FAST observations of ion solitary waves
FAST observations of ion solitary waves
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DOI：
10.1029/2002ja009485
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发表时间：
2003-04-30
影响因子：
2.8
通讯作者：
Moebius, E
中科院分区：
文献类型：
--
作者：
McFadden, JP;Carlson, CW;Moebius, E
来自FAST航天器的测量结果显示，在加速区下缘观察到的离子孤立波的传播速度比相关的极光质子束要快。平行相速度与质子束参考系中的声速一致，强烈表明这些波是离子声模式。它们的高相速度使它们在离子束种群之外，并排除了离子两流不稳定性作为它们的来源。这些低空结构可能是由加速度区下缘产生的湍流引起的。它们在FAST高度的优先观测可能是由于它们的高速度与弱朗道阻尼相结合，这种弱朗道阻尼仅限于损耗锥附近的脆弱热等离子体片离子。三种不同的方法估计这些结构的速度进行了审查。对于FAST天线配置，发现在电流模式或电压模式下工作的Langmuir探针之间的信号延迟不能提供有效的速度估计。相反，速度是通过测量孤波负势阱内电子分布中的能量位移来估计的。利用测得的波势和电场，计算出结构的尺度尺寸和速度。不对称孤立波，有时被描述为弱双层，也被检查并显示没有显著的净势。这些新的速度估计与基于维京号观测的报告形成鲜明对比，与最近从极地观测推断出的估计相差约2倍。这些结果将在以前的估计以及可能的误差来源的背景下进行讨论。
[1] Measurements from the FAST spacecraft are used to show that ion solitary waves observed at the lower edge of the acceleration region travel at velocities faster than the associated auroral proton beams. The parallel phase velocity is consistent with the acoustic speed in the reference frame of the proton beam, strongly suggesting these waves are an ion acoustic mode. Their high phase velocity places them outside the ion beam population and rules out the ion two-stream instability as their source. These low-altitude structures may arise out of turbulence generated at the lower edge of the acceleration region. Their preferential observation at FAST altitudes may result from their high velocity combined with weak Landau damping that is restricted to the tenuous hot plasma sheet ions near the loss cone. Three different methods for estimating the velocity of these structures are examined. For the FAST antennae configuration it is found that signal delays between Langmuir probes operated in either current mode or voltage mode cannot provide valid estimates of the velocities. Instead, velocities are estimated by measuring the energy shift in the electron distribution within the negative potential well of the solitary wave. Using the measured wave potential and electric field, the scale size and velocity of the structures are calculated. Asymmetric solitary waves, sometime described as weak double layers, are also examined and shown to have no significant net potential. These new velocity estimates contrast sharply with reports based upon Viking observations and differ by about a factor of 2 from recent estimates deduced from Polar observations. These results are discussed in the context of previous estimates along with possible sources of error.