Stellar Magnetic Storm Induced Magnetospheric Polarity Reversals: A Potential Diagnostic for Distinguishing Unmagnetized and Magnetized (Exo)planets

Abstract

(Exo)planetary environments are forced by stellar activity, which manifest through variable radiation, particle and magnetic fluxes, stellar winds, flares, and magnetic storms known as coronal mass ejections (CMEs). Recent studies have shown that planets with intrinsic magnetic fields respond differently to stellar wind associated interplanetary magnetic fields (IMFs) compared to planets that lack intrinsic magnetism, as observed in solar system planets. However, detailed investigations that aim to uncover subtle ways in which transient CMEs impact exoplanets are still at a nascent stage. Here, we utilize 3D magnetohydrodynamic simulations to investigate the impact of stellar CMEs on Earth-like (exo)planets with different magnetic fields. Our results, gleaned from 16 different combinations of planetary and IMFs show that planetary atmospheric mass-loss rates are dependent on the relative orientation of stellar wind and planetary magnetic fields, with significantly higher losses when the CME and planetary magnetic fields are oppositely oriented—favoring enhanced magnetic reconnections. In contrast, for unmagnetized planets, the mass-loss rate does not strongly depend on the orientation of the stellar ejecta. We also find that induced magnetospheres in unmagnetized and weakly magnetized planets are prone to global polarity reversals following reversals in the CME-associated stellar wind magnetic field, however, magnetized planets preserve their intrinsic magnetospheric topology. Our analysis of the magnetotail current density dynamics during polarity reversal is qualitatively consistent with observations of Venus. This differential response of induced and intrinsic magnetospheric behavior may provide a potential diagnostic for detecting and characterizing exoplanetary magnetic fields.