Data‐Based Modeling of the Magnetosheath Magnetic Field

A quantitative model of the magnetosheath (MS) magnetic structure is developed, using a multi‐year set of Geotail, Themis, Cluster, and MMS magnetometer and plasma instrument data. The MS database is created using an identification algorithm, based on observed magnetic field magnitudes and proton densities, normalized by their concurrent interplanetary values, followed by additional filtering with the help of standard bow shock (BS) and magnetopause (MP) models. The model architecture is based on the toroidal/poloidal formalism and a coordinate system that naturally accounts for the tailward flaring of both boundaries. The magnetic field expansions include 960 free coefficients, derived by fitting the model to a grand data set, split into independent training and validation subsets with 1,291,380 and 411,933 1‐min records, respectively. The model faithfully reproduces basic types of the interplanetary magnetic field (IMF) wrapping around the MP. Regular IMF sectors result in strongly dawn‐dusk asymmetric draping, with much larger magnitudes at the quasi‐perpendicular dusk side of BS, and weaker at the quasi‐parallel dawn side, where the MS field lines are bent and dragged tailward. Except in the case of the flow‐aligned IMF orientation, the subsolar field steadily grows toward the MP, and the effect is clearly IMF Bz‐dependent: the field and its gradient are larger (smaller) for northward (southward) IMF Bz, implying a pile‐up of the magnetic flux in the first case and stronger reconnection in the second. Model distributions of the MS field magnitude reveal local depressions, associated with polar cusps near the high‐latitude limits of data coverage. Plain Language Summary The terrestrial magnetosheath is a relatively wide transition region, separating our planet’s magnetosphere from the undisturbed flow of magnetized solar wind. Due to the sudden compression of the incoming plasma flow at the bow shock and high conductivity of the magnetosheath medium, the relatively weak magnetic field of interplanetary origin undergoes abrupt compression and drapes around the magnetosphere boundary, the magnetopause. Huge amounts of archived spacecraft data accumulated in the world data centers during past decades of space flight made it possible to develop quantitative models of the magnetosheath magnetic field, based on direct in situ observations. This paper presents first results of such a modeling study, providing a closed analytical representation of the magnetosh