Ring current at Saturn: Energetic particle pressure in Saturn s equatorial magnetosphere measured with Cassini/MIMI

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L09102, doi: /2006gl029223, 2007 Ring current at Saturn: Energetic particle pressure in Saturn s equatorial magnetosphere measured with Cassini/MIMI N. Sergis, 1 S. M. Krimigis, 1,2 D. G. Mitchell, 2 D. C. Hamilton, 3 N. Krupp, 4 B. M. Mauk, 2 E. C. Roelof, 2 and M. Dougherty 5 Received 8 January 2007; revised 20 March 2007; accepted 29 March 2007; published 3 May [1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft provides measurements of the energetic ion population within the magnetosphere of Saturn. Energetic ion directional intensities, energy spectra and ion composition, are measured by the Charge Energy Mass Spectrometer (CHEMS) over the range 3 to 236 kev per charge and by the Low Energy Magnetospheric Measurements System (LEMMS) for ions in the range < E < 18 MeV. This work reports preliminary results of partial particle pressure distributions throughout the equatorial magnetosphere and comparison with in situ measurements of the magnetic pressure provided by Cassini s magnetometer. The results cover 11 passes from late 2005 to early 2006, when the spacecraft was particularly close to the nominal magnetic equator in the range 5 < R < 20 R S and can be summarized as follows: (1) the plasma b (particle pressure/magnetic pressure) profile increases radially outward to maximum values of 1 at L >10 R S ; (2) most particle pressure is contained in the range of 10 < E < 150 kev; and (3) in the high beta region 10 < L < 19, where the apparent ring current resides, oxygen generally contributes more than 50% of the total particle pressure. The results demonstrate that typical assumptions of MHD models, whereby particle pressure is presumed to reside with the cold plasma, are not supported by the data. Citation: Sergis, N., S. M. Krimigis, D. G. Mitchell, D. C. Hamilton, N. Krupp, B. M. Mauk, E. C. Roelof, and M. Dougherty (2007), Ring current at Saturn: Energetic particle pressure in Saturn s equatorial magnetosphere measured with Cassini/MIMI, Geophys. Res. Lett., 34, L09102, doi: / 2006GL Office for Space Research and Applications, Academy of Athens, Athens, Greece. 2 Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA. 3 Department of Physics, University of Maryland at College Park, College Park, Maryland, USA. 4 Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany. 5 Space and Atmospheric Physics Group, Imperial College, London, UK. Copyright 2007 by the American Geophysical Union /07/2006GL029223$ Introduction [2] Following Saturn orbit insertion on July 1, 2004, Cassini has been providing in situ measurements of the Saturnian magnetospheric environment. In this study we use energetic particle and magnetic field measurements to investigate the particle pressure and plasma b distribution over the equatorial magnetosphere and delineate the distribution of the Saturnian ring current, inferred from Voyager 1 and 2 magnetic field measurements [Connerney et al., 1983] and confirmed from particle measurements [Krimigis et al., 1981a; Krimigis et al., 1983; Mauk et al., 1985]. A planetary plasma sheet was identified by the Low Energy Charged Particle (LECP) and plasma (PLS) instruments onboard Voyager [Krimigis et al., 1982, Bridge et al., 1981] and more recently investigated in greater detail with Cassini [Krupp et al., 2005; Sittler et al., 2006]. Despite the limited range of compositional measurements of the Voyagers instruments, it was evident that in some parts of the magnetosphere the particle pressure was comparable to the magnetic pressure and moreover, the presence of oxygen ions could play a significant role in the Saturnian plasma sheet [Krimigis et al., 1983]. [3] Extending to 20 R S as a ±5 R S thick, disk-shaped equatorial layer, the plasma sheet is characterized by comparatively high ion intensities, with maximum fluxes recorded mostly inside 10 R S [Krupp et al., 2005]. Ion and electron intensities provided by the LEMMS sensor of MIMI have been sufficient to identify and distinguish the plasma sheet from the surrounding low intensity lobe region. The overall Saturnian magnetosphere has also been studied through an MHD modeling approach, based on early Cassini measurements [Hansen et al., 2005]. [4] In this work, we map the equatorial distribution of the particle pressure, using ion and magnetic field data obtained along 11 near-equatorial orbits of Cassini. We investigate the distribution of the plasma b value over particle energies and we further focus on the relative contribution of O + to the total plasma pressure. The results reveal a clearly delineated ring current extending from 10 R S out to 19 R