Interplanetary conditions and magnetospheric dynamics during the Cassini orbit insertion fly‐through of Saturn's magnetosphere

Abstract

The primary purpose of this paper is to discuss the interplanetary conditions that prevailed during the Saturn orbit insertion (SOI) fly‐through of Saturn's magnetosphere by the Cassini spacecraft in June–July 2004 and the consequent magnetospheric dynamics. We begin by examining concurrent interplanetary magnetic field (IMF) and Saturn kilometric radiation (SKR) data from Cassini together with images from the Hubble Space Telescope (HST) from an interval in January 2004, which show the effect of the arrival at Saturn of a corotating interaction region (CIR)‐related compression region. We then examine the IMF data obtained over five solar rotations bracketing the SOI fly‐through and show that a similar CIR compression and embedded crossing of the heliospheric current sheet (HCS) is expected to have impinged on Saturn's magnetosphere at some time during the fly‐through. Examination of the IMF direction on either side of the fly‐through confirms the HCS crossing. Observations of SKR show relatively weak emissions modulated at the planetary rotation period on the SOI inbound pass. Strong bursts extending to low frequencies, which are not in phase with the previous emissions, were observed on the outbound pass, similar to the CIR‐related SKR bursts seen in the January data. We thus suggest that the inbound pass occurred under uncompressed conditions of SKR and auroral quiet, while much of the outbound pass occurred under compressed conditions of SKR and auroral disturbance, probably of the same general character as observed in association with CIR compressions during the HST‐Cassini campaign in January 2004. We also examine the in situ magnetic field data observed outbound by Cassini in the predawn sector and find that the largest emission bursts are associated with concurrent variations in the predawn magnetic field, which are indicative of the injection of hot plasma at the spacecraft. Specifically, after an initial field strength increase, the field becomes depressed in strength and highly variable in time. These observations are consistent with an injection of hot plasma into the nightside magnetosphere from the tail, which we suggest is connected with auroral processes of the same nature as observed by the HST during the January 2004 campaign. The injection may be associated with compression‐induced reconnection in the tail, as has been proposed to explain the auroral signatures observed in the HST‐Cassini campaign data.