Periodicity in Saturn s magnetosphere: Plasma cam

Transcription

1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L14203, doi: /2009gl039043, 2009 Periodicity in Saturn s magnetosphere: Plasma cam J. L. Burch, 1 A. D. DeJong, 1 J. Goldstein, 1 and D. T. Young 1 Received 4 May 2009; revised 25 June 2009; accepted 30 June 2009; published 30 July [1] Plasma ion data from the Cassini Plasma Spectrometer (CAPS) are examined for all orbits from October 25, 2004 through Dec. 26, To eliminate effects of incomplete angular coverage, data are only used from the CAPS anode that is closest to viewing into the corotational flow and within 20 of that flow. The data are plotted in the SKRbased SLS3 longitude system. The result is a cam-shaped distribution in radial distance and SLS3 that has an outer lobe extending beyond 20 R S at SLS3 longitudes in the range The western edge of this outer lobe maps to the inner extent of a previously observed spiral pattern of periodic ion enhancements, which had the magnetic signature of plasmoids at distances >35 Rs. The plasma cam and the plasmoid spiral emanating from it are responsible for plasma periodicities observed at radial distances beyond 15 Rs in Saturn s magnetosphere. Citation: Burch, J. L., A. D. DeJong, J. Goldstein, and D. T. Young (2009), Periodicity in Saturn s magnetosphere: Plasma cam, Geophys. Res. Lett., 36, L14203, doi: / 2009GL Southwest Research Institute, San Antonio, Texas, USA. Copyright 2009 by the American Geophysical Union /09/2009GL039043$ Introduction [2] One of the most striking and consistent features of Saturn s magnetosphere, and one of its enduring mysteries, is the periodicity that is observed in its magnetic fields, plasmas, energetic particles, and wave emissions. The diurnal peaks in Saturn Kilometric Radiation (SKR) first reported by Desch and Kaiser [1981] from Voyager radio wave data provided the first measure of the rotation rate of this gas giant planet. The period reported by Desch and Kaiser was hours, while more recent measurements by Cassini [Kurth et al., 2008] show a period of 10.8 hours. The fact that the rate is gradually but measurably slowing down indicates from inertial considerations that the SKR rotation period cannot be the rigid rotation period of the planet [e.g., Goldreich and Farmer, 2007]. As discussed by Gurnett et al. [2007], while planetary rotation is the main driver of the observed periodicities, there apparently is some slippage due to electromagnetic forces in the magnetosphere and ionosphere that results in a small deficit to perfect corotation. [3] Numerous studies of Saturn s periodic plasma and field phenomena have been reported from Voyager and more recently from Cassini. For example, periodicities in the Voyager magnetic-field data were analyzed by Espinosa and Dougherty [2000] and Espinosa et al. [2003a, 2003b]. Espinosa et al. [2003b] first used a camshaft analogy to explain periodic magnetic perturbations observed by Voyager with the cam action being supposed to result from a magnetic anomaly fixed in the planet s frame of reference. But with the realization that the rotation rate is slowing down, attention has turned to a longitudinally concentrated outflow of plasma from Enceladus [Gurnett et al., 2007], from other possible ionospheric or inner magnetospheric sources [Carbary et al., 2007], or from plasma asymmetries originating farther out in the ring current [e. g., Khurana et al., 2009] as the driving force for the periodic phenomena. An important consideration in the Carbary et al. and Khurana et al. studies is the tilt of Saturn s rotational axis with respect to the solar-wind flow, which should cause a bending of the plasma disk at distances beyond about 20 R S. [4] Both Goldreich and Farmer [2007] and Southwood and Kivelson [2007] note that the rotating nonaxisymmetric magnetic-field signal observed by Cassini inside of about 15 R S must be supported by a longitudinally localized system of field-aligned currents linked to the ionosphere. Like Gurnett et al. [2007], Goldreich and Farmer [2007] suggest a corotating two-cell convection system of the type proposed by Hill et al. [1981] for Jupiter as the source of localized outflowing plasma. Goldreich and Farmer [2007] refer to the localized outflowing plasma as a plasma tongue, which is connected to the ionosphere by the field-aligned currents responsible for the nonaxisymmetric magnetic field signal. [5] Another important consideration reported by Arridge et al. [2008] is their observation that the magnetospheric and magnetotail