Saturn s magnetodisc current sheet

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007ja012540, 2008 Saturn s magnetodisc current sheet C. S. Arridge, 1,2,4 C. T. Russell, 3 K. K. Khurana, 3 N. Achilleos, 2,4,5 S. W. H. Cowley, 6 M. K. Dougherty, 4 D. J. Southwood, 7 and E. J. Bunce 6 Received 14 May 2007; revised 1 October 2007; accepted 6 November 2007; published 12 April [1] Pioneer 11 observations from the dawn flank of Saturn s magnetosphere provided evidence for a thin equatorial current sheet. Magnetometer data from the Cassini orbiter have also revealed the presence of this current sheet and shown it to be a consistent feature of the dawn flank. Arridge et al. (2007) recently investigated the stress balance in this current sheet and showed that the centrifugal force dominated the mechanical stresses beyond a radial distance of R S from Saturn. These observations led Arridge et al. (2007) to interpret this current sheet as a magnetodisc, similar to that in the jovian magnetosphere but strongly asymmetric in local time. In this paper we present a survey of these thin current sheets in Saturn s magnetosphere and show that they have been observed at all local times where Cassini has explored the outer magnetosphere beyond 15 R S. We interpret the observations as evidence for Saturn s magnetodisc current sheet and show that under certain conditions the magnetodisc is not as asymmetric as Arridge et al. (2007) first suggested. Under low solar wind dynamic pressures where the subsolar magnetopause standoff distance is >23 R S, Saturn s ring current dominates over Saturn s internal field and produces a magnetodisc. Under compression the dayside field becomes more dipolar and the magnetodisc is only present on the nightside and flanks of the magnetosphere. Hence in contrast to Jupiter s magnetodisc, the kronian magnetodisc is highly sensitive to the upstream solar wind conditions. We also present theoretical stress balance calculations supporting the existence of a magnetodisc at Saturn, where we show that the centrifugal force is plausibly sufficient to produce the observed magnetodisc. Citation: Arridge, C. S., C. T. Russell, K. K. Khurana, N. Achilleos, S. W. H. Cowley, M. K. Dougherty, D. J. Southwood, and E. J. Bunce (2008), Saturn s magnetodisc current sheet, J. Geophys. Res., 113,, doi: /2007ja Introduction [2] Jupiter s middle magnetosphere between 20 R J and R J is dominated by a thin washer-shaped sheet of electric current and energetic particles known as the magnetodisc. In stark contrast to the terrestrial magnetosphere, the field due to this ring current severely distorts the magnetic field around Jupiter and stretches out the field into a disc-like configuration. In the magnetodisc the stretched field is reminiscent of the field geometry in the Earth s magnetotail with stretched field lines around a thin 1 Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, UK. 2 Centre for Planetary Sciences, University College London, London, UK. 3 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA. 4 Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College London, London, UK. 5 Atmospheric Physics Laboratory, University College London, London, UK. 6 Department of Physics and Astronomy, University of Leicester, Leicester, UK. 7 European Space Agency, Paris, France. Copyright 2008 by the American Geophysical Union /08/2007JA012540$09.00 current sheet. At radial distances smaller than 20 R J the perturbation field due to the ring current is small compared to the field due to the planet and so the observed magnetic field remains quasi-dipolar. [3] Prior to the arrival of Cassini, observations at Saturn did not reveal such a strongly distorted magnetosphere. Observations on the dayside by Pioneer 11 and the two Voyager spacecraft revealed a quasi-dipolar magnetosphere where the ring current was not sufficiently intense to distort the field into the disc-like structure observed at Jupiter [Acuña et al., 1983]. In contrast to the dayside, the dawn flank observations ranging between 0300 and 0600 Saturn Local Time (SLT) showed evidence of a thin current sheet and stretched field configuration, similar to that observed on the outbound pass of Pioneer 10 at Jupiter. [4] Smith et al. [1980] discussed this field geometry at Saturn and provided several interpretations. In support of a magnetodisc interpretation they pointed out that dayside observations at Jupiter under highly compressed magnetospheric conditions show the magnetodisc to be much weaker and poorly ordered, presumably due to the effect of the solar wind and proximity to the magnetopause on the dayside, and suggested that this might provide an explanation for the quasi-dipolar dayside and stretched out dawn flank. Smith et al. [1980] also suggested that the thin current sheet could be associated with either a planetary wind or 1of9

