The low-frequency interplanetary radiation
Observations of the low-frequency interplanetary radiation have continued, although no major activity has begun since mid-1991. Theoretical work has been completed which suggests that the long-speculated source at the termination shock is feasible and could at least qualitatively account for several aspects of the observations, although some difficulties exist. The Ulysses encounter with Jupiter occurred in February 1992 and has renewed discussion of a Jovian source for the emissions. We reconsider the arguments used previously to rule out Jupiter as a source and conclude that the existence of a relatively high-density boundary to the heliosphere renders most of these arguments ineffective in eliminating Jupiter as a source. We conclude, therefore, that Jupiter must be considered as likely a candidate source as the termination shock for the low-frequency emissions and, given that Jupiter is a known source of emissions in the few-kHz range, perhaps Jupiter is the more likely candidate. Even with a Jovian source, however, the low-frequency emissions continue to be useful sounders of the heliosphere which can provide information on the size of the heliospheric cavity. No conclusive evidence is available which identifies either Jupiter or the termination shock as the source.