MHD simulations using average solar wind conditions for substorms observed under northward IMF conditions

Substorms are known to sometimes occur even under northward interplanetary magnetic field (IMF) conditions. In this paper, we perform three‐dimensional global magnetohydrodynamic simulations to examine dayside reconnection, tail, and ionospheric signatures for two cases of substorm observations under prolonged northward and dawnward IMF conditions: (1) a strongly northward/dawnward IMF case with B IMF  = (0, −20, 20) nT; (2) a weakly northward/dawnward IMF case with B IMF  = (0, −2, 2) nT. Throughout the simulations, we used the constant solar wind conditions to reflect the prolonged solar wind conditions around the substorm times. We found that, in both cases, the tail reconnection occurred after the usual high‐latitude reconnection on the dayside, providing a possible energy source for later triggered substorm observations under northward IMF conditions. The presence of an equal amount of IMF B y allows the high‐latitude reconnected magnetic field lines to transport to the tail lobe, eventually leading to the tail reconnection. The simulation results also revealed the following major differences between the two cases: First, the reconnection onset (both on dayside and in the tail) occurs earlier in the strongly northward IMF case than in the weakly northward IMF case. Second, the polar cap size, which is finite in both cases despite the northward IMF conditions and thus supports the lobe energy buildup needed for the substorm occurrences, is larger in the strongly northward IMF case. Accordingly, the polar cap potential is far larger in the strongly northward IMF case (hundreds of kilovolt) than in the weakly northward IMF case (tens of kilovolt). Third, in the strongly northward IMF case, the strong earthward tail plasma flow appears to be caused by the enhanced convection (so enhanced duskward E y ) due to the tail reconnection. In contrast, in the weakly northward IMF case, the earthward tail plasma flow increases gradually in association with a modestly increased duskward electric field. In addition, the inner plasma pressure and the cross tail current near the reconnection site increase significantly in the strongly northward IMF case but less significantly in the weakly northward IMF case after the onset of the tail reconnection. In conclusion, the simulation results support observations of the substorms under northward IMF conditions in the presence of an equal amount of IMF B y by demonstrating the energy input via dayside reconnection and the subsequent occurrence of the tail reconnection.