Magnetospheric state of sawtooth events
Magnetospheric state of sawtooth events
复制标题
锯齿事件的磁层状态
DOI：
10.1002/2016ja022693
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Xia Cai
中科院分区：
文献类型：
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作者：
S. Fung;J. A. Tepper;Xia Cai
磁层锯齿事件于20世纪90年代初首次被发现，其特点是在地球同步区出现多个缓慢减少的准周期间隔，随后质子微分能通量急剧增加。连续的质子通量振荡被解释为夜间地磁场的伸展和双极化的重现。由于锯齿波的间隔通常延长到2-10个 周期，所以锯齿波有时被称为磁层模式。虽然过去二十年来对锯齿事件的研究已经产生了关于这类事件的大量信息，但锯齿事件发生的磁层状态条件以及锯齿振荡如何依赖于磁层状态条件仍然不清楚。在这项研究中，我们研究了与锯齿现象发生之前、期间和之后的间隔相关的特征磁层状态条件(由PSW行星际磁场(IMF)Btot、IMF Bz Vsw、AE、Kp和Dst指定，所有这些条件都彼此时移)。应用以前发展的统计技术，我们确定了最可能的磁层状态，分别有利于锯齿事件的发展和发生。统计上确定的锯齿形磁层状态也通过使用样本外事件进行了验证，证实了锯齿形间隔可能代表磁层的特定全球状态的概念。我们认为，磁层的“锯齿状态”可能是一种边缘稳定状态，在这种状态下，一个原本连续的加载过程的加载速率稍有提高，就会使磁层进入边缘不稳定的区域，使其迅速释放有限的能量，并随着加载过程的继续返回到边缘稳定的区域。当磁层在边缘稳定(加载)和不稳定(卸载)阶段之间切换时，就会产生锯齿状振荡。
Magnetospheric sawtooth events, first identified in the early 1990s, are named for their characteristic appearance of multiple quasiperiodic intervals of slow decrease followed by sharp increase of proton differential energy fluxes in the geosynchronous region. The successive proton flux oscillations have been interpreted as recurrences of stretching and dipolarization of the nightside geomagnetic field. Due to their often extended intervals with 2–10 cycles, sawteeth occurrences are sometimes referred to as a magnetospheric mode. While studies of sawtooth events over the past two decades have yielded a wealth of information about such events, the magnetospheric state conditions for the occurrence of sawtooth events and how sawtooth oscillations may depend on the magnetospheric state conditions remain unclear. In this study, we investigate the characteristic magnetospheric state conditions (specified by Psw interplanetary magnetic field (IMF) Btot, IMF Bz Vsw, AE, Kp and Dst, all time shifted with respect to one another) associated with the intervals before, during, and after sawteeth occurrences. Applying a previously developed statistical technique, we have determined the most probable magnetospheric states propitious for the development and occurrence of sawtooth events, respectively. The statistically determined sawtooth magnetospheric state has also been validated by using out‐of‐sample events, confirming the notion that sawtooth intervals might represent a particular global state of the magnetosphere. We propose that the “sawtooth state” of the magnetosphere may be a state of marginal stability in which a slight enhancement in the loading rate of an otherwise continuous loading process can send the magnetosphere into the marginally unstable regime, causing it to shed limited amount of energy quickly and return to the marginally stable regime with the loading process continuing. Sawtooth oscillations result as the magnetosphere switches between the marginally stable (loading) and unstable (unloading) phases.