Plasma conditions at Europa's orbit

With attention turned to Europa as a target for exploration, we focus on the space environment in which Europa is embedded. We review remote and in situ observations of plasma properties at Europa's orbit, between Io's dense, UV-emitting plasma torus and Jupiter's dynamic plasma sheet. Where observations are limited (e.g. in plasma composition), we supplement our analysis with models of the neutral and plasma populations from Io to Europa. We evaluate variations and uncertainties in plasma properties with radial distance, latitude, longitude and time.

</ce:displayed-quote></ce:para>If we consider only the EUV channel of UVIS, the spectral emissions model concludes that O(III) is the dominant ionization state of oxygen in the Io torus. This unphysical result occurs because the model maximizes the amount of O(III) in order to minimize the model/spectrum discrepancy at 702 Å. With the inclusion of the FUV channel, there are two additional O(III) spectral lines located at 1661 and 1666 Å. These lines, first detected in the Io torus by Moos et al. (1991), place a strong constraint on the amount of O(III) present in the torus. Unfortunately, they are relatively faint and barely above the level of noise in the UVIS spectra. Therefore, the values we derive for the mixing ratio of O(III) or O(II) as a function of radial distance should more properly be thought of as an upper or lower limit on the actual value. With this caveat in mind, there is still significantly more O(III) and less O(II) compared to the Voyager model of Bagenal (1994). The [O(II)]/[O(III)] ratio, averaged over 6.2-8.8 R_J, is 3.7 - less than half the corresponding value of 8.8 from Bagenal (1994). The value of this ratio generally decreases with increasing radial distance, which is consistent with the observed increase in electron temperature.</ce:para>Note that the Bagenal (1994) oxygen composition came from Bagenal et al. (1992), which was based on the limited spectral range of the Voyager UVS observations. The average charge state of oxygen ions at 8.8 R_J is reported by Steffl et al. (2004b) to be 1.2-1.3, corresponding to the abundance ratio OII/OIII between 2.0 and 3.3. Shemansky et al. (2014), however, argue for OIII > OII. We return to this issue of the charge state of oxygen ions in Section 4.</ce:para>While emissions from the torus plasma describe the conditions near Io, they do not extend much beyond about 7-8 R_J (Herbert and Sandel, 1995; Steffl et al., 2004b; Yoshioka et al., 2014). Thus, extrapolating conditions to the orbit of Europa requires combining Io plasma torus data with models of radial transport and physical chemistry (discussed in Section 4).</ce:para>