In this paper, the Space Weather Modeling Framework (SWMF) is used to simulate the real-time response of the magnetosphere to a solar wind event on June 5, 1998, in which the interplanetary magnetic field shifted its direction from north to south. Since most current models do not take into account convective effects of the inner magnetosphere, we first study the importance of Rice Convection Model (RCM) in the global model. We then focus on the following four aspects of the magnetosphere’s response: the magnetosphere’s density distribution, the structure of its magnetic field lines, the area of the polar cap boundary, and the corresponding ionospheric current change. We find that (1) when the IMF changes from north to south in this event, high magnetosheath density is observed to flow downstream along the magnetopause with the solar wind; low-latitude reconnection at dayside occurs under the southward IMF, while the magnetic field lines in the tail lobe caudal, caused by the nightside high latitude reconnection, extend into the interplanetary space. Open magnetic field lines exist simultaneously at both high and low latitudes at the magnetopause; (2) the area of the polar cap is obviously increased if the IMF turns from the north to the south; this observation is highly consistent with empirical observations; (3) the ionospheric field align current in the northern hemisphere is stronger than in the southern hemisphere and also increases as the IMF changes from north to south. SWMF with the Rice Convection effect provides reliable modeling of the magnetospheric and ionospheric response to this solar wind variation.
Simulation results from a global magnetohydrodynamic (MHD) model are used to examine whether the bow shock has an indentation and characterize its formation conditions, as well as its physical mechanism. The bow shock is identified by an increase in plasma density of the solar wind, and the indentation of the bow shock is determined by the shock flaring angle. It is shown that when the interplanetary magnetic field (IMF) is southward and the Alfvén Mach number (Mα) of solar wind is high (> 5), the bow shock indentation can be clearly determined. The reason is that the outflow region of magnetic reconnection (MR) that occurs in the low latitude area under southward IMF blocks the original flow in the magnetosheath around the magnetopause, forming a high-speed zone and a low-speed zone that are upstream and downstream of each other. This structure hinders the surrounding flow in the magnetosheath, and the bow shock behind the structure widens and forms an indentation. When Mα is low, the magnetosheath is thicker and the disturbing effect of the MR outflow region is less obvious. Under northward IMF, MR occurs at high latitudes, and the outflow region formed by reconnection does not block the flow inside the magnetosheath, thus the indentation is harder to form. The study of the conditions and formation process of the bow shock indentation will help to improve the accuracy of bow shock models.
We present a statistical study of “trunk-like” structures observed in He+ and O+ in the inner magnetosphere. The main characteristic of these structures is that the energy of the peak flux decreases earthward. Using observations from the Helium Oxygen Proton Electron (HOPE) instrument onboard Van Allen Probe A, we identify the trunks observed from November 2012 to June 2019 and obtain the universal time, L shell, magnetic local time (MLT), and energy information of each trunk’s root and tip. We then investigate the behavior of trunks in terms of their frequency of occurrence, temporal evolution, spatial and energy distribution, and trunk dependence on different geomagnetic indices. We find that (1) the trunks are always located at L = 1.5−4.0 and have a preferential location mainly concentrated at MLT = 18−24, (2) the sector within MLT = 14−16 is a forbidden zone without trunk roots, and (3) the energy of He+ trunks is the largest near dusk and gradually decreases in the counterclockwise direction, whereas the energy of O+ trunks is relatively evenly distributed with MLT and L. The differences between He+ and O+ trunks are probably due to the different charge exchange and Coulomb collision lifetime. The dependence on different geomagnetic indices indicates that the trunk structures occur more frequently during relatively quiet periods.