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

Abstract

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 BIMF = (0, −20, 20) nT; (2) a weakly northward/dawnward IMF case with BIMF = (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 By 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 Ey) 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 By by demonstrating the energy input via dayside reconnection and the subsequent occurrence of the tail reconnection.

3D GUMICS Simulations of Northward IMF Magnetotail Structure

“…Moving on to comparisons with other recent results, there are some similarities between the features of our simulation run and the results of Li et al (2021), but there are also some interesting differences. Initially, one might expect the closed wedge to contract and be associated with strong earthward flows, similar to those traditionally associated with northward IMF substorm conditions as discussed by Lee et al (2010), Miyashita et al (2011), Park et al (2015, Li et al (2021), and references therein. However, in our simulation, it is clear (from the middle panels in Figure 2) that the wedge increased in length toward more negative X-values rather than forming and contracting earthward.…”

The formation of superdense plasma sheet in association with the IMF turning from northward to southward

“…Substorm, the main process of solar wind entry during southward IMF condition, was not supposed to occur during northward IMF condition. However, recent observations have shown that substorms did occur during northward IMF condition [ Du et al , ; Lee et al , ; Park et al , ; Peng et al , ; L. Q. Zhang et al , ; Park et al , ]. Such substorms may significantly facilitate solar wind entry in the magnetotail during northward IMF condition.…”

Bursty bulk flows at different magnetospheric activity levels: Dependence on IMF conditions

“…Now there are growing evidences that the tail X line also occurs for NW-IMF [e.g., Park et al, 2015;Zhang et al, 2015c]. According to Zhang et al [2015c], the near-Earth X lines occur for SW-IMF as well as NW-IMF, but more frequently for SW-IMF.…”