  Solar type III radio bursts contain a wealth of information about the
dynamics of electron beams in the solar corona and the inner heliosphere;
currently unobtainable through other means. However, the motion of different
regions of an electron beam (front, middle and back) have never been
systematically analysed before. We characterise the type III burst
frequency-time evolution using the enhanced resolution of LOFAR in the
frequency range 30 to 70 MHz and use this to probe electron beam dynamics.
Methods. The rise, peak and decay times with a 0.2 MHz spectral resolution were
defined for a collection of 31 type III bursts. The frequency evolution is used
to ascertain the apparent velocities of the front, middle and back of the type
III sources and the trends are interpreted using theoretical and numerical
treatments. The type III time profile was better approximated by an asymmetric
Gaussian profile, not an exponential as previously used. Rise and decay times
increased with decreasing frequency and showed a strong correlation. Durations
were smaller than previously observed. Drift rates from the rise times were
faster than from the decay times, corresponding to inferred mean electron beam
speeds for the front, middle and back of 0.2, 0.17, 0, 15 c, respectively.
Faster beam speeds correlate with smaller type III durations. We also find type
III frequency bandwidth decreases as frequency decreases. The different speeds
naturally explain the elongation of an electron beam in space as it propagates
through the heliosphere. The rate of expansion is proportional to the mean
speed of the exciter; faster beams expand faster. Beam speeds are attributed to
varying ensembles of electron energies at the front, middle and back of the
beam.
