A model has been proposed to explain the ionospheric effects of the sudden commencements as measured with a Doppler sounder. The model consists of two different sources of electric fields generated in the ionosphere during the sudden commencement: a westward electromagnetically induced electric field generated in the F region due to the change of magnetic field and an eastward dynamo electrostatic field set up in the dynamo region and transferred along the geomagnetic field lines to the F region. The change of Doppler frequency shifts during the sudden commencement (ssc) is due to the vertical motion of the charged particles forced by these two opposing electric fields. The theoretical Doppler shifts were calculated on the basis of this model, and four observed ssc's and scf's were used to check the validity of the model.