Assessment of the magnetospheric contribution to the suprathermal ions in Saturn s foreshock region

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006ja012084, 2007 Assessment of the magnetospheric contribution to the suprathermal ions in Saturn s foreshock region M. F. Thomsen, 1 J. P. DiLorenzo, 1 D. J. McComas, 2 D. T. Young, 2 F. J. Crary, 2 D. Delapp, 1 D.B. Reisenfeld, 3 and N. Andre 4,5 Received 15 September 2006; revised 1 December 2006; accepted 2 February 2007; published 26 May [1] Nineteen months of Cassini Plasma Spectrometer measurements are surveyed for episodes of suprathermal ions upstream from Saturn s bow shock. A total of 45 hours of mass-resolved observations are obtained. Suprathermal ions (between 3 and 50 kev/q) in Saturn s foreshock are found to be dominantly comprised of H + and ions with m/q = 2, presumably solar wind He ++, with no detectable contribution from magnetospheric water group ions. In light of the dominant contribution of water group ions to the hot plasma of the outer magnetosphere, it thus appears that magnetospheric leakage is not a significant source of upstream ions in this energy range. One implication is that the more energetic O + ions reported recently most likely originate from direct leakage of already-energized magnetospheric particles, rather than from their upstream acceleration by bow shock-related processes. Citation: Thomsen, M. F., J. P. DiLorenzo, D. J. McComas, D. T. Young, F. J. Crary, D. Delapp, D. B. Reisenfeld, and N. Andre (2007), Assessment of the magnetospheric contribution to the suprathermal ions in Saturn s foreshock region, J. Geophys. Res., 112,, doi: /2006ja Introduction [2] The region upstream from the Earth s bow shock that is magnetically connected to the shock itself is populated by a diversity of suprathermal (few kev-10 s kev) and superthermal (>few 10 s kev) ions [e.g., Thomsen, 1985, and references therein]. Magnetic field lines that thread this foreshock region extend into the shocked plasma of the magnetosheath and potentially up against the magnetopause boundary of the magnetosphere. There has thus been a longstanding debate about the extent to which magnetospheric leakage contributes to the foreshock ion populations, as opposed to shock-related processes that accelerate them out of the incident solar wind plasma. Important evidence implicating the Earth s magnetosphere as a source of the upstream ions included an observed correlation between the appearance of the ions and the occurrence of geomagnetic activity [e.g., Mitchell and Roelof, 1983; Baker et al., 1988; Kudela et al., 1990, 1994]. However, the most convincing evidence that the Earth s magnetosphere does contribute was the observation of significant fluxes of singly ionized 1 Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 2 Instrumentation and Space Research Division, Southwest Research Institute, San Antonio, Texas, USA. 3 Department of Physics and Astronomy, University of Montana, Missoula, Montana, USA. 4 Mullard Space Science Laboratory, University College London, Holmbury-St. Mary, UK. 5 Now at Research and Scientific Support Department, European Space Agency, Noordwijk, Netherlands. Copyright 2007 by the American Geophysical Union /07/2006JA oxygen, which is absent in the solar wind but present in varying amounts within the magnetosphere [e.g., Möbius et al., 1986; Christon et al., 2000; Posner et al., 2002; Keika et al., 2004] (see also Baker et al. [1984] and Krimigis et al. [1985] for similar observations at Jupiter). These O + observations were either obtained at energies above 50 kev or were integrated over the range kev, but no compositional evidence has been reported specifically in the suprathermal range below 50 kev/e, except for studies of the He ++ /H + ratios [Fuselier and Thomsen, 1992; Fuselier et al., 1995]. One reason for this is that O + may be difficult to detect in the foreshock region because it is typically a minor ion even within the magnetosphere. [3] At Saturn, although the upstream solar wind conditions are somewhat different than at Earth, an entirely analogous bow shock and foreshock region exists, with similar questions regarding possible magnetospheric contributions to the upstream ions. As at the Earth, energetic particle observations show the presence of significant fluxes of energetic ( kev) O + ions [Krimigis et al., 2005]. One major difference with the Earth, however, is that Saturn s magnetospheric ion population is dominated by water group ions (denoted W + ) produced in the inner magnetosphere by pickup of ionized water products from the rings and satellites, especially Enceladus [e.g., Richardson, 1986; Young et al., 2005; Leisner et al., 2006]. This large W + content suggests that a more robust signature of suprathermal heavy magnetospheric ions might be present in Saturn s foreshock. [4] The Cassini spacecraft has now been in orbit around Saturn since July 2004, during which it has spent a considerable amount of time upstream from Saturn s bow shock in the noon-to-predawn sector. During these upstream 1of7

