Ulysses observations of Jupiter's 10 h modulation in interplanetary space in 2004

Abstract

Since the Pioneer 10/11 mission, Jupiter is known as a dominant and almost constant source of MeV electrons in the inner heliosphere. An interesting feature of the Jovian electron source is the fact that Jupiter's rotation period (~10 h) can frequently be recovered in the energy spectrum of Jovian electrons in the vicinity of the planet. However, this modulation has never been convincingly reported to exist beyond ~1 AU away from the planet. In order to search for the 10 h modulation in the heliosphere, we reexamined Ulysses MeV‐electron data from the High Energy Telescope (HET) and Kiel Electron Telescope (KET) for the second Jupiter flyby using a Lomb‐Scargle spectral analysis. From days 140 to 147 of 2004, when Ulysses was ~1.2 AU away from the planet at low latitudes during the spacecraft's second flyby, a clear 10 h modulation signature was found when electrons were streaming away from Jupiter. We present data of an event that suggests the presence of the 10 h modulation even at ~2.2 AU.