Periodic tilting of Saturn s plasma sheet

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L24101, doi: /2008gl036339, 2008 Periodic tilting of Saturn s plasma sheet J. F. Carbary, 1 D. G. Mitchell, 1 P. Brandt, 1 E. C. Roelof, 1 and S. M. Krimigis 1 Received 14 October 2008; accepted 14 November 2008; published 16 December [1] From the vantage of the dawn sector, the INCA instrument on Cassini imaged neutral hydrogen atoms (20 50 kev) emitted from the center of the Saturn s plasma sheet for five days during late Points along the center of the plasma sheet were found from contoured images projected onto the noon-midnight plane; points within 20 R S of Saturn were fitted to straight lines, and the slopes of these lines were examined as a function of time at one hour resolution. The slopes vary between 17 and 24 with a period of hours, the same as that of Saturn kilometric radiation (SKR). This periodic tilting of the plasma sheet is in phase with SKR radiation in the sense that the maximum tilt angle occurs when the maximum in the SKR power occurs, and the tilt angle periodicity has a phase angle of 47 in SLS-3 longitude. Citation: Carbary, J. F., D. G. Mitchell, P. Brandt, E. C. Roelof, and S. M. Krimigis (2008), Periodic tilting of Saturn s plasma sheet, Geophys. Res. Lett., 35, L24101, doi: /2008gl Introduction [2] The magnetosphere of Saturn exhibits a multitude of periodicities. A period near hours appears in energetic charged particles, both ions and electrons [Carbary and Krimigis, 1982; Carbary et al., 2007a], in low energy plasma [Gurnett et al., 2007], in the energetic neutral atoms [Paranicas et al., 2005; Carbary et al., 2008], and in magnetic field perturbations [Espinosa and Dougherty, 2000; Giampieri et al., 2006]. Saturn s kilometric radiation ( khz) shows an especially pronounced periodicity [Desch and Kaiser, 1981; Gurnett et al., 2005] upon which longitude systems may be based [Davies et al., 1996; Kurth et al., 2008]. However, the radio period does not remain constant and small variations of 1% have been detected over long times of several months [Galopeau and Lecacheux, 2000; Gurnett et al., 2005; Kurth et al., 2008]. [3] The source of these periodicities remains obscure. Saturn s magnetic axis is aligned within 1 of its rotation axis [Davis and Smith, 1990; Dougherty et al., 2005; Nichols et al., 2008], so periodicities at Saturn cannot arise from a significant magnetic tilt such as they do at Jupiter. Originally proposed to explain periodicities at Jupiter [e.g., Dessler and Hill, 1975], a rotating anomaly is often invoked to address Saturn s periodicities. The anomaly may result from some centrifugally-driven convective instability, which results in outflow at a particular longitude [Gurnett et al., 2007; Goldreich and Farmer, 2007], or it may be caused by a longitudinally asymmetric arrangement of 1 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. Copyright 2008 by the American Geophysical Union /08/2008GL field-aligned currents [Southwood and Kivelson, 2007]. Once the anomaly is established, it may cause a corotating or partially corotating bulge in Saturn s inner magnetosphere that propagates radially outward in the form of a spiral wave [Espinosa et al., 2003; Carbary et al., 2007c]. A wavy magnetotail can be generated by the sliding effects of the anomaly as its bulge rotates non-concentrically about Saturn [Carbary et al., 2007b]. Alternately, magnetospheric waves could be produced by solar wind pressure that alternately lifts the light side of the anomaly and drops the heavy side [Khurana et al., 2008]. The asymmetric slide model predicts that Saturn s plasma sheet would remain at a fixed angle relative to the solar wind flow, while the asymmetric lift model predicts that the plasma sheet angle would vary with the rotation period of the anomaly. [4] The Ion-Neutral CAmera (INCA) onboard the Cassini spacecraft affords a unique perspective with which to observe the plasma sheet. INCA observes energetic neutral atoms (ENA) generated by the collision of energetic ions (>10 kev) in Saturn s magnetosphere with cold neutral atoms in the neutral cloud surrounding the planet, and can provide images of the energetic plasma coming from the plasma sheet. During December 2004, Cassini moved outward along the dawn flank of Saturn s magnetosphere and afforded INCA several days of observing the plasma sheet from along the dawn side. This unique dataset has been used to measure periodicities in the ENA emissions and characterize the shape of the plasma sheet [e.g., Paranicas et al., 2005]. Here, the same data can be used to show time variations in the motion of the plasma sheet. 