Plasma convection in Saturn s outer magnetosphere determined from ions detected by the Cassini INCA experiment

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L04102, doi: /2007gl032342, 2008 Plasma convection in Saturn s outer magnetosphere determined from ions detected by the Cassini INCA experiment M. Kane, 1 D. G. Mitchell, 2 J. F. Carbary, 2 S. M. Krimigis, 2 and F. J. Crary 3 Received 13 October 2007; revised 24 December 2007; accepted 2 January 2008; published 20 February [1] The Ion and Neutral Camera (INCA), one of three sensors comprising the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft, measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere. The measured intensity spectrum and anisotropy of hot hydrogen and oxygen ions may be used to deduce the spectral parameters and the velocity of the ion population. The anisotropies are frequently convective in nature, allowing for the determination of a bulk velocity. Under the frozen in assumption, this is also the velocity of the cold plasma of magnetospheric ions. Initial analysis of selected measurements of nightside ion populations with strong anisotropies indicates nearly rigid corotation of magnetospheric plasma interior to Titan s orbit. Beyond this distance, these measurements infer that the plasma maintains at best a constant rotation velocity, falling farther behind the rigid corotation rate at increasing distance. Citation: Kane, M., D. G. Mitchell, J. F. Carbary, S. M. Krimigis, and F. J. Crary (2008), Plasma convection in Saturn s outer magnetosphere determined from ions detected by the Cassini INCA experiment, Geophys. Res. Lett., 35, L04102, doi: / 2007GL Introduction [2] Voyager 1 and 2 encountered Saturn s magnetosphere during 1980 and 1981 and measured the velocity of cold plasma using data from the Plasma Science (PLS) experiments [Richardson, 1986, 1998]. During the inbound passes, velocities were calculated from a distance of 17 R s (Voyager 1, 1 R s = 60,330 km) and 20 R s (Voyager 2) to the region just outside the orbit of Enceladus (4 R s ). In both cases, the trend was for rigid corotation within 6 R s and sub-corotation beyond, although with significant scatter in the Voyager 2 data beyond 14 R s. Generally, PLS measurements indicated magnetospheric motion was dominated by corotation within the orbit of Titan, consistent with earlier conclusions based on analysis of Pioneer data [Thomsen et al., 1980]. [3] At higher energies, ion anisotropies were investigated using the ion channels of the Low Energy Charged Particle (LECP) detector on Voyager 1 and 2 [Carbary et al., 1983]. That investigation detected differences between the typical dayside and nightside anisotropies. During the inbound encounter within the dayside magnetosphere, distributions 1 RCG, Inc., Bel Air, Maryland, USA. 2 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 3 Southwest Research Institute, San Antonio, Texas, USA. Copyright 2008 by the American Geophysical Union /08/2007GL were typically trapped and contained both first and second order anisotropies. On the nightside, however, the distributions were frequently convective, with the first order anisotropy dominant. Carbary et al. [1983] concluded from the size of first order anisotropies that the dawn side magnetosphere sampled by Voyager was in partial rotation with Saturn, fully corotational within about 20 R s (dipole L of 27 R S ) but falling behind the rigid rotation rate beyond that point. [4] This work extends the previous work of Kane et al. [1995, 1999] by applying the techniques used to quantify the anisotropies of hot ions at Jupiter in order to calculate the velocity of plasma in the magnetosphere of Saturn. The Voyager LECP detector contains a scanning telescope that stepped through 8 look directions in one 360 scan cycle in the Jovian equatorial plane. The Voyager 2 analysis of Kane et al. [1995] used this broad angular coverage of ion intensities to constrain the flow velocity calculation. In the case of the Galileo Energetic Particles Detector (EPD), which contained a particle telescope with a 15 opening, the spacecraft was spinning and the detector platform scanned perpendicular to the spin plane. The result was nearly complete sky coverage of 3 ion species but at low spatial resolution. Kane et al. [1999] derived distribution function parameters (temperature, density, velocity, spectral index) for hot ion populations in the Jovian magnetosphere from this EPD data set. EPD data have also been used extensively to characterize plasma flows in the Jovian magnetosphere [e.g., Krupp et al., 2001]. 