Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation
Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation
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DOI：
10.1029/2019ja027158
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发表时间：
2019-12-04
影响因子：
2.8
通讯作者：
Oimatsu, S.
中科院分区：
文献类型：
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作者：
Yamamoto, K.;Nose, M.;Oimatsu, S.
用货车艾伦探针A在径向距离约为5.8 R-E，磁地方时约为13 hr的磁赤道附近探测到两个波频率约为7 mHz的二次谐波极向Pc 4波。同时还测量了具有能量色散的质子蝶形分布，10-30 keV的质子通量以与Pc 4波相同的频率振荡。利用离子探测技术，我们发现Pc 4波向东传播时，每个波包的方位波数（m数）分别约为220和260。这种向东传播的高m（m > 100）波在以前的研究中很少报道。漂移反弹共振的条件是很好地满足估计的m数在这两个事件。质子相空间密度也被检查，以了解波激发机制。我们得到了质子相空间密度的能量和径向梯度的时间变化，发现径向梯度的时间增强可以产生两个波包。在本次事件中，航天器远地点附近的冷电子密度>100 cm（-3），因此Pc 4波的本征频率变低。这导致满足10-30 keV质子的漂移-反弹共振的共振条件和满足由于回旋效应而不稳定的条件的m数的增加。
Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of similar to 7 mHz were detected by Van Allen Probe A at a radial distance of similar to 5.8 R-E and magnetic local time of 13 hr near the magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10-30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of similar to 220 and similar to 260 for each wave packet, respectively. Such eastward propagating high-m (m > 100) waves were seldom reported in previous studies. The condition of drift-bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm(-3) in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift-bounce resonance for 10-30 keV protons and meets the condition for destabilization due to gyrokinetic effect.