Ongoing Experiments and Future Opportunities of Radio Occultations of Jupiter with the Juno Spacecraft

Abstract

Since 2023, NASA’s Juno mission has been conducting radio occultations of Jupiter’s atmosphere, ionosphere, and aurorae. Radio occultations are enabled by the natural orbital evolution of the trajectory, placing Jupiter between the Juno spacecraft and Earth. As of September 2026, data from over two dozen occultation pairs have been analyzed to determine atmospheric temperature/pressure profiles and ionosphere electron density profiles over a wide range in latitudes, including the polar regions where auroral activity is present. Measurements of the radio frequencies at X-band (8.4 GHz) and Ka-band (32 GHz) are inverted into refractivity through classical numerical ray tracing techniques. From the refractivity, atmospheric temperature/pressure and ionosphere electron density are derived. These data reveal temperature distribution of Jupiter’s atmosphere in regions never before sampled and provide unique data on the polar structure. Future radio occultation opportunities will continue to probe the northern high latitude and equatorial regions.

Background

Jupiter’s atmosphere was sampled by the Pioneer and Voyager missions in the 1970s through radio occultation and the Galileo probe. Radio occultations are key to providing direct measurement of the temperature structure and electron density measurements. Juno’s extended mission offers dozens of occultation observation opportunities due to the natural orbital evolution of the spacecraft’s trajectory. These observations are distributed over a wide range of latitudes of the planet, from the equatorial regions to the north and south poles. These data are complementary to other data including Juno’s Microwave Radiometer (MWR), plasma wave instrument (Waves), and visible & infrared imaging (from both Juno instruments and ground-based observations).

Observation

Juno Gravity Science Instrument (Asmar et. al. 2017) is a radio science instrument which utilizes dual-frequency X-band (8.4 GHz) and Ka-band (32 GHz) radio links between the Juno spacecraft and the Earth-based observing stations of NASA’s Deep Space Network (DSN). Although designed to measure gravity fields, the instrumentation is also excellent for radio occultations (Buccino et. al., 2022). As Juno passes behind Jupiter during closest approach, the received frequency is affected by refraction. The depth the occultation reaches is limited by spacecraft pointing: when the signal refracts outside the main beam of the antenna, the observation is concluded at a depth of approximately ~300 mbar. Although Juno is not capable of limb-tracking maneuvers as was done on Voyager, small offsets to the pointing enable a deeper probe depth than otherwise would be achieved (~500 mbar).

Figure 1. Map of the conducted (as of August 2026) and future experiment opportunities for radio occultation locations on Jupiter with the Juno spacecraft. Triangles indicate an ingress occultation, and circles indicate an egress occultation. Overlaid is the magnetic field model (JRM33) and the atmospheric basemap, along with the auroral ovals (white lines).

Regions of Interest

Juno’s extended mission allowed for probing of the atmosphere and ionosphere at a wide range of latitudes beginning with PJ-53 (July 2023). As the occultation season progresses, the latitude becomes more poleward allowing probing of the circumpolar cyclones. A maximum of 88 N occurred on PJ-86 (August 2026), within the polar cyclone. In the south, a maximum of -88 S was achieved on PJ-83. As occultation season reaches its end on PJ-97 (August 2027), opportunities exist for mid-latitude and equatorial regions, including a near-equatorial occultation on PJ-90 (December 2026).

Results

The atmosphere and ionosphere perturb the radio link causing measurable changes in frequency. Following the methodology in Schinder et. al. 2015, a numerical ray tracing technique is used to invert these frequency changes, along with the spacecraft’s trajectory and planetary ephemeris, into a refractivity profile. The first set of Jupiter occultations resulted in detailed temperature-pressure profiles from mid-latitude regions to the upper-latitude regions (Caruso et. al., 2025 and Smirnova et. al., 2025). These revealed cooler stratosphere and warmer troposphere at the equator. Further studies (Smirnova et. al., 2026) showed the polar stratospheric vortex is indeed cooler with a sharp temperature change at 65°N. Ionosphere electron density profiles of the same regions (Coffin et. al., 2025) show high variability in electron density layers, suggesting a complex interaction with the magnetosphere.

Acknowledgements

The work of DB, MP, RP, and SL was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged. AC, LGC, PT, and MZ are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2022-16-HH.0, No. 2023-6-HH.0, and No. 2024-5-HH.0. PS was supported by NASA Contract NNM06AA75C from the Marshall Space Flight Center under subcontract 699054X from Southwest Research Institute.

© 2026 California Institute of Technology. Government sponsorship acknowledged.

References

• Asmar, Sami W., et al. "The Juno gravity science instrument." Space Science Reviews1 (2017): 205-218.
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• Caruso, Andrea, et al. "Probing Jupiter's atmosphere through Juno Radio occultations: Methodology and initial observations." Geophysical Research Letters22 (2025): e2024GL113231.
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• Smirnova, Maria, et al. "Juno radio occultations reveal the structure of Jupiter's cold northern polar vortex." Astronomy & Astrophysics706 (2026): A109.
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