LOW-FREQUENCY ELECTROMAGNETIC-WAVES AND INSTABILITIES WITHIN THE MARTIAN BI-ION PLASMA

The Martian environment is characterized by the presence of heavy (oxy gen) 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 (LFE W) in the proton gyrofrequency range are studied. The excitation of th ese waves results from the relative motion between the solar wind prot ons and planetary heavy ions, which are considered as unmagnetized and , therefore, may act like a beam in the solar wind. The model takes in to account the small extension of the Martian magnetosphere compared w ith the pickup gyroradius of an exospheric ion. From the dispersion an alysis it was found that the most unstable waves with relatively high growth rates propagate oblique to the ambient magnetic field. For smal l propagation angle to the magnetic field these are right-hand polariz ed whistler waves in the solar wind frame, and due to Doppler shift th ey appear near to the proton cyclotron frequency as left-hand polarize d waves in the beam (spacecraft) frame. We suggest that the sporadic L FEW emission as seen in the upstream region of Mars by Phobos-2 may in dicate the existence of localized ''heavy ion bunches'' whose origin i s relatively unclear, but a possible relation to the Martian moons can not be excluded. Especially, the so-called Phobos events marked by spe ctral peaks around the proton cyclotron frequency may be interpreted a s 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 cyclotro n emission was observed as well. Another important aspect of LFEW exci tation is its role in proton deceleration and heating upstream the bow shock where turbulent processes may provide a strong momentum couplin g between the solar wind and the newly generated ions of planetary ori gin.