2 magnetotail currents. The planetary wind interpretation was rejected since the field lines appeared to be closed in the current sheet region. Since the magnetic field lines were being swept in the antisunward direction, Smith et al. [1980] concluded that the magnetotail interpretation was the most likely scenario. [5] Clearly, single spacecraft observations are insufficient to conclude that at Saturn the magnetosphere is quasidipolar on the dayside when it may only have been observed in a relatively compressed state. Furthermore, the arguments in favor of the near magnetotail on the dawn flank rest on the lagging field configuration. We now understand this sweepback at Jupiter in terms of radial currents flowing in the equatorial magnetospheric plasma, which transmit torque from the planet and exert an azimuthal force on the plasma to try to keep the magnetosphere rotating with the planet [e.g., Vasyliunas, 1983]. Since Saturn has strong internal plasma sources [Blanc et al., 2002], one might also expect such a lagging configuration, which thus provides an alternative explanation for this field configuration. [6] This thin current sheet is also quite clear in Cassini magnetometer data and its physical origin was studied by Arridge et al. [2007]. In that work the stress balance in the current sheet was examined and found to be dominated by the centrifugal force beyond R S. Hence the authors suggested that the current sheet was the kronian analogue of Jupiter s magnetodisc, albeit highly asymmetric in local time due to the confining effect of the solar wind. Thus these findings were in support of Smith et al. s [1980] earlier discussions regarding a magnetodisc origin for the current sheet observed in the Pioneer 11 magnetometer data set. The work of Arridge et al. [2007] clearly suggested that a largescale examination of the field data at Saturn was required to examine the variability of the dayside ring current. [7] In this paper we present magnetometer data from the Cassini magnetic field experiment [Dougherty et al., 2004] which demonstrates that the field can become stretched out over the dawn, nightside, and dayside sectors and hence that the field can adopt a magnetodisc configuration. We find that under compressed magnetospheric conditions the field does not stretch out near the noon meridian whilst apparently the magnetosphere is disc-like over other local time sectors. Hence the magnetodisc has a strong local time asymmetry under compressed conditions. We interpret this quasi-dipolarization of the dayside in terms of the confining effect of the Chapman-Ferraro current system. To support the identification of the magnetodisc we use a simple stress balance calculation to show that centrifugal stresses are plausibly sufficient to produce this disc-like configuration, at a distance that is consistent with the observations. 2. Observations [8] Figure 1 shows magnetometer data from the dawn flank of Saturn s magnetosphere, from Cassini s Rev 3 orbit of Saturn, and ranges from periapsis to the first outbound magnetopause crossing (indicated by the vertical dashed line). Beyond 15 R S,theB z component of the magnetic field becomes very small, around 1 nt or less, and the radial and azimuthal components of the field are dominant, consistent with Cassini s location under a current sheet where the magnetic field has a lagging configuration. The measured field strength is also larger than that due to Saturn s internal field, indicating that the observed field is dominated by the external current systems. This is reflected in Figure 1g, which shows the ratio between the perturbation field strength (i.e., due to external currents) and the internal field. In the stretched current sheet region this ratio is larger than unity. [9] Two angles are also plotted on Figure 1. Figure 1e shows the angle between the perturbation field and the internal field of the planet. This slowly rotates as Cassini moves outbound from being 180 (antiparallel to the internal field) near Saturn to being almost 90 beyond 15 R S. Figure 1f shows the angle between the total observed field and the spin axis of Saturn, hence when this angle is 90 the field lines point radially away from or toward Saturn. Similarly to Figure 1e, this angle rotates as Cassini moves outbound, becoming close to 90 beyond around 15 R S. Hence beyond 15 R S the field adopts a distorted stretched out current sheet configuration. [10] To examine the spatial distribution and variability of these current-sheet-like fields, we surveyed the magnetometer data for current-sheet-like fields from day 180 of 2004 to day 228 of 2006 (29 orbits, Saturn Orbit Insertion to revolution 27). To objectively carry out the survey, we adopted a set of criteria that the field must meet in order to be classified as current-sheet-like. [11] First, the angle between the perturbation field and the internal field ff(db, B INT ) must lie within 60 and 120, to