REFLECTED IONS NEAR MARS - PHOBOS-2 OBSERVATIONS

When the PHOBOS-2 spacecraft entered the Martian ion foreshock region and was magnetically connected with the bow shock, the mass spectrometer ASPERA recorded enhanced proton fluxes in the perpendicular and antisunward directions. The third elliptical orbit on February 8, 1989 has been analysed in detail in this respect and we have concluded that the observed disturbances might have resulted from gyrating reflected ions and field aligned beams propagating upstream from the bow shock. Comparison with a simple model of specular reflection can partially explain observations. Disturbances recorded inside the foot seem to result from kinetic processes mentioned by Moses et al. [1988]. The density of the backstreaming ions at the Martian bow shock reached 0.4 cm-3 and could account for 60% of the observed solar wind velocity decrease just upstream of the shock. The Martian upstream region is very compact and that results in a shortage of space for different instabilities to develop. So no reflected beam disruptions were observed, only beam-like distributions were recorded.