S with b 1 and dominated by O Observations [5] Eleven Cassini passes from day 265 of 2005 to 182 of 2006 were very close to the nominal magnetic equatorial plane of Saturn and within ±3 R S of the magnetic equator, as identified by the most recent global magnetic field model for Saturn [Khurana et al., 2004]. We have used ion intensity data, obtained by all three MIMI sensors [Krimigis et al., 2004], namely LEMMS, INCA and CHEMS, along with in situ magnetic field measurements provided by the spacecraft s magnetometer [Dunlop et al., 1999] for dipole L-values between L = 5 and L = 20. The particle energies covered, ranged between 3 and 4000 kev for protons, (combined CHEMS and LEMMS measurements) and 9 to 236 kev/e for O + plus water products and He + ions (CHEMS). High energy (>236 kev) O + ions were locally sampled using the ion mode of INCA. Both high energy O + L of6

2 Figure 1. (top) Intensity and (bottom) pressure, integrated per each of 32 MIMI/CHEMS channels, presented as color spectrograms for a representative equatorial pass (days 78 to 82 of 2006). The peak pressures extend to considerably higher energies (20 to 200 kev) compared to the maximum intensity region (<30 kev). and He + ions were found to be insignificant as pressure contributors as are the electrons and were ignored in this study. From the CHEMS sensor, only telescope 2 was utilized, as it is nearly aligned to the LEMMS telescope. For the common CHEMS and LEMMS energy range ( kev), CHEMS measurements were employed for two reasons: (1) CHEMS, unlike LEMMS kev channels, is practically free of light contamination; and (2) CHEMS can compare H + to O + pressures, as both ion species are measured separately by the same instrument. Considering that our main objective is to reveal the global particle pressure regime, rather than study in detail any individual dynamical features, a 5-min time resolution was chosen, since it offers reliable statistics for CHEMS measurements for almost the entire region of interest, while some of the dynamical characteristics of the Saturnian magnetosphere can still be observed. [6] We have used the observed intensities as representative of those perpendicular to B, even though some were at pitch angles considerably off of 90. We did so because the scanning mechanism of LEMMS failed in the first year of the mission, so that its field of view is thereafter fixed. Consequently, it samples varying pitch angles throughout an orbit, depending upon the spacecraft s orientation. We estimate that the error introduced by the method used is 30%, meaning that the particle pressure may be underestimated by that amount. The 30% estimate has been checked by evaluating pitch angle distributions when LEMMS was rotating early in the mission, when the orbit was significantly off the nominal magnetic equatorial plane. [7] Cassini s closest approaches to Saturn during the period under study were concentrated in the local afternoon sector. As a result, the spacecraft was unable to sample the sector defined by local times between 1200 hrs and 1800 hrs and L > 10. The upper proton energy limit for the data used was 4 MeV, which has proved more than adequate because the pressure contribution by energies > 4 MeV is negligible. 3. Results [8] The combined particle intensity and magnetic field data (CHEMS, LEMMS and MAG measurements) made possible the calculation of the partial particle pressure and plasma b throughout the trajectory of the spacecraft, as introduced by Krimigis [Krimigis et al., 1981b] for a more limited set of Voyager data: P i ¼ 8p 3 ZE i max E i min E i de j i ðeþ ) P part: ¼ 8p u i 3 X DE i i E i u i j i ðeþ ð1þ b part: ¼ P part: ¼ P part: P mag: B 2 ð2þ 8p 2of6

3 Figure 2. (top, middle) Partial particle pressure (in dyne/ cm 2 ) and plasma b distribution over the L-Local Time space for 11 equatorial passes of Cassini. The particle pressure was calculated for H + ions between 3 and 4000 kev and O + ions between 9 and 236 kev/e (combined MIMI/CHEMS and LEMMS data), while the magnetic pressure comes from Cassini/MAG measurements. (bottom) Partial plasma b distribution over the nominal magnetic equatorial plane. where P i is the pressure supplied by the i-energy channel, P part. and P mag. are the partial particle pressure and the magnetic pressure, respectively, E i-min and E i-max are the lower and upper limits of each i-energy channel of central energy E i (geometrical mean) and energy width DE i, while j i is the intensity and u i the velocity of either of the ion species (H + or O + in our case) in a particular i-channel. [9] The representative example presented in Figure 1 illustrates the intensity distribution (Figure 1, top) and the corresponding partial particle pressure (Figure 1, bottom), both