Neptune's Dark Vortex NDS-2018 ("NDS-2018")

A Northern Dark Spot discovered in 2018 (NDS-2018) was detected in the annual observations of Neptune from the OPAL program. Once 2019 OPAL data showed that the spot was drifting very close to the equator, where numerical simulations find that spots may perhaps cause planetary-scale disturbances as they disrupt themselves, additional observing programs were conducted to understand how the dark vortex was changing on shorter time scales, and to capture finer detail by taking a higher number of blue (F467M) images in each HST orbit. Dark spots require high spatial resolution at blue wavelengths to be observed, so no ground-based observatory has yet published a detection of one. Only the Voyager 2 spacecraft and Hubble have made published observations of dark vortices on Neptune. Wong et al. (2022) reviews the overall collection of HST imaging data from the WFC3/UVIS instrument. These have been processed to correct for geometric distortion, cosmic ray hits, and fringing (at narrowband red wavelengths), then navigated to define latitude, longitude, emission, and incidence angles for each pixel as described in Wong et al. (2020). Frames have been corrected for loss of contrast due to scattered light from the wings of the point spread function, and limb darkening and large-scale zonal brightness variation were removed by constructing difference images. The team prepared stacks of coadded difference images to maximize PSF sampling and to reduce noise from pixel instability.