Hiroyasu Tadokoro

Variation of Jupiter's aurora observed by Hisaki/EXCEED: 1. Observed characteristics of the auroral electron energies compared with observations performed using HST/STIS

Temporal variation of Jupiters northern aurora is detected using the Extreme Ultraviolet Spectroscope for Exospheric Dynamics (EXCEED) on board JAXAs Earth-orbiting planetary space telescope Hisaki. The wavelength coverage of EXCEED includes the H2 Lyman and Werner bands at 80–148 nm from the entire northern polar region. The prominent periodic modulation of the observed emission corresponds to the rotation of Jupiters main auroral oval through the aperture, with additional superposed −50%–100% temporal variations. The hydrocarbon color ratio (CR) adopted for the wavelength range of EXCEED is defined as the ratio of the emission intensity in the long wavelength range of 138.5–144.8 nm to that in the short wavelength range of 126.3–130 nm. This CR varies with the planetary rotation phase. Short- (within one planetary rotation) and long-term (> one planetary rotation) enhancements of the auroral power are observed in both wavelength ranges and result in a small CR variation. The occurrence timing of the auroral power enhancement does not clearly depend on the central meridian longitude. Despite the limitations of the wavelength coverage and the large field of view of the observation, the auroral spectra and CR-brightness distribution measured using EXCEED are consistent with other observations.

Test‐particle simulation of energetic electron‐H2O elastic collision along Saturn's magnetic field line around Enceladus

We examine the variation of energetic electron pitch angle distribution at the magnetic equator and loss rate of precipitated electrons into Saturns atmosphere through pitch angle scattering due to elastic collisions with neutral H2O along Saturns magnetic field line around Enceladus. To examine the variation of those, we perform one-dimensional test-particle simulation when the co-rotating electron flux tube passes through the dense H2O region in the vicinity of Enceladus (~6.4 min). We focus on 1 keV as a typical kinetic energy of the electrons in this study. The initial pitch angle distribution is assumed to be isotropic. Results show that the equatorial electron pitch angle distribution near the loss cone ( 160°) decreases with time through pitch angle scattering due to elastic collisions. It is found that the electrons of ~11.4% to the total number of equatorial electrons at the initial condition are lost in ~380 s. The calculated loss time is twice faster than the loss time under the strong diffusion.