2 Figure 1. Cassini orbital coverage (in Saturn Solar Equatorial coordinates) from 26 July 2004 until 1 March The dashed line shows the average bow shock position predicted by Slavin et al. [1985], scaled by a factor of 0.7. Diamonds indicate locations where suprathermal foreshock ions were detected. intervals, the Cassini Plasma Spectrometer (CAPS) instrument observed many episodes of suprathermal foreshock ions. CAPS measurements combine E/q analysis with timeof-flight determination to identify with good resolution the mass of detected ions. This paper reports the results of CAPS measurements of the composition of suprathermal ion populations observed upstream from Saturn s bow shock during the first 19 months after Cassini s insertion into orbit around Saturn until the evolution of the orbit prevented further entries into the upstream region. We find that the contribution of W + to the foreshock suprathermal ions (3 50 kev) is very much less than the W + contribution to the outer magnetospheric plasma and is in fact probably below the limits of detectability for the measurements to date. This suggests that the energetic water group ions reported by Krimigis et al. [2005] leak directly from the magnetosphere and are not produced by energization of lower-energy particles within the shock/foreshock region. Evidence for such direct leakage at Jupiter has been reported by Krupp et al. [2002]. 2. Observations [5] Figure 1 illustrates the Cassini orbital coverage projected onto Saturn s equatorial plane from 26 July 2004 (shortly after orbital insertion) until 1 March Also shown in the figure is the average bow shock position predicted by Slavin et al. [1985], scaled by a factor of 0.7 to put it inside most of the foreshock events we have found (see below). As indicated in the figure, the apoapsis portions of the Cassini orbit gave good coverage to the dawnside foreshock region. Since the typical Parker-spiral interplanetary magnetic field direction at Saturn is nearly perpendicular to the radial, Cassini should have had many opportunities for magnetic connection to the bow shock, at a variety of angles between the field and the shock normal. [6] The CAPS instrument consists of three separate analyzers: an ion beam spectrometer (IBS), intended to measure intense beams of ions such as the solar wind and the rammed Titan ionosphere; an electron spectrometer (ELS), measuring electrons between 0.6 ev and 28 kev; and an ion mass spectrometer (IMS), which measures the mass-resolved hot-ion distribution from 1 ev to 50 kev. The fields of view of the analyzers are swept back and forth across the sky by the physical rotation of the instrument on an actuator platform. During full actuation, the fields of view cover 50% of the unit sphere. The details of the CAPS instrument design are discussed in much greater depth by Young et al. [2004]. [7] The present study uses measurements from the IMS, which consists of an electrostatic analyzer (to determine E/q, the ion energy per charge), followed by a time-of-flight chamber (to determine particle speed, which combined with E/q yields the mass per charge). On entering the time-offlight chamber, an ion passes through a thin carbon foil, generating secondary electrons that create the start signal for the time-of-flight measurement and enable determination of the direction of incidence of the original ion. This start signal can be used alone to monitor the total directional ion flux ( singles data, SNG). After passing through the foil, the original ion moves through a region containing a linearly varying electric field. Particles emerging from the foil with a positive charge are turned around in this field and detected at a stop microchannel plate, yielding a very high mass resolution measurement [Nordholt et al., 