Local time dependence of the dominant frequency of Pi2 pulsations at mid- and low-latitudes
Local time dependence of the dominant frequency of Pi2 pulsations at mid- and low-latitudes
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中低纬度 Pi2 脉动主频率的局地时间依赖性
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发表时间：
2002
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通讯作者：
J. Bitterly
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作者：
K. Kosaka;T. Iyemori;M. Nosé;M. Bitterly;J. Bitterly
我们利用三个地面站获得的地磁场数据对 Pi2 脉动进行了统计分析。在夜侧发现了 Pi2 主频率的局部时间依赖性。中纬度Pi2脉动的频率在黄昏一侧低于黎明一侧。这种趋势归因于等离子体球的形状向黄昏一侧凸出。已证实 Pi2 频率还取决于用 Kp 指数测量的地磁活动。在扰动期间，Pi2脉动的频率高于安静期间的脉动频率。这种依赖性被解释为是由等离子体层顶的尺寸引起的，其在受干扰的条件下比在安静的条件下更小。低纬度地区 Pi2 脉动的主频率在午夜后达到峰值，并且也观察到与中纬度地区类似的 Kp 依赖性。然而，低纬度Pi2的结果与中纬度Pi2的结果不同。我们认为中纬度Pi2的主导机制是等离子体层表面波。为了检验等离子体层顶表面波是中纬度 Pi2 脉动的主要机制的观点，我们使用包含等离子体层凸起影响的等离子体层模型估计了等离子体层顶表面波的共振频率。估算的黎明侧面波频率高于黄昏侧，与观测结果基本一致。安静条件下（Kp≤3）的预测频率几乎等于中纬度地区观测到的Pi2频率。这些结果表明，中纬度地区 Pi2 脉动的主频率取决于等离子体层顶的结构。
We carried out a statistical analysis of Pi2 pulsations using the geomagnetic field data obtained at three ground stations. A local time dependence of the dominant frequency of Pi2 was found on the nightside. The frequency of mid-latitude Pi2 pulsations is lower on the dusk side than that on the dawn side. This tendency is attributed to the shape of the plasmasphere which bulges out to the dusk side. It was confirmed that the Pi2 frequency depends also on the geomagnetic activity measured with Kp index. During the disturbed periods, Pi2 pulsations have higher frequency than that in the quiet periods. This dependence is interpreted to be caused by the size of the plasmapause which is smaller under the disturbed conditions than that under the quiet conditions. The dominant frequency of Pi2 pulsations at lower latitudes has a peak in post-midnight, and a Kp dependence similar to that at mid-latitudes is also observed. However, the result for low-latitude Pi2’s is different from that for mid-latitude Pi2. We consider that the dominant mechanism of mid-latitude Pi2 is the plasmaspheric surface wave. In order to examine the idea that the surface wave on the plasmapause is the dominant mechanism of Pi2 pulsations at mid-latitudes, we estimated the resonance frequency of the surface wave on the plasmapause using a plasmaspheric model which includes the effect of the plasmaspheric bulge. The estimated frequency of the surface wave is higher on the dawn side than that on the dusk side, which is essentially consistent with the observational results. The predicted frequency under quiet conditions (Kp ≤ 3) is nearly equal to the observed Pi2 frequency at mid-latitudes. These results suggest that the dominant frequency of Pi2 pulsations at mid-latitudes depends on the structure of the plasmapause.