Global asymmetric distributions of the low frequency whistler-mode waves in the Martian induced magnetosphere

¹ Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, 264209, PR China
² State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China

^★ Corresponding authors: tamin@sdu.edu.cn; bsc@sdu.edu.cn

Received: 26 April 2024
Accepted: 5 March 2025

Abstract

Whistler-mode wave are vital electromagnetic waves that exist universally in the solar wind, shock, comet, and magnetosphere of magnetized celestial bodies. Recent studies have found that they can also be observed and locally generated in the induced magnetosphere of unmagnetized planets such as Mars. Whistler-mode wave distributions in the magnetosphere of magnetized celestial bodies are typically linked to the intrinsic dipole field morphology and solar wind-magnetosphere interaction. However, the global distribution pattern of these waves in the induced magnetosphere of Mars, an unmagnetized body, remains unclear. In this study, using observations from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we for the first time find that the low frequency (f < 16 Hz) whistler-mode waves in the Martian induced magnetosphere show a hemisphere asymmetric distribution and B-minimum preference in the Mars-Solar-Electric (MSE) coordinate system. The wave occurrence rate is ∼ 1% in the vicinity of the center of the magnetotail in the –E hemisphere (Z_MSE < 0) for X_MSE < 0, and it is approximately ten times higher than that in the +E hemisphere (Z_MSE > 0). Wave instability analyses based on the linear theory suggest that the global non-uniform background magnetic field and plasma density in the Martian induced magnetosphere caused by solar wind and Mars interactions can affect the wave growth rate, leading to a significant difference in wave occurrence between the ±E hemispheres. These wave properties are naturally distinct from the whistler-mode waves in the terrestrial magnetosphere with an intrinsic global dipole magnetic field. This study provides new insights for studying whistler-mode waves on unmagnetized celestial bodies with similar interactions between interstellar wind and ionosphere across the universe.