4d~. Space Res. Vol ~. No. 1. pp 301—314. 1986 073—11~7 86 S0.l~) .50 Pnnted in Gmat Brttain. All rtnhts resersed. Copsnght © COSPAR THE MARTIAN MAGNETOTAIL 0. Vaisberg and V. Smirnov Space Research institute, U.S.S.R. Academy of Sciences, 117810 Moscow, U.S.S.R. ABSTRACT There is evidence of a strong influence of an atmospheric (cometary) interaction on the Martian tail formation: small total magnetic flux in the tail, the existence of plasma flow of apparently planetary origin, interplanetary magnetic field control of magnetic field orientation in the tail and other evidence. At the same time the large radius of the Martian magnetotail (about 2 planetary radii) can be considered as a strong evidence for the existence of a planetary magnetic field. Plasma and magnetic field properties in the Martian tail are in many respects similar to the ones observed in the tail of Venus. The limited amount of near-Mars measurements leads to some reservations in coming to definite conclusions. A combined magnetosphere of Mars is suggested that consists of two polar-tied magnetic tubes connected to the tail and an equatorial Venus-type interaction region in- between. INTRODUCTION From the study of different magnetospheric systems we know two clear-cut cases: one that arises from solar wind interaction with (nearly orthogonal to the flow) a magnetic moment of the planet, i.e., terrestrial magnetosphere (see for example /1,2/), and another one resulting from solar wind interaction with a planetary atmosphere, that, for the case of Venus, is the limiting case of a cometary type interaction /3,4/. Any obstacle to solar wind flow creating a substantial disturbance in this flow develops the tail or wake with characteristics determined both by the nature of obstacle and by parameters of the external flow. A crucial role in the determination of the topology of the tail is played by the magnetic field. For a strong enough planetary field two lobes of the tail are tied to the polar regions of the planet while the tail of a field—free planet rotates around the solar wind flow direction with changes in the IMF direction /3/. From the point of view of solar—planetary studies, Mars is an unlucky planet. In spite of the significant number of spacecraft launched to Mars in the first two decades of the space era, we do not know very much about the Mars environment. Previous Soviet and American space programs did not provide enough room for measurements of particles and fields. We spent much effort during the preparation of Mars 2-3 /1971-1972/ and Mars 4-7 /1973/ programs advocating before the planetary community the importance of studies of the Mars environment. There were similar but unsuccessful attempts by American scientists to include a magnetometer in the Viking program /1976/. Nevertheless, one fly—by mission, three orbiters and two landers had instrumentation for the measurement of particles and fields in the Mars environment. Besides that several spacecraft were used to investigate the ionosphere of Mars by radio—occultation technique. Yet the data we have are relatively scarce. The orbits of Mars—bound satellites were chosen without allowance for magnetospheric studies. The instrumentation of the seventies was not sophisticated enough and the telemetry rate was insufficient. These are the reasons why when discussing the Martian tail we are often forced to use indirect arguments. Pri- marily, we do not know much about the magnitude and orientation of Martian magnetic moment. The plan of this paper is the following. After a brief description of the experiments per- formed we will discuss what we know about the Martian magnetic field. Then in succession, magnetic and plasma measurements in the Martian tail will be considered. Finally, we will discuss the present state of knowledge and some speculations about solar wind—Mars interactions. 301