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Mars' dayside ionosphere is maintained primarily by ionization from solar ultraviolet photons and subsequent chemical reactions, with small contributions from other mechanisms such as impact ionization and charge exchange. In December 2023, the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed the impact of an interplanetary coronal mass ejection (ICME) on Mars' ionosphere, including strongly enhanced fluxes of suprathermal electrons. We show that this enhancement in suprathermal electron fluxes increased ion production from electron impact, so that dayside electron impact ionization rates exceeded photoionization rates during the ICME. This change in ion production mechanisms led to unusually high densities of the minor ions C+${\\mathrm{C}}^{+}$and O++${\\mathrm{O}}^{++}$ . Space weather events are known to increase ion escape rates, so changes in ion composition during space weather events have important implications for atmospheric evolution. We show that scaling nominal loss rates to account for space weather may underestimate carbon loss from Mars' atmosphere. Plain Language Summary Dayside planetary ionospheres are primarily produced through interactions between atmospheric neutral gases and sunlight. Impact by energetic particles, especially electrons, typically only contributes a small amount of plasma. We observed unusually high densities of the minor ions C+${\\mathrm{C}}^{+}$and O++${\\mathrm{O}}^{++}$in Mars' ionosphere during a space weather event in December 2023, when a large bubble of magnetized plasma launched from the Sun impacted the planet. This plasma bubble compressed the Mars magnetosheath, pushing suprathermal electrons to lower altitudes, where they impacted the atmosphere and significantly increased plasma production through impact ionization. Changes in the production mechanisms of the ionosphere during space weather events lead to changes in its density and composition. This is important because space weather events are known to increase the amount of atmospheric gas escaping from a planet, which can have important implications for how the atmosphere evolves over millions of years. Key Points Unexpected increases in C+ and rarely detected O++ densities were observed during the December 2023 space weather event at Mars These ions were produced by electron impact ionization from the intense electron fluxes associated with the space weather event Electron impact ionization exceeded photoionization during the event, which temporarily altered the composition of ions escaping Mars

Corotating interaction regions (CIRs) are long-lasting solar wind structures that persist over multiple solar rotations. These structures accelerate particles throughout the heliosphere and can impart significant energy onto planetary ionospheres and magnetospheres. Understanding how CIRs and their associated energetic particles evolve radially with heliocentric distance is of great interest and can give insight into acceleration mechanisms that occur within these structures. CIR solar wind and particle properties have been examined at numerous heliocentric distances but have been largely unexplored at Mars. We examine the properties of a CIR observed over two Carrington rotations by the Solar Terrestrial Relations Observatory (STEREO)-A at 1 au and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at 1.5 au. This CIR was observed during near-radial alignment of the spacecraft, allowing for the exclusion of significant longitudinal variations in the CIR’s properties. We find that during both rotations, the scaled solar wind density and interplanetary magnetic field are slightly higher at MAVEN than measurements at STEREO-A, and the CIR compression occurs over similar periods at both spacecraft during each event. Suprathermal particle enhancements are observed at both spacecraft during both passes of the CIR. Variations in the particle spectra between observations indicate suprathermal particle acceleration at both spacecraft, with higher-energy populations observed at 1.5 au compared to 1 au. Spectral anisotropies observed at MAVEN also indicate a combination of local and nonlocal particle acceleration, further demonstrating that the acceleration mechanisms associated with CIRs are complex and evolve with heliocentric distance.