1998]. Particles emerging from the foil as neutrals or with negative charge move through the field region and are detected at another stop microchannel plate, yielding a lower-resolution determination of the mass. Since the charge-state fractionation in the foil typically favors the formation of neutral atoms, the sensitivity of the low-mass-resolution measurement ( straight-through, ST) is higher than that for the high-resolution measurement ( linear electric field, LEF). The SNG data yield an even higher sensitivity but without mass resolution. For our study we focus on ST measurements of the composition, with SNG observations used to identify intervals of suprathermal foreshock ions. We use ELS observations to confirm that the spacecraft was in the upstream solar wind. [8] Figure 2 shows an example of CAPS observations of suprathermal foreshock ions. The top shows the electron energy spectrum as a function of time for a 6-hour interval on 10 September 2005, and the bottom shows the corresponding SNG ion spectra. The narrow intense population of ions seen at 1 kev/e in the bottom of Figure 2 is the incident solar wind H + distribution, and the extension of that population up to 3 kev/e is the solar wind He ++ distribution (with the same flow speed, the alpha particles appear at an energy-per-charge of twice that of the solar wind hydrogen). The faint, hot population at energies above a few kev/e comprises the foreshock ion distribution. In addition to the particle measurements, the figure is useful in illustrating some of the viewing complications for CAPS on Cassini. The periodic modulation of the solar wind fluxes (7 min) is caused by the actuator motion sweeping the 2of7

3 Figure 2. Example of suprathermal ion event in Saturn s foreshock region. (top) Color-coded CAPS ELS electron counts per A-cycle (an A-cycle corresponds to 32 sec, during which counts are accumulated for a total of 0.5 s at each of the 64 energy levels) as a function of energy and time for a 6-hour interval on 10 September 2005 (DOY 253). Spiky counts above a few tens of ev are foreshock electrons. (bottom) Similar ion count rate spectrogram from the SNG measurements of the CAPS IMS. The fluxes vary significantly with look direction. After 2020 UT, the instrument was viewing the incident solar wind, which shows up as the intense population between 1 and 3 kev/e. Suprathermal foreshock ions can be identified as the distinct low-intensity population above 3 kev/e. field of view back and forth across the narrow beam. In addition, there is a longer-period variation caused by different spacecraft rotations during this interval. [9] Figure 3 shows time-of-flight measurements from the IMS-ST summed over the entire 6-hour interval illustrated in Figure 2. The TOF data product is a matrix of observed counts, binned in 32 E/q by 512 TOF channels (at lower telemetry rates, there may be only 256 or 128 TOF bins). Each matrix is accumulated for 256 or 512 s, depending on the telemetry mode. Thus each returned matrix represents a sum over all the viewing directions seen during that interval, as well as a sum over the eight different polar angle detectors. Because TOF data products are not returned for telemetry rates less than 1 kbps, Figure 3 represents a total integration time of only 68.3 min. The red dashed lines in Figure 3 are drawn to guide the eye toward the suprathermal regions of the matrix that are occupied by different species. The intense fluxes of solar wind H + are clearly seen at E/q 1000 and TOF channel 125. At E/q 2500 and TOF channel 175, the solar wind alpha particles can be seen. Additionally, there is a broad streak across all TOF values at the solar wind E/q. These counts are caused by random TOF coincidences between protons (or alpha particles) that may create a start signal but no stop signal and others that may create a stop without having made a start signal. These coincidences are only problematic if the fluxes are quite high. Another artifact visible in Figure 3 is the peak near TOF channel 275, which is produced by second- Figure 3. CAPS IMS counts binned by E/q and time of flight for the 6-hour interval shown in Figure 2. This matrix is compiled from the IMS TOF-ST data product, comprising TOF-identified counts from the Straight-Through (ST) MCP. The red dashed lines show the regions in E-versus- TOF space that should be occupied by different ion species [cf., Young et al., 2004]. The species regions are truncated at 3 kev/e to show the portions of the matrix over which the counts were summed to produce the suprathermal ion statistics discussed in the text. 3of7

