Whistler‐Mode Waves Observed in the Martian Ionosphere

Abstract It is well‐established that whistler‐mode waves can play a fundamental role in mediating energy transfer through wave‐particle interactions in planetary magnetospheres. In this study, we present in situ observations of whistler‐mode waves within the Martian ionosphere using the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft measurements. We find that these emissions are associated with the solar wind electron penetration along open field lines, which is characterized by the pronounced distribution of one‐sided loss cones in the outward direction for hot ( E > 50 eV) electron fluxes. Analyses of wave instability show that these anisotropic electrons can serve as the primary free energy source for substantial local wave amplification through cyclotron resonance. Our findings reveal that the Martian ionosphere with relatively weak magnetic fields and solar wind intrusions facilitates the local wave excitation. The results of this study provide new insights into wave‐particle interactions on unmagnetized planets.