Observation Of Ion-Scale Magnetic Flux Ropes In Earth’s Magnetosheath

When the solar wind passes over the Earth,it creates the bow shock a dozen Earth radii upstream of the Earth.The magnetosheath is located between the bow shock and the Earth’s magnetopause,and there is often a variety of plasma instability inside it,which will further develop into turbulence.It can trigger magnetic reconnection and form a variety of magnetic structures such as magnetic holes,magnetic flux ropes,magnetic bottles and so on.Among them,the flux rope(FR)structure has helical magnetic field lines,which are observed as an increase in axial magnetic field components and bipolar changes in toroidal components.The flux ropes in magnetosheath are related to the magnetic reconnection process occurring in magnetosheath and magnetopause.In particular,the ion-scale flux ropes are often observed in the reconnection diffusion region and outflow region and play an important role in the energy conversion process from large scale to small scale.Using high-precision data from Magnetospheric Multiscale Mission(MMS)satellites,this article is conducted on ion-scale flux rope structures in the magnetosheath of the Earth.The main contents and conclusions are as follows:1.A new kind of ion-scale flux rope was found within the reconnection separatrix region on one side of the magnetosheathUsing the electron diffusion region(EDR)observation list,it was found that a new type of ion-scale magnetic flux rope exists within separatrix region surrounding the EDR.This new type of ion-scale flux rope has an axial direction parallel to the background magnetic field line and approximately perpendicular to the reconnection X-line.Both magnetosheath electrons and magnetosphere electrons exist inside the structure.At the same time,common structures inside the separatrix region like electron hole(EH;Also known as electrostatic solitary wave because of its electric field performance)or double layer(DL)can be observed,indicating that the flux rope is located in the separatrix region.Through joint analyses of the electron motion direction,particle energy range,the polarity of parallel current,and current-induced magnetic field twisting inside the flux rope,it was found that the ion-scale flux rope is generated by parallel current carried by low-energy magnetosheath electrons flowing into the reconnection X-line.2.Statistical study of ion-scale flux ropes in Earth’s magnetosheathUsing survey mode and burst mode data from MMS satellite,ion-scale flux ropes within the magnetosheath were searched for respectively,and the spatial distribution,evolution and occurrence factors of flux ropes were analyzed statistically.The statistical results show that the normalized distribution density of ion-scale flux ropes is higher near the bow shock and lower near the magnetopause.The formation of structures may be closely related to the bow shock magnetic reconnection.The ion-scale flux ropes are generated near the bow shock and propagates to the downstream of the magnetosheath.There is an exponential relationship between the scale and the number of flux ropes,which conforms to the model of flux rope growth.The plasma parameters do not change significantly because they are all ion-scale before and after evolution.Ion-scale flux ropes are more easily generated under the conditions of strong solar wind dynamic pressure and high Mach number.