Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales
Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales
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DOI：
10.1029/2010ja015437
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发表时间：
2010-09-25
影响因子：
2.8
通讯作者：
Cummer, Steven A.
中科院分区：
文献类型：
--
作者：
Han, Feng;Cummer, Steven A.
夜间D区显著的时间变异性是众所周知的，但它的不可获取性意味着时间尺度、幅度和这种变异性的来源还没有得到很好的理解。通过测量闪电发射的高功率宽带甚低频（VLF）信号在地球电离层波导中传播，探测电离层D区。我们分析了由位于杜克大学附近的传感器记录的2005年7月和8月的宽带sic数据，通过将测量的sic光谱与模型结果进行比较，并提取了每次测量的假设指数电子密度剖面的高度。夜间测得的D区电子密度剖面高度在某些夜晚表现出几公里的大变化，而在其他夜晚表现出相对稳定的行为。260小时内的每小时平均高度介乎82.0至87.2公里，平均值为84.9公里，标准差为1.1公里。5 h的最大变化约略高于4.0 km， 1 h的最大变化约为1.3 km，在较短的时间内观测到更大的梯度。在某些夜晚，我们还观测到5个纬度上的空间变异性高达2.0 km，而在其他夜晚则没有空间变异性。在某些夜间，时间变率与探测区雷击发生率密切相关。这表明闪电放电与下部电离层之间的直接能量耦合可能是D区变率的一个重要来源。然而，在其他夜晚，测量到的高度时间变率显示与当地闪电、移位闪电（如闪电引起的电子降水）或地磁活动的相关性很弱或没有相关性。这些测量结果表明，夜间D区变化可能由许多来源驱动。
Significant temporal variability of the nighttime D region is well known, but its inaccessibility means that the time scales, magnitudes, and sources for that variability are not well understood. We probed the ionospheric D region by measuring the high-power broadband very low frequency (VLF) signals launched by lightning and propagating in the Earth-ionosphere waveguide. We analyzed broadband sferic data of July and August 2005 recorded by our sensors located near Duke University by comparing measured sferic spectra to model results and extracted the height of an assumed exponential electron density profile for each measurement. The measured nighttime D region electron density profile heights showed large temporal variations of several kilometers on some nights and relatively stable behaviors on others. The measured hourly average heights in 260 h ranged between 82.0 and 87.2 km, with a mean value of 84.9 km and a standard deviation of 1.1 km. The maximum variation in the 5 h period was around just above 4.0 km and the maximum variation in the 1 h period was around 1.3 km, with sharper gradients observed over shorter time periods. We also observed spatial variability as large as 2.0 km over 5 latitudes on some nights and no spatial variability on others. On some nights, the temporal variability exhibited a close correlation with the occurrence rate of lightning discharges under the probed region. This suggests that the direct energy coupling between lightning discharges and lower ionosphere can be a significant source of the D region variability. However, on other nights, the measured height temporal variability showed weak to no correlation with local lightning, displaced lightning (as would be expected for lightning-induced electron precipitation), or geomagnetic activities. These measurements suggest that nighttime D region variability may be driven by many sources.