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.
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