Abstract
AbstractRecent estimates of the plasma mass‐loss rates by the formation and down‐tail propagation of plasmoids observed in the plasma sheet in Jupiter's and Saturn's magnetosphere fall short of inner moon source rates by at least an order of magnitude. Here we argue that on the time scale between large‐scale disconnection events, ~15 h at Jupiter and ~45 h at Saturn, mass‐loaded closed flux tubes will typically have stretched out a few hundred planetary radii down tail at speeds ~100–200 km s−1. Consequently, the “plasmoids” of order ~10 planetary radii in length observed at closer planetary distances represent only a small planetward portion of the overall structure that is disconnected and lost down tail. Plasmoid mass‐loss estimates are then revised upward by around an order of magnitude, becoming comparable to the moon source values. Additional “hidden,” e.g., small‐scale, mass‐loss processes of comparable strength may not then be required. The essentially continuous azimuthally flowing source plasma in the dusk sector is shown to correspond to a plasma sheet layer adjacent to the magnetopause of width typically ~10% of the distance to the magnetopause in that local time sector. This physical picture also provides a simple explanation for the asymmetry in the plasmoid bipolar field signature observed at both Jupiter and Saturn and predicts that the apparent plasmoid length will increase with distance down tail to a limit beyond a few hundred planetary radii where the full ~100–200 planetary radii structures will be observed.
Highlights
A principal feature of the outer environments of Jupiter and Saturn are the sources of gas and plasma formed by the moons Io and Enceladus, respectively, which orbit deep within the equatorial quasidipolar magnetospheres
The generally closed nature of the nightside flux tubes to radial distances of at least ~60 RS observed by Cassini [Jackman and Arridge, 2011], and the generally subcorotational nature of the plasma flow to similar distances [Thomsen et al, 2014], it seems likely that the Dungey cycle flow may be overemphasized in these models, such that the dusk outflow region may extend over a greater width of the more distant tail with the Dungey cycle confined to a narrower region near dawn
Typical values are estimated as ~30 kg sÀ1 for Jupiter based on large-scale structures that recur on time scales of ~15 h, and ~7 kg sÀ1 for Saturn based on large-scale structures that recur on time scales of ~45 h
Summary
A principal feature of the outer environments of Jupiter and Saturn are the sources of gas and plasma formed by the moons Io and Enceladus, respectively, which orbit deep within the equatorial quasidipolar magnetospheres. Vasyliunas [1983] proposed that closed mass-loaded flux tubes could stretch out down tail following subcorotating transport to the nightside though the dusk sector and would eventually pinch off via reconnection within the plasma layer, forming a large-scale tailward traveling closed-loop plasmoid In principle this process can occur in a steady state as originally envisaged but more likely proceeds in an episodic time-dependent manner, either on large spatial scales involving a significant sector cross tail or as a phenomenon operating on smaller length and time scales [Kivelson and Southwood, 2005]. From a detailed plasmoid survey at Saturn, Jackman et al [2014] estimate an associated mass loss rate of ~3 kg sÀ1, compared with a likely Enceladus plasma source of typically ~50–150 kg sÀ1 In both cases, these estimates fall short of the total mass-loss requirements by at least an order of magnitude. We begin by briefly reviewing the above recent mass-loss rate determinations and the assumptions on which they are based
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