Analysis of Io’s far-ultraviolet emission morphology using HST STIS spectral imaging data from 1997 to present

The volcanism on Jupiter’s moon Io – generated by huge tidal forces exerted by the gas giant – is the most active in the whole solar system. It feeds a thin and short-lived atmosphere, consisting of mostly SO₂, S₂, O, SO and S. This sublimation-driven atmosphere loses around 1000 kg s^-1 to the Jovian magnetosphere. The processes that drive this atmospheric escape of neutrals and the formation of neutral clouds within the plasma torus are not fully understood yet. The observation of far ultraviolet (FUV) emissions of Io’s atmosphere and its environment provide an opportunity to study the atmospheric escape. In this study, spectral data from the Space Telescope Imaging Spectrograph (STIS) instrument of the Hubble Space Telescope (HST) are analyzed. STIS observed Io for the first time in 1997, and until today a total amount of 122 datasets with exposure time > 500s is available. This large number of datasets and the ongoing HST campaign allow long-term studies of the FUV emissions. In these observations, STIS is used with a 52 times 2 arcseconds large slit capturing the complete ~1 arcsecond wide disk of the moon. The captured photons pass a grating, such that the 25 times 25 arcseconds large detector image contains both, spatial and spectral information and consists of one dispersion and one cross-dispersion axis. Figure 1 shows such a raw dataset of an observation carried out in October 2024. The corresponding observation geometry is displayed in Figure 2, including the Jovian magnetic field, the plasma torus and incoming solar radiation. The most prominent features are the HI-1216Å-line and the OI-1304Å-line, although the photons captured here do not originate from Io. These are foreground geocoronal emissions, i.e., scattered light from H and O atoms within Earths exosphere that need to be removed from the raw data. To analyze Io’s atmosphere, reflected sunlight from the Io disk needs to be removed as well. To do so, a synthetic model of reflected sunlight is generated, using a daily solar spectral model and the point-spread-function (PSF) of STIS. To obtain the albedo, the synthetic reflected sunlight is compared to the data between 1520Å and 1640Å, where no Io-genic emissions are expected. The wavelength-dependency of the albedo is neglected. The observed emissions – in all relevant wavelengths – consists of three main features: Bright equatorial spots that vary their position with the orientation of the Jovian background field, the limb glow and emissions from Io’s extended exosphere. The analysis’ focus lies on the long-term brightness variability in neutral and sulfur ion auroral ultraviolet emissions from Io's equatorial spots and the limb glow. Furthermore, emissions from Io's extended exosphere are evaluated to figure out spatial brightness variations along the instrument's slit. Since these extended exosphere emissions are proportional to the line-of-sight (LOS) column density, a simple analytical density model of Io’s escaping atmosphere proportional to r^-2 is applied and compared to the extended emission profiles. The results are investigated regarding to correlations of neutral aurora, neutral extended emissions and ion emissions and compared to plasma torus density models and aurora models.

Figure 1: Detector raw image of an observation on 2024-10-22. Several O and S emission lines are displayed to indicate wavelengths where to expect Io-genic emissions.

Figure 2: Local geometry of STIS observing Io within the Jovian system (in scale) in LOS coordinates (y from HST towards Io, z towards Io North projected in the LOS plane). Top: 3D-image, bottom: slices along xy-, xz- (LOS), and yz-plane. STIS’ field of view is displayed in grey, the Jovian background magnetic field lines including the dipole axis and the magnetic equator in orange, the Io plasma torus in red and incoming solar radiation in yellow.