GEOPHYSICAL RESEARCH LETTERS, VOL. 20, NO. 2, PAGES 165-168, FEBRUARY 5, 1993 PHOBOS EVENTS AS PRECURSORS OF SOLAR WIND-DUST INTERACTION Konrad Sauer, KlausBaumgfirtel Max-Planck-Institut ftir extraterrestrische Physik, AuBenstelle Berlin, Germany Uwe Motschmann Institutftir Geophysik und Meteorologie, TU Braunschweig, Germany Abstract Significant magnetic field perturbations in the solarwind were observed duringthe Phobos-2 elliptical orbits around Mars when the spacecraft crossedthe Phobos orbit. As remarkable feature, these so-called Phobos events were detected in all casesonly on the upstream side of the Phobos orbit. A simple one- dimensional fluid model is developed which shows that a thin charged dust cloud with an extension of no more than 100 km is able to producephase-standing, right-hand polarized precursorwaveswith a spatial periodicity of about the proton skin length. This is the same type of upstream waves which is generated by oblique, subcritical shocks. The measured period of the magnetic variations (•40 s) maybe explained as Doppler shift owing to the spacecraft motion throughthe standing wave structure. The fluid results are confirmed by hybrid code simulations. In conclusion, we suggest the Phobos events to be precursor waves resulting from the interaction of the solar windwith charged dust rings along the Phobos orbit. Introduction Magneticdisturbances in the solar wind when the Phobos spacecraft crossed the orbit of the moonPhobos in its first elliptical orbit around Mars were firstly reported by Riedler et al. (1989). A similar, but not so strong event was seen also on the third orbit. Later, plasma measurements using the Plasma Wave System (Grard et al., 1991) have indicated spacecraft potential variations at nearly the same positions where the MAGMA/FGMM magnetometershave localized the so-called Phobos events.Additionally, a third event of similar kind was found between the orbits of Phobos andDeimos (seealso Russel et al., 1990).In a paper of Dubininet al. (1990), the observation of the Phobos events was seen as an indirect evidence for the existence of a gas/dust torus along the Phobos orbit. But no theoretical explanation wasgiven how a localized charged dustring may produce the observed solar wind perturbations which extend in upstream direction over 1000km. In a paperby Russel et al. (1990) the Phobos events are interpretedas usual foreshock turbulenceowing to diffuse ions for special situations when the spacecraft and the bow shock are magnetically connected. A crucial pointis that the Phobos Copyright 1993 by the American Geophysical Union. Paper number 93GL00236 0094-8534/93/93 GL-00236503.00 events were detected only in the upstream region of the Phobos moonorbit.FGMM magnetic field data (Roatsch et al., 1990) clearly indicate that the magnetic perturbations abruptly disappear after the spacecraft has crossed the orbit. This is obviously not consistent with the scenario of foreshock turbulence. Alternatively, we followed the idea, that an obstacle formed by an almost immobile ion cloud (charged dustparticles) may have a similar effect on the super-Alfvenic solar wind as a subcritical shock. The latter is known to be a source of phase-standing upstream waves (precursors). The character of these shock-associated waves wasstudied by Mellott and Greenstadt (1984),seealso Mellott (1985). This paper studies phase-standing waves (PSW) in a magnetized plasmaflow and its excitation by a charged dust cloud. Starting from the generaldispersion relation for low-frequency waves in the proton gyrofrequency regime, a criterion for the presence of upstream/ downstream PWS is derived, and their wave lengths are calculated as a function of the Alfven Mach number with the plasmabeta as parameter.Then, for a given dust density distribution the spatial structure of upstream PSW is determined by integrating the stationary system of fluid equations. Finally, hybrid code simulations are used to control the fluid calculations. Comparingthese results with the propertiesof the observed Phobosevents,we come to the conclusion that thin (o<100km) dust rings along the the Phobos orbit with a charge density comparable to the solar wind proton densitymay be responsible for the observed magnetic field and plasma variations upstream the Phobos orbit. A similar approach to the solar wind interaction with the Phobos dust torus is contained in a recent publication (Krymskii et al. 1992).This work, however, concentrates on global aspects of the disturbed magnetic field downstream of the torus. Observation of Phobos events During the ellipticalorbitsof the Phobos-2 spacecraft around Mars in 1989, magnetic disturbances were detected when the spacecraft crossed the orbit of the Phobos moon (Riedler et al., 1989). Especially clear events were measured during the firstandthirdorbits. 10-min plotsof FGMM data with a resolution of 2.5 s including these two Phobos events (PE1, PE2) are shown in Figures 1 a,b.The zeros on the time scale mark the points of minimum distance between the moon orbit and the spacecraft trajectory (see alsoGrard et al., 1991).There is a third 165