calculated per each CHEMS energy channel, for a typical Cassini pass through the Saturnian magnetosphere and provides clear evidence of the importance of energetic (E > 50 kev) particles as a pressure contributor. It can be easily seen that a significant part of the plasma pressure is located well above 50 kev, corresponding to relatively low intensities, as expected because dp / E 1/2 jde, whereas dj = jde. [10] In Figure 2 we present the particle pressure and plasma b distribution for each pass, both in the magnetic equatorial plane and over the local time-dipole L space. It can be seen that outside about L = 11 for the dayside and L = 9 for the night sector, b approaches or exceeds unity, indicating that the particle plasma pressure dominates over the magnetic pressure. This regime seems to extend practically unchanged up to at least L = 15, where the pressure becomes more variable, while b values begin to decrease. The plasma b does not seem to have a strong dependence on local time, although the noon to dusk equatorial region has yet to be sampled by the spacecraft. [11] Figure 3 illustrates the particle pressure and b distributions over dipole L with a 5-min time resolution for all measurements that Cassini obtained while moving in the equatorial magnetosphere of Saturn for 5 < L < 20 and all sampled local times. The particle pressure appears enhanced in the region between L = 8 and L = 14, slightly decreasing and becoming scattered outside L = 15. The plasma b rapidly increases with L, reaching and often exceeding b = 1 outside approximately L = 10, but with substantial scatter at L > 15. The red line is a polynomial fit that describes the median of the distribution; the upper and lower curves are similar fits that bracket most of the observations. In light of the variability [e.g., Mauk et al., 2005], one could view the lower curve as a quiescent ring current, while the upper curve could be described as a disturbed ring current, as labeled on Figure 3. These nominal fits could be used in simulation models [e.g., Hansen et al., 2005]. [12] The CHEMS sensor of the MIMI instrument made available for the first time the separate identification of the charge state of heavier ions, among which O + is expected to be the most significant contributor to the total particle pressure. Figure 4 summarizes the distribution of the H + and O + partial pressures for the ring current region. It is apparent that, in terms of pressure, O + ions are most of the time equally important as protons; their contribution to the total pressure rapidly rises with increasing b, becoming dominant for b > 1. The overall distribution is furthermore characterized by substantial scattering, as also seen in Figure 3. [13] The results presented in Figure 5 clearly reveal that, even though the particle density drops radically above 3of6

4 L09102 SERGIS ET AL.: SATURN RING CURRENT: ENERGETIC PARTICLES L09102 Figure 3. (top) Partial particle pressure distribution with dipole L (5 < L < 20). The high particle pressure region is located between L 8 and L 14. Outside L 15 the distribution appears quite scattered. (bottom) Plasma b distribution with dipole L (5 < L < 20). Notice the radially outward increase to maximum values of b 1 for 10 < L < 15, where the nominal ring current is located. Outside L 15 b generally decreases, while its distribution also becomes much more scattered. 3 kev (decreasing by a factor of 150 between 3 and 50 kev), the corresponding differential pressure decreases only by a factor of 4. It is noteworthy that approximately 50% of the plasma pressure is supplied by 10% of the particles concentrated above E = 10 kev (see Figure 5, bottom), providing explicit evidence that energetic particles, even though less dense, play a dominant role as pressure contributors. [14] The O+ pressure distribution over the CHEMS energy channels (not shown here), suggests that a possibly important, but not large part of O+ pressure, could lie below the CHEMS energy threshold for O+ (9 kev), mostly for high b values, in the range between L = 11 and L = 14. Comparison with the Cassini Plasma Spectrometer (CAPS) measurements during Saturn Orbit Insertion (SOI) [Sittler et al., 2006] shows that the b due to particles below the CHEMS threshold (cold plasma) does not exceed b = 0.06 for L < 10, progressively decreasing to < 0.01 at L < 3. During the same interval, MIMI/CHEMS pressures exceeded those of CAPS by a factor of 10 at L 10 when b 1, with the two pressures being about equal at L 7 when b Thus, the combination of the MIMI detectors provides nearly full coverage of the particle energies important to the plasma pressure, with detailed energy and species resolution. Hence, we are able to assess the particle energy range where the main part of the plasma Figure 4. Relative O+ and H+ ion contribution to the total particle pressure for 10 < L < 15, where the most intense part of the ring current resides. Each point represents a 5-min time interval, while the highlighted data points (red) correspond to b > 1. It is evident that, in high pressure regions, O+ becomes an important and often dominant pressure contributor. 4 of 6

