Formation Of The Bow Shock Indentation: MHD Simulation Results

The coupling of solar wind and magnetosphere is one of the most critical factors in the interaction of the solar wind and the Earth's space environment.The bow shock,a collisionless standing shock,is a very important part of this coupling,which is formed by the interaction between the supersonic solar wind and the Earth's magnetosphere.When the upstream solar wind passes through the bow shock,it flows to the downstream of the magnetopause.This process effectively protects the Earth.Therefore,the study of the bow shock is greatly helpful to understanding the interaction between the solar wind and the Earth's magnetic field.In this paper,simulation results from the global magnetohydrodynamic(Space Weather Modeling Framework)model are used to examine whether the bow shock has an indentation and its formation conditions,as well as its physical mechanism.The main results are as follows:(1)From the results of the simulation,the definition of the bow shock indentation is studied.First,the bow shock is identified by the plasma density of the solar wind,and the indentation of the bow shock is determined by the shock flaring angle.The bow shock indentation under different solar wind conditions is analyzed,and the definition of the bow shock indentation is determined.When the difference between the minimum value and the average value(0?X?5)of the bow shock angle parameter?is greater than 0.3,the existence of the bow shock indentation can be determined;(2)The physical mechanism of the bow shock indentation is studied.Under the southward interplanetary magnetic field,the outflow region of the magnetopause reconnection in the low latitude region disturbs the flow structure of plasma in the magnetosheath.It forms a special structure,which contains the high velocity region and low velocity region of plasma flow.This special structure hinders the flow around the magnetosheath and widens the bow shock behind the structure.At the same time,the bow shock in front of the structure is not affected,so the bow shock indentation is formed between the widened bow shock and the unaffected bow shock;(3)The conditions for the formation of the bow shock indentation are analyzed.When the interplanetary magnetic field is southward and the Alfvén Mach number(M_?)of solar wind is high(>5),the bow shock indentation can be clearly determined.When the interplanetary magnetic field is southward and the Alfvén Mach number of the solar wind is low(?3.5),the magnetopause reconnection at low latitudes disturbs the flow in the magnetosheath.At the same time,a high velocity region and a low velocity region are formed.When the Alfvén Mach number of the solar wind is low,the magnetosheath is thicker.The blocking effect of this special structure is not obvious.Therefore,the bow shock indentation is not appeared.When the interplanetary magnetic field is northward,the magnetopause reconnection occurs at high latitudes.At this time,the outflow region of the magnetic reconnection does not form the high velocity region and low velocity region of plasma flow which are upstream and downstream of each other in the magnetosheath.Therefore,there is no blocking effect in the magnetosheath and the bow shock indentation cannot be formed.