Estimation of the drift rate and intensity of Neptune's storm by Pirika telescope
Authors: Sato Yuki , Takahashi Yukihiro , Sato Mitsuteru , Takagi Seiko , Imai Masataka , Ono Tatsuharu 
Published: October 2020

The outermost planet Neptune is known to have a giant storm system as Jupiter's Great Red Spot (GRS). However, there are only a few observations of Neptune's storm, and the structure, formation mechanism, and lifecycle of these giant storms are poorly understood. Voyager 2 observed Neptune on May 24, 1989, and discovered the Earth-sized Great Dark Spot (GDS) with 13,000 km. GDS was located in the southern hemisphere, but GDS became extinct when the Hubble Space Telescope observed it in 1994 (Hammel et al., 1995). It is unknown whether it is a sudden thing or storms such as GDS always occur in Neptune. A huge storm of 9,000 km at the equator was observed on July 2 and June 26, 2017, by Keck observatory (Edward et al., 2019). It's considered that Neptune storms occur at mid-latitudes in the north and south that an ascending air occurs. However, this huge storm occurred near the equator. A rotation axis of Neptune is 29.6&#176;, and the storm possibly occurred near the equator because of seasonal change. Neptune is a 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 is not easy to use those telescopes for long-term continuous monitoring. In order to investigate the temporal evolution of GDSs and storms in Neptune, 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 a long-term on a short time scale. The purpose of this study is to understand the atmosphere convection structure related to Neptune's storm and its temporal evolution. We observed Neptune by using 1.6 m Pirka telescope operated by the Faculty of Science in Hokkaido University from October 22, 2018, to November 26, 2018. The wavelength is 890, 855 nm. From this analysis, we can retrieve a weak absorption at 890 nm because the altitude of storms is higher than the surrounding areas. In addition, 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 effect of the earth atmosphere and calculated the relative intensity's theoretical values by the rotation. The bottom left figure shows the profile of the theoretical line. Here, we defined the longitude difference between the observer longitude and the storm's longitude. We assumed the storm's area and longitude of the storm at the start of our observation, and fit the observed values with the theoretical values in the method of least squares to estimate the drift rate and 890 nm albedo inside the storm. The fitting result is shown in the bottom right figure. We estimated that the drift rate of the storm is 24.6&#176;/ day, and the 890 nm albedo is 0.055.&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;