徑向行星際磁場事件之特性及其對磁層之影響;The characteristic of the radial IMF events and its inference for the magnetosphere

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Title:	徑向行星際磁場事件之特性及其對磁層之影響;The characteristic of the radial IMF events and its inference for the magnetosphere
Authors:	畢可為;Pi, Gilbert
Contributors:	太空科學研究所
Keywords:	太陽風;徑向行星際磁場;磁場結構;磁層頂;solar wind;radial IMF;magnetic field structure;magnetopause
Date:	2016-11-10
Issue Date:	2017-01-23 16:07:37 (UTC+8)
Publisher:	國立中央大學
Abstract:	長持續時間之徑向行星際磁場事件在行星際空間中並不常見，且屬於相對平靜之太陽風條件，但是其特殊之磁場指向，會導致磁層產生許多特殊的結構。本篇論文利用OMNI資料庫以及THEMIS任務所提供之觀測資料，針對此類事件在太陽風中之特性以及對磁層頂附近結構之影響作了完整之討論。長持續時間之徑向行星際磁場的發生頻率與太陽活動週期並沒有明顯關聯。當徑向行星際磁場事件發生時，其結構會在徑向方向延伸，造成整個結構擴張，同時也使得磁場強度、密度以及溫度下降。與年平均值相比，其下降比例分別為17.5%、21.9%以及35.8%。而太陽風動壓也下降27.8%。由於動壓的下降會使得磁層頂往外移動。另一方面，當行星際磁場為徑向時，快速磁聲波之馬赫數也會較低，因此，船艏震波也會因而向內移動。徑向行星際磁場穿過船艏震波後，BY與BZ都會被明顯放大，並且BX會隨著電漿流慢慢轉向另外兩個分量，進而包覆整個磁層頂，造成磁層頂外之磁場指向呈現南北不對稱之結構。此一不對稱之結構亦導致磁重聯發生之位置不對稱，磁重聯發生後，磁鞘中的磁場結構會被改變，造成磁層頂外受到北向磁場影響的區域變大，同時產生外低緯度邊界層。;Long-duration radial interplanetary magnetic field (IMF) events are one of the special solar wind conditions when the orientation of the IMFis aligned with the solar wind velocity. The radial IMF events usually are regarded as quiet solar conditions, but the unique orientation of the IMF can lead to some consequences in the magnetospheric system. In this thesis, we used the OMNI and THEMIS data to investigate the characteristics of the radial IMF events and its influence on the magnetospheric system. During the events, the IMF magnitude, solar wind speed, density, and especially its temperature are depressed in comparison with their yearly averages. In contrast to previous studies, we have found that the total time of the radial IMF per year does not change with solar activity. MHD simulation models failed to predict the location of the magnetopause under the radial IMF condition. A part of the inaccuracy is due to the use of assumed solar wind parameters in the simulations. Here we provide MHD modelers with the real solar wind parameters for simulations of the radial IMF. When the magnetic field transited through the bow shock, we compared the simultaneous data from OMNI database, THEMIS-B in the solar wind, and THEMIS-C in the magnetosheath to investigate the relationship between the magnetic field structures in the solar wind and in the magnetosheath. We found that the magnetic field will be enhanced in all components, especially in By, under the radial IMF conditions. After that, Bx will divert to the Y and Z components, but the diversion to By will be larger than that to Bz in this case. In the magnetosheath region near the magnetopause, the magnetic field will drape around the magnetopause, having an asymmetric magnetic field orientation in different hemispheres. It will lead to an asymmetric location of reconnection under radial IMF conditions and rearrange the structure of the magnetic field near the magnetopause. Once the reconnection takes place, the plane separating different orientations of the magnetic field in the magnetosheath shifts. It will lead to an enlargement of the region near the magnetopause affected by the positive magnetosheath Bz and enhance the positive Bz in the magnetosheath near the subsolar magnetopause.
Appears in Collections:	[Graduate Institute of Space Science] Department of Earth Sciences

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