Upstream proton cyclotron waves at Mars during the passage of solar wind stream interaction regions

¹ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Changping District, Beijing 102206, PR China
e-mail: jpguo@bnu.edu.cn
² Planetary and Space Physics Group, Department of Astronomy, Beijing Normal University, No.19, Xinjiekouwai Street, Haidian District, Beijing 100875, PR China
³ Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
⁴ Heilongjiang Mohe Observatory of Geophysics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
⁵ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China

Received: 20 February 2023
Accepted: 19 April 2023

Abstract

Proton cyclotron waves (PCWs) upstream from Mars are generated by ion–ion instabilities due to the interaction between the solar wind and the pickup protons that originate from the extended hydrogen (H) exosphere of Mars, indicating a loss of H to interplanetary space. Their occurrences and properties associated with nominal solar wind conditions have been widely studied, but relatively little is known about PCW activity under extreme solar wind conditions. In this work we characterize the upstream PCWs during the passage of solar wind stream interaction regions (SIRs) utilizing in situ observations by the Mars Atmosphere and Volatile EvolutioN spacecraft from October 2014 to August 2021. The 46 analyzed SIR events are a subset of the events that occurred in the perihelion season of Martian years 32–35. We find that the PCW occurrence rate is increased by a factor of about 1.8 during the SIR phase relative to the pre-SIR and post-SIR phases. Furthermore, the PCW activity tends to occur more frequently in the leading portion of the SIR. The PCWs detected during the SIR phase have more pronounced wave characteristics, namely larger wave amplitudes, smaller propagation angles with respect to the background magnetic field, and closer-to-circular polarization.