First look at Jupiter's synchrotron emission from Juno's perspective

Abstract

Since August 2016, measurements of Jupiter's microwave emissions at six wavelengths ranging from 1.3 cm to 50 cm have been made with the Juno Microwave Radiometer. In this paper, we introduce the first systematic set of in situ observations of synchrotron radiation in a polar plane while describing the modeling approach we use to analyze this data (collected 27 August 2016). Time series of brightness profiles at all six frequencies present similarities that are explained by the presence of known regions of intense synchrotron radiation. Our model predictions, though limited for now to the total intensity of the radiation, reproduce (qualitatively) the observation of temporal variations and allow to disentangle the synchrotron emission from the atmospheric emission. The discrepancies seen between the data and simulations confirm that physical conditions close to Jupiter affecting synchrotron emission (electron energy spectra, pitch angle distributions, and the magnetic environment) are different than we anticipated.

Angular Dependence and Spatial Distribution of Jupiter's Centimeter‐Wave Thermal Emission From Juno's Microwave Radiometer

“…To account for synchrotron radiation, we use a semi-empirical model developed by (Adumitroaie et al, 2016;Connerney et al, 2018;Levin et al, 2001;Santos-Costa et al, 2017) which computes the synchrotron emission produced by the motion of electrons trapped in the Jovian magnetic field, as it is experienced following a particular line of sight. Originally applied to model Earth-based observations, the model has been modified for the Juno mission to simulate the synchrotron emission as seen from an arbitrary spacecraft position in the vicinity of Jupiter and has been further enhanced to allow ingestion of externally produced electron distributions, such as those obtained through higher fidelity physics-based transport simulations (Santos-Costa & Bolton, 2008).…”

“…Indeed, the greatest source measured by channel 1 is due to synchrotron emission in the equatorial region with maximal values ranging between 4000 K and 6300 K depending on orbit, far dwarfing Jupiter's nadir thermal emission of 800–900 K. In such cases (generally when Juno is within about 30° of the equator), the effect of synchrotron emission cannot be merely screened but must be accounted for explicitly by subtracting its contribution from the measured antenna temperature. To account for synchrotron radiation, we use a semi‐empirical model developed by (Adumitroaie et al., 2016; Connerney et al., 2018; Levin et al., 2001; Santos‐Costa et al., 2017) which computes the synchrotron emission produced by the motion of electrons trapped in the Jovian magnetic field, as it is experienced following a particular line of sight. Originally applied to model Earth‐based observations, the model has been modified for the Juno mission to simulate the synchrotron emission as seen from an arbitrary spacecraft position in the vicinity of Jupiter and has been further enhanced to allow ingestion of externally produced electron distributions, such as those obtained through higher fidelity physics‐based transport simulations (Santos‐Costa & Bolton, 2008).…”

The in-situ exploration of Jupiter’s radiation belts

“…Table 1 lists Earth-based observation methods that offer context information for Jupiter's radiation belts. Synchrotron emissions, currently available also from the vantage point of Juno [146], offer global views for Jupiter's electron belts, but achieve little in terms of energy resolution and provide no data on energetic ions. Monitoring of the Io plasma torus, a product of the moon's volcanic activity, offers insights about magnetospheric flow fields which control the circulation of radiation belt particles [65,115], but only where the torus is present.…”

“…Day-night asymmetries cannot be resolved by Earth-based observations, but may be constrained by Juno (e.g. [146]), and based on future findings, priorities for local time coverage should be modified accordingly.…”

Residual Study: Testing Jupiter Atmosphere Models Against Juno MWR Observations

“…Most previous works have focused on similarly clear and dry downwelling regions with depletions of ammonia and water, where the probe entered and where spectroscopic measurements are taken (Bjoraker et al, 1986; Grassi et al, 2017). Earth‐based radio observations (e.g., de Pater et al 2016, 2019) give global coverage but require assumptions about limb darkening because viewing angle is correlated with latitude and are limited by the foreground synchrotron radiation emitted by high‐energy electrons gyrating around Jupiter's intense magnetic field (Burke & Franklin, 1955; Santos‐Costa et al, 2017). Juno's orbit takes it beneath the radiation belts, largely alleviating the limitations imposed by synchrotron emission and allowing finer spatial resolution than most Earth‐based radio observations while observing each location from multiple viewing angles.…”