Planetary and Space Science 192 (2020) 105058 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Variability of Io’s poynting flux: A parameter study using MHD simulations A. Blocker€ a,*, L. Roth a, N. Ivchenko a, V. Hue b a KTH Royal Institute of Technology, Space and Plasma Physics, School of Electrical Engineering, Stockholm, Sweden b Southwest Research Institute, San Antonio, TX, United States ARTICLE INFO ABSTRACT Keywords: Io’s plasma interaction creates an electromagnetic coupling between Io and Jupiter through Alfven waves trig- Io gering the generation of auroral footprints in Jupiter’s southern and northern hemispheres. The brightness of Io’s Sub-alfvenic plasma interaction footprints undergoes periodic variations that are primarily modulated by Io’s local plasma interaction through the Io footprint ∘ ∘ Poynting flux radiated away from the moon. The periodic pattern with two maxima near 110 and 290 Jovian MHD simulations fl longitude where Io crosses the dense plasma sheet is generally understood. However, some characteristics, like Poynting ux ∘ the 2–4 times stronger brightening of the southern footprint near Jovian longitude 110 or the lack of response to Io’s eclipse passage, are not fully understood. We systematically study variations in Io’s plasma interaction and the Poynting flux using a 3D magnetohydrodynamic model, performing a series of simulations with different upstream plasma conditions and models of Io’s atmosphere. Our results indicate that the strong Jovian magnetic ∘ field near 110 plays a more important role than previously estimated for the strong brightening there. We find that the Poynting flux is not fully saturated for a wide range of possible atmospheric densities (6 Â 1018–6 Â 1021 À m 2) and that density changes in the atmosphere by a factor of > 3, as possibly happening during Io’s eclipse passage, lead to a change of the Poynting flux by > 20%. Assuming that these expected changes in Poynting flux also apply to the footprints, the non-detection of a dimming in the footprint during the eclipse by Juno-UVS suggests that Io’s global atmospheric density decreases by a factor of < 2.5. We show that for smaller atmo- spheric scale heights (i.e. a more confined atmosphere), changes in the atmospheric density have less effect on the Poynting flux. The missing response of the footprint to the eclipse hence might also be consistent with a density decrease by a factor of > 3, if the effective atmospheric scale height is small (< 120 km). Finally, we provide new analytical approximations that can be used for analyzing the effect of the local interaction responsible for the footprint variability in future studies. 1. Introduction and collisions within Io’s atmosphere modify the plasma environment and drive large currents through the moon’s ionosphere which are Jupiter’s moon Io is embedded in a sub-Alfvenic flow of Jupiter’s coupled to Jupiter’s upper atmosphere by Alfven wing currents. In the far corotating magnetospheric plasma, which constantly overtakes the moon field, Alfven wing currents cause acceleration and precipitation of elec- and interacts with Io’s atmosphere. From this interaction, different wave trons in Jupiter’s ionosphere, leaving imprints in Jupiter’s southern and modes are excited with the Alfven mode being an especially important northern hemisphere in the form of auroral footprints (see reviews by mode as it can transport momentum and energy along the background Kivelson et al., 2004; Saur et al., 2004; Clarke et al., 2004). magnetic field over large distances. The Alfven waves travel along the The Io footprint was the first detected satellite footprint identified by fi þ magnetic eld lines north and south from Io. During their propagation, its H3 emissions in the infrared (IR) by Connerney et al. (1993) and then they experience filamentation and partial reflection at the density gra- confirmed in the Far ultraviolet (FUV) with HST observations (Prange dients between different plasma regions in the torus (e.g., Wright and et al., 1996; Clarke et al., 1996). Jupiter’s UV aurora is the result of in- Schwartz, 1989; Chust et al., 2005; Jacobsen et al., 2007; Hess et al., elastic collisions of magnetospheric energetic electrons with atmospheric fi 2010). In the reference frame xed with Io, the Alfven waves form the molecular hydrogen (H2, H, and Lyman-α) whereas the IR aurora is so-called Alfven wings which are tilted towards the downstream direc- þ mostly due to thermal emissions from the H3 molecular ion that originate tion of the plasma (e.g., Drell et al., 1965; Neubauer, 1980). Ionization from higher altitudes (1000 km) compared to the UV (250 km) (Radioti * Corresponding author. E-mail address: [email protected] (A. Blocker).