Steady-state of the Martian induced magnetosphere and its rapid response to interplanetary magnetic field rotation

Abstract

The induced magnetosphere in non-magnetized planets or moons, formed by the interaction between their atmosphere and stellar wind or planetary wind, is generally modulated by external magnetic field.   The magnetic field in the induced magnetosphere is believed to be dominated by the draped field. The direction of such draped field is theoretically expected to align with the y-z direction of the interplanetary magnetic field. However, observations show the opposite direction of magnetic field in the induced magnetospheres from the interplanetary magnetic field direction. Using joint observations from Tianwen-1 and MAVEN, we obtain the averaged magnetic field map of the Martian induced magnetosphere in the accurate MSE coordinate system and calculated its standard deviation. The standard deviation confirms that the averaged magnetic field distribution is consistent with the steady state assumption. The magnetic field map illustrates a clockwise rotation of the averaged magnetic field in the y-z plane, occurring in both the dayside and nightside in the Martian induced magnetosphere. According to the magnetic induction equation, this clockwise rotation of the magnetic field occurs when a difference in the speed of plasma flow exists within the magnetosphere. It should be noted that the induced magnetospheres of the other non-magnetized planets exhibit similar qualitative properties to that of Mars, suggesting that they share comparable magnetic field characteristics.   Observations of the response process of induced magnetosphere to external magnetic field are significant for understanding global dynamical processes in non-magnetized planets, and yet such observations are quite scarce. Using simultaneous observations from Tianwen-1 and Mars Atmosphere and Volatile EvolutioN (MAVEN), we report for the first time the dynamic response of the Martian induced magnetosphere to the rotation of interplanetary magnetic field from the Sun. The magnetic field in the Martian induced magnetosphere deflected as the interplanetary magnetic field rotated suddenly, and eventually stabilized (< 3.5 minutes). The convective electric field rotated in response to the interplanetary magnetic field rotation, and the pick-up oxygen ion plume emerged in minutes (< 3 minutes). These quite short recovery timescales indicate that the induced magnetosphere is a rapidly dynamic system, and is highly sensitive to external magnetic field. It cautions us that change of interplanetary magnetic field should be considered as one of the general types of space weather on Mars, and it is essential of monitoring and short-term forecasting of interplanetary magnetic field upstream of Mars.