Variations in the Density Distribution of the Io Plasma Torus as Seen by Radio Occultations on Juno Perijoves 3, 6, and 8

Abstract

The atmosphere of the Jovian satellite Io is constantly being lost to the surrounding magnetosphere of Jupiter. The material is ionized and then distributed by Jupiter's magnetic field into a torus around Jupiter called the Io plasma torus. This plasma affects radio signals as they propagate from the Juno spacecraft to Earth during the spacecraft's perijove passes. During Perijoves 3, 6, and 8 we determine the total electron content in the Io plasma torus using two‐way tracking data from Juno. We find that the location of the torus is displaced from predictions that use the VIP4 offset tilted dipole approximation. The displacements are consistent with those found in ground‐based observations. The peak total electron content and scale height are found for two different regions of the torus, the cold inner torus and a warmer torus beyond 5.5 RJ. Properties of the cold torus vary appreciably with System III longitude, but properties of the torus beyond 5.5 RJ do not.

Where Is the Io Plasma Torus? A Comparison of Observations by Juno Radio Occultations to Predictions From Jovian Magnetic Field Models

“…(2018) and Phipps et al. (2019) for four Juno perijoves (PJ). These are PJ1 ( λ III = 184°, 27 August 2016), PJ3 ( λ III = 60°, 11 December 2016), PJ6 ( λ III = 213°, 19 May 2017), and PJ8 ( λ III = 10°, 1 September 2017).…”

“…(2018) and Phipps et al. (2019), the observed differential Doppler shift is integrated with respect to time to yield an initial time series of the TEC along the Juno‐Earth line of sight. This contains several contributions from sources other than the IPT, such as Earth's ionosphere and the solar wind, which should be removed from the initial time series of TEC.…”

“…The fitted locations and their uncertainties from Phipps et al. (2018, 2019) are shown in Table 2. IPT properties from later perijoves have not yet been determined.…”

“…This convention is adopted for consistency with the reference Io torus models of Phipps and Withers (2017) and Phipps et al. (2018, 2019). The JRM09 dipole parameters are stated for reference only.…”

Resolving the Latitudinal Short‐Scale Gravity Field of Jupiter Using Slepian Functions

“…The dual‐frequency link on both communication legs allows for the cancellation of up to 75% of plasma noise (50% if the uplink is single‐frequency), due, among other effects, to the crossing of the Io Plasma Torus by the radio signals around perijoves (Phipps et al., 2019). Other sources of noise are from the Earth's troposphere and ionosphere.…”