Estimation of the drift rate and intensity of Neptune’s storm in 2018－2020

Abstract:

A storm more than 4,000 km in diameter occasionally occurs in Neptune. Since Neptune is great distance away from the Earth, storms in Neptune can be resolved only by using large telescopes such as Keck observatory and the Hubble Space Telescope. However, it isn't easy to always use those telescopes for Neptune observation. Therefore Neptune is not observed for the long term on a short time scale. We developed the technique to estimate the drift rate and intensity of storms by observing Neptune's whole spectrum in this study. When seeing is bad, it's possible to observe and acquire Neptune's observation data for the long term on a short time scale. The purpose is to deepen the understanding of Neptune's atmosphere convection structure by chasing the detailed change of storms. In this study, we observed Neptune by using 1.6 m Pirka telescope that Hokkaido University owns. The observation time is from October 22, 2018, to November 26, 2018, from July 8 to November 12, 2019, and we started observation on July 20 in 2020. The wavelengths are 890, 855 nm. The apparent size of storms from the observation point changes by the rotation of Neptune, so an 890 nm flux changes by the rotation. We took the ratio of an 890 nm flux and an 855 nm flux to correct the atmosphere's effect of the earth and calculated the theoretical values of the relative intensity by the rotation. We assumed storm's size and fit the observed values with the theoretical values in the method of least squares to estimate the drift rate and 890 nm reflectance inside storms. We estimated that the drift rate and the 890 nm reflectance are 24.6°/ day and 0.055 in 2018, respectively. In 2019, we assumed that there are a few storms and estimated that the drift rate and 890 nm reflectance of the first storm is 22.8°/day and 0.128, those of the second one is 39.6°/ day and 0.081, those of the third one is 51.9°/ day and 0.149, respectively. In 2018, Simon et al. (2019) discovered a new northern Great Dark Spot (NDS-2018) located at 23°N. NDS2018 drifted westward at 2.46°/hr in November 2018. However, NDS-2018 could not be seen because it was located on the night side during our observation, and it is considered that we observed a different storm. We will continue observation in 2020, compare with other researchers and amateur observations, and have a discussion in the future.