency within error bars of the estimation. Considering the group‐standing condition, we suggest that the narrowband waves observed in the spacecraft frame are originated from oblique whistler mode waves in the frequencies near the lower hybrid frequency, which are possibly generated by reflected ions from the lunar magnetic anomalies.