current sheets are warped into the northern hemisphere at all local times, creating a bowl-shaped geometry. This geometry forms the basis for the model of Khurana et al. [2009] who propose that it is distorted by the loading of plasma in a certain longitudinal sector. [6] The purpose of our study was to examine a large set of ion data to determine the overall distribution of plasma in Saturn s magnetosphere and possible causes of periodic phenomena. The results are relevant to the previously proposed plasma sources, specifically to the plasma tongue of Goldreich and Farmer [2007] and the outflowing inner-magnetospheric plasma of Gurnett et al. [2007] and Carbary et al. [2007]. [7] Ion data from CAPS (the Cassini Plasma Spectrometer) [Young et al., 2004] were analyzed on a statistical basis for the period October 25, 2004 through Dec. 26, An important constraint on the study was the incomplete angular coverage of the measurements. The field-of-view of the CAPS Ion Mass Spectrometer (IMS) covers a fan of 8 160, and this fan is moved both by the CAPS actuator and by frequent spacecraft maneuvers (the latter mostly in connection with imaging operations). These motions could certainly impact the results of a long-term statistical study. However, the observation that to a close approximation plasma corotates with the planet [Wilson et al., 2008] allowed us to evaluate statistically the ion fluxes as func- L of5

2 Figure 1. Daily average Cassini locations for this study. Averages were computed only when the spacecraft radius was 30 R S or less and the Kronographic latitude was between +10 and 10. (top) Average Kronographic latitude. (middle) Average local time. (bottom) Fraction of each day when Cassini was in the region specified above. tions of radial distance and SLS3 longitude, which is based on the modulation period of Saturn kilometric radiation [Kurth et al., 2008], by selecting only those data acquired with particular sensors that were viewing closely into the corotational flow direction, thereby effectively removing effects of spacecraft and actuator motions. [8] An important result of the study is a cam-shaped locus of plasma that fills a circular region inside L15 R s for most longitudes but displays an outer lobe that extends beyond 20 R s in the SLS3 longitude range between 270 and 50. The outer lobe of the cam is observed to overlap the region identified by Burch et al. [2008] as the inner locus of periodic ion fluxes observed by CAPS over the time period Dec. 29, 2005 and Sept. 7, These periodic fluxes were observed by Burch et al. [2008] to extend along a spiral path in R and SLS3 longitude that reached nearly 50 R s and along which magnetic signatures consistent with plasmoids were observed within the periodic ion fluxes at all distances beyond about 35 R s. Other similar observations of plasmoids in this region have been reported by Jackman et al. [2007] and Hill et al. [2008]. Hill et al. provided a more complete description of the plasmoids by computing the plasma flow velocities associated with them. Together the plasma cam and the spiral path of ion events observed beyond the cam can explain periodic plasma phenomena observed at radial distances greater than 15 R s. [9] The spatial overlap of the plasma cam with the spiral path of ion events and (at greater distances) plasmoids reported by Burch et al. [2008] supports their suggestion that magnetic flux tubes heavily laden with plasma stretch into the night side, ultimately leading to magnetic reconnection on closed field lines in the tail region of Saturn s magnetosphere as suggested for Jupiter s magnetosphere by Vasyliunas [1983] and Kivelson and Southwood [2005]. 2. Observations [10] The data were selected and averaged in the following manner. First, only Saturn Kronographic latitudes between 10 and +10 were considered. Next, for each four-second energy sweep of the IMS, samples were chosen to have energies near the corotation energy for water-group ions (18 amu) but falling below it at the larger distances (e.g., 0.6 times the corotation energy at 25 R S ) as determined by fitting the measured energy distribution versus L shell for all longitudes combined. All eight IMS anodes (which together Figure 2. Total number of samples for each bin in radius versus SLS3 longitude. Bin size is 1 R S by 5 inside 10 R S and 2 R S by 5 outside 10 R S. The contiguous white pixels near the origin represent no data. 2of5

