Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave-particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvenic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program (Ronmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40 degrees W longitude from the sub-Jupiter meridian, this density is found to be greater than or equal to 0.5% of the local plasma density (similar to 4000 cm(-3)), namely greater than or equal to 20 cm(-3). Assuming a stationary pickup process and a r(-n) distribution of pickup protons within several Io radii of Io's wake, this implies that more than 10(26) protons/s are created around Io. The ultimate origin of these protons is an open issue. (C) 1999 Elsevier Science Ltd. All rights reserved.