Properties of Plasma Waves Observed Upstream From Mars

Abstract

We describe a new method to analyze the properties of plasma waves and apply it to observations made upstream from Mars by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The slow measurement cadence of most charged particle instrumentation has limited the application of analysis techniques based on correlations between particle and magnetic field measurements. We show that we can extend the frequency range of applicability for these techniques, for a subset of waves that remain nearly coherent over multiple wave periods, by subsampling velocity distribution function measurements and binning them by the wave phase. This technique enables the computation of correlations and transport ratios for plasma waves previously inaccessible to this technique at Mars. By computing the cross helicity, we find that most identified waves propagate upstream in the plasma frame. This supports the conclusions of previous studies but enables a clearer determination of the intrinsic wave mode and characteristics. The intrinsic properties of observed waves with frequencies close to the proton cyclotron frequency have little spatial variability but do have large temporal variations, likely due to seasonal changes in the hydrogen exosphere. In contrast, the predominant characteristics of waves at higher frequencies have less temporal variability but more spatial variability. We find several indications of the presence of multiple wave modes in the lower frequency wave observations, with unusual wave properties observed for propagation parallel to the magnetic field and for background magnetic fields nearly perpendicular to the solar wind flow. Plain Language Summary The interaction between the solar wind and planetary environments leads to the growth of a variety of plasma waves. These waves serve as a sensitive tracer of the physical interactions between the solar wind and local charged particle populations. Plasma waves reveal the conversion of energy between kinetic and electromagnetic forms, as well as mediating the exchange of energy between different particle populations. However, identifying the intrinsic characteristics of low-frequency plasma waves often proves quite challenging, because the solar wind sweeps them downstream, altering their properties as measured by an orbiting spacecraft. By combining magnetic field and charged particle data, one can remove some of the resulting ambiguities. However, the low cadence of many charged particle observations limits the applicability of such analyses. In this work, we describe a new technique to eke higher time resolution out of the charged particle measurements and apply it to observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, opening a new window on the properties of plasma waves at Mars.

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

“…This trend is consistent with the result reported by Halekas et al. (2020) based on MAVEN observations. In contrast, Figure 7b shows the HR amplitude curves do not necessarily decrease with larger altitudes from Mars.…”

“…PCWs are additionally observed for slower SW speeds (v SW < 400 km s −1 ), with a decreasing trend in occurrence rate for faster speeds. This is consistently seen for all HR groups and the entire span of available MAVEN data, confirming the results from Halekas et al (2020) between September and November 2018. Although this trend disagrees with linear wave growth theory (Gary et al, 1986), Delva et al (2015) also reported a similar case for PCWs near Venus from the Venus Express mission.…”

“…In addition, MGS did not possess an onboard SW ion detection instrument to study SW properties during PCW events. As MAVEN provides both magnetic field and SW measurements, this mission presents an excellent opportunity to perform studies focused on these low frequency plasma waves (e.g., Andrés et al., 2020; Halekas et al., 2020; Liu et al., 2020). In this work, we present an extensive study with MAVEN observations covering almost 3 Martian years, identifying PCW events with strict criteria, to analyze the occurrence rate variability, main wave properties, and SW conditions that favor their presence.…”

“…In addition, MGS did not possess an onboard SW ion detection instrument to study SW properties during PCW events. As MAVEN provides both magnetic field and SW measurements, this mission presents an excellent opportunity to perform studies focused on these low frequency plasma waves (e.g., Andrés et al, 2020;Halekas et al, 2020;Liu et al, 2020…”

Variability of the Solar Wind Flow Asymmetry in the Martian Magnetosheath Observed by MAVEN

“…In particular, newborn planetary protons (H + ) and oxygen ions (O + ) are picked up at these locations and in the Martian magnetosheath by the magnetized SW, as reported by Barabash et al (1991), Curry et al (2015), Dubinin et al (2006), Rahmati et al (2015, 2017), Yamauchi et al (2015), and Dong et al (2015). In addition to contributing to planetary escape, such picked‐up ions are responsible, among other outcomes, for the presence of different plasma instabilities (e.g., Andrés et al, 2020; Bertucci et al, 2013; Brain et al, 2002; Halekas et al, 2020; Liu et al, 2020; Mazelle et al, 2004; Romanelli et al, 2013, 2016; Ruhunusiri et al, 2015, 2016; Russell et al, 1990).…”