4 Table 1. Suprathermal Foreshock Ion Events Year Day of Year Start HH:MM Stop HH:MM :00 6: :00 24: :00 24: :00 6: :00 24: :45 18: :30 21: :00 18: :00 7: :00 2: :50 21: :45 21: :00 3: :10 6: :15 18: :00 10: :15 23: :10 24: :06 21: :00 18: :00 19: :30 17: :30 18: :00 24: :10 6: :00 6: :45 12: :00 18: :00 4: :00 4: :50 18: :00 20: :00 24: :15 18: :00 24: :00 2: :00 24: :15 5: :30 12: :00 18: :00 20: :00 6: :00 8: :00 18: :00 24: :00 1: :00 12: :00 13: :30 24: :00 3:30 which upstream ions were identified. The total observation time represented by this figure is 45.2 hours. The figure shows very clearly the suprathermal populations of H + and He ++, but there is still no discernible suprathermal heavy-ion population. For comparison, Figure 4b shows a similar plot for a similar amount of observing time within Saturn s outer magnetosphere, which for the purposes of this study we define as beyond 12 Saturn radii from the planet. For this plot we again conducted a visual survey of the SNG spectrograms and identified a set of intervals in which the fluxes of ions above 3 kev were nearly as low as those in the foreshock ion events. The presence of hot W + ions is quite clear in Figure 4b, along with hot H + and a population of hot ions with m/q = 2 (while this could be He ++, as seen in the solar wind interval, it is probably dominantly H 2 +, which is seen abundantly within the inner magnetosphere). Note that there are two distinct populations identified as W + in the figure: The population at longer times of flight corresponds to water group ions that emerge from the foil as neutral atoms, whereas ary electrons created on the LEF grid by H + ions (see Young et al. [2004] for additional discussion). At energies above 3 kev, there is a very faint indication of suprathermal ions along the H + and He ++ traces, but no heavier particles can be discerned above the noise level. [10] The event shown in Figures 2 and 3 illustrates that the low fluxes of suprathermal ions in Saturn s foreshock produce few counts in the IMS, and so it is necessary to integrate the observations over many such intervals in order to build up the statistics of the measurement. For the present study, we have conducted a visual survey of SNG energy-time spectrograms such as those shown in Figure 2 to identify intervals in which upstream suprathermal ions were present. We identified a total of 50 events, which are listed in Table 1 and indicated by the open diamonds on the orbit traces in Figure 1. [11] Figure 4a shows an E/q versus TOF plot like that of Figure 2, now integrated over the complete set of intervals in Figure 4. E/q versus TOF matrix similar to Figure 3, summed over (a) the full set of suprathermal foreshock ion events and (b) a comparable observing time from within the outer region of Saturn s magnetosphere (r > 12 R s ), from intervals where the fluxes above 3 kev were low. The magnetospheric material shows the clear presence of heavy water group ions above 3 kev, whereas there is essentially no evidence for such ions in the upstream population. 4of7

5 Table 2. Summed Suprathermal Ion Counts Above 3 kev/e (Background-Corrected) Foreshock Outer Magnetosphere Total time, min H + counts ± ± 303 (m/q = 2) counts 1282 ± ± 225 W + counts 86 ± ± 96 (m/q = 2)/H ± ± W + /H ± ± the shorter times of flight correspond to water group ions that emerge from the foil as negatively charged ions and are subsequently accelerated toward the ST stop MCP within the linear electric field chamber, thus arriving earlier than similar particles that emerge from the foil as neutrals. Both populations are counted in the tally of W + ions. [12] Comparison of Figures 4a and 4b makes it clear that, while there are abundant water group ions present in Saturn s outer magnetosphere with energies above 3 kev, we find no convincing evidence for W + ions upstream from Saturn s bow shock. Not only are there very few counts within the W + region of the E-TOF spectrum in Figure 4a, but those counts are not located in the characteristic twopeak pattern seen in Figure 4b. Qualitatively, one would conclude from Figure 4a that the counts in the W + region are consistent with the background. [13] To assess quantitatively the contribution of W + ions to the