Juno-UVS approach observations of Jupiter's auroras

Abstract

Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Prior to the bow shock crossing on 24 June 2016, the Juno approach provided a rare opportunity to correlate local solar wind conditions with Jovian auroral emissions. Some of Jupiter's auroral emissions are expected to be controlled or modified by local solar wind conditions. Here we compare synoptic Juno-UVS observations of Jupiter's auroral emissions, acquired during 3-29 June 2016, with in situ solar wind observations, and related Jupiter observations from Earth. Four large auroral brightening events are evident in the synoptic data, in which the total emitted auroral power increases by a factor of 3-4 for a few hours. Only one of these brightening events correlates well with large transient increases in solar wind ram pressure. The brightening events which are not associated with the solar wind generally have a risetime of ~2 h and a decay time of ~5 h.

Keywords: Juno; Jupiter; aurora.

Figure 1

The location of the Juno spacecraft during approach as seen from Earth, with x and y axes parallel to ecliptic longitude and latitude, respectively, in units of Jupiter radii (the dots mark every 6 h; the asterisks mark each day). The nearly contiguous Juno‐UVS synoptic auroral observations from 3 to 29 June 2016 are indicated in blue, while Jupiter is outlined in green, and the orbit of Callisto is shown in red. During the synoptic observations, the spacecraft range to Jupiter center dropped from 302.0 to 73.4 R _J, so that the angular diameter of Jupiter as seen from Juno increased from 0.38° to 1.56°.

Figure 2

(a) (left) Juno‐UVS brightness image and (right) corresponding color ratio image of Jupiter, for 1 h of elapsed time (~1.3 s integrated time) starting at 12:34:28 spacecraft UT on 21 June 2016. The Juno range and subspacecraft system III longitude and latitude are indicated, along with nominal L = 6 and L = 30 auroral ovals. The larger and smaller white ovals at the north and south poles in the brightness image indicate the regions included in estimating the northern and southern emitted auroral power, respectively. Brightness (and color ratio) pixels have an angular size of 0.04° × 0.04°, which considerably oversamples the instrument spatial point‐spread function of along slit full width at half maximum FWHM ~ 0.20°, cross slit FWHM ~ 0.25° [Greathouse et al., 2013]. The color ratios are only shown for pixels where the brightness both exceeds 80 kR and is larger than 25% of the peak brightness. See supporting information for an animation of the entire observation period. (b) (left) Juno‐UVS brightness map and (right) corresponding color ratio map of Jupiter, for 30 h of elapsed time (~38 s integrated time) starting at 12:00:00 spacecraft UT on 21 June 2016. The L = 6 and L = 30 auroral ovals are indicated. The color ratios are only shown for pixels where the brightness both exceeds 80 kR and is larger than 25% of the peak brightness.

Figure 3

Estimated total emitted power observed with Juno‐UVS from Jupiter's northern aurora (red) and more poorly observed southern aurora (blue), averaged over 1 h intervals during ~64 contiguous rotations of Jupiter over 3–30 June 2016 (day of year 155–182). Occasional data dropouts of ~10 h were due to periodic repointing of Juno's antenna toward Earth. The total solar wind ram pressure measured by JADE is shown for comparison (green, with the vertical spread indicating ±1σ errors), along with HST‐STIS estimates of total emitted power from the northern aurora (black asterisks). The single approach‐phase bow shock crossing on 24 June 2016 at 08:16 UT is indicated by a vertical purple line and subsequent magnetopause crossings by vertical orange lines.

Figure 4

Profiles of total emitted power from Jupiter's northern aurora observed by Juno‐UVS, averaged over 1 h intervals during four major brightening events. The events each had peak emitted powers in the 9–15 TW range but have been normalized to a peak of 12 TW in this plot and overlaid by start time in order to compare their time dependencies. For comparison, the dashed black line shows an exponential increase from the baseline emitted power of 3 TW up to a peak of 12 TW (with a 1/e risetime of 2 h) and a decay back to the baseline emitted power (with a 1/e dimming time of 5 h).