Abstract
Long-term analysis of the scintillation rate of the radio star Cassiopeia A at a frequency of 50 Mc./s. has been carried out using data obtained at Ottawa and Saskatoon. It is found that the variation of scintillation rate with sidereal time can be explained by assuming the ionospheric irregularities responsible for the scintillations to be greatly extended along the direction of the earth's magnetic field, and to have a drift motion whose average direction appears to be around an axis passing through the earth's dip-pol