Auroral Acceleration Generates Electron Beams in Jupiter's Middle Magnetosphere

Abstract

Observations made by the Juno spacecraft above Jupiter's polar regions have revealed that electrons accelerated toward Jupiter, which contribute to auroral emissions, are frequently accompanied by electrons accelerated away from Jupiter. These electrons should be observable as narrow electron beams in the middle magnetosphere, in accordance with the principles of adiabatic particle motion. The existence of such beams has been previously reported using data from the Galileo mission, and their relation to auroral processes has been hypothesized. In the present study, we analyze electrons measured by Juno's JEDI instrument in the middle magnetosphere between 13 RJ and 50.5 RJ radial distance and within energies of 30-1,200 keV. The pitch angle distributions of potential electron beams are fitted with an intensity 'beamness' function. The presence of narrow beams is demonstrated throughout the observation range. The energy fluxes of auroral and equatorial electron beams are compared by including pitch angle scattering processes along the magnetospheric field lines. This is achieved by solving the pitch angle diffusion equation for different sets of diffusion coefficients. The statistical occurrence distribution and the energy fluxes of the beams are consistent with auroral upward accelerated electrons observed in studies of the polar space environment. This finding provides further support for the hypothesis that the electron beams observed in the middle magnetosphere originate from the auroral acceleration region.

Figures (10)

• Figure 1: A schematic of the Jovian magnetosphere. The field-aligned electrons measured above the main aurora are represented by blue arrows based on Figure 9 from mauk2020. They describe upward, bidirectional and downward accelerated electrons, organized according to the results from salveter2022. The bidirectional electron beams measured in the middle magnetosphere are shown as purple arrows.
• Figure 2: Trajectory of Juno orbits 8 and 30 in the $\rho$-$z$ plane. The coordinate system of the trajectory is described by a tilt of the System III axes by the VIP4 connerney1998 dipole tilt of $9.5\degree$ towards longitude $201\degree$. The magnetic field lines are calculated with the JRM33 connerney2022 and CON2020 model connerney2020. The data used in this analysis, which magnetically map to a maximum $M$-shell of $M=50.5$ are marked as pink orbit segments.
• Figure 3: (Top) An electron intensity-pitch angle distribution (dots) fitted with the beam intensity function (solid lines). The crosses are the counts for the two data points closest to the magnetic field direction. This distribution is measured in energy channel $E_2$ with central energy $\bar{E_2} =32 ~\mathrm{keV}$ on day 36 of 2018. (Bottom) The beam intensity function for different example beamness parameters $m$, with $A=100$ and $B=0$.
• Figure 4: The pitch angle diffusion process is demonstrated for a diffusion coefficient $D_1 = 10^{-5}\mathrm{s}^{-1}$ from li2021 for $\bar{E_1}=32~\mathrm{keV}$. The initial condition, a Gaussian peak with $2~\sigma$ corresponding to the mean loss cone in the middle magnetosphere, is shown in pink. The solution functions are shown in purple for three example times ($10 ~\mathrm{s}$, $20 ~\mathrm{s}$, $30 ~\mathrm{s}$), with the last solution fitted with a Gaussian function. The resulting $2~\sigma$ position at $\alpha= 2.81\degree$ is marked as dashed black line. In addition the solution for a higher energy channel diffusion coefficient $D_7 = 10^{-6}\mathrm{s}^{-1}$ for $\bar{E_7}=755~\mathrm{keV}$ is shown as turquoise line.
• Figure 5: Example overview plot for day 253 of 2019. (1) (2) Beam detection classifications of pitch angle distributions per energy channel and 30-s measurement interval for the parallel and anti-parallel directions, respectively. The beamness parameter is color coded, with narrow beams with $m\geq4$ displayed in pink and scattered beams with $m<4$ displayed in purple. Intervals that did not meet the beam detection criteria, representing insufficient pitch angle coverage (Low Coverage) and low field-aligned intensity (Isotropic) are displayed in gray and blue, respectively. (3) The electron intensity-pitch angle spectrogram averaged over 30‐s intervals and over energies from $\sim30$ to 1,200 keV. (4) Measured total magnetic field strength $B$ in nT, the modeled $M$-shell in $\mathrm{R_J}$ and the absolute magnetic latitude in degrees.