Spatially Asymmetric Increase in Hot Electron Fraction in the Io Plasma Torus During Volcanically Active Period Revealed by Observations by Hisaki/EXCEED From November 2014 to May 2015

Abstract

The satellite Io, which has volcanoes and is located at 5.9 RJ from the center of Jupiter, is a powerful plasma source in the magnetosphere. The heavy ions originating from Io form a torus‐like structure and emit radiation. The pickup energy and hot electrons are believed to power the Io plasma torus. Voyager data showed that a trace amount of hot electrons (at several hundreds of eV) exist in the torus. The origin of hot electrons, that is, plasma heating and/or transport mechanisms, have been mentioned in previous research. However, the contribution of each mechanism toward supplying hot electrons remains poorly understood. To address this issue, we explored the time variation and spatial structure of hot electrons by spectroscopic observations using the Hisaki satellite. In this study, the radial distributions of plasma densities and temperatures were derived from the emission line intensities in the extreme ultraviolet range of day of year (DOY) 331 in 2014 to DOY 134 in 2015, which includes the Io's volcanically active period. We found that hot electrons inside the torus began to increase particularly on the duskside ~40 days after the onset of volcanic activity. This result suggests that the mass increase in the torus with volcanic activity enhanced the plasma transport from the outside within a specific region or via a local heating process.

Combining UV Spectra and Physical Chemistry to Constrain the Hot Electron Fraction in the Io Plasma Torus

“…Yoshioka et al () and Tsuchiya et al () have found fractions of hot electrons on the order of a few percent inconsistent with our steady state physical chemistry model. More recent work from Yoshioka et al () and Hikida et al () have found values for the fraction of hot electrons (less than 1%) from spectral analysis at the orbit of Io more consistent with our physical chemistry modeling for steady state Io plasma torus conditions. Hikida et al () in their recent paper shows evidence for an enhancement of the fraction of hot electrons during transient times possibly due to volcanic eruptions on Io.…”

Morphology of the Io Plasma Torus From Juno Radio Occultations

“…These events were observed alongside the decrease of the rotation period of IPT and increase in thermal electron temperature. Furthermore, the increased activity observed in the early 2015 was concurrent with an increased hot electron fraction (Hikida et al., 2020), which can affect the ionization of the main elements in the IPT (Steffl et al., 2006) and potentially its electron content as a consequence. Yoshioka et al.…”

“…These events were observed alongside the decrease of the rotation period of IPT and increase in thermal electron temperature. Furthermore, the increased activity observed in the early 2015 was concurrent with an increased hot electron fraction (Hikida et al, 2020), which can affect the ionization of the main elements in the IPT (Steffl et al, 2006) and potentially its electron content as a consequence. Yoshioka et al (2018) retrieved the radial profile of the electron and ion densities that matched the spectrum observed by the EXCEED instrument, onboard Hisaki, in the same period, showing that the electron peak density at ∼6R J E can increase up to  E 20%.…”

The Space Environment of Io and Europa

“…In particular, Yoshioka et al (2018) compared the Hisaki torus emissions at the peak of the enhancement (DOY 50 2015) with quiettime (late 2013) to which they applied a physical chemistry model to derive a factor 4.2 increase in neutral source, a ~2–4 times faster radial transport rate, and a minor increase in hot electron fraction (from 0.4% to 0.7%). Hikida et al (2020) argue that the hot electron fraction was particularly enhanced on the dusk side. Yoshioka et al (2018) also found a reduction in the O/S ratio of the neutral source (from 2.4 to 1.7), consistent with the analyses of an active period in the Galileo ‐era by Nerney et al (2017) and by Herbert et al (2001).…”

“…The Japanese space agency (JAXA) put the Hisaki satellite into orbit around Earth in September 2013 (Kimura et al, 2019; Yoshikawa et al, 2014, 2016; Yoshioka et al, 2013). Hisaki 's UV spectrometer has been observing emissions from the Io plasma torus, determining composition (Hikida et al, 2018; Yoshioka et al, 2014, 2017), mapping the oxygen neutral cloud (Koga et al, 2018a, 2018b), and measuring temporal variations (Hikida et al, 2020; Kimura et al, 2018; Koga et al, 2019; Tsuchiya et al, 2015, 2018, 2019; Yoshikawa et al, 2017).…”