ensure that the field due to the magnetospheric current systems largely points away from or toward the planet. Near the equatorial plane the internal field is mostly oriented N-S but there are a number of orbits where Cassini is located some degrees from the equator. However, at the relatively large radial distances involved, the internal field does not deviate by more than 10 from the N-S direction. [12] Second, the angle between the total field and the spin axis ff(b, W) must also lie within 60 and 120, to ensure that the field lines point largely radially away from or towards Saturn. These first and second criteria do not always mean the same thing; for example, when the magnetosphere is compressed the field lines may be highly dipolar, thus not meeting the second criteria, but the perturbation field may still be nearly perpendicular to the internal field. Hence these two criteria are not always equivalent. [13] The angular ranges of 90 ± 30 specified above were established based upon an examination of the ranges of angles observed on the dawn flank and reflect a purely empirical understanding of the dawnside data. [14] Third, the magnitude of the perturbation field must be greater than or equal to the planetary field, i.e., the field due to the magnetospheric current systems should compete with or dominate the internal field of Saturn. Since we seek to establish the gross field configuration, small-scale features such as transient current sheet crossings do not meet our criteria. In these cases such small-scale departures are ignored for the purposes of our survey. [15] The effect of these criteria can be observed in Figure 1. From Figure 1g the third criterion is met at 15 R S when the observed field strength (solid line) is equal to the magnitude of the internal field contribution (dashed 2of9

3 Figure 1. Magnetometer data from the outbound leg of Cassini s Rev 3, showing (a c) three components of the magnetic field in cylindrical polar coordinates, (d) the observed and internal (dashed) field magnitudes, (e) the angle between the perturbation (B OBS B INT ) field and the internal field, (f) the angle between the total observed field and the spin axis of Saturn, and (g) the ratio between the perturbation and internal field strengths. At the bottom of the figure the first two digits of the year and the doy, the cylindrical radial distance of Cassini, the vertical distance from the magnetic equator (offset by 0.04 R S from the rotational equator), and Cassini s local time are shown. line). Since Cassini is at such low latitudes on this pass and the field is highly stretched out the two angular criteria are equivalent. From this plot we see that the angular criteria are not met until around 0.6 R S later than the field strength criteria. Hence all three criteria are met at 15.6 R S and the field is current-sheet-like beyond this distance. [16] The results of this survey are presented in Figure 2 and show that stretched current-sheet-like fields have been observed in the outer magnetosphere at all local times where Cassini has explored radial distances greater than 15 R S away from Saturn (i.e., all local times with the exception of the interval from 11 to 20 hours). Hence the field can adopt a disc-like magnetodisc configuration reminiscent of that in the jovian magnetosphere. Using these measurements, the mean of the distance to the inner edge of the current-sheetlike fields is 16.2 R S with a standard deviation of 2.37 R S. The term inner edge is used purely to refer to the point at which the field is strongly stretched out and is not meant to represent any particular discontinuity in the azimuthal currents responsible for the current sheet. Clearly, the currents in the magnetodisc region must merge with those in the quasi-dipolar region inside of 16 R S. [17] Our perturbation field strength criterion ensures that our analysis is sensitive to examining the outer extent of the ring current region. Here the ring current perturbation field dominates over the internal field of the planet in determining the geometry of the magnetic field. In the quasi-dipolar region inside of 16 R S, but sufficiently far from the inner edge of the ring current, there will no doubt be a perturbation field which is quasi-radial. However this perturbation field is not sufficient to strongly affect the shape of the total field. There is no essential conflict between this analysis of the outer extent of Saturn s ring current and models of the 3of9

