The Io torus and highlights from 10 years operation of the Hisaki mission

Abstract

The Io plasma torus (IPT), located in the Jovian inner magnetosphere (6-8 RJ from the planet), is filled with electrons and heavy ions such as sulfur and oxygen, a significant portion of which originates from the volcanoes on Io. The IPT serves as a crucial region connecting the primary plasma source (Io) with the middle and outer magnetosphere, where highly dynamic phenomena occur. Understanding the behavior of plasma in the IPT is essential for discussing the plasma dynamics in the whole Jovian magnetosphere. A comprehensive understanding of the IPT can be achieved through spectral analysis of ion emissions, which are generated by electron impact excitation. This method is called 'plasma diagnostics.' The emission lines from ions in the IPT are mainly in the extreme ultraviolet (EUV) region. Therefore, EUV spectroscopic data are important for the study of Jupiter’s inner magnetosphere. Hisaki, an Earth-orbiting spacecraft equipped with the extreme ultraviolet spectroscope EXCEED, has been providing high-resolution spectra of the IPT from 2013 to 2023. Here we present a summary of 10 years of operations for IPT spectroscopic observation by Hisaki. Ion and electron density variation, the relationship between the IPT and auroras, and responses to volcanic activity will be discussed.