2. Instrument and Data Set [5] The Magnetospheric IMaging Instrument on the Cassini spacecraft consists of the Low Energy Magnetospheric Measurement System (LEMMS), the CHarge Energy Mass Spectrometer (CHEMS), and the Ion Neutral CAmera (INCA). The complete instrument is described by Krimigis et al. [2004]. [6] This paper relies on data from the INCA sensor. INCA measures energetic neutral atoms (ENA) from 7 kev to 3 MeV/nucleon with time resolution of 85 seconds to 6 minutes. The sensor uses a time-of-flight method that can separate energetic neutrals in energy and species by use of a thin-foil and fan technique. With a field of view of , INCA provides images with 64 64, 32 32, or pixels. The lower resolution images contain information on ENA energies at the cost of low spatial resolution, while the higher resolution images contain information on ENA spatial distribution at the cost of energy information. This investigation uses pixel images from the kev hydrogen channel, which represents a compromise between energy and spatial resolution. L of5

2 [7] INCA images from day 352 (December 17) through 356 (December 21) 2004 were used for this study. INCA made unique observations during this time for three reasons. First, the spacecraft was moving outbound along the dawn flank, near the equatorial plane at radial distances greater than 20 R S, and remained in a constant orientation with INCA looking toward Saturn. Second, INCA made nearly continuous observations throughout the period. The unique combination of viewing geometry and continuous measurement allowed INCA to monitor the entire Saturnian magnetosphere from one side. Third, at this time, the equatorial plane of Saturn (at noon) was inclined 23 relative to the Saturn-Sun-Orbit plane [Acton, 1996; Arridge et al., 2008]; the planet had just passed its southern hemisphere solstice. Therefore, Cassini had the opportunity to observe the plasma sheet from an edge-on perspective when the sheet was tilted at an extreme angle to the solar wind flow. The exceptional INCA images from this time have been used previously to examine ENA periodicity and plasma sheet warping [Paranicas et al., 2005]. 3. Analysis Technique [8] To improve the statistics, INCA images were first averaged into one-hour time periods for the five days in the observing interval. Because the INCA orientation and range to Saturn were essentially constant for each hour interval, the images suffered no degradation from motion of the spacecraft. After 5 5 pixel smoothing, the images were then projected onto the noon-midnight plane of the Saturn- Sun-Orbit (SSO) coordinate system and the intensities were corrected for slant viewing. Figure 1 shows a sample projected image. The +X SSO axis points toward the Sun and the X SSO axis toward the magnetotail. The spin axis of Saturn is tilted about 23 from the vertical, and the bright central region defines the plasma sheet. [9] The center of the plasma sheet can be determined by simple image processing. For each projected image, 32 contours were constructed using the CONTOUR procedure in the Interactive Data Language (IDL) software. Each contour line consists of a set of positions {X SSO,Z SSO } in SSO coordinates. The centroids of each set define the center of the contour (X C = SX SSO /N, Y C = SY SSO /N, where N = number of points in each contour set), and the range points (R = [(X SSO X C ) 2 +(Y SSO Y C ) 2 ] 1/2 ) define the distances of each point from the center. The two maxima for each set of R points determines the center of the of the plasma sheet for a particular contour. The red triangles in Figure 1 indicate the maxima from the contours. The locus of these points defines the central plasma sheet in profile as viewed from the edge-on geometry. [10] In profile, the plasma sheet center appears as a roughly straight line between approximately 20 R S and +20 R S.In Figure 1, a thick red line shows this linear fit. The tilt angle of the plasma sheet is the slope of this straight line, which is 22.9 in the example. The tilt angle is found for each hour by fitting straight lines to the contour maxima from each image between 20 R S and +20 R S. The