C. M. Jackman

Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics

The first extended series of observations of Saturns auroral emissions, undertaken by the Hubble Space Telescope in January 2004 in conjunction with measurements of the upstream solar wind and interplanetary magnetic field (IMF) by the Cassini spacecraft, have revealed a strong auroral response to the interplanetary medium. Following the arrival of the forward shock of a corotating interaction region compression, bright auroras were first observed to expand significantly poleward in the dawn sector such that the area of the polar cap was much reduced, following which the auroral morphology evolved into a spiral structure around the pole. We propose that these auroral effects are produced by compression-induced reconnection of a significant fraction of the open flux present in Saturns open tail lobes, as has also been observed to occur at Earth, followed by subcorotation of the newly closed flux tubes in the outer magnetosphere region due to the action of the ionospheric torque. We show that the combined action of reconnection and rotation naturally gives rise to spiral structures on newly opened and newly closed field lines, the latter being in the same sense as observed in the auroral images. The magnetospheric corollary of the dynamic scenario outlined here is that corotating interaction region-induced magnetospheric compressions and tail collapses should be accompanied by hot plasma injection into the outer magnetosphere, first in the midnight and dawn sector, and second at increasing local times via noon and dusk. We discuss how this scenario leads to a strong correlation of auroral and related disturbances at Saturn with the dynamic pressure of the solar wind, rather than to a correlation with the north-south component of the IMF as observed at Earth, even though the underlying physics is similar, related to the transport of magnetic flux to and from the tail in the Dungey cycle.

Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics

[1]xa0We study the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a ∼6.5-month interval when the spacecraft was approaching Saturn at heliocentric distances between ∼8.5 and ∼8.9 AU. It is shown that the structure of the IMF is consistent with that expected to be formed by corotating interaction regions (CIRs) during the declining phase of the solar cycle, with two sectors during each solar rotation, and crossings of the heliospheric current sheet generally embedded within few-day higher-field compression regions, separated by several-day lower-field rarefaction regions. This pattern was disrupted in November 2003, however, by an interval of high activity on the Sun. These data have then been employed to estimate the voltage associated with open flux production at Saturns magnetopause using an empirical formula adapted from Earth. The results show that the CIR-related structuring of the IMF leads to corresponding structuring of the interplanetary interaction with Saturns magnetosphere and hence also to intervals of very different dynamical behavior. During few-day compression regions where the IMF strength is ∼0.5–2 nT, the average Dungey cycle voltage is estimated to be ∼100 kV, such that the open flux produced over such intervals is ∼30–40 GWb, similar to the typical total amount present in Saturns magnetosphere. The magnetosphere is thus significantly driven by the solar wind interaction during such intervals. During some rarefaction intervals, on the other hand, the field strength remains ∼0.1 nT or less over several days, implying reconnection voltages of ∼10 kV or less, with negligible production of open flux. The magnetosphere is then expected to enter a quiescent state, dominated by internal processes. Overall, ∼100 GWb of open flux is estimated to be produced during each ∼25-day solar rotation, about 3 times the typical flux contained in the tail, and sufficient to drive three to five substorms. We point out, however, that CIR-related variations in solar wind dynamic pressure will also occur in synchronism with the field variations, which may also play a role in modulating the open flux in the system, thus reinforcing the synchronization of the pattern of growth and decay of open flux to the CIR pattern. Estimates of open flux production associated with the period of strong solar activity indicate that major magnetospheric dynamics were excited by reconnection-mediated solar wind interaction during this interval.

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

[1]xa0The primary purpose of this paper is to discuss the interplanetary conditions that prevailed during the Saturn orbit insertion (SOI) fly-through of Saturns 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 Saturns 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.

Implications of rapid planetary rotation for the Dungey magnetotail of Saturn

[1]xa0Employing our current understanding of the structure and dynamics of Saturns magnetosphere, we present a time-dependent model of the kronian Dungey cycle magnetotail, which is based upon a modification of a similar model developed for Earths magnetotail (Milan, 2004a). The major difference arises due to the rapid rotation of Saturn and the partial corotation that this imposes on the open field lines threading the polar cap. This results in twisted tail lobes, with the form of concentric cylinders of oldest to newest open flux from the inside out. The oldest, and hence longest, open field lines form the backbone of a highly extended magnetotail. Surrounding this are bundles of field lines disconnected by tail reconnection, propagating down-tail at the solar wind speed. Owing to the twisted nature of the tail, these bundles remain entangled with the lobe cores to form “exterior flux ropes.” In the limit that the addition and removal of open flux from the magnetosphere by magnetic reconnection can be treated as a last-in-first-out system, we formulate a description of the flux transport within the tail and drive this with estimated dayside reconnection voltages deduced from Cassini observations of the IMF made upstream of Saturn (Jackman et al., 2004).