4 permeability of free space, and we have z B r DB r /D where DB r is the radial field outside of the current sheet and D is the sheet half-thickness. [21] Estimates for the half-thickness of the current sheet in the literature range between 1.5 and 3 R S [Giampieri and Dougherty, 2004; Dougherty et al., 2005] and in particular modeling from Cassini SOI showed that the thickness of the current sheet in the magnetodisc region was approximately 1.8 R S [Dougherty et al., 2005; G. Giampieri, private communication, 2005]. Hence a sheet half-thickness of 2 R S was assumed, consistent with the range of estimates in the literature. [22] With these definitions the equation for stress balance in the radial direction can be written as rw 2 r ¼ j f B z ¼ B zdb r m 0 D ð1þ Figure 2. Intervals of data containing stretched currentsheet-like fields, illustrated by bold curves aon the trajectory of Cassini. The trajectory is projected onto the KSM X-Y plane with a model magnetopause curve corresponding to a subsolar standoff distance of 26 R S [Arridge et al., 2006]. whole ring current, as demonstrated in the companion paper to this report [Bunce et al., 2008]. 3. Interpretation [18] We interpret the stretching of the magnetic field lines into a current-sheet-like configuration as the formation of a magnetodisc similar to that in the jovian magnetosphere. Because we have employed single-spacecraft observations it is clearly not possible to show that a disc-like configuration is present in different local time sectors, or longitudinal hemispheres, simultaneously. However, on the basis of theoretical arguments we can show that mechanical stresses in Saturn s magnetosphere are sufficiently strong to produce a magnetodisc. In particular we will show that the centrifugal force in Saturn s magnetosphere might plausibly produce the observed magnetodisc. [19] The importance of centrifugal stress in Saturn s ring current was demonstrated by McNutt [1984] and Mauk et al. [1985] using published Voyager particle and field data. These studies showed that the pressure gradient force was negligible beyond approximately 14 R S. More recently with Cassini data, the centrifugal force has also been shown to be dominant in the magnetodisc region beyond R S and is equally as important as the pressure gradient force at 19 R S [Arridge et al., 2007]. [20] We start with the equation of stress balance between the centrifugal force and the radial component of the Maxwell stress about a thin current sheet. Here r is the mass density, W is the angular velocity of the plasma (which we assume is corotating with Saturn), r is the (cylindrical) radial distance, j f is the azimuthal volume current density, B z is the axial field through the current sheet, m 0 is the In a magnetodisc or tail-like configuration DB r B z [e.g., Vasyliunas, 1983], hence we can use (1) to examine this inequality as a function of radial distance and in particular find the distance at which DB r = B z where the field should be strongly distorted. Solving (1) for DB r and substituting into the inequality we obtain: n c ¼ B 2 z m 0 Dm p M i W 2 r where M i is the mass of a particular ion, or the average mass for a particular ion composition (in proton masses), and m p is the mass of a proton. This number density n c is the critical number density above which the field is distorted into a disc-like configuration. We assume that B z is the mainly 3 dipolar field of Saturn with a dipole moment of nt R S [e.g., Davis and Smith, 1990]. [23] Figure 3 presents the results of this calculation for two different ion species, water group ions with M i = 18 and protons M i = 1. Clearly, a higher number density of protons can be contained by the Maxwell stress simply because they are 18 times less massive than water group ions. [24] Superposed on these theoretical curves are observed ion number densities from Voyager [Richardson, 1995] and model electron number densities from Cassini [Persoon et al., 2005]. The observed ion number densities are obtained from Table 1 of Richardson [1995] and we plot both alternatives 1 and 2. The oscillation between high and low density regions reflects the immersion of Voyagers 1 and 2 in hot tenuous and cold dense regions of the magnetosphere. Since the magnetodisc is the symmetry plane for the magnetosphere one would expect the cold plasma to lie close to the magnetodisc. Hence we search for intervals where the theoretical curves cross the higher density values from Richardson [1995]. Furthermore, these densities are in agreement with electron densities determined from the electron spectrometer instrument on Cassini [Young et al., 2005] (assuming quasi-neutrality) and published in the work of Arridge et al. [2007]. [25] Figure 3 shows that the critical number density is not reached until around 16.5 R S where the observed ion number densities are larger than the water group curve. Hence these calculations are reasonably consistent with the ð2þ 4of9

