Morphology of the ultraviolet Io footprint emission and its control by Io's location

Abstract

[1] A total of 74 images of the ultraviolet footprint of the Io flux tube (IFT) on Jupiter's upper atmosphere made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have been analyzed to characterize their location, morphology, and brightness distribution. The observations cover a wide range of central meridian Jovian longitudes and Io orbital positions and include north and south footprint emissions. Comparing the location of the IFT with that expected from the VIP4 model of the Jovian magnetic field, we find that the lead angle is generally not significantly different from zero in the System III longitude sector 125°-195°. Instead, the lead angles reach about 8°in the 50°sector, coinciding with a region of possible magnetic anomaly. We observe that the brightness of the main footprint shows intrinsic intensity changes that appear to be controlled by the system III longitude of Io and its position above or below the center of the torus. The size of the primary spot magnetically maps into a region varying from 1 to over 10 Io diameters in Io's orbital plane. Multiple footprints are observed with varying brightness relative to the mean spot. The number of spots is found to increase as Io gets closer to the torus outer edge facing the spots. The separation between the first and second spots is typically 1°-3°of longitude and increases when Io is displaced from the torus center in the direction of the IFT signature. These features confirm that Alfvén waves play an important role and generate energization of precipitated electrons. However, the observed variation of the FUV spot structure with Io's position appears inconsistent with models where reflections of Alfvén wings occur between the torus boundary and Jupiter's ionosphere. Instead, the multiple spots apparently correspond to electron precipitation generated by Alfvén waves reflected inside the plasma torus.

Magnetic energy fluxes in sub-Alfvénic planet star and moon planet interactions

“…80-120 nm) is included in addition to the FUV, the conversion factor is ∼20% (Gustin et al 2012;Bonfond et al 2013). Io's main footprint displays a brightness in the FUV in the range of 5-700 kR (e.g., Clarke et al 1996Clarke et al , 1998Gérard et al 2006;Wannawichian et al 2010). The required electron input power to generate these footprint fluxes was derived to lie in the range 4-300 × 10 9 W (e.g., Prangé et al 1996;Clarke et al 1996;Gérard et al 2006).…”

“…120-180 nm) typically lie on the order of several 100 kR, where 1 kR = 10 3 Rayleigh correspond to 10 13 photons m −2 s −1 into 4π steradians (Clarke et al 2002). An emission output in the FUV of 1 kR requires an input energy flux by energetic electrons of ∼10 −4 W m −2 (e.g., Gérard et al 2006). In the FUV, an emission of 1 kR corresponds to an energy flux of ∼10 −5 W m −2 .…”

UV Io footprint leading spot: A key feature for understanding the UV Io footprint multiplicity?

“…[8] The recent ACS observations complete the partial scheme of the UV footprint morphologies shown in Figure 5 of Gérard et al [2006]. We extracted 21-pixel wide stripes from the background subtracted images and stretched them in order to display the footprint shape as a function of the longitude mapped to Io's orbital plane.…”

Solar wind effects on Jupiter non-Io DAM emissions during Ulysses distant encounter (2003–2004)

“…Jovian emissions, when observed from Earth, are dependent on the central meridian longitude (CML) of the observer and on the phase of Io (Carr et al 1983). The moon Io exerts a strong electrodynamic influence on the Jovian magnetospheric activity, including its high latitude radio emission (Bigg 1964;Goldreich & Lynden-Bell 1969;Crary & Bagenal 1997;Gerard et al 2006). As a consequence, part of this emission is controlled by the geometry of Io in relation to Jupiter and to the Earth, resulting in preferred pairs of CML-Io phase values for the occurrence of Io emissions (Carr et al 1983;Boischot et al 1987;Genova et al 1989;Queinnec & Zarka 1998).…”