Observations of MeV electrons in Jupiter's innermost radiation belts and polar regions by the Juno radiation monitoring investigation: Perijoves 1 and 3

Abstract

Juno's “Perijove 1” (27 August 2016) and “Perijove 3” (11 December 2016) flybys through the innermost region of Jupiter's magnetosphere (radial distances <2 Jovian radii, 1.06 RJ at closest approach) provided the first in situ look at this region's radiation environment. Juno's Radiation Monitoring Investigation collected particle counts and noise signatures from penetrating high‐energy particle impacts in images acquired by the Stellar Reference Unit and Advanced Stellar Compass star trackers, and the Jupiter Infrared Auroral Mapper infrared imager. This coordinated observation campaign sampled radiation at the inner edges of the high‐latitude lobes of the synchrotron emission region and more distant environments. Inferred omnidirectional >5 MeV and >10 MeV electron fluxes derived from these measurements provide valuable constraints for models of relativistic electron environments in the inner radiation belts. Several intense bursts of high‐energy particle counts were also observed by the Advanced Stellar Compass in polar regions outside the radiation belts.

Intervals of Intense Energetic Electron Beams Over Jupiter's Poles

“…Since JEDI can directly detect electrons to about 1.2 MeV, the discussion above suggests that electrons in the beam with energies between 1 and 15 MeV are not uncommon. This seems to agree well with the radiation monitoring system analysis of >5 and >10 MeV electrons presented in Becker et al (), see below. Our analysis does not extend to a more rigorous accounting of the likely energy spread at each time.…”

“…The band around 160 keV offers some guidance that these beams must contain high energy components. Additionally, radiation monitoring data presented by Becker et al () show a period of >10 MeV electrons that agrees well with the times of the intense beam predicted by JEDI on the inbound leg of PJ3.…”

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

“…The main difference between the MWR data and our simulations is a discrepancy in the brightness profiles (up to a factor of 3). The discrepancies between the data and simulations, combined with Juno's field and particle measurements [see, e.g., Bolton et al, 2017, Becker et al, 2017b, confirm that physical conditions close to the planet affecting synchrotron emission (electron energy spectra, pitch angle distributions, and the magnetic environment) are different than we anticipated. Model improvements and results on polarization will be the topic of a follow-up paper.…”

“…Another challenge is related to the fact that the radiation depends on the electron energy spectrum and scales in proportion to B × N (equations (1)- (4)). Large errors in our simulations can then be easily introduced from small uncertainties on B and N. Juno's first perijoves have confirmed strong discrepancies between magnetic and particle measurements and models, with differences of ∼2-3 Gauss for the magnetic field and by up to an order of magnitude for the particle observations [Bolton et al, 2017;Becker et al 2017b]. Knowing the current inaccuracy of magnetic field and electron distributions at Jupiter inside 1.5 R J and the fact that the synchrotron radiation is highly beamed, our simulations of polarized radiation are indeed difficult to both carry out and validate (which are the focus of ongoing modeling work).…”

Juno's first glimpse of Jupiter's complexity

“…This changing geometry of the orbit provides a unique opportunity to sample a range of magnetospheric regions as well as view the planet from different angles. While the radiation environment on first passes has been somewhat weaker than anticipated [ Becker et al, ], the spacecraft will begin to dip deeper into the population of >10 MeV electrons that are trapped in Jupiter's strong magnetic field. These first glimpses of Jupiter from Juno gathered in this special issue suggest that the next couple years will provide much to think about.…”