5 displace the inner edge of the magnetodisc toward the planet. [29] Recent work has shown significant pressure in energetic particles in the range of 5 20 R S [Sergis et al., 2007] but the contribution to the total current density has not yet been evaluated. The observational work to date has shown that inertia appears to dominate the outer parts of the ring current as discussed in this paper. More detailed numerical work is in progress to include this additional contribution and to evaluate the development of the magnetodisc. We emphasize that this calculation merely provides evidence that there are mechanical stresses sufficient to produce a magnetodisc and that we do not particularly seek evidence that the centrifugal force is the dominant mechanical stress. Figure 3. Comparison of (a) calculated critical ion number densities from equation (2), assuming proton or water group composition (solid curves), (b) equatorial electron number densities from a model [Persoon et al., 2005] (thin solid curve), and (c) measured ion number densities from Richardson [1995] (filled circles and open squares). observed location of the stretched field region and thus support our association of these current-sheet-like fields with the disc-like configuration of a magnetodisc. [26] It is important to point out that this calculation assumes full corotation of the magnetospheric plasma which is not the case in reality. Any plasma subcorotation will tend to push outward the location of the transition point between dipolar and current-sheet like fields. Modeling of the corotation lag from Voyager data by Saur et al. [2004] shows that at 15 R S the plasma velocity is (4/5)v COR, and at 20 R S it is (2/3)v COR, where v COR = W r is the full corotation velocity at that radial distance. We can account for this by inserting the square of these factors, 16/25 and 4/9 respectively, in the denominator of equation (2). By applying this correction the critical point, where the critical ion number density crosses the observed ion number density, shifts outwards to 18 R S and 19 R S, respectively, still in reasonable agreement with the transition point observed in the magnetometer data. [27] We also investigated the effect of varying the halfthickness of the current sheet. The half-thickness was varied between 1.5 and 3.0 R S and produced a change of ±1 R S in the location of the predicted magnetodisc inner edge. Exploring both changes in thickness and in fractions of subcorotation, between 1.5 D 3.0 and 0.66 v/v COR 1.0 we obtained a range of predicted inner edges between and 20.5 R S. Hence our observed inner edge R S lies within the range presented by the theoretical calculations. [28] It should be emphasized that forces due to pressure gradients have been neglected in the above analysis. Beyond a reasonable distance from the inner edge of the ring current these pressure gradients should be directed planetward and as such should produce an outward force. Thus any pressure gradients in the main region of the ring current will enhance the effect described above and will 4. Dayside Disruption of the Magnetodisc [30] The results of our survey show that both quasidipolar and current-sheet-like fields can be observed on the dayside and hence that the magnetodisc can be disrupted on the dayside. Figures 4 and 5 show magnetometer data from two adjacent passes of Cassini on the dayside, revolution 13 and 14, respectively, where the subsolar magnetopause was located at 24.8 R S and 16.8 R S respectively. [31] The magnetometer data from rev 13 in Figure 4 shows that the magnetodisc forms at a radial distance of 19.5 R S and is sustained until around 2 R S before the magnetopause crossing. At the outer edge of the current sheet it is the angular criteria which break down, most noticeably the perturbation field angles which can be understood in terms of the Chapman-Ferraro currents straightening out the field lines. At the inner edge the two angular criteria are met right to the end of the plot; it is, however, the field ratio criterion that identifies the magnetodisc. [32] The compressed case on rev 14 in Figure 5 presents a very different picture. Both angular criteria are met between 8 and 15 R S but this does not result in the identification of the magnetodisc as the field ratio criterion is not met. Adjacent to the magnetopause the external currents do determine the field structure more strongly, but the orientation of both field angles suggest that this is the result of the Chapman-Ferraro currents. The total field angle criterion is met in the inner magnetosphere because of Cassini s relatively high latitude (10 ) and is approximately consistent with a dipolar field at that radial distance and latitude. [33] These two inbound orbit legs show the very different field structure that can be adopted on the dayside (between 0900 and 1000 h SLT) during compressed and during expanded conditions. During expanded conditions the field can stretch out much more readily and form the magnetodisc. Smith et al. [1974] discussed the variation of the jovian ring current strength with system size, speculating that the ring current should be strongest in the outer magnetosphere and most intense when the magnetosphere was in an expanded state corresponding to relatively low solar wind pressures. Hence the field should be the most distorted under expanded conditions. Bunce et al. [2007] have recently confirmed this hypothesis for Saturn s ring current. [34] This is also a plausible explanation for the occurrence of a quasi-dipolar field on the dayside at Saturn. As we have seen, the quasi-dipolar field on rev 14 was 5of9

