Outer magnetospheric structure: Jupiter and Saturn compared

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A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 contribution from those involved in the Dungey cycle [Cowley et al., 2003] being significant only during periods of enhanced solar wind activity [Badman and Cowley, 2007]. The resulting circulation pattern is described in detail by Kivelson and Southwood [2005] and well illustrated by Figure 7 of that paper. [6] Saturnian equivalents of the inner magnetosphere and magnetodisk have recently been reported by Arridge et al. [2008a] but are observed only when the magnetosphere is in an expanded configuration. In this paper we complete the comparison with Jupiter by considering evidence for an outer magnetospheric cushion region at Saturn, paying particular attention to those expanded passes where the magnetodisk, which forms a vital component of the cushion region’s definition at Jupiter, is expected to be present. We begin with a review of the extensive observations made at Jupiter. Figure 1. Schematic representation of the Jovian magnetosphere with (top) the noon meridian viewed from dusk and (bottom) the equatorial plane viewed from above. In both cases the Sun is to the left. The inner (blue), middle (yellow), and outer (green) magnetospheres are not shown to scale. Figure adapted from Smith et al. [1976]. there is now growing evidence for a Saturnian circulation pattern consistent with the Vasyliũnas cycle [André et al., 2007; McAndrews et al., 2009] although, in contrast to the Jovian dynamics discussed above, the solar wind appears to retain considerable influence on the outer magnetosphere [McAndrews et al., 2008], magnetotail [Bunce et al., 2005] and aurora [Bunce et al., 2008]. The complex interaction between solar wind and internally driven transport processes at Saturn is thus a topic of much research. [5] Returning our attention to Jupiter, the dayside magnetosphere is traditionally divided into three qualitatively distinct spatial regions (Figure 1) as discussed by Smith et al. [1976]. The inner magnetosphere (R ] 10–20 R J ) is dominated by Jupiter’s strong internal dipole with a smoothly varying, southward directed field close to the equator. Further out, the increasing significance of centrifugal forces leads to a radially stretched magnetodisk (20 ] R ] 60 R J ) where the ballooning instability dominates the magnetic field geometry. Finally, between the magnetodisk and magnetopause, the more dipolar yet disordered outer magnetosphere or cushion region (described by Kivelson [1976] as “a layer of magnetic turbulence”) is suggestive of reduced centrifugal stresses and plasma‐depleted flux tubes. Kivelson and Southwood [2005] interpret these flux tubes as those involved in the final (mass release) stages of the Vasyliũnas cycle with a secondary 2. The Cushion Region at Jupiter 2.1. Typical Characteristics: Ulysses [7] A typical inbound pass through the Jovian magnetosphere was made by the Ulysses spacecraft [Wenzel et al., 1992] along a low‐latitude ( ∣B R ∣ while radially stretched, nondipolar field lines are associated with ∣B R ∣ > ∣B ∣. Away from the equator the changing direction of dipolar magnetic field lines invalidates this interpretation and the angular parameter INT must be used to characterize the field instead. A critical angle ( INT = 50°) is defined above which the magnetic field will be considered significantly nondipolar and if, in addition to this, ∣B R ∣ > ∣B ∣, we describe the resulting field configuration as a radially stretched magnetodisk. [9] With these points in mind the Ulysses data of Figure 2 can be seen to show three distinctly different magnetospheric regions. A quasi‐dipolar inner magnetosphere, shaded blue, exists close to the planet where INT < 50°. Between distances of 30–70 R J Ulysses explores a radially stretched region, shaded yellow, where ∣B R ∣ > ∣B ∣ and INT reaches values close to 90°. This is the Jovian middle magnetosphere or magnetodisk which, on closer inspection, is found to consist of a relatively thin current sheet (R C
A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 Figure 2. Ulysses observations in the Jovian magnetosphere. (a) Jovian System III magnetic field components (B R ,red;B , blue or white; B ,green)and±∣B∣ (black). (b) Normalized poloidal field components (∣B ∣/∣B∣, blue or white; ∣B R ∣/∣B∣, red). (c) Angle INT between the observed magnetic field, B OBS ,and the internal magnetic field, B INT . Horizontal dashed lines denote the critical magnetodisk angles of 50° and 180 − 50 = 130°. (d) Thirty‐minute normalized magnetic field RMS fluctuation. (e) SWOOPS thermal electron density (blue or white) and temperature (red). Vertical dashed lines denote local minima in absolute magnetic latitude, ∣l M ∣,which beyond 50 R J corresponds to l M = 0°. The inner magnetosphere (blue), magnetodisk (yellow), transition region (white), cushion region (green), boundary layers (cyan), magnetopause crossings (red), and magnetosheath (grey) are shaded. The radial distance, planetocentric latitude, and local time of the spacecraft are shown along the x axis. due to the rocking motion of the rotating planetary dipole which is inclined by ≈10° to the rotation axis. Beyond the magnetodisk the field direction rotates slowly and, for R > 83 R J , Ulysses explores a third magnetospheric region which is once again dipolar. This is the outer magnetosphere or cushion region, shaded green, separated from the magnetodisk by a roughly 14 R J transition region of intermediate properties, shaded white. In the transition region the disturbed magnetic field rotates through ∼90° and the two poloidal field components are comparable in magnitude. [10] The cushion region electron density (measured by the SWOOPS instrument [Bame et al., 1992]) is ∼10 −2 cm −3 while, in contrast, the first crossing of the magnetodisk plasma sheet in the middle magnetosphere is associated with an electron density almost an order of magnitude higher. At the radial distance of this crossing (≈68 R J ) the electron density observed in the magnetodisk lobes is comparable to that seen previously in the overlaying cushion region. Both the plasma sheet and magnetodisk lobe density decrease upon approaching the planet, a counterintuitive observation interpreted by Phillips et al. [1993] as an effect associated with Ulysses unusual trajectory. In order to launch Ulysses into a polar orbit around the sun, the planetocentric latitude of the spacecraft increased significantly on approaching Jupiter. This is clearly evident from the top right panel of Figure 3 where the meridional projection of Ulysses’s Jupiter flyby trajectory is shown in red. The high planetocentric latitude also explains the B R dominated field seen in the quasi‐dipolar ( < 50°) inner magnetosphere. As can be seen from Figure 2e, where density is plotted in blue/white and temperature in red, a transient density (temperature) increase (decrease) is observed at roughly 24 R J . This feature is interpreted by Phillips et al. [1993] as a spacecraft traversal of high‐latitude open field lines. [11] The highly disturbed nature of the transition region field is clear when the RMS fluctuation (calculated from the 3of14
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Outer magnetospheric structure: Jupiter and Saturn compared

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