University of Colorado at Boulder University of Colorado CU Home Search A to Z Index Map
Laboratory for Atmospheric and Space Physics

Galileo EUVS

Galileo Extreme Ultraviolet Spectrometer

Recent re-analysis can be found in:
Nerney, E. G., Bagenal, F., Steffl A. J. (2017). Io Plasma Torus Ion Composition: Voyager, Galileo, Cassini, J. Geophys. Res., Jan 2017, DOI: 10.1002/2016JA023306


The Extreme Ultraviolet Spectrometer (EUVS) was mounted on the spinning part of the orbiter with the field of view perpendicular to the spin axis. The EUVS is a Voyager design objective grating spectrometer, modified to cover the wavelength range from 54 to 128 nm with wavelength resolution 3.5nm for extended sources and 1.5nm for point sources and spatial resolution of 0.87 deg x 0.17 deg. (Hord et al. 1992)


The data was provided by Wayne Prior. The data is from 1996 to 2002 and contains torus and auroral observations. The raw data, calibration curve and wavelength file are available here for download:Data Counts per SecondCalibration CurveWavelength File


The observations were analyzed using our spectral emission model that simulates a spectrum given specific plasma conditions. We used atomic cross sections and radiative decay rates from the CHIANTI database version 8.0 in simulating the emission. We then treated every discrete emission as a normalized gaussian at the fwhm of the instrument (35 Angstroms) and added up all the gaussians to create simulated spectrum.

Documentation about how we simulate a spectrum using CHIANTI is provided here: CHIANTI_Emiss_Documentation

All the necessary IDL code to simulate a spectrum, with the exception of the CHIANTI database version 8.0.7 and accompanying routines, can be found here:

The CHIANTI database version 8.0.7 and accompanying routines can be found here: CHIANTI

The Galileo EUVS data were initially in units of counts over variable integration times (mostly in 47—95 seconds range). We then summed the spectra over the spatial direction, limiting the spectral range to wavelengths short of 950 Angstroms. We subtracted the background signal, divided by the appropriate integration time, and used the calibration curve and bin sizes from Hord et al. (1992) to put the GLL EUVS data in units of Rayleighs/Angstrom. The spectrum shown is from combining the median values of the counts in each spectral bin from data obtained between DOY 171—174 in June 1996.

The quality of the Galileo UVS data does not allow detailed analysis but we have taken the ion composition and electron properties consistent with the Cassini UVIS data (Steffl et al. 2004b) and used CHIANTI 8.0 to produce a synthetic spectrum for comparison. Figure 7 shows the data plus synthetic spectrum using Cassini composition as well as a best fit to the data. The lower emission in the 833/834 A region suggests a lower abundance of oxygen during the Galileo epoch (1996) than at the time of Cassini flyby (2000). A better match is found by decreasing the amount of oxygen to 10%.


Screen Shot 2017-02-07 at 11.43.48 AM

Below is a look at the normalized and background subtracted GLL EUVS spectra from 1996 and 2000.


Available for download here is a pdf book of the GLL-EUVS observations.

PDF Book

Here is a link to the UV Torus emissions bibliography