[P-EM09] 宇宙天気・宇宙気候

[PEM09-P06] Two-dimensional local modeling of thermospheric heating and neutral mass density enhancement driven by ionospheric Alfvén resonator

大井川 智一¹、品川 裕之²、*田口 聡¹ (1.京都大学大学院理学研究科地球惑星科学専攻地球物理学教室、2.情報通信研究機構)

キーワード：中性大気質量密度異常、カスプ、アルフベン波

In the cusp region neutral mass density enhancement is commonly observed at altitudes near 400 km. This enhancement greatly exceeds the predictions of the empirical atmospheric model, and is considered to be anomalous. Several recent studies have shown that quasi-static processes imposed by a static moderate electric field in the cusp cannot reproduce the typical magnitude of the mass density enhancement adequately. In order to understand how the Joule heating and resultant mass density enhancements are generated in the F region of the ionosphere by time-varying processes associated with Alfvén waves, we have developed a new two-dimensional model that can incorporate the ionospheric Alfvén resonator process. In this model Alfvén waves were solved as frequency-domain boundary value problems. The result from the modeling runs shows that the ionospheric Alfvén resonator modes generate significant neutral upwelling at altitudes near 300 km, which creates a cell of the neutral mass density enhancement at altitudes near 400 km. This cell becomes evident roughly 1 hour after the ionospheric Alfvén resonator modes are set up, and this region continues to exist stably for two more hours. A fractional mass density enhancement at 400 km altitude reaches 33% three hours after the ionospheric Alfvén resonator modes having an Alfvénic field-aligned current of 20 μA/m² are set up, which explains the result obtained from previous satellite observations.