tilt angles are then displayed as a function of time to determine possible variations in the plasma sheet orientation. [11] A JPEG animation (Animation 1) has been generated to show the temporal variation of the plasma sheet and its tilt. 1 Each frame of Animation 1 represents an hour average of the projected ENA images from kev hydrogen in the same format as Figure 1 except the contours arenot plotted. The straight white lines indicate fits to the contour maxima. Two regular variations appear in Animation 1. The first is a rotational motion of a bright spot (or blob ), which moves left to right along the central plasma sheet (see Carbary et al. [2008] for a discussion of blob motion), and the second is the tilting motion of the central plasma sheet. This paper concerns the variations in the latter. 4. Periodic Behavior and Relation to SKR [12] The top plot of Figure 2 presents the tilt angle as a function of time for days 352 to 356. The angle s signal exhibits a regular modulation with a period very close to the period of SKR radiation during this time [Kurth et al., 2008]. A tick scale in the top plot of Figure 2 emphasizes this. The angle varies from 17 to 24, with a mean uncertainty of 0.5 in the angle. The amplitude of the variation is irregular, although it may be decreasing with time as suggested by the dashed line envelope in Figure 2. [13] The period of this modulation can be confirmed by a Lomb periodogram, as shown in the bottom plot of Figure 3. The periodogram was limited to period between 5 and 15 hours, which is the region of interest for SKR periodicity. In that range, the tilt signal has only one very strong peak at 10.8 hours. The signal-to-noise ratio of the peak is 11.3 (i.e., peak value to mean value of secondary peaks). [14] A precise relationship between the plasma sheet tilt and the SKR radiation can be demonstrated. A cross correlation analysis was performed between the tilt angle and the logarithm of SKR power ( khz) summed into 1-hour time bins without correction. The correlation coefficient between the angle and the SKR power was computed for hourly time shifts between 10 hours and +10 hours. The top frame of Figure 3 indicates the results. Although the peak correlation coefficient is only r = 0.4, the probability p ffiffiffiffiffiffiffiffi that the two signals are not correlated is only p = erfc(r N=2 )= [e.g., Press et al., 1992]. The SKR power reaches a maximum at the same time that the angle reaches a maximum. That is, when the plasma sheet tilts highest on the dayside (and lowest on the nightside), SKR power is maximized. [15] Finally, the phase of the tilt angle can be examined as a function of SLS-3 longitude, as defined by Kurth et al. [2008]. The SKR longitude was computed for the noon meridian at 20 R S, where the tilt angle is measured. The bottom of Figure 3 indicates the resulting phase relation between the tilt angle and SLS-3. If the tilt angle q is fit to the function cos(8 8 o ), where 8 is the SLS longitude and 8 o is the phase, the phase is found to be 46. If the tilt is caused by a weighting of the plasma sheet, then the sheet would be lightest at this longitude and heaviest at = 226 in SLS-3 longitude. 5. Discussion [16] This analysis tacitly assumes that the intense region of ENA emission in the equatorial plane represents the 1 Animations are available in the HTML. 2of5

3 Figure 1. Sample linear fit determining the tilt of the plasma sheet. Hour averages of INCA images of neutral hydrogen (20 50 kev) were projected onto the noonmidnight plane in Saturn-Sun-Orbit coordinates (SSO). Contours of the image are shown as white lines; the range maxima of these contours appear as red triangles. The triangles within ±20 R S (dotted red lines) generally lie in a straight line and are subject to a linear fit, shown as the heavy red line. In this case, N = 36 points were used for the fit, which had a standard deviation of 0.5 R S ; the fitted line had a tilt angle of Figure 3. Comparison of the tilt angle signal with SKR signal (sum of radio power within khz band). (top) Correlation coefficient vs. offset time between tilt angle and SKR signal. (bottom) Tilt angle as function of SLS-3 longitude. The solid line represents a cosine fit to the tilt angle in SLS-3 longitude system. The dashed vertical line indicates the phase of this fit is Figure 2. (top) The slopes of linear fits are shown as a function of time