Propagation to the ground at high latitudes of auroral radio noise below the electron gyrofrequency
Propagation to the ground at high latitudes of auroral radio noise below the electron gyrofrequency
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低于电子陀螺频率的极光无线电噪声在高纬度地区传播到地面
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
R. Horne
中科院分区：
文献类型：
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作者：
R. Horne
利用HOTRAY射线追踪码研究了高纬度地区电离层电子陀螺频率（ƒce≈1.4 MHz）以下频率的极光无线电噪声的传播。描述了两种机制，据此，该频率范围内的能量可以进入地面。根据Wu et al.（1989）的理论，假设辐射是由电离层以上电子密度耗尽区域的上升损耗锥产生的。结果表明，由于这些波是在电离层中与ƒpe ƒce产生的，并且对于偶极磁场，Z模式波在上层电离层中反射，并且无法进入地面。然而，Z模式波可以进入第二个无线电窗口，在那里能量可以被模式转换成传播到地面的哨声模式波。通常，初始波法向角的范围是几度，因此在地面上发射的纬向扩散是几度。这表明，在地面上观测到的任何排放都应严格限制在产生区域的纬度范围内。对于整个电离层中ƒpe < ƒce的密度模型，表明Z模式波可以通过电离层传播，因此对于ƒLcut < f < ƒce，其中ƒLcut是左手截止频率。由于折射，这些波在纬度上传播了几十度，但传播应该随着频率的降低而减少。这表明该机制存在低频截止，低于此截止，波只能通过模式转换到达地面。
The propagation of auroral radio noise at frequencies below the ionospheric electron gyrofrequency (ƒce ≈ 1.4 MHz) at high latitudes is investigated using the HOTRAY ray tracing code. Two mechanisms are described whereby energy in this frequency range may access the ground. It is assumed that the radiation is generated by an upgoing loss cone in regions of depleted electron density above the ionosphere according to the theory of Wu et al. (1989). It is demonstrated that since the waves are generated with ƒpe ƒce in the ionosphere and for a dipole magnetic field, Z mode waves are reflected in the topside ionosphere and do not have access to the ground. However, Z mode waves can access the second radio window where energy can be mode converted into whistler mode waves which do propagate to the ground. Typically, the range of initial wave normal angles is a few degrees and the resulting latitudinal spread of emissions on the ground is a few degrees. This suggests that any emissions observed on the ground should be closely confined in latitude to the generation region. For a density model where ƒpe < ƒce throughout the ionosphere it is shown that Z mode waves can propagate through the ionosphere and hence to the ground for ƒLcut < ƒ < ƒce where ƒLcut is the left-hand cutoff frequency. Due to refraction these waves are spread over a few tens of degrees in latitude but the spread should decrease with decreasing frequency. This suggests there is low-frequency cutoff to this mechanism below which waves can only reach the ground by mode conversion.