2. Ion Anisotropies at Saturn [5] In addition to remotely sensing source distributions of neutral atoms, the INCA detector, part of the MIMI subsystem of detectors [Krimigis et al., 2004] that includes the Low Energy Magnetospheric Measurements System (LEMMS) and the Charge-Energy-Mass-Spectrometer (CHEMS) detector, is capable of acquiring ions by turning off the high voltage to its charged particle deflection plates. INCA differentiates heavy ions (C, N, O) from hydrogen via pulse height analysis and employs time of flight to measure fluxes in eight energy channels for protons (5 360 kev) and nominal oxygen ions ( kev). Based on CHEMS analysis [e.g., Krimigis et al., 2005] we know that oxygen dominates the Z 6 INCA response within the Saturnian magnetosphere. [6] When INCA is in ion mode, it produces ion flux data arranged in a array that maps to a field of view, reporting ion intensities at 256 positions in the sky for 7 energies and 2 species. At oblique angles of incidence, the energy loss of an incident ion penetrating the outer foil of the detector is more uncertain, so the ions counted at the L of5

2 Figure 1. INCA solar wind velocity calibration. During a shock in 2001, Cassini was oriented in a manner favorable for obtaining CAPS solar wind velocities. These were used to adjust the INCA response function. The response function, generated using a flat fielding approach near the Saturn bow shock, was refined so that a fit to a convected power law distribution matched the CAPS derived velocities. edges of the detector (i.e., the outer rows and columns of pixels) are not used. When the instrument (by means of spacecraft rotation) is in spin mode, we may use all columns of pixels parallel to the spacecraft spin axis. In this mode, the result is an array of averaged counts in pixels fixed in space. We are able to produce a nearly all sky map of counts during spin mode, which is ideally suited for anisotropy studies. However, the resulting array of counts in spin mode is derived from averaging counts in a row of 16 detector pixels. [7] Our methodology is similar to that developed previously by Kane et al. [1995, 1999] for Voyager LECP observations at Jupiter and in the heliosphere, and for Galileo EPD measurements within the magnetosphere of Jupiter. We assume a priori that the hot plasma distribution is either a convected power law or a convected kappa distribution, isotropic in its rest frame. Assuming initial values for the parameters that characterize the distributions (convection velocity, spectral index, temperature, and a normalization constant) we use the geometric factors of each pixel and channel efficiencies to convert the intensity of the distribution into a model array of detector counts. The geometric factors were determined by calculation and preflight calibration. The efficiencies were generated over a time period during Saturn Orbital Insertion when the spacecraft was at the nose of the magnetosphere and we assumed, based on observation, the plasma was stationary to a level of 10 s of km/sec. A flat fielding approach was used, where the array of efficiencies for each of the 7 TOF channels was adjusted to yield uniform intensities. [8] A least squares algorithm minimizes the difference between the model and the observed counts in each pixel in the middle 5 channels used (the highest channel was removed because of noise in the TOF circuit, the lowest because the hydrogen/oxygen separation is poor at the lowest energies). Avoiding edge effects, we are left with a pixel array in each of 5 channels, and subtraction of 4 corner pixels (which are under sampled because of the instrument design) yields a total of 960 points. For spin data, we are able to use the edges parallel to the spin axis but not the 2 rows adjacent to the other edges. The resulting array is for 4 sectors and 5 channels for a total of 3840 points. [9] With the instrument response function refined, we investigated a post-shock event in the solar wind after the encounter with Jupiter during The solar wind energetic ions downstream of an interplanetary shock are frequently isotropized in the solar wind frame, and are thus ideally suited to test our methods. The results are displayed in Figure 1. The Cassini Plasma Spectrometer (CAPS [Young et al., 2004]) determined cold plasma moments such as the velocity when the orientation of the spacecraft is favorable. The solid line represents solar wind velocities for the period determined by CAPS. Shock passage on DOY 227 coincided with a short period in the figure where CAPS was not in a position to measure the solar wind velocity. [10] In addition to