Energetic Electron Distributions Near the Magnetic Equator in the Jovian Plasma Sheet and Outer Radiation Belt Using Juno Observations

Abstract

We present the average distribution of energetic electrons in Jupiter's plasma sheet and outer radiation belt near the magnetic equator during Juno's first 29 orbits. Juno observed a clear decrease of magnetic field amplitude and enhancement of energetic electron fluxes over 0.1-1,000 keV energies when traveling through the plasma sheet. In the radiation belts, Juno observed pancake-shaped electron distributions with high fluxes at ∼90° pitch angle and whistler-mode waves. Our survey indicates that the statistical electron flux at each energy tends to increase from 𝐴𝐴 𝐴𝐴 = 80 to 𝐴𝐴 𝐴𝐴 = 8 . The equatorial pitch angle distributions are isotropic or field-aligned in the plasma sheet and gradually become pancake-shaped at 𝐴𝐴 𝐴𝐴 𝐴 15 . The electron phase space density gradients at 𝐴𝐴 𝐴𝐴= 100 − 30, 000 MeV/G are relatively small at 𝐴𝐴 𝐴𝐴 𝐴 30 and become positive over 𝐴𝐴 8 < 𝑀𝑀 < 30 , suggesting the dominant role of adiabatic radial transport at higher 𝐴𝐴 𝐴𝐴 shells, and the possible loss processes at lower 𝐴𝐴 𝐴𝐴 shells.

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

“…Juno‐JEDI also confirmed the Galileo result of Tomás et al. (2004) regarding the prevalence of field‐aligned distributions near the magnetic equator in the M>∼12 region for <1 MeV electrons (Ma et al., 2021). However, previous Juno work did not investigate if these field‐aligned distributions observed near the magnetic equator are scattered beams or true beams.…”

“…Juno-JEDI discovered the prevalence and persistence of upward electron beams on field lines connected to the main auroral ovals and polar cap of Jupiter (Allegrini, Mauk, et al, 2020;Mauk et al, 2017bMauk et al, , 2017cMauk et al, , 2020. Juno-JEDI also confirmed the Galileo result of Tomás et al (2004) regarding the prevalence of field-aligned distributions near the magnetic equator in the M>∼12 region for <1 MeV electrons (Ma et al, 2021). However, previous Juno work did not investigate if these field-aligned distributions observed near the magnetic equator are scattered beams or true beams.…”

“…">MeV electron PADs exhibit a transition from stable pancake distributions at R = 9 Rj to variable and diverse PAD shapes outward of 15 Rj. A similar transition has been previously reported in the Jovian magnetosphere for keV to hundreds of keV electrons based on Galileo and Juno measurements (Tomás et al., 2004; Ma et al., 2021), and for keV to tens of MeV electrons at Saturn (e.g., Clark et al., 2014).…”

“…Garrett and Jun (2021) have shown with Galileo measurements that the long-term trend of MeV electron PSDs is a positive slope PSD radial profile, which would lead to inward radial diffusion. At lower kinetic energies, Ma et al (2021) have shown with Juno data that electron PSD profiles are almost flat at M > 15, so one cannot conclude if the net effect of radial diffusion would be inward or outward transport. However, the reported long-term trends do not preclude the existence of transient negative slope PSD profiles that can lead to transient outward radial diffusion.…”

“…At lower kinetic energies, Ma et al. (2021) have shown with Juno data that electron PSD profiles are almost flat at M > 15, so one cannot conclude if the net effect of radial diffusion would be inward or outward transport. However, the reported long‐term trends do not preclude the existence of transient negative slope PSD profiles that can lead to transient outward radial diffusion.…”