foreshock suprathermal ion population, we have summed the counts from within the E/q-TOF regions indicated by red dashed lines in Figures 4a and 4b, which are drawn to encompass the bulk of the observed H +, m/q = 2, and W + ions above 3 kev. The threshold value of 3 kev is chosen to be above the energies likely to be contaminated by coincidences due to solar wind particles. Moreover, it represents a rough lower limit to the energy needed for a water group ion to escape upstream from the shock, as follows: The minimum escape energy depends on the geometry of the magnetic field, flow, and shock normal [e.g., Schwartz et al., 1983]. As noted by Schwartz et al. [1983], in order to escape from the shock, particles must have at least a speed equal to the dehoffman-teller speed, which from equation (3) of that paper is v HT ¼ V sw 1 þ cos2 q Vn cos 2 2 cos q Vn cos q VB ; ð1þ q Bn cos qbn where the angles between the shock normal (n), magnetic field direction (b), and upstream flow direction (v) are defined by Schwartz et al. [1983], and V sw is the solar wind speed. For Saturn s foreshock region, we consider two limiting cases: (1) A Parker-spiral field that is essentially normal to the solar wind flow velocity, and (2) a field that is parallel to the shock normal. Considering only the situation where n, v, and b all lie in Saturn s ecliptic plane, case 1 is equivalent to [q VB = 90, q Bn = 90 q Vn ], for which equation (1) gives E o þ > 1 2 m wvsw 2 1 sin 2 ; ð2aþ q Vn where m w is the water-group ion mass. Similarly, case 2 is equivalent to [q Bn =0, q VB = q Vn ], which yields E o þ > 1 2 m wvsw 2 sin2 q Vn ð2bþ For a typical solar wind speed of V SW = 400 km/s and for q Vn 20 35, which is appropriate to the locations indicated in Figure 1, this gives O + escape energies ranging from kev (situation 2) to kev (situation 1). [14] Table 2 summarizes quantitatively the comparison between the suprathermal (>3 kev) content of Figures 4a and 4b. Note that the summed counts listed in Table 2 are proportional to the integrated energy flux in the relevant energy range, which is related to but not the same as the density of that population. However, our primary interest is in the relative composition (i.e., W + /H + ), so the distinction is not critical for our present purposes. [15] The summed counts in Table 2 have been corrected for background, energy level by energy level, as illustrated in Figures 5a and 5b, which show cuts through the E/TOF matrices of Figures 4a and 4b at E/q = 3168 ev. For the light-ion peaks, which are clearly well above background, the background is estimated by linearly interpolating between the counts observed at the two edges of each peak. For the W + region, the count rates in the foreshock region are quite low, and the background is estimated by performing a linear fit of counts versus TOF channel for the TOF channels above those included in the W + sum, then extrapolating that linear fit back across the W + region. [16] Table 2 shows that while the total counting time and the total H + counts for the foreshock measurements significantly exceed the corresponding values for the outer magnetosphere, the total W + counts from the upstream suprathermal ion population are very much less than in the outer magnetospheric ion sample. The contribution of m/q = 2 ions to the foreshock population is also significantly less than the corresponding value in the outer magnetosphere, although the difference is not nearly as dramatic. [17] The uncertainties listed in Table 2 represent only the statistical uncertainties propagated through the summation and ratio. There are additional uncertainties associated with the background subtraction, which are more difficult to quantify. We have tried several alternative methods of estimating the background for the W + summation, and we get somewhat different numbers for W + /H +. This sensitivity suggests that the W + counts are dominated by background, as discussed qualitatively above, and we conclude that the W + /H + ratio in the suprathermal upstream ions is probably consistent with zero. [18] As mentioned above, we have included all the intervals containing identifiable suprathermal foreshock ions, with no attempt to sort them according to the type of velocity space distribution (e.g., field-aligned beam, intermediate ions, gyrating ions, or diffuse ions [c.f., Thomsen, 1985]). In light of the restricted viewing of the CAPS instrument, however, we suspect that our intervals are dominated by diffuse ion events. A more detailed examination of the correspondence between the suprathermal ions 5of7

