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Solar type III radio bursts contain a wealth of information about thedynamics of electron beams in the solar corona and the inner heliosphere;currently unobtainable through other means. However, the motion of differentregions of an electron beam (front, middle and back) have never beensystematically analysed before. We characterise the type III burstfrequency-time evolution using the enhanced resolution of LOFAR in thefrequency 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 weredefined for a collection of 31 type III bursts. The frequency evolution is usedto ascertain the apparent velocities of the front, middle and back of the typeIII sources and the trends are interpreted using theoretical and numericaltreatments. The type III time profile was better approximated by an asymmetricGaussian profile, not an exponential as previously used. Rise and decay timesincreased with decreasing frequency and showed a strong correlation. Durationswere smaller than previously observed. Drift rates from the rise times werefaster than from the decay times, corresponding to inferred mean electron beamspeeds 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 typeIII frequency bandwidth decreases as frequency decreases. The different speedsnaturally explain the elongation of an electron beam in space as it propagatesthrough the heliosphere. The rate of expansion is proportional to the meanspeed of the exciter; faster beams expand faster. Beam speeds are attributed tovarying ensembles of electron energies at the front, middle and back of thebeam.

Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport