Statistics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..501m&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Statistics
Scientific paper
Introduction The Fast Imaging Plasma Spectrometer (FIPS) component of the Energetic Particle and Plasma Spectrometer (EPPS) [1] on the MESSENGER spacecraft has provided evidence of a multi-species magnetospheric plasma at Mercury [2]. In addition, relatively low values of mass-per-charge in the compositional spectrogram are indicative of multiple ionizations of some species, in turn, a feature diagnostic of relatively high electron temperatures. While there is no means of directly assessing how closely the conditions of local thermodynamic equilibrium (LTE) apply, testing this assumption for consistency with the measured data can potentially help advance our understanding of both the composition and temperature of this plasma. Poor particle statistics (low counting rates) necessitated the summing of data over the entire magnetospheric pass to provide a statistically significant mass-per-charge spectrum. Hence, we make the ab initio assumption that time-aliasing is not an issue, i.e., the ionic composition of the magnetosphere is approximately constant along the spacecraft trajectory. Such multicomponent plasmas, with multiply charged but kinetically cold ions, are well known in Jupiter's magnetosphere, with their compositional origins at Io and their (multiple) charge states resulting from the electrons in the hot, Io plasma torus [3,4,5]. Observations FIPS provides both energy-per-charge and velocity measurements, allowing for the mass per charge of detected components to be determined. Within Mercury's magnetosphere, ions in the mass/charge range 1-56 amu/e were detected with a time resolution of 8 s and within an energy range 0.1-13.5 keV/e. An overview of the data and compositional inferences from the signals corresponding to singly ionized species have already been discussed [2]. The multiply charged ions (mass-per-charge ~12 amu/e and less) are likely the result of impact ionization by hot electrons in the magnetosphere, most likely from the plasma sheet. Both apparent diamagnetic depressions in the measured magnetic-field magnitude [7] and X-rays observed by MESSENGER's X-Ray Spectrometer are consistent with electron temperatures as high as 1 - 10 keV [8]. Cursory examination of the (time-integrated) mass-per-charge spectrum also suggests specific species with mass-per-charge values between ~4 and ~10. There are suggestive peaks at some values and absences of peaks at others. A model can be developed to analyze such higher-charge ions. For the purpose of this discussion, we assume species at the following approximate peaks: 4, 4.5, 5, 6, 6.5, 8, and 9.5. We also assume no signals at either 7 or 10.5. After a detailed discussion of this scenario, we also discuss the effects of lower and higher electron temperatures on the FIPS measurements. First-Order Analysis An attempt to match up these peaks with predicted ionization states from equilibrium is tempting, but lacking justification, given our current sparse knowledge of the system. However, observations in the Jovian system suggest that there should be a consistent effective electron temperature providing consistent ionization states across different species. Hence, by considering inferences of singly ionized species that are present, a consistency argument can be made of an effective electron temperature. On this basis, possibilities for the observations include: He+ and/or C+3 (if there is indeed carbon present) for the mass-per-charge peak of 4. The peak at ~5 could be from O+3, that at 6 from C+2 (again, if carbon is present), 6.5 from K+6, 8 from O+2 (but also potentially K+5, or Na+3). The peak at 9.5 could be K+4. Given these possibilities, the next question is whether any of these give a plausible (effective) electron temperature. The key is to consider the various ionization potentials for the corresponding precursor ions. Potassium Ions Potassium is a known constituent of Mercury's exosphere, although it was not detected during the recent MESSENGER flyby [10]. The