Locally Generated ULF Waves in the Martian Magnetosphere: MAVEN Observations

Abstract

We investigate Martian ultralow frequency (ULF) electromagnetic waves generated by local plasma instabilities below the Martian bow shock. Recent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown that ULF waves generated upstream of the Martian bow shock can propagate down to the upper ionosphere, possibly facilitating heavy ion escape from Mars by heating the ionospheric plasma. In contrast to the upstream waves oscillating near the upstream proton cyclotron frequency, we identify narrow band ULF magnetic field fluctuations with frequencies near the local proton cyclotron frequency (f cp(local) ) from MAVEN data. In addition to expected proton cyclotron waves locally generated in the magnetosheath, we newly identify compressional narrow band emissions near f cp(local) (and its harmonics for some cases) in the dayside upper ionosphere and in the nightside magnetotail. The dayside waves are preferentially observed for high solar extreme ultraviolet (EUV) conditions and are often associated with ring/shell-like, hot protons of magnetosheath origin in the presence of cold, dense ionospheric protons. The nightside waves exhibit distinct preference for high-solar-EUV, strong-solar-wind conditions, under which both warm and cold protons are enhanced. The observed properties of these compressional waves are generally consistent with a proton Bernstein mode instability driven by a positive perpendicular slope in proton velocity distribution functions. The excited waves can cause perpendicular heating of thermal protons, thereby transferring energy from precipitating hot protons to cold ionospheric protons.

Statistical Properties of Plateau‐Like Turbulence Spectra in the Martian Magnetosheath: MAVEN Observations

“…Harada et al. (2019) find that the occurrence of PCWs in the Martian magnetosheath decreases with solar EUV irradiance. The density of pickup ions and their relative density ratio are sensitive to the variation of EUV irradiance.…”

“…The properties of pickup ions and the growth of ion anisotropy instabilities depend on the local plasma parameters, such as the pickup angle ( α BV , the angle between the local plasma velocity, V i , and the background magnetic field, B ), the local number density n i and β i , where β i = 2 μ 0 ( n i k B T i )/| B | 2 , μ 0 is the vacuum permeability, k B is the Boltzmann constant, T i the local ion temperature, and μ 0 is the magnetic field strength (Cowee & Gary, 2012; Gary et al., 1993). The ion ring‐beam instability generates significant magnetic‐field fluctuations at frequencies below the proton cyclotron frequency in the form of proton cyclotron waves (PCWs), which have been frequently observed in the Martian environment (Andrés et al., 2020; Brain, 2002; Delva et al., 2015; Harada et al., 2019; Romanelli et al., 2013). If created by anisotropy instabilities, PCWs propagate predominantly in the direction parallel to the background magnetic field with a left‐hand polarization (Gary & Madland, 1988; Huddleston et al., 1999).…”

Micro‐Scale Plasma Instabilities in the Interaction Region of the Solar Wind and the Martian Upper Atmosphere

“…Here, we focus on observations obtained by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft (Jakosky et al., 2015) that show the existence of intense wave activity in conjunction with the initial stage of ion acceleration at Mars. The observed waves are at much higher frequencies than the Ultra Low Frequency (ULF) waves previously studied using MAVEN data (e.g., Collinson et al., 2018; Fowler et al., 2018, 2020; Harada et al., 2019; Ruhunusiri, Halekas, Connerney et al., 2015; Ruhunusiri, Halekas, Espley et al., 2017). Important aspects of the observations presented here are nonlinear waves and solitary structures that are presumably produced in the saturation phase of instabilities in the ion‐acoustic frequency range.…”

Space Weather Observations With InSight

“…Furthermore, during the night‐time between 20:00 and 5:00 hr local time, waves with periods on the order of tens of minutes are observed (Figure 4). These could be related to the interaction of the fluctuating IMF field lines predicted by the ENLIL model (Figures 1c and 1d; see also the GIF in Movie ) with the martian obstacle, although other sources cannot be ruled out (Harada et al., 2019). Enhanced fluctuations on sols 724–726 are observed in the time series after subtracting a 60‐min running mean to highlight variability at sub‐hourly periods (Figure 4a).…”