Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?
Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?
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DOI：
10.1029/2018ja025684
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发表时间：
2019-11
期刊：
影响因子：
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通讯作者：
O. Kozyreva;V. Pilipenko;D. Lorentzen;L. Baddeley;M. Hartinger
中科院分区：
文献类型：
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作者：
O. Kozyreva;V. Pilipenko;D. Lorentzen;L. Baddeley;M. Hartinger
Pc5 - 6波段（约3 - 20分钟）的地磁脉动是昼侧高纬度ULF活动的持续特征。磁层顶表面本征模可能是这些脉动的潜在机制。人们可能期望这些模式的地面响应接近开闭场线边界（OCB）的电离层投影。利用位于斯瓦尔巴特群岛的仪器的数据，我们研究了磁鞘等离子体脉冲“入侵”到昼侧磁层的瞬态地磁响应。这些侵入是由行星际磁场向南的适度变化触发的，并被观测为向赤道的红色极光边界突然向低纬度移动和绿色线发射增强。每次极光扰动都伴随着~1.7 - 2.0 mHz的地磁脉动爆发。ULF脉动的近尖点纬度结构与红极光向赤道边界的瞬时位置进行了比较，假设是OCB的代理。光学OCB纬度已确定使用的数据从子午线扫描光度计。瞬变地磁响应的纬向最大值往往位于受扰OCB代用品附近，与光度计和磁力计方法的误差在1 °~2 °范围内。记录的瞬态脉动可能与磁层顶表面模式谐波的地面图像有关。理论研究表明，在初始激发后，表面大尺度模式转换为局部阿尔文振荡，因此只能存在有限的时间。因此，实际非均匀等离子体中的MHD表面模不能孤立地考虑，而是作为一个具有离散谱和连续谱的模的组合系统，它们之间具有不可逆变换。
Geomagnetic pulsations in Pc5‐6 band (~3–20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open‐closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive “intrusion” of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of ~1.7–2.0‐mHz geomagnetic pulsations. Near‐cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error ~1°–2° of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large‐scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them.