Variability of Upstream Proton Cyclotron Wave Properties and Occurrence at Mars Observed by MAVEN

Abstract

The presence of plasma waves upstream from the Martian bow shock, with frequencies near the local proton cyclotron frequency in the spacecraft frame, constitutes, in principle, an indirect signature for the existence of planetary protons from the ionization of Martian exospheric hydrogen. In this study, we determine the "proton cyclotron wave” (PCW) occurrence rate between October 2014 and February 2020, based on Magnetometer and Solar Wind Ion Analyzer measurements from the Mars Atmosphere and Volatile EvolutioN mission. We characterize its dependence on several wave and solar wind (SW) properties, and solar longitude ranges. We confirm a previously reported long‐term trend with more PCWs near perihelion, likely associated with changes in exospheric hydrogen density. Furthermore, we report for the first time a decrease in median PCW amplitude for each consecutive Martian perihelion. Such variability cannot be attributed to differences in the distribution of SW conditions. This trend could be associated with changes in solar inputs, foreshock effects, and asymmetries due to the SW convective electric field influencing newborn protons. In addition, we observe PCWs more frequently for low to intermediate interplanetary magnetic field (IMF) cone angles, slower SW speeds, and higher SW proton densities. The IMF cone angle preference likely results from the trade‐off between associated linear wave growth rates, wave saturation energies, and pick‐up proton densities. Moreover, the dependencies on SW speed and density indicate the importance of the characteristic SW transit timescale and the charge exchange process coupling SW protons with the hydrogen exosphere.

Variability of the Incompressible Energy Cascade Rate in Solar Wind Turbulence Around Mars

“…By analyzing MAVEN MAG observations between October 2014and March 2016, Romanelli et al (2016 reported that the PCW occurrence rate upstream of the Martian bow shock varies with time and takes higher values near the Martian perihelion. Romeo et al (2021) confirmed such annual periodicity of PCWs occurrence rate upstream from the Martian bow shock for three Martian perihelia, analyzing MAVEN MAG and Solar Wind Ion Analyzer (SWIA) data between October 2014 and February 2020. This long-term trend has been associated with the seasonal variability of the Martian H exosphere, that extends well beyond the Martian bow shock, and provides the source of newborn planetary protons (e.g., Bhattacharyya et al 2015;Chaffin et al 2014;Clarke et al 2017;Yamauchi et al 2015;Rahmati et al 2017;Halekas 2017;Halekas et al 2020;Halekas & McFadden 2021).…”

“…It is worth emphasizing that, although this term (proton cyclotron waves) has been used very often to make reference to them, they are not associated with the ion cyclotron wave mode. Temporal variability in the PCWs occurrence rate has been reported based on Mars Global Surveyor magnetic field observations (Bertucci et al 2013;Romanelli et al 2013) and more recently with MAVEN Magnetometer (MAG) measurements (Connerney et al 2015;Jakosky et al 2015;Romanelli et al 2016;Romeo et al 2021). By analyzing MAVEN MAG observations between October 2014and March 2016, Romanelli et al (2016 reported that the PCW occurrence rate upstream of the Martian bow shock varies with time and takes higher values near the Martian perihelion.…”

“…In this context, proton cyclotron waves are ultra-low frequency electromagnetic plasma waves with observed frequencies (in the spacecraft frame) near the local proton cyclotron frequency and whose most likely source are newborn planetary protons, a result from the ionization of the Martian extended hydrogen (H) exosphere (e.g., Russell et al 1990;Brain 2002;Mazelle et al 2004;Romanelli et al 2013;Ruhunusiri et al 2016;Romanelli et al 2016Romanelli et al , 2018Romeo et al 2021). It is worth emphasizing that, although this term (proton cyclotron waves) has been used very often to make reference to them, they are not associated with the ion cyclotron wave mode.…”

Plasma Waves in the Distant Martian Environment: Implications for Mars' Sphere of Influence

“…These cyclotron waves have well‐defined frequencies at $${f}_{ci}=\frac{qB}{2\pi {m}_{i}}$$, where q is the charge of an electron, B is the magnetic field, and m i is the mass of the ion species i . We note that in this study, the term ICW, when at the proton gyrofrequency, is not the same as proton cyclotron waves, which are related to newborn pickup protons (e.g., Halekas et al., 2020; Romanelli et al., 2016; Romeo et al., 2020; Ruhunusiri et al., 2016). B is observable using the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission magnetometer (MAG) (Connerney et al., 2015; Jakosky, Grebowsky, et al., 2015; Jakosky, Lin, et al., 2015).…”

“…same as proton cyclotron waves, which are related to newborn pickup protons (e.g., Halekas et al, 2020;Romanelli et al, 2016;Romeo et al, 2020;Ruhunusiri et al, 2016). B is observable using the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission magnetometer (MAG) (Connerney et al, 2015;Jakosky, Lin, et al, 2015).…”