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In September 2017, an active region of the Sun produced a series of strong flares and a coronal mass ejection that swept past Mars producing enhanced ionization and heating in the upper atmosphere. Emissions from atmospheric hydrogen Lyman-alpha were also enhanced at Mars. Temperatures derived from neutral species scale heights were used in conjunction with the H Lyman-alpha observations to simulate the effects of this space weather event on Martian hydrogen properties in the exosphere. It was found that hydrogen abundance in the upper atmosphere decreased by similar to 25% and that the H escape rate increased by a factor of 5, mainly through an increase in upper atmospheric temperature. This significant escape rate variation is comparable to seasonally observed trends but occurred at much shorter timescales. Such solar events would statistically impact extrapolation of Martian water loss over time.

Plain Language Summary The upper atmosphere of Mars is the region where its atmosphere can escape to outer space. Atmospheric escape has been linked to seasonal changes as Mars' distance to the Sun varies along its orbit. A strong solar storm impacted Mars in September 2017. During that time, the upper atmosphere of the planet was heated on short timescales. Observations and analysis have shown that atmospheric escape of hydrogen, a key element in water, during the solar storm was comparable to the seasonal escape over a Martian year. This impacts understanding of primordial water content on Mars when extrapolating back in time.