€ https://doi.org/10.1016/j.pss.2020.105058 Received 11 March 2020; Received in revised form 26 June 2020; Accepted 20 July 2020 Available online 8 August 2020 0032-0633/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). A. Blocker€ et al. Planetary and Space Science 192 (2020) 105058 et al., 2013). Radioti et al. (2013) present a comparative study of the UV from 1997 to 2009 with the HST. The brightness of the spots follows a and IR images of Jupiter’s auroral emissions obtained from HST and high quasi-sinusoidal modulation with an amplitude of about Æ30% of the performance ground-based telescope on one rare occurrence. They average and strongly varies over short timescales (2–4 min). For the showed that the Io footprint and tail are co-located in UV and IR, but the derived maximum vertical brightness, they calculate precipitating elec- variation of the footprint brightness compared to the main emission is tron energy fluxes between 250 and 2000 mW/m2 for the MAW spot. The significantly larger in the UV than in IR. Both UV and IR images show that ratio between the precipitated and the emitted power lies around 20%. the footprint consists of several individual spots and a fainter tail which Moreover, Bonfond et al. (2013) found an asymmetry between the are associated with the direct Io-Jupiter Alfven wing coupling, as well as northern and the southern spots’ brightness. Emissions of the spots in the reflection of traveling Alfven waves between Io and the Jovian atmo- southern hemisphere are on average twice as bright as in the northern sphere (e.g, Connerney et al., 1993; Connerney and Satoh, 2000; Clarke hemisphere. This asymmetry is still under investigation. The footprint et al., 2002; Gerard et al., 2006; Mura et al., 2018). Investigations of the emitted power and brightness peak near the plasma sheet crossing, i.e. ∘ ∘ structure of the Io footprint suggest three Io footprint spots with an System-3 longitude (λIII ) 110 and 290 , where Io’s orbit intersects the extended downstream tail (Clarke et al., 2002; Gerard et al., 2006; plasma torus plane. While Io is at the same centrifugal latitude, i.e. Bonfond et al., 2008). Substructures in the Io footprint similar in similarly in the center of the plasma torus, the brightness of the southern ∘ appearance to a von Karman vortex street are seen in high-resolution IR spot at λIII 110 is much larger than at 290 . Bonfond et al. (2013) images obtained with the Juno spacecraft (Mura et al., 2018). The conclude that the centrifugal latitude of Io is not the only parameter brightest spot is called the Main-Alfven-Wing (MAW) spot and is gener- controlling the spots’ brightness, but that other processes such as the ated directly by the main Alfven wing. It coincides with the location modulations of the power transmission along the Alfven wing, of the where the moon’s Alfven wing intersects Jupiter’s upper atmosphere. power transfer to the precipitating electron, or the size of the loss cone Some of the Alfven waves are reflected at the torus boundary where a also play a major role. Hess et al. (2013) developed a model for the power latitudinal density gradient exists. The reflected waves which escape the transfer between the local plasma interaction at Io and the UV emissions torus create the reflected Alfven wing spots (RAW). The reflection ge- taking into account the acceleration mechanism and the Alfven wave ometry strongly depends on the upstream plasma density at Io and thus propagation effects. With their model, they can explain the average on Io’s location in the torus (Bonfond et al., 2008, e.g.). The trans- brightness of the spots. But the model does not reproduce the peak of ∘ hemispheric electron beam spots (TEB) are generated by electrons brightness for Io’s λIII longitudes close to 110 . They conclude that Alfven accelerated away from Jupiter in one hemisphere and precipitating in the wave reflections, magnetic mirroring of the electrons, local plasma opposite hemisphere (e.g., Bonfond, 2012). The electron beams propa- interaction at Io, and kinetic effects close to Jupiter act together and gate from one hemisphere to the opposite without being affected by the determine the brightness of the spots but they are not able to explain all density gradient at the torus boundary (Bonfond et al., 2008). The rela- the details of the variations of the footprint brightness with λIII longitude. tive distance between the three spots systematically changes with respect Io’s plasma interaction is strongly controlled by Io’s atmosphere. The to the λIII longitude of Io. The brightness ratios between the spots global atmosphere is persistently maintained by strong volcanic outgas- significantly vary and therefore relate the generation of the spots to sing and sublimation of SO2 frost and is known to undergo seasonal different processes (Bonfond et al., 2013).