Outer magnetospheric structure: Jupiter and Saturn compared

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A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 (decreasing UT time for an inbound pass) on the x axis. The final magnetopause crossing was made at a radial distance of 45.6 R S (R SS = 28.5 R S ) on the equatorial dawn meridian. The spacecraft then proceeded inward, moving through the dayside magnetosphere over a period of 10 days, to a 5.6 R S periapsis near the dusk terminator. As in section 2 for Jupiter, the qualitative structure of the observed magnetic field is most readily apparent when we consider the ratio of the poloidal field components to the total field magnitude (∣B ∣/∣B∣, ∣B R ∣/∣B∣) and the angle, INT , between the observed magnetic field and the field associated with the Burton et al. [2009] internal dipole. [26] Two magnetospheric regions are apparent on this pass (c.f. the three observed at Jupiter) with a quasi‐dipolar region close to the planet, shaded blue, and a radially stretched ( INT > 50°, ∣B R ∣ > ∣B ∣) region, shaded yellow, from 25 R S out to the magnetopause. These two regions are interpreted as Saturnian equivalents to the Jovian inner magnetosphere and magnetodisk, respectively, separated by a roughly 7 R S “transition region,” shaded white in Figure 5, where the mean field rotates through ∼30° and has properties intermediate between the two adjacent regions. It should be noted, however, that this is a very different type of transition region to the one described at Jupiter as it involves two qualitatively different magnetospheric regions. The absence of an equivalent transition region for the Ulysses pass at Jupiter is most likely due to differences in the spacecraft trajectories and the speed at which each spacecraft made transition into the quasidipolar region. Unlike the Ulysses observations made at Jupiter there is no evidence for a third, quasi‐dipolar region between the magnetodisk and magnetopause which might be interpreted as a Saturnian equivalent of the cushion region. [27] Both the inner magnetosphere and magnetodisk show variability with a period close to the approximately 10h30m planetary rotation period, however, unlike the periodic phenomena seen in the Jovian magnetosphere, these variations cannot be interpreted as a rotational flapping of the magnetosphere due to the small ( ∣B ∣) were inspected for evidence of a cushion region. [30] For the 12 passes on which an unambiguous magnetodisk was identified, spacecraft observations reveal an extremely dynamic magnetosphere varying over multiple time scales of order minutes to days. The position and/or orientation of the magnetodisk appear to vary both during and between spacecraft passes at Saturn, as does the mean angle by which the magnetic field deviates from that of the Burton et al. [2009] dipole. In general, however, the magnetosphere is characterized by a two‐layered geometry (as observed on Cassini Rev 20; Figure 5) with a highly dipolar, B ‐ dominated region close to the planet, analogous to the Jovian inner magnetosphere, and a radially stretched magnetodisk ( > 50°, ∣B R ∣ > ∣B ∣) at large radial distances out to the magnetopause. There is no evidence for a third magnetospheric region, between the magnetodisk and magnetopause, which might be interpreted as a Saturnian equivalent to the cushion region seen at Jupiter. Rotational periodicities are often present at Saturn, both in the inner magnetosphere and magnetodisk, but the physical origin of these periodicities is different to those seen at Jupiter and crossings of the magnetodisk are rare. Finally, the field in the Saturnian magnetodisk is also “less stretched” than that at Jupiter, as evidenced by the two poloidal field components lying closer together in value. These observations are consistent with the magnetodisk modeling work of Achilleos et al. [2010a] which shows that field lines in the Saturnian magnetodisk have a much larger radius of curvature (>2 R S ) than their Jovian equivalents. [31] The remaining passes can be divided into two categories. In the noon sector the reduced distance to the 8of14
