Long term variability of Jupiter's northern auroral 8-micron CH4 emissions
We present a study of the long term variability of Jupiter's mid-infrared auroral CH4 emissions. 7.7 - 7.9 micron images of Jupiter recorded by Earth-based telescopes over the last three decades were collated in order to quantify the magnitude and timescales over which the northern auroral hotspot's CH4 emissions varies. We find that the ratio of the radiance of the poleward northern auroral emissions to a lower-latitude zonal mean, henceforth 'Relative Poleward Radiance' or RPR, exhibits a 37% variability over a range of timescales. We searched for patterns of variability in order to test whether seasonally-varying solar insolation, the 11-year solar cycle, or short-term solar wind variability at Jupiter's magnetopause could explain the observed evolution. The variability of the RPR exhibits a weak (r < 0.2) correlation with the solar insolation received at Jupiter's high-northern latitudes. This rules out the hypothesis suggested in previous work (e.g. Sinclair et al., 2017a) that shortwave solar heating of aurorally-produced haze particles is the dominant heating mechanism in the lower stratosphere. We also find the variability exhibits negligible (r < 0.18) correlation with the monthly-mean sunspot number, which rules out variability associated with the solar cycle. On shorter timescales, we find moderate correlations of the RPR with solar wind conditions at Jupiter in the preceding days before images were recorded. For example, we find correlations of r = 0.45 and r = 0.51 of the RPR with the mean and standard deviation on the solar wind dynamical pressure in the preceding 7 days. The moderate correlation suggests that either: 1) only a subset of solar wind compressions lead to brighter, poleward, CH4 emissions and/or 2) a subset of CH4 emission brightening events are driven by internal magnetospheric and independent of the solar wind.