6 Figure 4. Magnetometer data from the inbound leg of Cassini s Rev 13. The format is the same as Figure 1 except the times at the bottom of the figure also show HH:MM as well as the two digits of the year and the doy. In this example a thin current sheet is also evident from the magnetometer data, terminating (according to the criteria discussed in the text) several R S before the magnetopause. associated with a compressed magnetosphere. Do elevated solar wind dynamic pressures always result in a quasidipolar dayside, and low dynamic pressures result in a dayside magnetodisc? [35] To test for this effect we examined Cassini passes through the dayside magnetosphere (defined as between 0700 and 1700 h local time, although Cassini has not yet examined the dusk sector in the outer magnetosphere) which turned out to only include inbound legs of each orbit from Rev A (October 2004) through to Rev 20 (January 2006) and covered a total of 20 dayside passes. [36] For each pass the subsolar magnetopause stand-off distance was inferred using a magnetopause model [Arridge et al., 2006] and the last observed magnetopause crossing. The configuration of the magnetospheric magnetic field was also noted, quasi-dipolar or current-sheet-like, using the results of our survey. Hence the data set comprises a set of magnetopause stand-off distances for revolutions where the field was quasi-dipolar and for when the field was current-sheet-like. [37] From these data a histogram was produced showing the stand-off distances observed on the dayside, and the stand-off distances observed when a magnetodisc was observed inside the magnetosphere. This histogram is shown in Figure 6 and clearly shows that current-sheet-like configurations on the dayside are associated with more expanded magnetospheric conditions. [38] We also formally tested the hypothesis that the subsolar stand-off distance of the magnetopause affects the current-sheet-like or quasi-dipolar configuration of the dayside. The data were split by stand-off distance into three intervals: r 0 < 20 R S (compressed), 20 < r 0 < 23 R S (nominal), and r 0 >23R S (expanded), and the number of passes in quasi-dipolar and magnetodisc fields were noted. These data were used to construct a 2 3 contingency table presented on the right of Table 1. 6of9

7 Figure 5. Magnetometer data from the inbound leg of Cassini s Rev 14. The format is the same as Figure 4 and the data is from the adjacent revolution. [39] Under the null hypothesis that the subsolar stand-off distance of the magnetopause has no effect on the occurrence of a current-sheet-like or quasi-dipolar dayside we would expect this table to be uniformly distributed. The corresponding contingency table (on the left of Table 1) of expected values was constructed using the fraction of observations in each interval. For example out of the 20 dayside legs, 8 were during expanded conditions (40%) and dipolar fields were observed on 12 legs (60%), based on these statistics dipolar fields under expanded conditions should be observed = 4.8 times. [40] Under the assumption that each revolution was independent, Fisher s exact test was used to try to reject the null hypothesis. This test was used since the data were categorical in nature and the expected values were all less than 5, thus eliminating the possibility of using the simpler Pearson s chi square test. Since we expected an expanded magnetosphere to be associated with a larger subsolar standoff distance, we applied a one-tailed (left tail) test and found a probability of P = that our data could be obtained by chance alone. Hence there is significant evidence at the 95% level to reject the null hypothesis that there is no relationship between the configuration of the dayside field and the standoff distance of the magnetopause. [41] This analysis was robust to our choice of radial ranges separating compressed, nominal and expanded stand-off distances. The analysis was also carried out using just two size criteria: compressed (r 0 <21R S ) and expanded for (r 0 >21R S ) and found to be significant at the 99% confidence level. [42] The histogram in Figure 6 suggests that a subsolar stand-off distance of 23 R S is sufficient to prevent the formation of the magnetodisc on the dayside. Hence to a first order approximation solar wind pressures in excess of npa could prevent the formation of the magnetodisc (using the magnetopause model of [Arridge et al., 2006] with this stand-off distance). [43] There is some evidence for internal control of the structure of the dayside field in that there is one occasion where a magnetodisc was observed under compressed conditions, and one where a quasi-dipolar configuration was observed under expanded conditions. These departures 7of9