6 Figure 5. Cuts through the E/q versus TOF matrices from Figure 4, at 3168 ev/e, for (a) the foreshock suprathermal ion regions and (b) the outer magnetosphere. The grey-shaded regions show the TOF ranges that are included in the sums for the three species of interest (H +,He ++, and W + ). The heavy lines indicate the background levels adopted for each of the summation regions at this energy level. and magnetic field directions and wave properties would be desirable but is beyond the scope of this brief note. 3. Conclusion [19] Figure 4a and Table 2 show that the suprathermal ions (between 3 and 50 kev/q) in Saturn s foreshock are dominantly comprised of H + and ions with m/q = 2, presumably solar wind He ++, with no detectable contribution from magnetospheric water group ions. In light of the dominant contribution of W + to the hot plasma of the outer magnetosphere (Figure 4b), we conclude that magnetospheric leakage is probably not a significant source of upstream ions in this energy range. Thus the more energetic O + ions reported by Krimigis et al. [2005] most likely originate from direct leakage of already-energized magnetospheric particles, rather than from their upstream acceleration by bow shock-related processes. An alternative source might be solar wind pickup of energetic neutral atoms liberated from the magnetospheric ion population by charge exchange with neutral gas [e.g., Johnson et al., 2005; Mitchell et al., 2005]. Future examination of the magnetic field orientation during upstream energetic ion events, as well as the ion angular distributions, should help assess this possibility. [20] Acknowledgments. This study was supported by the NASA Cassini program. Work at Los Alamos was conducted under the auspices of the U. S. Department of Energy. [21] Amitava Bhattacharjee thanks Stanley Cowley and Cesar Bertucci for their assistance in evaluating this paper. References Baker, D. N., R. D. Zwickl, S. M. Krimigis, J. F. Carbary, and M. H. Acuna (1984), Energetic particle transport in the upstream region of Jupiter: Voyager results, J. Geophys. Res., 89, Baker, D. N., R. D. Belian, T. A. Fritz, P. R. Higbie, S. M. Krimigis, D. G. Sibeck, and R. D. Zwickl (1988), Simultaneous energetic particle observations at geostationary orbit and in the upstream solar wind: Evidence for leakage during the magnetospheric compression event of November 1, 1984, J. Geophys. Res., 93, Christon, S. P., M. I. Desai, T. E. Eastman, G. Gloeckler, S. Kokobun, A. T. Y. Lui, R. W. McEntire, E. C. Roelof, and D. J. Williams (2000), Lowcharge-state heavy ions upstream of Earth s bow shock and sunward flux of ionospheric O +,N +, and O +2 ions: Geotail observations, Geophys. Res. Lett., 27, Fuselier, S. A., and M. F. Thomsen (1992), He ++ in field-aligned beams: ISEE results, Geophys. Res. Lett., 19, 437. Fuselier, S. A., M. F. Thomsen, F. M. Ipavich, and W. K. H. Schmidt (1995), Suprathermal He ++ in the Earth s foreshock region, J. Geophys. Res., 100, 17,107. Johnson, R. E., M. Liu, and E. C. Sittler Jr. (2005), Plasma-induced clearing and redistribution of material embedded in planetary magnetospheres, Geophys. Res. Lett., 32, L24201, doi: /2005gl Keika, K., M. Nose, S. P. Christon, and R. W. McEntire (2004), Acceleration sites of energetic ions upstream of the Earth s bow shock and in the magnetosheath: Statistical study of charge states of heavy ions, J. Geophys. Res., 109, A11104, doi: /2003ja Krimigis, S. M., R. D. Zwickl, and D. N. Baker (1985), Energetic ions upstream of Jupiter s bow shock, J. Geophys. Res., 90, Krimigis, S. M., E. T. Sarris, D. G. Mitchell, D. C. Hamilton, C. Bertucci, and M. Dougherty (2005), Spatial distribution, composition and charge state of energetic ions upstream from the Kronian magnetosphere, Eos Trans. AGU, 86(52), Fall Meet. Suppl. Abstract SH53B-08. Krupp, N., et al. (2002), Leakage of energetic particles from Jupiter s dusk magnetosphere: Dual spacecraft observations, Geophys. Res. Lett., 29(15), 1736, doi: /2001gl Kudela, K., D. G. Sibeck, R. D. Belian, S. Fischer, and V. Lutsenko (1990), Possible leakage of energetic particles from the magnetosphere into the upstream region on June 7, 1985, J. Geophys. Res., 95, 20,825. 6of7

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