Variations in Jupiter's extended sodium nebula over two decades

We have been performing ground-based observations of Jupiter's sodium nebula longer than two decades. The sodium atoms in the nebula originate in Io's volcanic gas. The nebula distributes over 1,000 Jupiter's radii, and its angular size is approximately 10 degrees. Therefore, ground-based observations of D-line emissions in the sodium nebula can be made with small optics. The observation results indicate that the sodium D-line brightness in the nebula varies with various timescales like day-to-day through year-to-year. While it is naturally expected that variations in the brightness of the nebula reflect that of volcanism on Io, it may be influenced by locations of active volcanic plumes on Io. Sometimes, variations of the nebula differ between the eastern and western sides of Jupiter.  Details on the variations of Jupiter's extended sodium nebula will be shown. Especially, a period in which Juno spacecraft has been orbiting Jupiter will be focused.