Io-phase motion and jovian decametre source locations

Abstract

Io, Jupiter's innermost Galilean satellite, plays a major part in determining when jovian decametre storms are observed¹. Specifically, as a result of beaming of the radiation at a preferred angle to the local magnetic field, the most favourable satellite locations for the reception of emission are approximately 90° and 240° γ_Io (departure of Io from superior geocentric conjunction). Recently a second-order effect has been noted—a periodic displacement in the particular γ_Io phase at which the reception probabilities are observed to peak^2–4. The modulation has a peak-to-peak magnitude of 5° to 10°, a period of about 12 yr, and is most pronounced in the case of the jovian decametre storm sources Io–B and Io–C. (There is not yet general agreement as to the behaviour of Io–A.) The effect has been tentatively correlated with the ±3.5° variation in the jovicentric declination of the Earth, D_E; however, it has also been considered³ that it may be a consequence of changes intrinsic to the jovian activity cycle. Here we interpret this phenomenon simply in terms of the latitudinal variations inherent in the Earth–Jupiter viewing geometry as defined by the 12-yr cycle in D_E. Furthermore, we show that the phase of the Io–B and Io–C displacements relative to D_E phase uniquely defines the hemisphere, north or south, from which the radiation associated with each source is beamed. The results agree with, and lend credence to, the source positions as inferred from polarimetry^5,6 and more recently from considerations of planetary-limb shadowing⁷.