Characterising injection signatures in Jupiter's ultraviolet aurora using Juno observations

TL;DR

The study addresses whether Jupiter's ultraviolet injection signatures originate from magnetodisc scattering or high-latitude Alfvénic acceleration and whether outer arcs are related to injections. It combines automatic detection of discrete UV features from Juno-UVS with in-situ measurements (JEDI, MAG-FGM) to test mechanisms and to classify injections into dawn-storm and non-dawn-storm types, uncovering strong evidence for magnetodisc-pitch-angle scattering as the dominant driver and arsenals of energy-dependent drift shaping feature morphology. A key finding is that the hemispheric power ratio follows the isotropic-scattering relation $\log_{2}\left(\frac{P_{N}}{P_{S}}\right) = -\log_{2}\left(\frac{B_{N}}{B_{S}}\right)$ with substantial scatter partly explained by slight sub-corotation of injections (≈85%), and that arc-like outer features can be sequences of dawn-storm injections broadened by drift. Arc-like features and blob-like injections share similar particle/wave properties, suggesting a common origin, with arc formation arising from energy-dependent drift rather than a distinct acceleration mechanism, and non-dawn-storm injections contributing to a broad, often-dusk-dominated distribution of injections.

Abstract

Discrete features in Jupiter's ultraviolet aurora have been interpreted as signatures of plasma injections in the middle magnetosphere. There exists some ambiguity whether magnetodisc scattering or high-latitude Alfvenic acceleration best describes the observed properties of these injection signatures, and also to what extent arcs in the outer emission are related to injections. Many injection signatures are the result of the evolution of dawn storms; there is, however, limited evidence that non-dawn-storm injection signatures are sometimes present in the aurora. We use automatic detection of these discrete features, alongside data from Juno-UVS and in-situ measurements by other Juno instruments, to show that scattering likely accounts for most of the electron precipitation associated with injection signatures. Additionally, there is evidence that injection signatures can be classified into two types: dawn-storm and non-dawn-storm. Arc-like features in the outer emission show very similar properties to traditional blob-like injection signatures and may consist of sequences of injection signatures that have broadened into an arc via energy-dependent electron drift.

Figures (24)

• Figure 1: Exemplar map of UV brightness from PJ7-N, saturated to highlight injection signatures. Automatically detected features in the outer emission have been highlighted according to their feature type as defined in the text: red = small blob (SB), magenta = large blob (LB), green = arc. Gridlines are in jovicentric coordinates and are spaced by 15° in latitude and longitude.
• Figure 2: Histogram in radial distance and local time of the projected position in the equatorial plasma sheet of features in the outer emission. The mean-average location for each local-time bin is given by the red line; error bars denote the standard deviation. Histograms flattened in local time and radial distance are given to the bottom and right of the main plot, respectively.
• Figure 3: An injection signature observed 2017-05-19 by Juno-UVS during PJ6-S, highlighted in green. The brightness map is given on the left and the colour-ratio map on the right. The main emission is present to the right of the injection signature.
• Figure 4: Histogram in magnetospheric local time of blob-like injection features in the outer emission. Features are categorised by the magnetospheric longitude shift between the brightness peak and colour-ratio peak: no shift (<0.5°), slight shift (0.5° to 1°), and shift (>1°), from positive (red, bottom) through to negative (green, top) shifts.
• Figure 5: North-to-south UV auroral power ratio vs surface magnetic-field-magnitude ratio for injection signatures detected by Juno-UVS during the first 40 perijoves. The N$\rightarrow$S projections are denoted by black $\times$, and the S$\rightarrow$N projections by green $+$. The best-fit linear relation for all points is given by a solid red line, and separate fitted relations for the N$\rightarrow$S and S$\rightarrow$N case by dotted and dash-dot lines respectively. The theoretical pitch-angle-scattering relation is given by a dashed blue line.