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We simulate the space environment around AU Microscopii b and the interactionbetween the magnetized stellar wind with a planetary atmospheric outflow forambient stellar wind conditions and Coronal Mass Ejection (CME) conditions. Wealso calculate synthetic Ly$\alpha$ absorption due to neutral hydrogen in theambient and the escaping planetary atmosphere affected by this interaction. Wefind that the Ly$\alpha$ absorption is highly variable due to thehighly-varying stellar wind conditions. A strong Doppler blue-shift componentis observed in the Ly$\alpha$ profile, in contradiction to the actual escapevelocity observed in the simulations themselves. This result suggest that thestrong Doppler blue-shift is likely attributed to the stellar wind, not theescaping neutral atmosphere, either through its advection of neutral planetarygas, or through the creation of a fast neutral flow via charge exchange betweenthe stellar wind ions and the planetary neutrals. Indeed, our CME simulationsindicate a strong stripping of magnetospheric material from the planet,including some of the neutral escaping atmosphere. Our simulations show thatthe pressure around close-in exoplanets is not much lower, and may be evenhigher, than the pressure at the top of the planetary atmosphere. Thus, theneutral atmosphere is hydrodynamically escaping with a very small velocity($<15~km~s^{-1}$). Moreover, our simulations show that an MHD treatment isessential in order to properly capture the coupled magnetized stellar wind andthe escaping atmosphere, despite of the atmosphere being neutral. This couplingshould be considered when interpreting Ly$\alpha$observations in the context ofexoplanets atmospheric escape.

Space Weather-driven Variations in Ly$\alpha$ Absorption Signatures of Exoplanet Atmospheric Escape: MHD Simulations and the Case of AU Mic b