8 from a solar wind driven picture may be caused by the inaccuracy of our criteria for identifying current-sheet-like fields but internal mechanisms might play a role. If the magnetodisc is formed largely by centrifugal stresses, any variability in the mass-loading process in the inner magnetosphere, or any mass loss in the tail via the Vasyliunas cycle [Vasyliunas, 1983], will also affect the structure of the field and might account for these departures. 5. Summary [44] In this paper we have presented evidence for a magnetodisc in Saturn s magnetosphere. A simple stress balance calculation, under the assumption of purely centrifugal stresses, showed that a magnetodisc would naturally form around 16.5 R S, in good agreement with the observed inner edge of the magnetodisc. We have also shown that the magnetodisc can be disrupted in the noon sector when the solar wind dynamic pressure is high. [45] The stress balance calculation also allowed us to understand why the magnetodisc is disrupted near noon, and why Connerney et al. [1983] concluded that Saturn appeared to have a quasi-dipolar Earth-like magnetosphere. The inner boundary of the magnetodisc is relatively close to the nominal subsolar magnetopause, where the magnetopause straightens out the field lines and resists the stretching of the field. Thus under nominal or compressed conditions the disc is prevented from forming and the magnetosphere adopts a quasi-dipolar configuration on the dayside. Under expanded conditions this confining effect occurs at a much greater radial distance and the field can stretch out to form the magnetodisc under the centrifugal effect of the outer magnetospheric plasma. [46] Bunce et al. [2007] showed that the magnetic moment of the ring current varies by a factor of three between Table 1. Contingency Tables Containing the Null Hypothesis (Left) That the Solar Wind Does Not Affect the Formation of the Magnetodisc and the Actual Observations (Right) Disc-Like Quasi-Dipolar Disc-Like Quasi-Dipolar r 0 <20R S < r 0 <23R S r 0 >23R S compressed and expanded magnetospheric states. Hence the dynamics of the internal stresses responsible for the ring current play a strong role in producing the dayside magnetodisc under low solar wind dynamic pressures. Bunce et al. [2008] have shown that their model ring current produces a quasi-dipolar configuration when the magnetosphere is compressed, and a stretched-out configuration when the magnetosphere is expanded, thus supporting the results of our study. [47] The magnetodisc configuration in Saturn s magnetosphere must be taken into account in further studies of the kronian magnetosphere. For example, some models of the energization of particles in the jovian magnetosphere rely on non-adiabatic encounters with the magnetodisc current sheet in order to achieve high temperatures [e.g., Cheng, 1990]; the findings reported here also open up this possibility for Saturn. Also the presence of a magnetodisc implies that the background magnetic field at more distant moons such as Titan and Hyperion is significantly nondipolar. Moreover, the encounters with Titan on the dayside are complicated by the issues discussed in section 4. The location of Titan relative to the magnetopause becomes more important, and the question is no longer simply whether Titan is inside the magnetosphere, magnetosheath, or solar wind. The proximity to the magnetopause will vary the structure of the background field and this must be taken into account in modeling and data analysis. [48] Acknowledgments. CSA would like to acknowledge useful discussions with Tim Horbury. CSA was supported in this work by a PPARC Quota studentship at Imperial College and by the PPARC Rolling Grant to MSSL/UCL, MKD was supported by a PPARC Senior Fellowship, and KKK and CTR were supported by NASA. Work at Leicester was supported by PPARC grants PPA/G/O/2003/00013 and PP/E001130/1. SWHC was supported by a Royal Society Leverhulme Trust Senior Research Fellowship and EJB was supported by a PPARC Postdoctoral Fellowship PPA/P/S/2002/ [49] Zuyin Pu thanks the reviewers for their assistance in evaluating this paper. Figure 6. Histograms showing the inferred subsolar magnetopause stand-off distance for (top) all the dayside crossings and (bottom) only for those crossings where a magnetodisc configuration was detected inside the magnetosphere. In Figure 6 (bottom) the histogram from Figure 6 (top) is reproduced in gray to facilitate comparison. Typically, the disc-like configurations are observed when the magnetosphere is expanded. References Acuña, M. H., J. K. Alexander, R. 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