Low-frequency electromagnetic waves and instabilities within the Martian bi-ion plasma

Abstract

The Martian environment is characterized by the presence of heavy (oxygen) ions of planetary origin which strongly influence the solar wind dynamics, including the bow shock structure and position and may cause additional plasma boundaries in the magnetosheath. In this paper the dispersion characteristics of low-frequency electromagnetic waves (LFEW) in the proton gyrofrequency range are studied. The excitation of these waves results from the relative motion between the solar wind protons and planetary heavy ions, which are considered as unmagnetized and, therefore, may act like a beam in the solar wind. The model takes into account the small extension of the Martian magnetosphere compared with the pickup gyroradius of an exospheric ion. From the dispersion analysis it was found that the most unstable waves with relatively high growth rates propagate oblique to the ambient magnetic field. For small propagation angle to the magnetic field these are right-hand polarized whistler waves in the solar wind frame, and due to Doppler shift they appear near to the proton cyclotron frequency as left-hand polarized waves in the beam (spacecraft) frame. We suggest that the sporadic LFEW emission as seen in the upstream region of Mars by Phobos-2 may indicate the existence of localized "heavy ion bunches" whose origin is relatively unclear, but a possible relation to the Martian moons cannot be excluded. Especially, the socalled Phobos events marked by spectral peaks around the proton cyclotron frequency may be interpreted as signatures of the solar wind interaction with a tenuous gas torus. A comparable situation is known from the AMPTE Ba and Li releases where during the late stages of the experiments an enhanced proton cyclotron emission was observed as well. Another important aspect of LFEW excitation is its role in proton deceleration and heating upstream the bow shock where turbulent processes may provide a strong momentum coupling between the solar wind and the newly generated ions of planetary origin.

“Phobos events”—Signatures of solar wind interaction with a gas torus?

“…It results from the fact that the whistler branch approaches a finite frequency (Ω p ) in the k = 0 limit in the presence of an unmagnetized ion beam, which implies a singularity of the phase velocity in this limit (see Sauer et al, 1998). This long wavelength instability, however, is not on the focus of this study.…”

“…It is tempting to speculate that this signal results from the above instability because its properties just coincide with those predicted for the fastest growing mode under the conditions realized near Mars. Moreover, we suggest, that this instability comes into play not only in cases of a spatially limited neutral gas cloud but in any situation where the sw interacts with a relatively weak and homogeneous heavy ion source, e.g., at large distances from a comet or in the late stage of an AMPTE release (see, e.g., Sauer et al, 1998).…”

“…(10) and start from the cold dielectric tensor K(ω) of a two-ion plasma with relative drift using an appropriate limit for low-frequency waves (see, for example, Sauer et al, 1998). Then the dispersion relation results from the condition Fig.…”

Wavelet application to the magnetic field turbulence in the upstream region of the Martian bow shock

“…On the other hand, reflected protons are often absent, which suggests that other mechanisms of wave generation can take place (see, for example, Baumgärtel et al, 1998;Sauer et al, 1998). The detailed growth rate calculation, performed for conditions corresponding to the observation of these waves, allows us to identify them as L-H waves generated by reflected pickup ions with little Doppler shift in the s/c frame.…”

Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko

“…After initial arrival a new phenomenon was found, now called the "singing comet" ; ∼ 40 mHz waves generated by a cross-field current instability created by freshly ionized, not yet magnetized water ions within the Larmor sphere (sphere with radius of 1 Larmor radius, Sauer et al, 1998) of the comet. At that time, these newly created ions also indicated the "birth of a magnetosphere" for which the spatial distribution of the lowenergy plasma was discussed by Edberg et al (2015b).…”