Jupiter's Dark Polar Region as Observed by the Hubble Space Telescope During the Juno Approach Phase

We present an analysis of Jupiter's dark polar region, previously described as a crescent‐shaped region devoid of emission along the dawnside poleward of the main oval. Using ultraviolet images obtained by the Hubble Space Telescope between days of year 137 and 176, coupled with interplanetary data captured during the approach phase of the NASA Juno spacecraft, we examine the morphology and variability of the dark polar region and the influence of solar wind conditions on the region. We find that the dark polar region is more variable than previously thought and in particular shows significant variability during compressions of the magnetosphere by the solar wind. Duskward extensions of the dark polar region through the noon sector are often observed during periods of deep rarefaction in the solar wind, while a larger dark polar region width is observed nearer to local noon under solar wind compression. Intensity distributions of the dark polar region reveal a greater probability of lower emission intensity during deep rarefactions. Calculation of the magnetic flux associated with the dark polar region returns lower flux values than previous estimates of the total open flux at Jupiter. This may suggest that either the dark polar region does not account for the entirety of the open flux region, or the magnetotail is not composed of only open flux, as previously suggested.