Jovian active longitude derived from CML-Io phase diagram
Résumé
Recent investigations by Galopeau et al. (J. Geophys. Res. 112, 2007) have shown that some specific
Jovian ‘active’ longitudes favour the Io-controlled Jovian decameter radiation by considering that the
emissions are produced near the local gyrofrequency by the cyclotron maser instability (CMI), along an
active magnetic field line carried away by Io along its revolution around Jupiter, within a hollow cone of
constant angle θ. The active field line is supposed to present a lead angle δ relatively to Io’s position. This
model can explain why the occurrence probability of the radio emission is larger in some specific regions
of the central meridian longitude (CML)-Io phase diagram: the so-called Io-A, Io-B, Io-C and Io-D
‘sources’. Long term observations allow us to define four boxes representing the typical Io-controlled
source regions. The precise location of these four boxes strongly constrains the range of possible active
longitude for both northern and southern radio emissions. Our model also allows us to derive the half-
angle θ of the hollow cone describing the emission beaming. We analyse the effect of the emission
frequency and the value of the lead angle δ on the active longitude range and emission angle θ. Our study
shows that a common active longitude can only explain part of the emissions coming from restricted
zones of the CML-Io phase diagram. It is particularly clear for the Io-D source