3 Figure 3. Average ion counts from anodes closest to the corotational flow but within 20 of it at Kronographic latitudes from 10 to +10. Pixels are as defined in Figure 2. Energies for each L value are near the corotation energy for water-group ions (18 amu) but fall below it at the larger distances as determined by fitting the measured energy distribution versus L shell for all longitudes combined. The contiguous white pixels near the origin represent no data, while white pixels elsewhere represent an average of less than 2 counts per 62.5-ms sample. sample the fan) were evaluated as to their look directions with respect to corotation. Only those anodes viewing within 20 of the corotational flow were used. Of these, the anode viewing closest to the corotational flow direction was selected. These selected anodes provided the basic data set used in the analysis. [11] Another important consideration is the coverage in latitude, local time, and SLS3 longitude of the data set used in the study. For our large data set the statistics with respect to these quantities are extensive, though far from perfect. The coverage in latitude and local time is shown in Figure 1, in which one point per day is plotted except for those days in which Cassini stayed outside 30 R S or was more than 10 above or below the equator. The top and middle plots show the daily average Kronographic latitude and KSM local time, while the bottom plot shows the fraction of each day during which the above criteria were met. It is clear from Figure 1 that positive and negative latitudes are about equally represented and that the full local time range is covered, although there are significant gaps in the local time coverage for some periods. [12] Figure 2 shows the data coverage in dipole L value and SLS3 longitude, which is quite extensive and fairly uniform as expected as the planet rotates under the spacecraft. The smaller number of samples close to the planet result from the higher spacecraft velocities near periapsis. The bin sizes for the sampling procedure are 1 R S by 5 inside R = 10 R S and 2 R S by 5 outside 10 R S. [13] For the plot shown in Figure 3, average count rates (counts/62.5 ms) were computed in the bins defined for Figure 2. For an electrostatic analyzer such as CAPS-IMS the count rate is proportional to energy flux. Each average typically contains counts from several different anodes from different locations within the spatial bins depending on the individual look directions of the anodes. [14] The results in Figure 3 show a cam-shaped region with a lobe extending beyond 20 R s at SLS3 from about 285 to 50 while its more circular sector remains well inside 15 R S between about 90 and 225. It is perhaps notable that there are two peaks inside 5 R S one just after 225 and a second between 315 and 360. The second of these may well be related to the electron density peak observed by Gurnett et al. [2007] at 330. [15] In order to determine whether or not the sampling distribution (Figure 2) was in some way responsible for the cam feature, we performed a correlation between the samples shown in Figure 2 and the average counts plotted in Figure 3. The resulting correlation coefficient was 0.3, indicating a very slight anticorrelation, which can be explained at least partially by the fact that in the inner region (L < 10 R S ) the higher spacecraft velocities led to fewer samples while the density increased as the planet was approached. We conclude that sample aliasing could not have produced the cam feature or affected it significantly. Figure 4. Multi-color pixels show the plasma cam as in Figure 1 shown rotated at four positions during a single day. Green pixels show total counts per 4 seconds for all 8 anodes at midpoints of periodic plasma events from Burch et al. [2008, Figure 1]. Also plotted is the spiral-path fit to the total data set in Burch et al. The SLS3 = 100 location is noted in each plot. The magnetopause location is taken from Arridge et al. [2006]. 3of5

