Structure of the interplanetary magnetic field during the interval spanning the first Cassini fly-through of Saturn's magnetosphere and its implications for Saturn's magnetospheric dynamics

We examine the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a 5 month period spanning the first fly-through of Saturn's magnetosphere, this interval corresponding to six solar rotations at the spacecraft. It is shown that the structure of the interplanetary medium was consistent with expectations for the declining phase of the solar cycle, generally consisting of two IMF sectors and two corotating interaction region compressions during each solar rotation. Field strengths and consequent estimated reconnection voltages at Saturn's magnetopause were overall weaker by a factor of about two compared with those observed during the immediately preceding interval investigated by Jackman et al. (J. Geophys. Res., 109, A11203, doi:10.1029/2004JA010614, 2004). Specifically, during the four solar rotations immediately preceding the fly-through, it is estimated that the total open flux produced at Saturn's magnetopause was ∼60 GWb per solar rotation, compared with ∼100 GWb per solar rotation estimated similarly for the earlier interval. These values compare with estimates of ∼35 GWb of open magnetic flux typically present in Saturn's tail lobes and polar cap. However, in the solar rotation immediately following the fly-through, it is found that field and voltage values recovered to former overall values.