Abstract
Abstract. The interaction of the solar wind with a planetary magnetic field causes electrical currents that modify the magnetic field distribution around the planet. We present an approach to estimating the planetary magnetic field from in situ spacecraft data using a magnetohydrodynamic (MHD) simulation approach. The method is developed with respect to the upcoming BepiColombo mission to planet Mercury aimed at determining the planet's magnetic field and its interior electrical conductivity distribution. In contrast to the widely used empirical models, global MHD simulations allow the calculation of the strongly time-dependent interaction process of the solar wind with the planet. As a first approach, we use a simple MHD simulation code that includes time-dependent solar wind and magnetic field parameters. The planetary parameters are estimated by minimizing the misfit of spacecraft data and simulation results with a gradient-based optimization. As the calculation of gradients with respect to many parameters is usually very time-consuming, we investigate the application of an adjoint MHD model. This adjoint MHD model is generated by an automatic differentiation tool to compute the gradients efficiently. The computational cost for determining the gradient with an adjoint approach is nearly independent of the number of parameters. Our method is validated by application to THEMIS (Time History of Events and Macroscale Interactions during Substorms) magnetosheath data to estimate Earth's dipole moment.
Highlights
Planets with an intrinsically generated magnetic field, such as Earth or Mercury, interact with the solar wind
As the calculation of gradients with respect to many parameters is usually very time-consuming, we investigate the application of an adjoint MHD model
In this paper we investigate the applicability of an adjoint approach to a MHD simulation code using automatic differentiation (Wengert, 1964)
Summary
Planets with an intrinsically generated magnetic field, such as Earth or Mercury, interact with the solar wind. The BepiColombo mission is most suitable for determining the interaction because of simultaneous observations of the magnetic field distribution in the magnetosphere and the solar wind. In contrast to empirical models, a MHD simulation requires only parameters of the solar wind conditions, planetary magnetic field, and plasma properties. This approach calculates the interaction self-consistently and does not contain parameters to fit electrical currents. In its final application for the BepiColombo mission, spacecraft data of the entire interaction region including the magnetosphere will be taken into account to estimate the planetary magnetic field
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