[P-EM09] Dynamics in magnetosphere and ionosphere

[PEM09-10] プラズマ圏内に局在化したQuarter Wavesの分布について

*尾花 由紀¹、Colin L. Waters²、Murray D. Sciffer²、Frederick W. Menk²、Robert L. Lysak³、Mark B. Moldwin⁴、Ian R. Mann⁵、David Boteler⁶、Vassilis Angelopoulos⁷、Christopher T. Russell⁷ (1.大阪電気通信大学工学部基礎理工学科、2.The University of Newcastle、3.University of Minnesota、4.University of Michigan、5.The University of Alberta、6.Natural Resources Canada、7.UCLA, IGPP)

キーワード：ULF波, 磁気圏電離圏結合, プラズマ圏, 磁力線共鳴振動

The latitudinal distribution of quarter-wave mode ULF pulsations was investigated. We examined the diurnal variation of the local field line eigenfrequency over the latitude range L=1.7-6.8 using cross-phase analysis of magnetic data from the MEASURE, THEMIS, CANMOS, and CARISMA magnetometer arrays. The detected eigenfrequencies for L-shell in middle latitudes were remarkably low (1.5-2 times lower than usual daytime value) near the dawn terminator. This occurred when a field line was mapped into the plasmasphere, and one end of the field line was sunlit while the other end was in darkness. However, the eigenfrequencies for higher L-shells were not extraordinarily low. These results suggest that resonant quarter-wave modes were localized in the mid-latitude region, in the plasmasphere, but were not generated at high latitudes even though the ionospheric conditions were strongly asymmetric there. Our previous study showed that the mode transition from quarter wave to half wave depends on the ratio of ionospheric Pedersen conductances between sunlit and dark sides, and the typical value is approx. 10. The ionospheric conductances in this study seem to have satisfied this generation condition for stations at all latitudes. Therefore another condition may be necessary to explain the latitudinal localization of the waves. Some properties of quarter wave modes were examined using computer simulations. Our simulation results suggested that the resonance properties of heavily damped quarter mode waves may be masked by cavity mode energy when the cavity resonance is effective. This may control localized distribution of the quarter waves.