Abstract
The first extended series of observations of Saturn's 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 Saturn's 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.
Highlights
Initial results have demonstrated that both UV auroras and Saturn kilometric radiation (SKR) emissions respond strongly to the shock compressions that are associated with interplanetary corotating interaction regions (CIRs) [Clarke et al, 2005; Crary et al, 2005; Kurth et al, 2005]
[26] The first detailed sequence of images of Saturn’s UV auroras were obtained by the HST in January 2004, in coordination with Cassini observations of the upstream interplanetary medium and SKR emissions. These observations show that the auroras and related SKR emissions respond strongly to CIR-related compression regions in the interplanetary medium, the auroras becoming first significantly enhanced and broadened in the dawn sector, before assuming a pronounced spiral form, lasting a few tens of hours after the onset [Clarke et al, 2005; Crary et al, 2005; Kurth et al, 2005]
Related effects have been reported by Prangeet al. [2004], associated in this case with a CME shock tracked to Saturn from the Sun via Earth and Jupiter. We suggest that these effects represent the auroral counterparts of the strong solar wind dynamic pressure modulations of SKR emission found previously in Voyager data by Desch [1982] and Desch and Rucker [1983]
Summary
[2] the Saturn flybys by the Pioneer 11 and Voyager 1 and 2 spacecraft in the interval 1979 – 1981 provided much information about the large-scale structure of the magnetic field and plasma populations in Saturn’s magnetosphere [e.g., Smith et al, 1980; Ness et al, 1981, 1982; Behannon et al, 1983; Sittler et al, 1983; Richardson, 1986; Richardson and Sittler, 1990], relatively little information has been available until recently concerning the dynamics of the magnetosphere and the nature of its interaction with the interplanetary medium. [5] Figure 1e shows the interplanetary magnetic field (IMF) strength observed by Cassini over the relevant interval (24 – 30 January), together with the estimated reconnection voltage (open flux production rate) at Saturn’s dayside magnetopause, derived using the algorithm of Jackman et al [2004] This voltage is taken to be given by. $0100 UT on 28 January (Figure 1c) corresponds to an interval $40 hours after the arrival of the shock, some hours after the peak fields had arrived at the planet, during an interval of $100– 200 kV estimated dayside driving It shows a pronounced ‘‘spiral’’ auroral pattern, with strongest emissions in the dawn sector [Clarke et al, 2005]. We suggest that these effects are connected with reconnection dynamics in a rotating magnetosphere, as we go on to discuss
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