Detection of Zwan-Wolf effect in the ionosphere of Mars

At planets that possess strong dipole magnetic fields, the Zwan-Wolf effect acts to squeeze plasma along magnetic flux tubes, aiding in the deflection of the solar wind flow about the planet. While the effect has been most studied at Earth, candidate observations have also been made at the outer planets. Here we present observations of the Zwan-Wolf effect occurring at Mars, an unmagnetized planet that lacks a dipole magnetic field. Our analysis of observations made by NASA's Mars Atmosphere and Volatile EvolutioN spacecraft suggest that while the Zwan-Wolf effect is likely continuously active within the Martian ionosphere, it operates below detection thresholds of typical plasma analyzers most of the time. However, an interplanetary coronal mass ejection impact at Mars in December 2023 greatly enhanced the Zwan-Wolf effect within the ionosphere, allowing it to be observed, and highlighting the importance of space weather events for these unmagnetized planetary systems.