M8: UV Observations of the Io Plasma Torus From New Horizons and Rosetta

1. Abstract During the New Horizons flyby of Jupiter in February 2007, the Alice UV spectrograph obtained numerous high-quality spectra of the Io torus. These spectra were obtained in observations of Jupiter and the Galilean satellites, in which the Io plasma torus appears as "background" emission. As New Horizons flew down the Jovian magnetotail, the Alice instrument on that spacecraft was not able to observe the Io torus due to solar elongation constraints. However, a nearly-identical Alice UVS instrument aboard the Rosetta spacecraft was able to observe the Io plasma torus and the Jovian aurora for a total of 378 hours between 27 February 2007 and 08 May 2007. Although there is no spatial information in the Rosetta Alice data (since Rosetta was near Mars, roughly 4.2 AU from Jupiter) these observations show the temporal variability of the Io torus and Jovian aurora on the timescale of days to weeks. We present spectra of the Io plasma torus obtained from both Alice instruments and show a time series of emission intensity observed by Rosetta Alice. Compared to the Cassini epoch, we find emissions from the Io torus were fainter with a relative increase in emissions from higher ionization states. The Alice UV Spectrometers Alice is a light weight, low-cost, and low-power UV Spectrometer. Two Alice spectrometers are presently in flight: R-Alice aboard Rosetta and P-Alice aboard New Horizons. Two additional “Alice” spectrometers are being built: LAMP for LRO and UVS for Juno. Mass: 3 kg (R-Alice); 4.4 kg (P-Alice) Power: 4 W (R-Alice); 4.4 W (P-Alice) Wavelength Range: 700-2050Å (R-Alice); 520-1870Å (P-Alice) Spectral Resolution: ~5Å FWHM (point source); ~10Å (Filled-slit) Dispersion: ~1.8 Å/pixel Detector: 1024x32-pixel Double Delay Line (DDL) Photocathode: <1180Å: KBr 1180-1250Å: No photocathode >1250Å: CsI Spectrum extracted from here Iogenic neutral emissions ESA’s Rosetta spacecraft flew past Mars on 2007-02-25 (DOY 056) at 01:54:13 UTC. Three days later, R-Alice began observations of Jupiter and the Io plasma torus in support of the New Horizons mission. Observations continued intermittently until 2007-05-08 (DOY128). In total, 378 hours of integration were acquired. From a distance of ~4.2 AU, both Jupiter and the Io plasma torus are effectively point sources for R-Alice, so no spatial information is available. Observed count rates are rather low, as shown below. Shown below is a schematic and picture of P-Alice. R-Alice is largely similar, except that it lacks the Solar Occultation Channel and pick-off mirror. The R-Alice Jupiter spectrum has three components: emission from the Io plasma torus, emission from the Jovian aurora, and sunlight reflected from Jupiter. Examples of these three components are labeled in the calibrated composite spectrum shown below. Subtraction of the background Lyman alpha emission from the Jovian spectrum is imperfect, so this region has been set to zero. The 10-hour running average of the total luminosity of the Io torus emission features at 834Å, 900Å and 1410Å and the Jovian auroral emission feature near 1610Å are shown below. Given the signal-to-noise of the data, no obvious temporal variations are present. Compared to the Cassini epoch, the torus emissions are nearly a factor of two fainter, although some of this may be due to calibration errors. R-Alice Observations of Jupiter Jovian Aurora Reflected sunlight Torus emissions Though not fully appreciated until the Jupiter encounter, P-Alice is quite sensitive to high-energy electrons. Generally, the Alice detector count rate is sampled at a rate of 1 Hz whenever the instrument is on. Alice count rates during the Jupiter encounter are shown in the above plot. At the start of the Jupiter encounter, Alice count rates with the aperture door closed averaged ~150 Hz. However, at DOY 56.75 (2007-02-25 18:00 UTC) at a distance of 67.3 RJ from Jupiter, the count rate suddenly increased by a factor of 50. Between DOY 57-60 the dark count rate often exceeded 15 kHz, causing the instrument to safe. The count rate exhibited spikes when New Horizons was at a System III longitude of 130° and 280°, suggesting particles in the Jovian current sheet were responsible for the increase. A comparison with data from PEPSSI (PEPSSI data have been graciously provided by R.L. McNutt, D.K. Haggerty, and the PEPSSI team) showed Alice count rates are highly correlated with the energetic electron flux, as shown in the two plots below. Many (if not most) of the Alice counts are likely due to secondary electrons. Although no spatial or spectral information about the energetic electron distribution is available from the Alice count rates, it is sampled at a higher rate than either of the nominal particles instruments, PEPSSI and SWAP. P-Alice as an Electron Detector • Conclusions • The Io plasma torus has been observed by Alice spectrometers on New Horizons and Rosetta • We find no significant evidence for a weeks-to-months long change in the composition of the torus plasma. • The overall luminosity of the Io torus may be up to a factor of two lower during the New Horizons epoch, as compared to the Cassini epoch. • Spectral modeling shows that the average charge state of the plasma has increased. • Tentatively suggests the neutral source rate was low during the New Horizons flyby • Stay tuned! While there are some deviations, as a whole, the model is a good match to the observed torus spectrum. The best-fit model torus spectrum, broken down by ion species, is shown above. Compared to the Cassini epoch, the average charge state of the torus plasma is higher (1.67 vs. 1.55 during Cassini). Together with the lower torus luminosity seen by Rosetta Alice, this may imply a lower neutral source rate. Mixing ratios (ion density/electron density) for the two epochs are shown in the table below. P-Alice Observations of the Io Torus Although Alice science operations during the New Horizons Jupiter flyby were optimized for observations of the Galilean satellites and the Jovian aurora, the large field of view of the instrument enabled observations of the Io plasma torus as well. The illustration to the left shows the Alice observing geometry for an Io observation taken on 2007-02-25. We fit the P-Alice spectrum of the Io torus with a spectral model based on the CHIANTI emissions database v5.2. The spectrum and model fit are shown below. The imaging capability of Alice combined with time-tagging of photons allows the creation of spatial- spectra maps. Shown to the left are two preliminary spectral maps showing the noon ansa of the torus at 680Å (S III) and 833Å (O II & O III). M8: UV Observations of the Io Plasma Torus From New Horizons and Rosetta J. Steffl (SwRI)*, N.J. Cunningham (SwRI), P. D. Feldman (JHU), G. R. Gladstone (SwRI), J. Wm. Parker (SwRI), K. D. Retherford (SwRI), S. A. Stern (NASA HQ) *To whom correspondence should be addressed: steffl@boulder.swri.edu