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.