high ionization states of K+5 (or KVI) and K+6 (KVII) would require ionization of the next lower states (KV and KVI) whose ionization potentials (IPs) are 82.7 and 99.4 eV, respectively [11]. Mass-per-charge peaks of 6 and ~5 would result from the next two higher states, whose precursors have IPs of 117.6 and 154.9 eV, respectively. Such a wide range of ionization states would require different time histories of the ions and their interaction with the impacting electrons. In addition, there is a local minimum in the mass-percharge spectrum at ~5. Oxygen Ions Oxygen is another expected neutral component of Mercury's exosphere, and the strong mass-to-charge peak at 8 is consistent with O+2 (OIII). Singly ionized oxygen ions have an IP of 35.1 eV. A peak at ~5.5 would be consistent with O+3 whose precursor ionizes at 54.9 eV. O+3 itself ionizes at 77.4 eV and would contribute to a peak at a mass-to-charge of 4. An effective temperature as high as ~120 eV (to produce K+7) would tend to produce O+4, contributing to the mass-per-charge peak at 4 but tending to remove any contribution at a maas-per-charge value of 8. Calcium Ions Calcium is present in the exosphere. At 120 eV, Ca+6 should be present (again, assuming the same time history), contributing to a mass-per-charge peak at ~6.5. This is, however, near a local minimum in the spectrum and can probably be ruled out for the same reason as N+2 (if N2 were present). The precursor of Ca+4 has an IP of ~67 eV, and that of Ca+5 an IP of 84.5 eV. Resulting mass-per-charge peaks would occur at 10 and 8, respectively, consistent with the assumptions of our scenario but with other species as well. Sodium Ions Neutral sodium is present throughout the Mercury system, and Na+ tends to dominate the ions observed. That ion has an IP of 47.3 eV, and Na+2 has an IP of 71.6. Corresponding mass-to-charges of Na+2 and Na+3 are 11.5 and 7.7, respectively. To go to the next higher ionization state with a mass-to-charge of 5.75, the effective electron temperature would have to exceed the NaIV IP of 98.9 eV. Again the mass-per-charge data may be consistent within our scenario with a value of ~11.5, marginally with 7.7, but not with 5.75. Magnesium Ions versus Carbon Ions The lack of an obvious reservoir of carbon at Mercury makes the identification of the mass-to-charge peak at 6 problematic. However, peaks around 24, 12, and 8 are consistent with Mg+, Mg+2, and Mg+3, and the peak at 6 would be consistent with Mg+4. Precursor IPs are 7.6 eV, 15.0 eV, 80.1 eV, and 109 eV. Corresponding precursor IPs for C+, C+2, and C+3 are 11.3 eV, 24.4 eV, and 47.9 eV Conclusions The first ion plasma measurements of Mercury's magnetosphere and ionized exosphere [2] have revealed many different ionized species, significantly more than found in Io's plasma torus and outdone only by the even richer ionosphere of organic-rich Titan [12]. In addition, the low mass-to-charge ratios of many of the components suggest that many species exist in multiply ionized states. Consistency arguments based upon remote, neutral, and in situ plasma observations of Mercury's exosphere and magnetosphere suggest an effective electron temperature of ~70 eV, about twice that of the hot Io plasma torus. References [1] Andrews, G. B. et al. (2007) Space Sci Rev., 131, 523. [2] Zurbuchen, T. H. et al. (2008) Science, in press. [3] McNutt, R. L., Belcher, J. W. and Bridge, H. S. (1981) J. Geophys. Res., 86, 8319. [4] McNutt, R. L., Jr. (1993) J. Geophys. Res., 98, 21221. [5] Bagenal, F. et al. (1992) Geophys. Res. Lett., 19, 79. [6] Mukai, T. et al. (2004) Adv. Space Phys, 33, 2166. [7] Anderson, B. J. et al. (2008), Science, in press. [8] Slavin, J. A. et al. (2008), Science, in press. [9] Zaghloul, M. R. (2004) Phys. Rev. E 69, 026702. [10] McClintock, W. E. et al. (2008), Science, in press. [11] Cox, A. N., ed. Allen's Astrophysical Quantities (2000). [12] Cravens, T. E. et al. (2006) Geophys. Res. Lett., 33, L07105, doi: 10.1029/2005GL025575.
Anderson Brian J.
Gloeckler George
Killen Rosemary Margaret
Koehn Patrick L.
Krimigis Stamatios M.
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