A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 Table 2. Summary of Low‐Latitude (l < 20°), Expanded (R SS ≥ 23 R S ) Dayside Passes at Saturn as Identified Using the Arridge et al. [2006] Magnetopause Model and the Innermost Magnetopause Crossing for Each Pass Pass Direction R SS /R S LT/Decimal Hours l/Degree Magnetodisk? Cushion? Voyager 2 OUT 23 6 0 YES None observed Cassini Rev 00B OUT 27 6 −5 YES None observed Cassini Rev 020 IN 29 6 0 YES None observed Cassini Rev 015 OUT 26 7 0 YES None observed Cassini Rev 009 OUT 27 7 −9 YES None observed Cassini Rev 016 OUT 29 7 0 YES None observed Cassini Rev 019 IN 32 7 0 YES None observed Cassini Rev 003 OUT 30 7 0 YES None observed Cassini Rev 018 IN 32 7 0 YES None observed Cassini Rev 000 IN 25 8 −15 NO N/A Cassini Rev 005 OUT 27 8 −4 YES None observed Cassini Rev 017 IN 24 8 0 YES None observed Cassini Rev 013 IN 25 9 −19 NO N/A Cassini Rev 008 IN 26 9 −18 NO N/A Cassini Rev 003 IN 27 9 −1 NO N/A Cassini Rev 012 IN 23 9 −19 NO N/A Cassini Rev 057 OUT 24 12 7 NO N/A Cassini Rev 048 OUT 23 12 0 NO N/A Cassini Rev 051 OUT 27 12 1 NO N/A Cassini Rev 052 OUT 28 12 2 NO N/A Cassini Rev 049 OUT 29 13 −2 YES None observed Cassini Rev 044 OUT 24 14 5 NO N/A Cassini Rev 052 IN 29 16 4 NO N/A Cassini Rev 051 IN 25 16 4 NO N/A Cassini Rev 050 IN 33 16 −6 NO N/A Cassini Rev 048 IN 25 17 0 NO N/A magnetopause prevents a magnetodisk from forming for all but the most expanded magnetospheric conditions (Cassini Rev 49 outbound, R SS >29R S ) and plasma‐depleted flux tubes, if present, are difficult to distinguish from their plasmaloaded counterparts. This fundamental ambiguity could not be resolved using CAPS/ELS plasma moments as the lack of clear magnetodisk crossings (discussed above for Rev 20) prevented us from obtaining the central plasma sheet density necessary for comparison. Here it is important to remember that, at Jupiter, the cushion region and magnetodisk lobes have essentially the same density; it is only the density at the magnetic equator (synonymous with the center of the plasma sheet in the magnetodisk) that changes upon entering the cushion region. The dusk sector field, in contrast, is typically disturbed such that a stable magnetospheric configuration, either magnetodisk‐like or dipolar, is difficult to define. Similar observations were made in the dusk sector of the Jovian magnetosphere where the plasma sheet becomes so thick [Kivelson and Southwood, 2005] that exploring spacecraft rarely leave the disturbed central region. In such cases a “disk‐like” interpretation of the magnetic field geometry is no longer applicable. 4. Discussion [32] The lack of a persistent, unambiguous region of quasidipolar, B ‐dominated field between the dayside magnetopause and magnetodisk at Saturn is in stark contrast to the ∼20 R J thick, local time dependent cushion region typically observed at Jupiter. The apparent implication of this discovery is that the Saturnian dayside magnetosphere typically lacks an outermost layer of plasma‐depleted flux tubes and that dynamical processes in the Saturnian outer magnetosphere manifest themselves in a very different way to those observed at Jupiter. 4.1. The Importance of Plasma Return Flows [33] Both the Jovian and Saturnian magnetospheres are associated with a constant time‐averaged magnetospheric mass content and, as a result, the time‐averaged rate at which plasma is generated in the inner magnetosphere (primarily by processes related to Io and Enceladus) must equal the timeaveraged rate at which plasma is lost through reconnective processes occurring in the magnetotail. The difference between the Jovian and Saturnian inner magnetosphere source rates is typically assumed to be around an order of magnitude [Vasyliũnas, 2008]; however, caution must be applied when using this figure for the following important reason. Inner magnetosphere source rates are typically derived from a measurement of the “momentum loading” of magnetospheric field lines and, as a result, involve a significant contribution from charge exchange processes which change the momentum of magnetospheric plasma while not significantly altering its total mass. The inclusion of such charge exchange processes in the momentum loading calculation thus results in an overestimation of the true magnetospheric source rate by an unknown factor that is difficult to determine through experimental means. [34] The modeling work of Delamere et al. [2007] suggests that charge exchange processes may be an order of magnitude more important at Saturn than at Jupiter such that the true difference between the Jovian and Saturnian inner magnetosphere source rates may then approach a factor of 100. From an order of magnitude perspective, the average mass content of flux tubes in the Jovian and Saturnian magnetospheres (Table 1) is comparable. This fact, combined with the idea that average mass input equals average mass output, then implies that flux tubes in the Jovian magnetotail lose mass roughly 100 times faster than their Saturnian equivalents. This is similar to the factor of 30 difference in the total 9of14
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Outer magnetospheric structure: Jupiter and Saturn compared

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