Proton Outflow Associated With Jupiter's Auroral Processes

Abstract

The source of protons in the Jovian magnetosphere has been historically difficult to determine experimentally. The volcanic moon Io is the primary source of plasma in the magnetosphere, inputting ∼10 3 kg s −1 of heavy ions (e.g., Bagenal & Delamere, 2011). Approximately 10% of the total magnetospheric number density is attributed to protons (e.g., Dougherty et al., 2017), which are not sourced by Io. The proton influx necessary to account for this population from 5 to 30 R J (Jovian radii) has been estimated to be 2.5-13 kg s −1 (Bodisch et al., 2017). There are at least three possible sources of protons in Jupiter's magnetosphere: solar wind, icy moons, and ionospheric outflow. Solar wind leakage across the magnetopause boundary is one possible source (e.g., Delamere et al., 2015), yet it is difficult to explain how such protons would be transported to the inner magnetosphere via this mechanism. Since Jupiter is likely not undergoing standard a Dungey cycle (McComas & Bagenal, 2007), it is also difficult to attribute magnetospheric protons to the solar wind, particularly in the innermost regions. Of the icy moons, Europa sustains a neutral torus of many species including hydrogen (e.g., Smith et al., 2019), which can charge exchange with the local plasma environment, however, at much lower quantities than necessary to source the magnetospheric proton population (e.g., Bagenal & Dols, 2020). The focus of this study is to constrain the ionospheric outflow source of protons. The outflow has been previously estimated to be 35 kg s −1 (Nagy et al., 1986) integrating over the entire polar region, and 18.7-31.7 kg s −1 (Martin et al., 2020) at high latitudes between Io's M-Shell and up to and including the auroral oval, with 4.3-8.5 kg s −1 estimated to specifically outflow from the auroral oval. Both theoretical estimates are sufficient to explain the source of protons in Jupiter's magnetosphere, but the mechanisms of ionospheric outflow have not been previously observed.

The High‐Latitude Extension of Jupiter's Io Torus: Electron Densities Measured by Juno Waves

“…Parallel electric fields that cause upward‐directed ion beams and conics in Earth's auroral cavity region have been found to be correlated with density depletions and are associated with auroral acceleration processes (Persoon et al., 1988). In fact, upward proton beams and ionospheric outflow associated with Jupiter's auroral emissions have been observed by Juno (Szalay et al., 2021).…”

Water‐Group Pickup Ions From Europa‐Genic Neutrals Orbiting Jupiter

“…Therefore, we anticipate an input rate of protons to the Jovian magnetosphere from Europa to be less than 0.1 kg s −1 . This value is at least an order of magnitude less input than the 2.5–13 kg s −1 required to sustain magnetospheric proton abundances inside 30 R J (Bodisch et al., 2017 ), further pointing to Jupiter as the likely source for magnetospheric protons, where Juno observations have shown proton outflow contributes at least 1–5 kg s −1 to the magnetosphere (Szalay et al., 2021 ).…”

Closed Fluxtubes and Dispersive Proton Conics at Jupiter's Polar Cap

“…Quasi‐static electrostatic potentials associated with Jupiter's polar regions have been observed as large as 100's keV to ∼1 MeV (Clark et al., 2017 , 2020 ; Clark, Mauk, Haggerty, et al., 2017 ; Mauk et al., 2017 ). Near the main auroral emissions, quasi‐static potentials have also been observed to inject substantial quantities of protons into Jupiter's magnetosphere (Szalay et al., 2021 ), where ionospheric outflow is also required to transport ionospheric protons to these potential structures. While ionospheric outflow and some small degree of acceleration may be the source of the core ∼1 keV population, 100's keV quasistatic potentials would produce field‐aligned proton beams peaking around a characteristic energy.…”

“…Since arriving at Jupiter, Juno has directly sampled the fields and particles connected to the polar cap, while also collecting infrared and ultraviolet (UV) images of these regions. When magnetically connected equatorward of the polar caps, Juno observed diverse ion and electron populations associated with both ionospheric and Io‐genic plasma sources (Allegrini et al., 2017 ; Clark et al., 2018 ; Mauk et al., 2017 , 2018 ; Szalay et al., 2017 , 2021 ; Valek et al., 2019 ). Energetic ion conics, likely due to perpendicular heating via wave‐particle coupling, were observed just poleward of the main emissions (Clark, Mauk, Paranicas, et al., 2017 ).…”