Closed Fluxtubes and Dispersive Proton Conics at Jupiter's Polar Cap

Abstract

Two distinct proton populations are observed over Jupiter's southern polar cap: a ∼1 keV core population and ∼1-300 keV dispersive conic population at 6-7 R_J planetocentric distance. We find the 1 keV core protons are likely the seed population for the higher-energy dispersive conics, which are accelerated from a distance of ∼3-5 R_J. Transient wave-particle heating in a "pressure-cooker" process is likely responsible for this proton acceleration. The plasma characteristics and composition during this period show Jupiter's polar-most field lines can be topologically closed, with conjugate magnetic footpoints connected to both hemispheres. Finally, these observations demonstrate energetic protons can be accelerated into Jupiter's magnetotail via wave-particle coupling.

Figure 1

(a) The Juno trajectory in a coordinate system with the z‐axis aligned with Jupiter's magnetic dipole. (b) The Juno footprint mapped to Jupiter using magnetic field models along with ultraviolet images of the auroral emissions, with labels for the distinct regions and solar direction. The grey boxes indicate where Juno observed dispersive proton conics.

Figure 2

Waves and particle data as a function of time with all five dispersive proton features marked with red triangles. Panel (a) shows the electric field spectral density. Panels (b) and (c) show H⁺ flux as a function of inverse speed and energy respectively. Lines in panel (b) show a dispersion relation for ions perpendicularly heated at 5.4 R_J planetocentric distance. Panels (d), (e), and (f) show H⁺ flux as a function of pitch angle (PA) for 50 to 1,000 keV, 10–50 keV and 0.1–2 keV, respectively. Each pixel in the Jovian Auroral Distributions Experiment (JADE) PA spectrograms (e) and (f) are 22.5° wide, which is the resolution of an individual JADE‐I anode. Horizontal dashed lines in panels (d–f) show expected PAs for perpendicularly heated ions originating at 2.9 R_J and 5.4 R_J. A schematic highlighting the acceleration mechanism is shown on the left.

Figure 3

Time‐of‐flight count rates observed by the Jovian Auroral Distributions Experiment as a function of mass per charge and energy per charge for (a) the dispersive conic events, (b) the magnetosheath, and (c) the magnetotail. The core and dispersive proton populations are marked in (a), where the lower count rate “fork” is an instrumental feature also due to protons. During this time, magnetospheric heavy ions (Oⁿ⁺, Sⁿ⁺) are observed above ∼5 keV, exhibiting significant similarities to the composition of the magnetotail. Panel (d) shows the magnetosphere in the noon‐midnight plane, with the locations of (a) and (c) and three ranges of magnetopause boundaries (Ranquist et al., 2020).