5 Figure 5. (top) The particle density (red) and pressure (blue) as a function of energy, calculated per each of 32 MIMI/ CHEMS energy channels, for one hour accumulation of CHEMS measurements at the specified L and b values. O + ion density and pressure were calculated for energies between 9 and 236 kev (CHEMS O + range) and extrapolated down to 3 kev assuming a constant O + to H + ratio for 3 to 9 kev, while for H + the entire CHEMS energy range (3 to 236 kev) was used. It is apparent that even though the density decreases strongly with increasing energy, the differential particle pressure changes by less than 10 over the entire energy range. (bottom) The corresponding percent fractional density and pressure for E < E i,e i being the geometrical mean of the i-chems channel. It is evident that nearly half of the total plasma pressure is above 10 kev, where only 10% of the particles are found. pressure is located, although the b calculated here should be seen nominally as a lower limit. 4. Discussion [15] The combined particle intensity (MIMI) and magnetic field (MAG) measurements on Cassini revealed the presence of a high b plasma region, extending from 9 R S out to 19 R S, with median values of b > 0.1. Actual values over this interval in L range from b as high as 10 to <0.01. We have bracketed the observations by an upper and lower envelope with average fits as given in Figure 3. Clearly, there does exist a ring current at Saturn, as inferred from Voyager 1 and 2 observations of both the magnetic field [Connerney et al., 1983] and energetic particles [Krimigis et al., 1983; Mauk et al., 1985]. The region, however, is not the neatly-modeled, symmetrical current sheet extending from 8 to15r S that fit the magnetic field data from the limited Voyager and Pioneer 11 coverage [Connerney et al., 1981]. The observations suggest that it is a highly variable region and could well be dominated by a series of injections [Mauk et al., 2005] and other dynamical features, as suggested by ENA observations [e.g., Paranicas et al., 2005; Mitchell et al., 2005]. By analogy with Earth, we suggest that the lower envelope in Figure 4 could constitute a quiet-time ring current, while the upper one could be described as disturbed-time ring current. [16] We have provided these designations and the associated fits as an aid to modelers of Saturn s magnetospheric environment and dynamics. In this context, we note that the observed particle pressures (Figure 2, top) are within the general range assumed by Hansen et al. [2005] in their MHD model of Saturn s magnetosphere. There is, however, a major inconsistency between modeling and observations that is common to virtually all MHD models. It is assumed that the principal component of the pressure resides with the cold plasma, where the density dominates. In fact, as shown in this study (cf. Figures 1 and 5), most of the pressure is supplied by the hot plasma, as is also the case at Jupiter. It is not clear to us how this experimental fact will affect the overall predictions of MHD models, but it must be clearly accounted for in future models and simulations. [17] Acknowledgments. We thank M. Kusterer (The John Hopkins University Applied Physics Laboratory, JHU/APL) for assistance with the data reduction. We are grateful to colleagues on the MIMI and MAG teams, who provided valuable comments that have improved the presentation. Work at JHU/APL was supported by NASA under contract NAS and at the Office for Space Research and Applications of the Academy of Athens by subcontract References Bridge, H. S., et al. (1981), Plasma observations near Saturn: Initial results from Voyager 1, Science, 212, Connerney, J. E. P., M. H. Acuña, and N. F. Ness (1981), Saturn s ring current and inner magnetosphere, Nature, 292, Connerney, J. E. P., M. H. Acuña, and N. F. Ness (1983), Currents in Saturn s magnetosphere, J. Geophys. Res., 88, Dunlop, M. W., M. K. Dougherty, S. Kellock, and D. J. Southwood (1999), Operation of the dual magnetometer on Cassini: Science performance, Planet. Space Sci., 47, Hansen, K. C., A. J. Ridley, G. B. Hospodarsky, N. Achilleos, M. K. Dougherty, T. I. Gombosi, and G. Tóth (2005), Global MHD simulations of Saturn s magnetosphere at the time of Cassini approach, Geophys. Res. Lett., 32, L20S06, doi: /2005gl of6

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