for five days in The tick marks appear at intervals to show SKR period; the dashed lines indicate an envelope within which the oscillations occur. (bottom) Lomb periodogram of the tilt angle signal. The dashed vertical line shows the SKR period. plasma sheet [e.g., Mitchell et al., 2003]. This assumption is reasonable because energetic neutral atoms originate from collisions between cold neutrals and energetic ions, both of which should be found in the equatorial plane. Another assumption is that the intensity contours of a projected ENA image can capture the plasma sheet emissions. Inherent in this assumption are sufficiently adequate statistics of the ENA and a benign geometry. Hour averaging and an optimal side view at near 90 help mitigate concerns about the second assumption. Finally, the analysis has examined only a few days in 2004 when Saturn was near solstice. Not examined here, solar wind conditions may have favored the expression of tilt periodicity at this time, so that such periodicity may not always be taking place at Saturn. Nevertheless, the results show that Saturn s plasma sheet can exhibit periodicity in its tilting at least some times. [17] Although Saturn s magnetic axis is aligned to within 1 of its spin axis, the planet can apparently generate latitudinal motion of its plasma sheet in a manner similar to that of Jupiter. In the case of Jupiter, whose magnetic axis is tilted, this motion has a constant amplitude of 10. At Saturn, the peak-to-peak amplitude varies from 6 to 4 and changes throughout the observing interval. The tilting of Saturn s plasma sheet was observed for several days in late 2004 when the planet was near its southern solstice. More 3of5

4 observations are required to confirm that this motion is a regular dynamical feature of Saturn s magnetosphere, or whether it appears only under certain conditions. The extended Cassini mission may offer opportunities for these observations. [18] The periodic tilting motion of Saturn s plasma sheet reflects periodic variations observed throughout the magnetosphere including those of the magnetic fields, charged particles, both ions and electrons, and the kilometric radiation. The periodic tilting does not reflect tilting of the neutral cloud itself, which must be confined by non-electromagnetic forces to the equatorial plane of Saturn [e.g., Richardson, 1998]. [19] The tilting motion of the plasma sheet suggests that the asymmetric sliding model of magnetospheric periodicity [Carbary et al., 2007b] is inappropriate for Saturn, at least at the time the observations shown here were made. In contrast, two other models can produce a periodic tilting of the plasma sheet. A system of rotating non-axisymmetric currents could produce an apparently oscillating dipole with a tilt amplitude of [Southwood and Kivelson, 2007], while the asymmetric lift model predicts a periodic tilting of the plasma sheet with an amplitude of 8 10 [Khurana et al., 2008]. In the latter model, solar wind pressure alternately raises and lowers the plasma sheet because of a longitudinal mass asymmetry. From the bottom plot of Figure 3, the light sector should occur near SLS-3 longitude of 47 and the heavy sector should occur near 227. The asymmetric lift model further suggests that the plasma sheet tilting should be affected by the solar wind ram pressure and the seasons at Saturn. As Saturn approaches its equinox, the model predicts that the modulations should be greatly reduced. [20] On the other hand, the observations do suggest disparities with the asymmetric lift model as presently configured. Most importantly, the model indicates that the light sector should become parallel to the solar wind flow on the nightside, and the observations presented here do not indicate this. Also, the amplitude of plasma sheet oscillation is only 2 3 whereas the model suggests a larger amplitude of 8 or more. 6. Conclusions [21] Imaged by the Cassini INCA detector during late 2004, neutral hydrogen atoms (20 50 kev) trace the center of the Saturn s plasma sheet, the center of which was determined by linear fits to contour maxima. When examined as a function of time, these slopes vary between 17 and 24 with a well-defined period of hours, the same period as that of Saturn kilometric radiation (SKR). The maximum tilt angle occurs when the maximum in the SKR variation occurs. 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