Study On The Influence Of Interplanetary Magnetic Field On The Position Of Martian Bow Shoc

The solar wind-planet interaction is one of the important studies in planetary space physics.Unlike the Earth,Mars does not have a global dipole magnetic field,resulting a planet with an atmosphere and a weak residual magnetic field.Therefore,the Martian ionosphere exposes directly in the moving magnetic field of the solar wind.Driven by the effects of the "pick-up" of Martian ions and the "mass-loading" of the solar wind,the interplanetary magnetic field(IMF)accumulates on the dayside of the Martian ionosphere,forming the Martian induced magnetosphere.The Martian bow shock is a standing shock due to the interaction of the solar wind and the Martian induced magnetosphere.The position of the bow shock is a very important parameter to characterize the interaction between the solar wind and the planet.Under such conditions,the IMF frozen in the solar wind affects the position of the Martian bow shock,and there are not many studies on the influence of the strength and direction of the IMF on the position of the Martian bow shock currently.Using data from the Mars Atmosphere and Volatile Evolution(MAVEN)mission,this work manually identifies a total of 10,283 Mars bow shock crossing events during the phase of decreasing solar activity(October 2014 to December 2020)and record the solar wind parameters upstream of the crossing events,such as solar wind velocity(V),solar wind proton density(N),and interplanetary magnetic field(IMF).Based on these parameters,this work focuses on the influence of the strength and direction of the IMF on the position of the Martian bow shock.The main results are as follows.(1)The bow shock in both the subsolar and flank regions of Mars vary linearly away from Mars with the increasing IMF strength,which is consistent with the theory and the results of magnetohydrodynamic(MHD)simulations.(2)We demonstrate for the first time that the bow shock locats closer to Mars in both the subsolar and flank regions under the radial IMF conditions compared to other IMF cases.We also offer mathematical relations to characterize the specific relationship between the two paraments.The phenomenon may be due to the weaker magnetic pile up process and the "low pressure magnetosheath" model in the radial IMF condition.(3)The cross-section of the Martian bow shock is elongated in the north-south direction when the Y component of the IMF is dominant,which is on account of the fast magnetosonic speed effect and verifies the elongation phenomenon of the terrestrial bow shock.In summary,the increase of IMF strength leads to an enhanced solar wind convective electric field and stronger magnetic field pile up effects,which can result in an increase in the region of the Martian induced magnetosphere and the location of the bow shock further away from Mars.In terms of IMF direction,the contribution of the radial IMF to the magnetic pile up process is smaller,which causes the bow shock to be closer to Mars.In addition,the contribution of the IMF direction to the bow shock is also affected by the fast magnetosonic speed,which reaches a minimum at the upstream IMF direction parallel to the shock normal.The reduced fast magnetosonic speed leads to the increase of magnetic Mach number and eventually results in the closer location of bow shock to the Mars.