DISTURBANCES IN MERCURYS MAGNETOSPHERE - ARE THE MARINER 10 SUBSTORMSSIMPLY DRIVEN

In addition to providing the first in situ evidence of a magnetosphere at Mercury, the first flyby by Mariner 10 inspired reports of Earth-l ike substorms. While the small scales at Mercury should make the subst orm timescale there much shorter than it is at the Earth, these early interpretations may have too readily assumed that the substorm require ment of an energy storage and release phase occurs. Instead, its size should make Mercury's magnetosphere especially prone to disturbances t hat are purely ''driven'' by the changing external boundary conditions on the magnetosphere imposed by the solar wind. These result simply f rom the magnetosphere's relatively unhindered reconfiguration in respo nse to the varying interplanetary parameters, including the IMF southw ard component. In this paper we demonstrate that the ''disturbed'' str ucture observed outbound from closest approach during the first Marine r 10 flyby can alternately be explained as a consequence of a typical period of rotating IMF. We use an appropriately modified IMF-dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field s ignature during the disturbed outbound pass segment. This result sugge sts that rapid restructuring of the small magnetosphere in response to changing local interplanetary conditions may dominate the magnetosphe ric dynamics at Mercury. Future Mercury magnetosphere missions should be instrumented to distinguish between this driven magnetospheric dyna mism and any internal Earth-like substorm processes. These results als o remind us that driven reconfigurations must always be considered in studies of transients in the terrestrial magnetosphere.