Leading edge and peak flux density exciter speeds for well connected type-III bursts Original Research Article

We derive the speed along the interplanetary field of the leading edge and peak flux density from dynamic WIND/WAVES spectrograms of interplanetary type-III fast-drift radio bursts associated with 52 beam-like near-relativistic electron events observed at 1 AU from 1997 to 2000. The speeds of the leading edge and peak are established by a best fit to time-frequency points obtained from different frequency cuts of the WAVES spectrograms. These are the points when the flux density first rises above the pre-event level or reaches the peak flux density, respectively. The peak flux speeds are usually <0.1c with a median of 0.065c. While the speeds assigned to the leading edge can sometimes exceed 0.3c, their median speed is 0.15c (electron excitation energy of 6 keV). We examine the speeds of the leading edge of the type-III bursts with respect to the delayed injection of the near-relativistic electrons measured at ACE to determine whether the speeds are correlated with the observed delays. We find a poor correlation and interpret this as no significant association between the low-energy electrons in the type-III leading edge and the population of near-relativistic electrons.