Relation between the short-term variation of the Jovian radiation belt and thermosphere derived from radio and infrared observations

Abstract

We report the first comprehensive observations of Jovian synchrotron radiation (JSR) and H₃⁺ emission from the Jovian thermosphere to investigate the generation process of short-term (days to weeks) variations in the Jovian radiation belt. The observations were made by the Giant Metrewave Radio Telescope and NASA Infrared Telescope Facility during November 2011. The total flux density of JSR increased by approximately 5% between 6–9 November and 12–17 November, associated with the increased solar UV/EUV flux. From 7 to 14 November, a possible rise in the infrared H₃⁺ emission was observed in the middle-latitude region, corresponding to a temperature variation of approximately 10 K. These results are consistent with the scenario that the solar UV/EUV heating causes variations in the thermospheric temperature and JSR. Radio images along the equatorial region showed that the JSR intensity decreased inside 1.5 Jovian radii (R_J) and the peak position shifted outward. This implies that energetic electrons are attenuated by some internal loss process, despite the simultaneous increase in radial diffusion. A physical model for the radiation belt shows that such an internal loss process can explain the observed variation of brightness distribution. Typical loss time scale is longer than strong diffusion limit, which suggests the existence of some pitch angle diffusion process such as wave-particle interaction. Thus, variations of the total JSR flux density and thermospheric temperature seem consistent with the scenario, and the brightness distribution of JSR can be explained by the increase in radial diffusion accompanied by internal loss processes.