# Energetic proton dropouts during the Juno flyby of Europa strongly depend on magnetic field perturbations

## TL;DR

This study analyzes how magnetic field perturbations influence energetic proton dropouts during Juno's flyby of Europa, revealing the dominant role of field disturbances at 100 keV and a combination of effects at 1 MeV.

## Contribution

It demonstrates that magnetic field perturbations caused by plasma-atmosphere interactions are key to understanding energetic proton dropouts, providing insights into Europa's electromagnetic environment.

## Key findings

- Field perturbations dominate proton loss at ~100 keV.
- A combination of field effects and surface absorption explains losses at ~1 MeV.
- Magnetic field modeling helps probe Europa's atmosphere and magnetic environment.

## Abstract

During Juno's only flyby of Europa, the Jupiter Energetic Particle Detector Instrument (JEDI) measured complex dropouts in the energetic ion flux in Europa's wake. We investigate the causes of these dropouts, focusing specifically on energetic protons of ~100 keV and ~1 MeV, using back-tracking particle simulations, a prescribed description of Europa's atmosphere and a three-dimensional single fluid magnetohydrodynamics (MHD) model of the plasma-atmosphere interaction. We investigate the role of magnetic field perturbations resulting from the interaction between Jupiter's magnetospheric plasma and Europa's atmosphere and the presence of field-aligned electron beams in Europa's wake. We compare the simulated effect of the perturbed fields on the pitch angle distributions of the ion losses to Juno-JEDI measurements. We find that at ~100 keV, field perturbations are the dominant factor controlling the distribution of the losses along the flyby, while at ~1 MeV a combination of field perturbations and absorption by the surface due to short half bounce periods is required to explain the measured losses. We also find that the effect of charge-exchange with Europa's tenuous atmosphere is weak and absorption by dust in Europa's environment is negligible. Furthermore, we find that the perturbed magnetic fields which best represent the measurements are those that account for the plasma interaction with a sub-/anti-Jovian asymmetric atmosphere, non-uniform ionization of the atmosphere, and electron beams. This sensitivity to the specific field perturbation demonstrates that combining observations and modeling of proton depletions constitute an important tool to probe the electromagnetic field and atmospheric configurations of Europa.