Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations

Abstract

We examine the final 44 orbits of the Cassini spacecraft traversing the midnight sector of Saturn's magnetosphere to distances of~21 Saturn radii, to investigate responses to heliospheric conditions inferred from model solar wind and Cassini galactic cosmic ray flux data. Clear storm responses to anticipated magnetospheric compressions are observed in magnetic field and energetic particle data, together with Saturn kilometric radiation (SKR), auroral hiss, and ultraviolet auroral emissions. Most compression events are associated with corotating interaction regions, producing~2-3.5 day intervals of magnetospheric activity that are recurrent with the~26 day solar rotation period (one or two such events per rotation), though one on the final pass is related to a nonrecurrent interplanetary shock possibly associated with an earlier X-class solar flare. The response to compressions is modulated by the concurrent relative phasing of the northern and southern planetary period oscillation (PPO) systems, with long (>1 planetary rotation) SKR low-frequency extension (LFE) intervals associated with strong field-aligned coupling currents being favored when the two PPO systems act together to thin and thicken the tail plasma sheet during each PPO cycle. LFE onsets/intensifications are then favored at thin plasma sheet phases most unstable to reconnection, producing energetic nightside particle injections and poleward contractions of dawn-brightened auroras. Correspondingly, solar rotation recurrent intervals of magnetospheric quiet conditions also occur with weak energetic particle fluxes and auroral emissions, associated with extended solar wind rarefactions. Overall, the results emphasize how strongly activity in Saturn's magnetosphere is modulated by concurrent heliospheric conditions.

Saturn's Nightside Ring Current During Cassini's Grand Finale

“…The best-fit modelled parameters are presented in Table 2. As described in Section 2 and tabulated in Table 1, the Revs have been classified into four separate color-coded categories according to solar wind and magnetospheric conditions determined by Bradley et al (2020), as summarised in Figure 12 of that paper. The results in Table 2, and throughout this paper, are color coded in the same manner as in Table 1.…”

“…In addition to the six model ring current and fringing field parameters, described in section 3.2, we also derive three additional parameters, the total ring current current IT , kRC the ratio of the magnetic moment of the ring current to the magnetic moment of the planetary dipole, and * the mean half thickness of the ring current D (equations 12-14). Figure 8, spaced at 26 day intervals, mark the approximate times of cyclical minima in the galactic cosmic ray flux observed in LEMMS data, and the approximate times of minima in the propagated solar wind dynamic pressure which result from heliospheric modulations at the solar rotation period (Bradley et al, 2020). It can be seen that with the exception…”

“…These systems give rise to intervals of thickening and thinning of the nightside plasma/current sheet during their rotational cycles, as well as north-south oscillations (Morooka et al, 2009;Provan et al, 2012;Ramer et al, 2017;. Bradley et al (2020) examined how Saturn's magnetosphere responds to solar wind compressions and rarefactions. They found that major compression-response events were triggered when the magnetosphere was compressed by the solar wind and when the two PPO systems were in antiphase, this PPO configuration giving rise to maximum variations in the thickness of the current sheet (see section 2).…”

“…Such events were first detected via enhancements in SKR power correlated with the solar wind dynamic pressure observed during approach to Saturn by the Voyager spacecraft in 1980/1981 (Desch, 1982;Desch & Rucker, 1983), thus being modulated on ~26 day solar rotation time scales by the structure of the interplanetary medium. Based on the SKR LFEs, Bradley et al (2020) categorized Saturn's magnetospheric response to 20 solar wind compressions as either a 'minor' or 'major' compressionresponse event. Major compression-response events are those with LFEs with durations greater than one planetary/PPO rotation period, while minor compression-response events either have no LFEs or LFEs lasting for less than one planetary period.…”

“…We wish to distinguish between intervals when the ring current responds to uniform solar wind and magnetospheric conditions, and intervals when the ring current reflects more variable conditions. In column 5 we also present the solar wind compression event nomenclature devised by Bradley et al (2020), where the letters in each label identify successive ~26 day solar rotation periods (with A…”