our calibration with CAPS results, we can also compare our separate results for hydrogen and oxygen (O 16 ) ions. The two ions have different band passes and will respond differently to convection due to their mass difference. We have assumed the same geometrical factors for both species. We note that the geometrical factor array has a built-in asymmetry that is assumed to be the result of non-uniformities in the detector gain rather than due to flow during the calibration period where the factors were obtained. There may have been some plasma flow during the flat-fielding interval that produced some of the anisotropy present in this array, so that there may be a small systematic error in the results. [11] We have investigated the anisotropies in the predawn and near-midnight sectors during selected orbits (9, 16, 18, 22, 23, and 25) of Cassini in 2005 and 2006 at plasma sheet crossings. The INCA detector during this period alternated between ion mode and neutral mode. During daily periods of several hours we analyzed the anisotropies of hydrogen and oxygen ions, measured simultaneously. The time resolution was typically about 6.4 minutes per sector. One sector was acquired within the array of pixels that span the sky in a space as discussed previously. When the spacecraft is in stare mode, we analyze a single array of data for each energy channel used. When the spacecraft is in spin mode, we combine four 90 sectors to create an array that covers all the sky, except a 30 cone at each end cap. The end caps are centered on the spacecraft spin axis. During a typical day, there are periods containing both spin and stare data. The stare data may or may not be useful depending on the attitude of the detector relative to the local plasma flow direction. The spin data is of more interest as it covers a large fraction of the sky and thus will generally produce a more reliable determination of the convection velocity based on the anisotropy measurements. [12] During much of 2005 and 2006, the INCA detector operated in a mode whereby data was accumulated alternately in neutral mode (high voltage deflection plates on) and ion mode. A typical 7 day period in 2005 shown in Figure 2 displays the pattern, the red lines bounding a time period where we have performed our analysis in detail. During this 11 hour period on day 286, the spacecraft was spinning for 4 hours with the remainder in stare mode. Since we have oxygen as well as hydrogen data, we were able to perform the analysis independently for both species and compare the results. Derived velocities tracked rigid 2of5

3 Figure 2. INCA time of flight activity during 2005 day The ion rejection plate high voltage (lower panel) was periodically switched, allowing detection of neutrals when on and ions when turned off. Ion activity in ion mode is from the 7 energy discriminated channels (3 220 kev, lowest energy has highest intensity) and a >220 kev channel. corotation during both periods, with the two species deviating from one another on average by 7% and 20% during spin and stare modes respectively. During this period, the averaged plasma speed was 179 ± 27 km/sec. Gradient anisotropies, time aliasing, transient disturbances, and other anisotropies in the convection frame may all contribute to the errors encountered while performing our calculations (e.g., the discrepancy in Figure 1). Our oxygen and hydrogen calculations for all periods studied deviated on average by <20% and is indicative of our estimate of the overall accuracy of this analysis. [13] A time series of angle-angle intensity plots from 8 oxygen channels acquired during this period appear in Figure 3. In this figure, within each white square (each Figure 3. INCA time of flight channels anisotropies (oxygen). Each box (eight shown for each channel) contains a array of counts. The spacecraft is spinning during this period of hour 12, day 286 of The most intense activity is seen while the upper edge of the detector is looking in the ram direction, in the general direction of anti-corotation. Cassini was located at 18 R S, 0.25 latitude, and 05 hours local time. 3of5

