Drift rates of major Neptunian features between 2018 and 2021

Using near-infrared observations of Neptune from the Keck and Lick Observatories, and the Hubble Space Telescope in combination with amateur datasets, we calculated the drift rates of prominent infrared-bright cloud features on Neptune between 2018 and 2021. These features had lifespans of ∼1 day to ≥1 month and were located at mid-latitudes and near the south pole. Our observations permitted determination of drift rates via feature tracking. These drift rates were compared to three zonal wind profiles describing Neptune’s atmosphere determined from features tracked in H band (1.6 μm), K’ band (2.1 μm), and Voyager 2 data at visible wavelengths. Features near −70deg measured in the F845M filter (845 nm) were particularly consistent with the K’ wind profile. The southern mid-latitudes hosted multiple features whose lifespans were ≥1 month, providing evidence that these latitudes are a region of high stability in Neptune’s atmosphere. We also used HST F467M (467 nm) data to analyze a dark, circumpolar wave at −60° latitude observed on Neptune since the Voyager 2 era. Its drift rate in recent years (2019–2021) is 4.866±0.009°/day. This is consistent with previous measurements by Karkoschka (2011), which predict a 4.858±0.022°/day drift rate during these years. It also gained a complementary bright band just to the north. •Late 2019 through 2020 showed a decrease in clouds at mid-latitudes.•The South Pole remained active during 2020 as the primary region of cloud activity.•At 1.6 μm a bright ring appeared at −66 degrees latitude.•The South Polar Wave (−60 deg) at 0.467 μm gained a bright band to the north.•The South Polar Wave’s speed is consistent with results from a decade before.