[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-06] Estimating the Probability of Upstream Proton Cyclotron Waves Driving ULF Waves in the Martian Ionosphere: Multi-Point Observations by MAVEN and MEX

*Kaworu Imada¹, Yuki Harada¹, Christopher Fowler², Glyn Collinson³, Jasper Halekas⁴, Suranga Ruhunusiri⁴, Gina DiBraccio³, Norberto Romanelli³ (1.Graduate school of Science, Kyoto University , 2. Department of Physics and Astronomy, West Virginia University, 3.NASA Goddard Space Flight Center, 4.Department of Physics and Astronomy, University of Iowa)

Keywords:Mars Atmosphere and Volatile EvolutioN (MAVEN), Mars Express (MEX), multi-point observation, Mars-solar wind interaction, proton cyclotron wave, ULF magnetosonic wave

Mars-solar wind interactions directly and significantly impact the plasma environment around Mars because the planet does not possess an intrinsic magnetic field of internal dynamo origin. Proton cyclotron waves (PCWs) are known to be one of the important processes driving the dynamic Martian plasma environment. PCWs are generated upstream of the Martian bow shock (BS), left-hand and elliptically polarized in the spacecraft reference frame, and have a frequency close to the local proton gyrofrequency. Since the first observation by Phobos-2 (Barabash et al., 1991; Riedler et al., 1989; Russell et al., 1990, 1992), the generation and propagation mechanisms as well as spatial distributions of the waves have been studied in detail based on observations of subsequent orbiters. For example, statistical analyses based on Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown that the waves occur over the wide upstream region (Brain et al., 2002; Romeo et al., 2021) and that their occurrence rates strongly vary as the solar longitude changes, peaking after the perihelion (Romeo et al., 2021). These waves are therefore thought to be generated from the cyclotron resonance with newborn pickup protons from the hydrogen corona, which extends well beyond the BS during the southern summer (Collinson et al., 2018; Romeo et al., 2021). Case studies based on MAVEN observations have revealed that they are advected back to the BS by the solar wind flow, and “ring” the induced magnetosphere as the associated pressure pulses which drive compressive magnetosonic waves in the ionosphere at an ultralow frequency (ULF) similar to that of the upstream PCW (Collinson et al., 2018; Fowler et al., 2018, 2021). These PCW-driven ULF waves are thought to be capable of heating planetary heavy ions (e.g., O⁺, O₂⁺, and CO₂⁺) via wave-particle interactions.
However, previous studies of these waves at Mars have been based almost exclusively on single-spacecraft observations (one simultaneous multi-point observation event has been reported by Collinson et al. (2018) with a limited discussion). As a result, it has been difficult to find out statistical properties of wave propagation, especially the probability at which upstream PCWs drive new ULF magnetosonic waves in the ionosphere (We define it as “ringing” probability) mainly due to the orbit limitation of the single spacecraft platform.
In this study, we estimated the “ringing” probability by analyzing quasi-simultaneous, multi-point observations of local magnetic fields based on MAVEN and Mars Express (MEX). First, we identified a number of events in which the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on board MEX operated in Active Ionosphere Sounding (AIS) mode in the Martian ionosphere, while MAVEN confirmed the PCWs with its magnetometers in the upstream region. Then, we estimated the local magnetic field magnitude at MEX for each event from so-called electron cyclotron echoes recorded on the ionograms obtained by MARSIS (Akalin et al., 2010; Gurnett et al., 2005), and automatically examined whether compressional fluctuations were detected in the ionosphere at frequencies close to those of the upstream PCWs, thereby estimating the “ringing” probability. The results show that the probability depends on various parameters such as the altitude of MEX and solar wind parameters. In particular, it was found the ringing probability is high in the dayside ionosphere (SZA<80°) and for high solar wind dynamic pressure conditions (P_dyn>3.0 nPa). In addition, during time intervals with upstream PCWs, the power of magnetic field magnitude fluctuations in the ionosphere was on average 2–3 times stronger in the PCW frequency range than that without upstream PCWs. These results suggest that the ringing processes are not uncommon and could be important in terms of long-term energy injection from the solar wind into the Martian ionosphere.