One‐Hertz Waves at Mars: MAVEN Observations

Abstract

We perform a survey of 1-Hz waves at Mars utilizing Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observations for a Martian year. We find that the 1-Hz wave occurrence rate shows an apparent variation caused by masking of the waves by background turbulence during the times when the background turbulence levels are high. To correct for this turbulence masking, we select waves that occur in time intervals where the background turbulence levels are low. We find that the extreme ultraviolet flux does not affect the wave occurrence rate significantly, suggesting that the newly born pickup ions originating in the Mars's exosphere contribute minimally to the 1-Hz wave generation. We find that the wave occurrence rates are higher for low Mach numbers and low beta values than for high Mach numbers and high beta values. Further, we find that a high percentage of 1-Hz waves satisfy the group-standing condition, which suggests that a high percentage of the waves seen as monochromatic waves in the spacecraft frame can be broadband waves in the solar wind frame that have group velocities nearly equal and opposite to the solar wind velocity. We infer that the wave occurrence rate trends with the Mach number and proton beta are a consequence of how the Mach numbers and beta values influence the wave generation and damping or how those parameters affect the group-standing condition. Finally, we find that the 1-Hz waves are equally likely to be found in both the quasi-parallel and the quasi-perpendicular foreshock regions. Key Points:• One-hertz waves at Mars show an apparent occurrence rate variation due to masking of the waves during times of high background turbulence levels • The wave occurrence rates vary significantly with Mach numbers and proton beta values but do not vary significantly with EUV flux levels • The 1-Hz waves are observed in both the quasi-parallel and quasi-perpendicular foreshock regions with comparable occurrence rates waves can also be seen far upstream of the bow shock as their group velocities can exceed the solar wind flow velocity enabling them to propagate upstream against the solar wind flow.

Properties of Plasma Waves Observed Upstream From Mars

“…The observed spacecraft frame polarizations in the higher frequency range vary strongly with the magnetic field direction, with near‐circular left‐handed polarizations for sunward and antisunward fields, near‐circular right‐handed polarizations for fields perpendicular to the sunward direction, and a mixed signal at intermediate field angles. This agrees with previous studies (Brain et al, 2002; Ruhunusiri et al, 2018) and supports an identification of whistler mode waves. These polarization trends result from the larger Doppler shift of waves traveling antiparallel/parallel to the field for sunward/antisunward background fields.…”

“…However, observations show that the observed polarization depends on the propagation angle with respect to the solar wind flow, in a manner consistent with intrinsic right‐handed polarization in the plasma frame (Fairfield, 1974). Similar waves exist in many planetary foreshocks, including those of Mercury and Venus (Orlowski et al, 1990), Saturn (Orlowski et al, 1992), and Mars (Brain et al, 2002; Ruhunusiri et al, 2018). At Mars, their amplitude decreases with distance from the bow shock, and most occur on field lines connected to the shock (Brain et al, 2002).…”

“…These waves, with spacecraft frame frequencies of ~0.5–3 Hz, occur commonly upstream from the Martian bow shock. Their occurrence rate has no reported seasonal dependence, and they do not correlate with the waves at the proton cyclotron frequency (Brain et al, 2002; Ruhunusiri et al, 2018). They can have a variety of polarization states in the spacecraft frame, depending on the angle between their propagation and the solar wind flow, consistent with predominantly upstream propagation of right‐hand polarized waves.…”

“…The individual waveforms have nearly circular left‐handed polarization, but with substantial variability in frequency and amplitude across the interval in question, which leads to the broadband signatures in the wavelets. The relatively sharp cutoff in wave power and polarization above ~1 Hz may result from a group standing condition (Ruhunusiri et al, 2018) or from a Doppler shift effect (Fairfield, 1974).…”

Plasma Turbulence at Comet 67P/Churyumov‐Gerasimenko: Rosetta Observations

“…A possibility for the observation of the lack of dominant injection at these transition times could be masking of the dominant injection processes by uniform injection processes that occur during these times. Such masking of monochromatic waves has indeed been observed at other planetary plasma environments (Ruhunusiri et al, 2018).…”