Influence Of Dipole Tilt Angle On The Bow Shock Shape And Location

The Earth’s bow shock is an important part of the solar wind-magnetosphere coupling system and it is formed by the interaction between the supersonic solar wind and the Earth’s magnetosphere. The upstream solar wind crosses the bow shock then bypasses the magnetopause and flows to the downstream. This mechanism plays a crucial role when protects the Earth from the harmful effect of the solar wind. In this work, the satellites’data is used to investigate the relationship between the dipole tilt angle and the subsolar standoff distance, the flaring angle, and the north-south asymmetry of the bow shock by fitting the bow shock shape and location. The results show that the subsolar standoff distance of the bow shock increases but the bow shock flaring angle decreases with the increasing absolute value of the dipole tilt angle, and the dipole tilt angle has different effects on the north-hemisphere and south-hemisphere bow shock, that is, the bow shock has a north-south asymmetry. This work lays the foundation for the establishment of a new bow shock model that contains the dipole tilt effects and all other parameters affecting the bow shock. The main findings are as follows:1. By using multiple satellites’ bow shock crossings data, we do statistical analysis to study the impact of the dipole tilt angle on the bow shock shape and location. Based on four satellites’ (IMP 8, Geotail, Magion 4, and Cluster 1) bow shock crossings from 1973 to 2002 and Cluster 2’crossings from 2012 to 2013, we make the whole fitting, piecewise fitting and the northern and southern hemispheres fitting to find out the quantitative relationship between the dipole tilt angle and the bow shock shape and location (the subsolar standoff distance and the flaring angle of the bow shock tail). It is shown:(1) The subsolar standoff distance increases with the increasing absolute value of dipole tilt angle, their relationship can be described as:(2) The flaring angle decreases with the increasing absolute value of dipole tilt angle increasing, their relationship can be described as:(3) When the dipole tilt angle changes from negative to positive, the dayside bow shock moves closer to the Earth and this movement can reach to 2.29RE;(4) The shape of the northern hemisphere bow shock and the southern hemisphere bow shock is different. For the north-hemisphere bow shock, with the dipole tilt angle negatively increasing; However, the flaring angle decreases, when the tilt angle increasing positively the flaring firstly increase and then decrease, overall remains the same. For the south-hemisphere bow shock, with the dipole tilt angle positively increasing, the flaring angle decreases, with the tilt angle increasing negatively the flaring angle increases, but the magnitude is small, the flaring angle remains the same.2. By using the SWMF model, we do the numerical simulation to further study the relationship between the dipole tilt angle and the bow shock shape and location. It is shown that:(1) The subsolar standoff distance increases with the absolute value of the dipole tilt angle increasing;(2) In GSM coordinate system, the impact of the positively increasing dipole tilt angle on one hemisphere bow shock is the same as the impact of the negatively increasing dipole tilt angle impact on the another hemisphere.