ULYSSES OBSERVATIONS OF WAVE ACTIVITY AT INTERPLANETARY SHOCKS AND IMPLICATIONS FOR TYPE-II RADIO-BURSTS

We present the first quantitative investigation of interplanetary type II radio emission in which in situ waves measured at interplanetary s hocks are used to compute radio wave intensities for comparison with t ype II observations. This study is based on in situ measurements of 42 in-ecliptic forward shocks as well as 10 intervals of type II emissio n observed by the Ulysses spacecraft between 1 AU and 5 AU. The analys is involves comparisons of statistical properties of type II bursts an d in situ waves, since the type II events are not related to particula r shock passages at Ulysses. Most of the 42 shocks are associated with the occurrence of electrostatic waves near the time of shock passage at Ulysses. These waves, which are identified as electron plasma waves and ion acoustic-like waves, are typically most intense several minut es before shock passage. This suggests that wave-wave interactions mig ht be of importance in electromagnetic wave generation and that type I I source regions are located immediately upstream of the shocks. We us e the in situ wave measurements to compute type II brightness temperat ures, assuming that emission at the fundamental of the electron plasma frequency is generated by the merging of electron plasma waves and io n acoustic waves or the decay of electron plasma waves into ion acoust ic and transverse waves. Second harmonic emission is assumed to be pro duced by the merging of electron plasma waves. The latter mechanism re quires that a portion of the electron plasma wave distribution is back scattered, presumably by density inhomogeneities in regions of observe d ion acoustic wave activity. The computed type II brightness temperat ures are found to be consistent with observed values for both fundamen tal and second harmonic emission, assuming that strong (similar or equ al to 10(-4) V/m) electron plasma waves and ion acoustic waves are coi ncident and that the electron plasma waves have phase velocities less than about 10 times the electron thermal velocity. Thus a single conve rsion mechanism provides a plausible scenario for generation of both f undamental and harmonic interplanetary type II emission.