Effects of Absorption by Io on Composition of Energetic Heavy Ions

In: Science 60,393-394, 18 October 1996. (SRL Publication 96-27). T. L. GARRARD, E. C. STONE, N. MURPHY.

T. L. Garrard and E. C. Stone, Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 USA.
N. Murphy, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099 USA.

Abstract

The Galileo Heavy Ion Counter is sensitive to ions with Z>= 6 and energies above ~ 6 MeV/nucleon. The observed composition of these heavy ions is consistent with two components: singly ionized iogenic O, Na, and S and highly ionized solar C, O, and Ne. The solar component is absorbed more strongly by Io because its gyroradius is smaller than Io's diameter.

The Voyager spacecraft encounters with Jupiter found that energetic heavy ions form a major component of the trapped radiation in the inner magnetosphere (1). A wide variation in elemental composition was observed, with O and S dominating in the inner magnetosphere while the middle and outer magnetosphere showed a more solar-like composition with substantial amounts of C, O, Ne, Mg, Si and Fe.

The Galileo Heavy Ion Counter (2) provides spectral and composition information for ions with Z>=6 and with energies from ~6 to >200 MeV/nuc. The data reported were taken near the orbit of Io on 7 and 8 December 1995 (3) and confirm that iogenic energetic particles have low ionization states that substantially affect their transport through Jupiter's magnetosphere.

It has been commonly assumed that the iogenic material is singly ionized, while the solar material is observed to be highly ionized (4). Because previous observations (5) have shown that the two components have different energy spectra, we report abundance ratios for S/O and C/O over a range of energies (6). Data were analyzed in two spatial regions: outside Io's orbit, from L = 7.6 to 6 inbound to Jupiter (near Io) and inside Io's orbit, from L = 5.85 to 4.39, where L is the McIlwain magnetic drift shell parameter (7). Io was at L ~ 5.95 during Galileo's flyby, but crosses L shells between ~5.9 and ~6.9 (at various magnetic latitudes) during a 10-hour rotation of Jupiter, due to the planet's dipole tilt. The region immediately surrounding Io shows a strong absorption feature and will not be discussed here.

The counting rates at fixed energies (E) in Figure 1 increase rapidly with decreasing L for 7.6>=L>=6.9 and 5.9>=L because they respond to particles diffusing inwards at constant magnetic moment (M=E/B) as the magnetic field strength increases (B~L-3). Changes in the ratios of the count rates inside of L~5.1 indicate a flattening of the spectrum for 6.5<=M<=8.6 GeV/G. The decreased count rates from L~6.9 into L~5.8 reflect absorption by Io.

Fig. 1
Counting Rates of energetic ions versus L. The rates are dominated by oxygen stopping in three different, successively deeper, detectors in the HIC E telescope. The 60-second averaging interval is ~3-6 spacecraft spin periods, concealing the substantial spin modulation of the rates. Black crosses: 2-detector events (O energies from ~16.2 to ~17.2 MeV/nuc). Red pluses: 3-detector events (~17.2 to ~27.3 MeV/nuc); Blue circles: 4-detector events (~29 to ~55 MeV/nuc).

Fig. 2
Distribution of estimated nuclear charge, Z. Three-detector events for the entire time interval l shown in Fig. 1 are histogrammed. The light blue lines are a 12.5x magnification. Note that the minimum energy of these nuclei ranges from 14.8 MeV/nuc for C to 24.5 MeV/nuc for S.

Outside Io's orbit, the C/O abundance ratio is 0.026 ± 0.007 at 16 to 17 MeV/nuc and <=0.05 at 6.5 to 14 MeV/nuc, which are significantly smaller than the solar wind or solar energetic particle (SEP) abundance ratios of ~0.4. These small C/O ratios indicate that the iogenic component of oxygen dominates. In the energy range 31 to 42 MeV/nuc, C/O rises dramatically to 0.29 ± 0.05, indicating that the iogenic O flux decreases more rapidly with increasing energy than does the solar component.

The S/O ratio outside Io's orbit is ~1.2 ± 0.2 in the four energy bins between 8.5 and 26.24 MeV/nuc, much larger than the SEP ratio of 0.04 and consistent with predominantly iogenic S and O. At 43 to 48.5 MeV/nuc, the S/O ratio decreases to ~0.38 ± 0.2, again suggesting a rapidly falling iogenic energy spectrum.

The C/O ratio was significantly smaller inside Io's orbit than outside it (~ 0.007 ± 0.003 at 16 to 17 MeV/nuc and ~ 0.04 ± 0.01 at 31 to 42 MeV/nuc inside versus 0.026 ± 0.007 at 16 to 17 MeV/nuc outside), showing that at these energies, Io absorbs the carbon ions more readily than oxygen ions as they diffuse inward. Stronger Io absorption of the fully ionized solar C is a consequence of its gyroradius being smaller than the diameter of Io. Conversely the S/O ratio is not significantly different inside and outside Io; the weaker absorption of the iogenic O and S indicates that they have significantly larger gyroradii resulting from a lower ionization state.

References and Notes

S. M. Krimigis et. al., Science 204, 998 (1979) , S. M. Krimigis et. al., Science 206, 977 (1979), R. E. Vogt et. al., Science 204, 1003 (1979), R. E. Vogt et. al., Science 206, 984 (1979).
T. L. Garrard, N. Gehrels, E. C. Stone, Space Sci. Rev, 60, 305 (1992).
T. V. Johnson et al., this issue of Science, (1996).
A. Luhn et al., Adv. Space Research 4, 161 (1984).
see for example, N. Gehrels, E. C. Stone, J. H. Trainor, J. Geophys. Res. 86, 8906 (1981), N. Gehrels, E. C. Stone, J. Geophys. Res. 88, 5537 (1983).
For this report we determined C/O abundance ratios at 8.5-11, 11-14, 16-17 and 31-42 MeV/nuc and S/O at 8.5-11, 11-14, 14-17, 24.7-26.24 and 43-48 MeV/nuc.
The L value for a drift shell in a centered dipole magnetic field corresponds to the equatorial distance (in planetary radii) of the shell from the center of the planet. The values used here were calculated by C. Paranicas and A. F. Cheng as described in J. Geophys. Res 99, 19433 (1994).
Acknowledgements: This work was supported in part by NASA under NAS7-918.

4 September 1996; accepted 23 September, 1996.

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