MAVEN Observations of Whistler-mode Waves Within the Magnetic Dips in the Martian Ionopause/Ionosphere

Abstract

Abstract Whistler-mode waves are natural and common electromagnetic emissions observed both surrounding planets and their moons with and without intrinsic magnetic field. Numerous observations have shown that the magnetic dip is a favorable region for the whistler-mode wave generation in the Earth’s magnetosphere. However, up to now, seldom observations of these waves have been reported in such regions at Mars. Based on the measurements from Mars Atmosphere and Volatile EvolutioN mission, quasi-parallel propagating whistler-mode waves are observed within magnetic dip structures in the Martian ionopause and ionosphere. Correspondingly, significant electron beams in the antiparallel direction are observed, and a linear instability analysis shows that f pe/f ce becomes extremely large (∼1500) inside the magnetic dip. Thus, the electron minimum resonant energy of whistler-mode waves decreases to several eV, which increases the number density of resonant beam electrons. Such beam electron distribution results in the necessary free energy for the whistler-mode wave growth. Our study indicates magnetic dips may be favorable regions for the whistler-mode wave excitation even in the Martian ionosphere, which has an extremely high f pe/f ce but does not have a global intrinsic magnetic field.