[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-10] Revisiting the Partial Ring Current Model: Stormtime Formation of the Dawnside Wedge Current System

*Shinichi Ohtani¹、Kareem Sorathia¹、Viacheslav G. Merkin¹、Harald U. Frey²、Jesper W. Gjerloev¹ (1.Johns Hopkins University Applied Physics Laboratory、2.Space Sciences Laboratory, University of California, Berkeley)

キーワード：Stormtime Magnetic Depression, Dawnside Current Wedge, Partial Ring Current

The dawn-dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. During the storm main phase, the horizontal ground magnetic component is more depressed at dusk than at dawn, which was originally explained by the partial ring current model. Recently Ohtani (2021) found that its magnitude is correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere-ionosphere coupling; the result is summarized in the attached figure. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four hypotheses. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, (c) during storms, the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each hypothesis, and therefore, none of these hypotheses can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (1) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (2) external compression probably explains only a small fraction (< 10%) of the events; (3) the westward AEJ starts to intensify in the midnight sector ~30 min before its dawnside intensification, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of substorm itself, but the results of previous studies suggest that such events are not very common.

Reference:
Ohtani, S. (2021). Revisiting the Partial Ring Current Model: Longitudinal Asymmetry of Ground Magnetic Depression during Geomagnetic Storms. Journal of Geophysical Research: Space Physics, 126, e2021JA029643. https://doi.org/10.1029/2021JA029643