Effects of stellar XUV spectra on atmospheric escape from a Mars-like planet orbiting inactive low-mass stars

ABSTRACT The influence of stellar extreme ultraviolet (XUV) spectral shape on ion escape from Mars-like planets is investigated using a multispecies single-fluid magnetohydrodynamic model coupled with thermospheric profiles derived from a one-dimensional model under various XUV conditions. Target stars include the Sun, HD85512, and GJ581 – relatively inactive, low-mass stars. Mars-sized planets are placed at orbital distances providing the same total XUV irradiance as present-day Mars. An additional case, XUV10 Mars, represents Mars under 10 times the current solar XUV flux, isolating the effects of spectral shape from energy input. Escape rates of $\mathrm{O_2^+}$, $\mathrm{O^+}$, $\mathrm{C^+}$, and $\mathrm{N^+}$ are calculated by integrating ion fluxes in the magnetotail. $\mathrm{O_2^+}$ escape dominates under present-day conditions, while $\mathrm{O^+}$ becomes more significant under enhanced XUV due to increased molecular ion dissociation. The highest $\mathrm{C^+}$ and $\mathrm{N^+}$ escape rates occur in the HD85512 case, and the lowest in the GJ581 system, reflecting differences in thermospheric temperature and ion source density. HD85512’s upper atmosphere attains the highest temperature, mainly due to strong heating from longer XUV wavelengths. Although $\mathrm{N^+}$ generally escapes more efficiently than $\mathrm{C^+}$, this trend reverses in HD85512 because of enhanced $\mathrm{C^+}$ density above $\sim$1000 km, where ions acquire tailward velocities. Among escape pathways, two dominant ion escape channels are identified: one linked to mass loading near the magnetic neutral plane, and another involving repeated flux-rope-like structures near the magnetic equator. The topside ionosphere structure, modulated by thermospheric conditions and stellar XUV characteristics, together with two escape channels, plays a key role in determining ion escape efficiency.