  During solar flares a large amount of electrons with energies greater than 20
keV is generated with a production rate of typically $10^{36}$ s$^{-1}$. A part
of them is able to propagate along open magnetic field lines through the corona
into interplanetary space. During their travel they emit radio radiation which
is observed as type III radio bursts in the frequency range from 100 MHz down
to 10 kHz by the WAVES radio spectrometer aboard the spacecraft WIND, for
instance. From the drift rates of these bursts in dynamic radio spectra the
radial propagation velocity $V_r$ of the type III burst exciting electrons is
derived by employing a newly developed density model of the heliosphere.
Calculations show that the radio radiation is emitted by electrons with
different $V_r$ and therefore by different electrons of the initially produced
electron distribution.
