A novel method to remotely analyse Jupiter’s ionospheric flows

Ground-based telescope observations of H₃⁺ are critical because they are currently the only way to remotely measure the ionospheric flows at the giant planets. These observations at Jupiter are critical for supporting space missions, such as Juno, because the IR instruments, such as JIRAM, lack the spectral resolution to measure the Doppler shift of the H₃⁺ spectra, from which the line-of-sight velocity can be derived and the ionospheric flows inferred. Furthermore, spacecraft can only provide information along the orbital path and swathes of observations of the aurora, however, ground-based observations can provide a global view of the ionospheric flows and aurora.

Past studies, using IR spectroscopic data, have identified several flows in Jupiter’s auroral regions. Rego et al. (1999), Stallard et al. (2001), and Johnson et al. (2017) have all observed sub-corotating H₃⁺ velocities the region of Jupiter’s main auroral emission, in line with corotation breakdown theory (e.g.: Hill, 2001, and Cowley and Bunce, 2001). Stallard et al. (2001) and Johnson et al. (2017) identified a stationary region in the magnetic pole reference frame situated in Jupiter’s polar aurora, which suggests a coupling to the solar wind. It is unknown whether the coupling is through a Dungey-like single-cell open field and return flow (Cowley et al., 2003) or Kelvin Helmholtz instabilities in viscous flow interactions on the dawn flank (Delamere and Bagenal., 2010). Wang et al. (2023) used simultaneous observations of H₂ and H₃⁺ IR emission to reveal the dynamics of the thermosphere and measure the effective ion drift for the first time. The effective ion drift was calculated from the relative velocity of the H₂ and H₃⁺ and showed two asymmetric ionospheric jets in Jupiter’s northern aurora. This implies a current system that is in line with Juno findings, which is that the main auroral emission is linked to both upward and downward currents (Mauk et al., 2020).

Although the ionospheric currents have been inferred from all these studies, to rigorously map the ionospheric flows, the true velocity vector is required, allowing us to move away from schematics and visualise the actual direction of the ionospheric flows. We have developed a novel analysis method using vector decomposition to derive the true velocity vector from the H₃⁺ line-of-sight velocity component. We used the VLT-CRIRES data taken on 31 December 2012 (Johnson et al., 2017), because this is the highest spatial resolution and spectral resolution data available. This dataset contains six complete scans of Jupiter’s northern auroral region, each with a different viewing angle, owing to Jupiter’s rotation over the night. By utilising the overlapping fields of view, we perform a vector decomposition analysis to derive the true velocity vector. The resulting map shows, for the first time, the true velocity vector of the H₃⁺ ions in Jupiter’s northern auroral region, and hence the direction of the ionospheric flows. These preliminary results not only act as a proof of concept but will provide new insight into the ionospheric flows and current systems in Jupiter’s northern auroral region.