4 Figure 4. Plasma convection speeds from Voyager 2 PLS (open symbols [Richardson, 1998]) and INCA anisotropies. The INCA data was obtained near dawn local time within 25 R S and near local midnight beyond 25 R S. The dashed line represents the rigid corotation rate. column of squares represents a 6.4 minute interval) is a pixel array of fluxes of oxygen ions. Since the spacecraft is spinning, a set of 4 such panels for each channel represents one complete rotational scan. The spacecraft spin axis, antisunward during this period, is vertically upward in the figure and at this local time (05 hours) looks generally into the ram direction. Thus for each energy a set of 4 squares is the intensity distribution of a part of the sky with a resolution of pixels respectively. We note that every 4 sectors the flux maximizes, when the upper edge of the detector is looking into the ram (anti-corotation) direction. At other times, the ram direction is not sampled. The centerline of the detector (center of any white square) views in a space from the ram direction. The flux modulation is strongest at lower energy, as we would expect with convection being the primary cause of the anisotropy. Anisotropies such as those shown in Figure 3 are frequently observed, so that convection is the primary cause of the ion anisotropies during such times. [14] We have selected and analyzed sample periods near dawn and near midnight local time. The most reliable results were typically found to be during periods where the anisotropy signature is strongest. These periods are represented in the plot of Figure 4. INCA derived azimuthal speeds (blue symbols) are near dawn local time within 25 R S of Saturn, while more distant analysis is from near midnight local time. We also derive a radial flow speed (in the equatorial plane). The overall average for the events studied was <10 km/sec inward, consistent with no net radial flow. A complete profile at specific local times will require more detailed analysis and will be part of a comprehensive work to follow. In this figure, we have added the results from calculations using Voyager 2 PLS inbound measurements [Richardson, 1998]. 3. Discussion [15] Previous analysis of anisotropies of magnetospheric ions based on Voyager LECP measurements used the k distribution [Vasyliunas, 1971] to fit spectra at 90 to the assumed corotation, then used the resulting spectral index to essentially calculate a velocity from the angular distribution [Carbary et al., 1983]. During the Voyager 2 outbound period, the Voyager spacecraft moved toward the dawn flank [Krimigis et al., 1983] near the local time of our measurements from Cassini during days of Our results thus far reflect the conclusions of Carbary et al. [1983] of plasma essentially in rigid corotation just within 20 R S but beyond the orbit of Titan falling significantly behind the rigid corotation rate. We note that further inward toward Saturn, the satellites Rhea and Dione appeared from Voyager analysis to cause major deviations from rigid corotation [Richardson, 1998]. From these observations models have been developed for the magnetospheric plasma [e.g., Richardson, 1998] with subsequent development to include the ion-neutral interaction and its resultant drag [Saur et al., 2004]. In addition, Mauk et al. [2005] have developed rotation curves for the inner magnetosphere based on the dispersion of hot particle injections that agree with Voyager PLS analysis [Richardson and Sittler, 1990]. Our outer region calculations agree with very recent results from the CAPS instrument [McAndrews et al., 2007]. [16] Within outer planetary magnetospheres, the k distribution function has been used to characterize hot ion observations in the magnetospheres of Jupiter [Krimigis et al., 1981], Saturn [Krimigis et al., 1983], Uranus [Mauk et al., 1987], and Neptune [Mauk et al., 1991]. The calibration and velocity calculations were similar when we used either a convected kappa distribution or a convected power law distribution. The a priori choice of the distribution function is not of critical importance in the determination of the primary parameters of interest (velocity, pressure, composition) as long as that function well describes the data. The k distribution temperatures for hydrogen and oxygen are mostly <3 kev and undifferentiated by species, unlike those at Jupiter [e.g., Kane et al., 1999]. At 18 R S, the corotation energy for oxygen ions is 3 kev, within the range of our calculated temperatures but well below the threshold of our lowest used channel (32 kev). At this distance, the oxygen temperature in our sample peaks at 3.8 kev, perhaps an indication that local pickup via charge exchange may be energizing the ion population here. Values for the spectral index (k) ranged from (H+) and (O+) with an average of 2.5 (H+) and 2.3 (O+) for all events analyzed. [17] Acknowledgments. We greatly appreciate the contribution by all working on the Cassini mission and the continuing support of NASA, without which this study would not be possible. 4of5

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