河野/英昭

講演予稿:
　極域磁気圏に対する substorm の影響を、人工衛星 POLAR の一年 間のデータを用いて統計的に調べた。substorm 中に POLAR が極 域磁気圏領域の外に出る例が全体の約 20 % で見られたが、これ らの例の発生は衛星位置及び太陽風動圧との相関が高く、 substorm の影響については更に調査が必要である。一方、それ以 外の例については、substorm の影響が明瞭に認められた。それは、 substorm の成長相において極域磁気圏磁場の強度が増大し、その 傾きの角度が減少する、というものである。このどちらの量も、 substorm の爆発相以降は静穏時の値に向け戻っていく。この現象 の成因について議論を行なう。

abstract:
　How the magnetic field perturbs in the polar magnetosphere during substorms has not been systematically surveyed in the past, because there was almost no satellite previously which stayed there for a long time. The purpose of this paper is to study it on a statistical basis by using data from the POLAR spacecraft, apogee of which is usually located well within the northern polar magnetosphere. For the interval of one year after the launch of POLAR in March 1996, we have sampled substorm events for which POLAR was located near its apogee (geocentric distance 9Re). As a result we have obtained 76 events. For more than ten out of the events, POLAR temporarily exited the magnetospheric domain and entered the cusp or magnetosheath. The occurrence of the exit is dependent on the spacecraft position (frequent at large X (GSM position)) and the solar wind dynamic pressure (frequent at a large pressure). Substorm may not be a major controlling factor for the exit, but further study is necessary before a final conclusion is drawn. A significant substorm-associated perturbation is found for the events without an exit The magnetic field strength increases during the growth phase and recovers during the expansion to recovery phases. The increase is interpreted to be the result of the pileup of the dayside-eroded magnetic flux over the polar magnetosphere. In addition, the tilt angle of the magnetic field at POLAR also shows a substorm-associated perturbation the field line tends to become less tilted during the growth phase and recovers during the expansion to recovery phases. This appears to be opposite to what is expected at first sight, because the tilt angle of the polar magnetopause is expected to increase as a result of the flux pileup. A possible explanation is the dragging effect of the ionosphere It tends to pin the magnetic field line at its footpoint, and when the field line is pushed inward and tailward by the piled-up flux, it becomes less tilted.

キーワード:
極域磁気圏、サブストーム、太陽風

Keywords:
polar magnetosphere, substorm, solar wind