Statistical Distribution of Magnetosonic Waves in the Martian Space

Martian space is rich in plasma waves generated by plasma instabilities in both the solar wind and the Martian environment. These waves interact with charged particles through wave–particle interactions, leading to the acceleration, heating, and scattering of particles. Such processes further influence the kinetic evolution of charged particles and cause variations in their spatial and energy distributions. Magnetosonic (MS) waves, a type of electromagnetic wave driven by proton ring-beam distributions, propagate nearly perpendicular to the background magnetic field with frequencies ranging from the proton gyrofrequency to the lower hybrid frequency. Based on their distinct origins, MS waves in Martian space can be categorized into two types: solar wind-originated MS waves and magnetosphere-generated MS waves, which can be distinguished by their frequency characteristics due to differences in the ambient magnetic field.

 

Using data from NASA’s MAVEN spacecraft collected between October 2014 and May 2023, we statistically analyze the occurrence rates and wave properties of both types of MS waves in Martian space. The results reveal that solar wind–originated MS waves exhibit higher occurrence rates (~27.6%) downstream of the dayside magnetic pileup boundary, with enhanced occurrence on the dusk side compared to the dawn side. These waves show larger amplitudes on the dayside, reaching maximum average values of ~2.5 nT. In contrast, magnetosphere-generated MS waves are preferentially observed in the nightside magnetic pileup region and particularly in the magnetotail (~8%), with a tendency toward the dawn side. Waves located within the Martian magnetosheath show amplitudes of approximately 0.5 nT. Further analysis indicates that increasing solar wind dynamic pressure significantly enhances the occurrence of solar wind-originated MS waves near the bow shock, in the magnetosheath, and in the magnetotail, while also increasing the occurrence of magnetosphere-generated MS waves in the magnetotail. With increasing solar EUV flux, the occurrence of solar wind-originated MS waves slightly increases in the magnetotail, whereas magnetosphere-generated MS waves increase markedly. Both types of MS waves are rarely observed in regions strongly affected by crustal magnetic fields, and their spatial distributions expand with altitude.

 

In summary, solar wind-originated MS waves exhibit higher occurrence rates and larger amplitudes overall. The spatiotemporal distribution characteristics of MS waves in Martian space provide important insights into their generation mechanisms, propagation behaviors, and wave-particle interaction processes. Moreover, an interesting case of simultaneous observation of solar wind-originated and magnetosphere-generated MS waves was identified. In this event, proton motions seem to be influenced and possibly modulated by solar wind-originated MS waves, and preliminary analyses of this wave event reveal additional intriguing features.