The excess electric noise component with a &ohgr;-1 power spectrum, previously identified on ISEE 3 below the plasma frequency, is quantitatively interpreted. The theory is based on quasi-thermal electron noise to which two effects are added: (1) ion quasi-thermal noise Doppler-shifted by the solar wind velocity Vsw, and (2) less important, particle impacts on and emission from the antennas. The first effect yields a component which is highly spin modulated with the electric field maximum when the antenna is perpendicular to Vsw. This result, which occurs in general when the parameter &ohgr;L/Vsw is large, contradicts a hand waving argument often found in the geophysical literature. This theory is in excellent agreement with the observations for the long S wire antenna (L=45 m) on which the impact noise is negligible. For the Z antenna which has a larger surface area, the contributions of both effects are necessary to interpret the measurements. These results could be used to give an independent measurement of the solar wind velocity magnitude and direction whenever the plasma frequency exceeds the lowest frequency channel (30 kHz) of the receiver.