Ganymede's Ionosphere Observed by a Dual‐Frequency Radio Occultation With Juno

Abstract

The Galilean moons of Jupiter are known to have atmospheres and ionospheres, detected with both ground-based observations and spacecraft data. An oxygen-hydrogen atmosphere was discovered on Ganymede with observations by the Hubble Space Telescope (Hall et al., 1998). Ganymede is a unique object in the solar system in that it has its own intrinsic magnetic field which interacts with the Jovian magnetosphere (Kivelson et al., 1997). Within the open field line regions at higher latitudes, sputtering generates an atmosphere of molecular oxygen subject to ionization and dissociated excitation from the Jovian magnetosphere (Eviatar et al., 2001). Within closed field line regions, it is expected the atmosphere is produced by sublimation (Eviatar et al., 2001). It is thought the ionosphere is generated from the neutral atmosphere via photoionization and electron impact from the Jovian magnetosphere (Carnielli et al., 2019). Prior to Juno's encounter with Ganymede, the only direct measurements of Ganymede's ionosphere were those acquired in-situ measurements from the Galileo particle detectors and by the Galileo radio occultation experiment. Due to the flyby distance of the in-situ spacecraft measurements, radio occultation data provide valuable information about the electron densities near the surface of Ganymede.The Galileo spacecraft executed a total of eight S-band radio occultations of Ganymede throughout its mission, resulting in five non-detections, two weak detections, and one strong detection of an ionosphere (McGrath et al., 2004). To the best of our knowledge, the Galileo radio science data at Ganymede were never archived. In particular with respect to Ganymede, only occultation profiles from the G8 encounter were ever published in scientific literature. The strong ionosphere detection occurred during the Ganymede G8 egress occultation resulting in a peak electron density of ∼5,000 cm −3 near the surface (Kliore, 1998). Initially, the lack of detection was surprising, but it was hypothesized that positive detections occurred where the trailing hemisphere (where the

In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere

“…Buccino et al. (2022) reported observations from a radio occultation experiment with the Juno spacecraft that during ingress an ionosphere with peak density 2,000 ± 500 (1 − σ ) cm −3 but no statistically significant signature was detected on egress. Observations of the Ganymede aurora were also captured during this flyby (Greathouse et al., 2022), where they report the emissions originate near the surface and extend to 25–50 km in altitude.…”

Juno's Close Encounter With Ganymede—An Overview

“…The ionosphere was probed by observing Juno's radio signal as the spacecraft disappeared behind Ganymede as seen from the Earth, and electron density in the ionosphere was determined (Buccino et al., 2022 ). The UVS mapped emissions from Ganymede's aurora that mark the boundary of the open and closed field lines of Ganymede's magnetosphere.…”

A Model of Ganymede's Magnetic and Plasma Environment During the Juno PJ34 Flyby

“…Analysis of Lyman‐ α emissions observed by HST also recently confirmed the presence of an extended hydrogen corona around the moon (Alday et al., 2017; Roth et al., 2023). Electron density profiles derived from plasma wave observations and radio occultation measurements suggest that Ganymede is surrounded by an ionosphere, consisting largely of $${\mathrm{O}}_{2}^{+}$$ and $${\mathrm{H}}_{2}^{+}$$ as well as minor contributions from O + and H + (e.g., Buccino et al., 2022; Carnielli et al., 2019; Eviatar, Vasyliunas, & Gurnett, 2001; Eviatar et al., 2000). The moon also exhibits auroral emissions driven by electron excitation of atmospheric molecules.…”

“…The electron density then dropped to 5 × 10 6 m −3 as Juno exited Ganymede's magnetosphere (Kurth et al., 2022). A radio occultation experiment conducted during the ingress detected signatures of Ganymede's ionosphere, finding a peak electron density of (2 ± 0.5) × 10 9 m −3 near the surface (Buccino et al., 2022). This value is approximately half of the previously established upper limit from Galileo's radio occultation observations at Ganymede (Kliore, 1998).…”