4 [16] Plots of L vs. SLS3 distributions (shown in auxiliary material) similar to that in Figure 3 show that the cam feature appears on the day side and the night side and for positive and negative latitudes, but inevitably with more limited statistics. 1 Figure 4 serves as a schematic representation of how the cam would rotate through the magnetosphere over one Saturn day, approaching the magnetopause for a several-hour period. Also shown in Figure 4 are green pixels that locate the midpoints of the periodic plasma events given by Burch et al. [2008, Figure 1]. Figure 4 also reproduces the spiral path in R vs. SLS3 coordinates derived by Burch et al. as a fit to a larger data set (as shown in their Figure 2), with those events lying beyond 35 R S exhibiting the magnetic signatures of plasmoids. The spiral path of the periodic plasma events is seen in Figure 4 to intersect the outer lobe of the plasma cam. 3. Discussion and Conclusions [17] A statistical study of plasma ion fluxes in Saturn s magnetosphere covering the period October 25, 2004 through December 26, 2007 has revealed a cam-shaped region of plasma when plotted in the L vs. SLS3 coordinate system (Figure 3). For most SLS3 longitudes the plasma fills a circular region well within 15 R S while in the SLS3 longitude range from 270 and 50 the plasma extends beyond 20R S. As the planet rotates, the cam intersects the magnetopause once per Saturn day (Figure 4). It is notable that when the SLS3 = 100 meridian is at noon, which coincides with the average peak in SKR intensity [Kurth et al., 2008], the plasma cam intersects the magnetopause near dusk. Since the SKR generation is known to be at low altitudes along magnetic field lines that intersect the magnetopause in the prenoon hours [Gurnett et al., 2007], it is unclear what, if any connection there is between the interaction of the plasma cam with the magnetopause and the generation of SKR. However, it can be stated that the plasma cam will produce periodic plasma events at all L shells between 15 R S and 25 R S and that the ion events lying along the spiral path identified by Burch et al. [2008] and plotted in Figure 4 will extend the periodic events outward to 50 R S on the night side. [18] Another outstanding question is what produces the observed plasma cam. An interchange-driven two-cell convection pattern locked in SLS3 as suggested by Gurnett et al. [2007] and Goldreich and Farmer [2007] is one possibility. In this context, the plasma cam would be the extension of the plasma tongue proposed by Goldreich and Farmer. [19] Burch et al. [2008] suggested that the ion events and plasmoids (R > 35 R S ) observed along the spiral path plotted in Figure 4 were produced by reconnection on closed field lines that are stretched outward by plasma loading in the longitude range lying at the base of the spiral. The overlap of the outer lobe of the plasma cam with the base of the spiral shown in Figure 4 provides further support for this suggestion with the plasma cam being responsible for the plasma loading. A further development of the conceptual model proposed by Burch et al. [2008] would then involve the following steps: 1 Auxiliary materials are available in the HTML. doi: / 2009GL [20] 1. Plasma loading of magnetic flux tubes occurs in a restricted range of SLS3 longitudes (source not yet known). [21] 2. These flux tubes rotate with the planet but with a lag as observed, e.g., by Wilson et al. [2008]. [22] 3. When the plasma-laden flux tubes rotate into the tail region their further stretching ultimately leads to magnetic reconnection of the type proposed by Vasyliunas [1983] for Jupiter, and this reconnection forms the plasmoids that were observed beyond 35 R S by Burch et al. [2008]. [23] 4. Corotation lag causes the ion events and associated plasmoids to lie along a spiral path in R vs. SLS3 coordinates, and this spiral path rotates with the planet. [24] 5. Rotation of the plasma cam and the spiral path of ion events and plasmoids are responsible for periodic plasma events (and perhaps magnetic-field events) that are observed beyond 15 R S in Saturn s magnetosphere. [25] Finally, the rotating acceleration events observed by Mitchell et al. [2009] in neutral atom images have been shown to approach the prenoon magnetopause at the times of strong SKR emissions, so the relationship between the cam outer lobe and the Mitchell et al. acceleration events will be important to determine in future studies. [26] Acknowledgments. This research was supported by JPL contract with Southwest Research Institute. Helpful discussions with Michelle Thomsen are gratefully acknowledged. References Arridge, C. S., N. Achilleos, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2006), Modeling the size and shape of Saturn s magnetopause with variable dynamic pressure, J. Geophys. Res., 111, A11227, doi: /2005ja Arridge, C. S., K. K. Khurana, C. T. Russell, D. J. Southwood, N. Achilleos, M. K. Dougherty, A. J. Coates, and H. K. Leinweber (2008), Warping of Saturn s magnetospheric and magnetotail current sheets, J. Geophys. Res., 113, A08217, doi: /2007ja Burch, J. L., J. Goldstein, P. Mokashi, W. S. Lewis, C. Paty, D. T. Young, A. J. 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