Outer magnetospheric structure: Jupiter and Saturn compared

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A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 rotational voltage across each magnetosphere (Table 1) and suggests that Vasyliũnas cycle reconnection is of a similar dynamical importance in each magnetosphere. Both systems should therefore contain a similar number of plasma‐depleted flux tubes (relative to their total flux content) and, all other things being equal, a Saturnian equivalent to the cushion region seen at Jupiter should be evident in the dayside outer magnetosphere. [35] Badman and Cowley [2007] estimate the mean reconnection voltage associated with the Dungey cycle to be 0.25 MV for Jupiter and 0.045 MV for Saturn. The difference between these two values is smaller than for the Vasyliũnas reconnection voltage however theoretical calculations [Badman and Cowley, 2007] suggest a layer of plasma‐depleted flux tubes, adjacent to the dawn magnetopause at Saturn, with typical thickness of 0.5–2 R S if the Vasyliũnas cycle is neglected. No evidence for such a layer was observed in this study. [36] Considering the Dungey and Vasyliũnas cycles as separate and distinct processes may be an oversimplification of reality as the microphysics of magnetotail reconnection at Jupiter and Saturn are poorly understood. Kivelson and Southwood [2005] suggest that the Dungey and Vasyliũnas cycles may close during the same reconnective episode in the magnetotail and, in this case, the total number of plasmadepleted flux tubes moving back around to the dayside will be altered from that expected when the two cycles are considered separately. Regardless of these details the above discussion makes it clear that plasma cycle return flows are likely to play an important role in the dynamics of Saturn’s outer magnetosphere and that plasma‐depleted flux tubes returning to the dayside should be ubiquitous in spacecraft observations. 4.2. Depleted Flux Tube Configuration [37] If there are plasma‐depleted flux tubes in the Saturnian outer magnetosphere, why are these flux tubes not associated with the quasi‐dipolar magnetic field configuration seen in the Jovian cushion region? The warped nature of the Saturnian magnetodisk will change the magnetic field configuration expected near the equator (particularly at large distances from the planet) in a way that is sensitive to the solar wind dynamic pressure and the instantaneous location of the spacecraft. However, the fact that the Jovian cushion region is observed even when the magnetodisk is tilted more than 10° away from the equator suggests that such warping is unlikely to account for the observed lack of a cushion region at Saturn. [38] Of more importance may be the dynamical motion of the magnetodisk at each planet. At Jupiter the magnetodisk flaps up and down (in the rest frame of the spacecraft) due to the 10° tilt between the planets dipole and rotation axes. The regular transition into different magnetospheric regions that results from this motion may make differences in magnetic field topology easier to detect. At Saturn the equivalent motion is far more complicated and, at present, poorly understood. However, should the magnetodisk at Saturn move in a more restricted fashion than at Jupiter, differences in topological regions may be less obvious in spacecraft observations. [39] Our analysis of Saturnian spacecraft data was limited to passes associated with a relatively expanded magnetosphere (R SS >23R S ) in order to ensure that the interior magnetodisk configuration, so essential for defining entry into the cushion region, was present. When the magnetosphere is in a compressed state the magnetodisk configuration vanishes and the entire dayside magnetosphere is associated with a quasi‐dipolar geometry very similar to that of the extensively studied terrestrial magnetosphere. Under such conditions it is very difficult to identify plasma‐depleted flux tubes based on their magnetic field configuration alone and a detailed study of the associated plasma data becomes important. However, because of the very low densities observed in the outer magnetosphere, the collection and interpretation of such measurements is fraught with difficulties and beyond the scope of this investigation. [40] The Dungey cycle contribution to the cushion region’s flux content is expected to increase during compressed magnetospheric conditions [Badman and Cowley, 2007] however the response of the dominant Vasyliũnas cycle is less certain. Although the steady state contribution of the Vasyliũnas cycle to the cushion region’s flux content must depend only on the internal mass loading rate, the physical mechanism by which this flux is added to the region may be controlled by the upstream solar dynamic pressure [Zieger et al., 2010] in a complicated fashion. However, even if the Vasyliũnas cycle is neglected, Badman and Cowley [2007] still predict a cushion region thickness of order 0.5 R S under expanded magnetospheric conditions. This suggests that restricting our analysis to the expanded magnetosphere will not significantly bias our conclusions in this study. [41] One potential explanation for the