[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-P19] Geometry of Dungey reconnection and its relation to the interchange cycle during periods of almost due northward interplanetary magnetic field

*渡辺 正和¹、蔡 東生²、熊 沛坤²、藤田 茂³、田中 高史⁴ (1.九州大学 大学院理学研究院、2.筑波大学 システム情報系、3.情報・システム研究機構 統計数理研究所、4.九州大学 国際宇宙惑星環境研究センター)

キーワード：Dungeyサイクル、交換サイクル、null-separator構造、MHDシミュレーション

When the interplanetary magnetic field (IMF) is southward, IMF-to-closed reconnection on the dayside followed by north lobe-to-south lobe reconnection in the magnetotail drive the so-called Dungey cycle in the magnetosphere, resulting in a well-known two-cell convection system in the ionosphere. In contrast, when the IMF is close to due northward, IMF-to-lobe reconnection in one hemisphere followed by lobe-to-closed reconnection in the opposite hemisphere produce the “interchange cycle” on the dayside and flanksides of the magnetosphere (Watanabe & Sofko, 2009). The latter reconnection processes are said to be interchange-type or interchange reconnection because the two topological regions to which reconnecting field lines belong interchange at the time of reconnection. The interchange cycle drives twin “reverse cells” at dayside high latitudes in the ionosphere circulating in the direction opposite to that of the Dungey cycle. In addition, for northward IMF, there appears an ionospheric convection cell on the nightside whose circulation direction is controlled by the IMF By component (Grocott et al., 2005). Recently, using magnetohydrodynamic (MHD) simulation, Tanaka et al. (2024) suggested that the nightside convection system for northward IMF is a consequence of Dungey reconnection processes (IMF-to-closed on the dayside and lobe-to-lobe on the nightside). A question then arises here. In what geometry dose Dungey reconnection occur during periods of close to due northward IMF? How can Dungey reconnection for northward IMF concur with the interchange cycle? The purpose of this paper is to answer these questions from a point of view of global magnetic field topology and geometry.

Using the Reproduce Plasma Universe (REPPU) code, we numerically reproduced a magnetosphere under the IMF conditions of B=6nT (field intensity) and θ=20° (clock angle). The global magnetic field topology of the magnetosphere consists of two magnetic nulls and two separators connecting them (the 2-null 2-separator structure). With the Geodesic Level Set method developed recently (Xiong et al., 2023), we extracted 2D separatrices emanating from the two magnetic nulls and determined the two separators. Due to the electric currents associated with lobe-to-closed interchange reconnection, both dayside and nightside separators exhibit hairpin bends in the vicinity of the nulls, showing outside-in approaches toward the nulls. With this geometry, field-aligned electric fields appear on the separators near the nulls in favor of Dungey reconnection, even for close to due northward IMF. Both dayside and nightside Dungey reconnection occur at high latitudes on the flanks of the magnetosphere, whose geometry is quite different from that for southward IMF. Dungey reconnection and interchange reconnection can coexist in this geometry. This geometry also indicates that the transition from the Dungey cycle to the Interchange cycle occurs not abruptly at a certain IMF clock angle but occurs gradually in a wide range of the IMF clock angle.

References
Watanabe & Sofko (2009). https://doi.org/10.1029/2008GL036682
Tanaka et al. (2024). Submitted to Journal of Geophysical Research
Grocott et al. (2005). https://doi.org/10.5194/angeo-23-1763-2005
Xiong et al. (2023). https://doi.org/10.1111/cgf.14994