observations described above lies in the physics of the magnetodisk itself. The disk is created as corotating magnetospheric plasma, strongly confined to the centrifugal equator, balloons outward under the influence of centrifugal forces. The frozen in magnetic field gets dragged out with the expanding plasma and eventually adopts the classic magnetodisk configuration. This expansion is generally thought to be restricted on the dayside by the dynamic pressure of the solar wind acting on the magnetopause such that only on the nightside of the planet can the expansion continue to the point of reconnection. Such reconnection typically removes mass from the system and keeps the total mass content of the magnetosphere in a state of long‐term quasi‐equilibrium. [42] Consider, however, a situation in which the dynamic pressure of the solar wind is particularly low such that the magnetopause retreats to large distances on the dayside. Is it possible, under these conditions, for the stretching of the magnetodisk to continue to the point of reconnection on the dayside too? And if so, what are the likely implications of this reconnection for the large‐scale structure and dynamics of the system? In the compressed state, where the magnetodisk extends all the way to the magnetopause, any plasmadepleted flux tubes present on the dayside will be draped over the magnetodisk and, as a result, retain a significant radial component. The north–south thickening of the current sheet due to this “draped magnetic flux” is likely to be negligible and probably undetectable. In the expanded state the radial distance at which magnetodisk reconnection occurs also represents the maximum distance at which a stable magnetodisk‐like configuration can be maintained. Beyond this distance corotation can no longer be enforced and plasmadepleted flux tubes will be able to relax into a more dipolar 10 of 14
A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 Figure 6. The critical ion density, N C , required for magnetodisk breakdown in the (top) Jovian and (bottom) Saturnian magnetospheres is shown in red. The measured thermal electron density is shown in blue, and the 1 s variability in the mean location of the Joy et al. [2002] (Jupiter) and Arridge et al. [2006] (Saturn) magnetopause is shaded in grey, mapped to the local time of the magnetopause crossing. Vertical black lines denote the actual location of the magnetopause observed by each spacecraft. configuration reminiscent of the cushion region. Small clumps of plasma containing closed loops of magnetic field may detach from the outer edge of the magnetodisk but, for the most part, this plasma will remain inside the magnetopause and eventually diffuse onto adjacent magnetospheric field lines. Eventually, this plasma will be lost in the magnetotail. [43] Can this effect explain the formation of the cushion region at Jupiter and, similarly, the absence of a cushion region at Saturn? The radial distance at which the magnetodisk breaks down can be estimated from the magnetospheric force balance condition (equation (1)) if we make the simplifying assumptions that m i m e and that the poorly constrained outward acting pressure gradient forces are equal to some multiple, k, of the centrifugal force. [44] Rearranging equation (1) for the field line radius of curvature, R C , under these assumptions we obtain B 2 R C ðk þ 1Þ 0 N i m i W 2 ; ð5Þ from which it is immediately apparent that, for increasing outward forces, the field line radius of curvature will decrease. This decrease cannot continue indefinitely, however, as the radius of curvature will eventually approach values comparable with the ion gyroradius. At this point the MHD assumptions upon which equation (1) is based will begin to break down and, for a low‐energy plasma, the magnetic field is likely to develop an x line. Reconnection at this x line may explain many of the magnetic nulls observed in the transition region at Jupiter by Haynes [1995]. [45] Assuming that ions have a kinetic energy E KE ∼ (3/2) k B T i , where T i is the mean ion temperature, the mean ion gyroradius, R g , for equatorially mirroring particles may be expressed as R g ¼ m i 2k B T 1=2 i ; ð6Þ jq i jB m i where q i is the mean electromagnetic charge on an ion. Setting R C = R g in equation (5), substituting for equation (6) and rearranging for N i gives jq i jB 3 m 1=2 i N C ; ð7Þ ðk þ 1Þ 0 m 2 i W2 2k B T i where N C is now the critical number density required for the ion gyroradius and field line radius of curvature to be equal. To estimate the distance at which this critical density is reached in each magnetosphere the value of N C must be compared with the observed number density measured by in situ spacecraft. This comparison is presented in Figure 6 for the Ulysses (inbound) pass at Jupiter and Cassini Rev 20 (inbound) pass at Saturn where it has been assumed that ion and electron number densities and temperatures are equal. This assumption is qualitatively consistent with the results of Thomsen et al. [2010]. 11 of 14
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Outer magnetospheric structure: Jupiter and Saturn compared

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