The Correlation of Radio Star Scintillations with Geomagnetic Disturbances

The correlation between the degree of scintillation of the radio source Cassiopeia A and the local magnetic K-index is studied. It is shown that the correlation coefficient between these quantities depends upon the epoch of the solar cycle at which the observations are made, on the position of the source in the sky, and on the radio frequency used. For observations on 38 Mc/s made at sunspot minimum, the correlation coefficient is small but positive for all positions of the source. At sunspot maximum, the correlation coefficient is positive when the source is near upper transit and negative when it is near lower transit; the average value of the correlation coefficient for all positions of the source is approximately zero. It is shown that these results can be explained when the finite angular diameter of the radio source is taken into account. For some conditions an increase in the degree of irregularity of the ionosphere can result in a decrease in the degree of scintillation which is observed. It is suggested, therefore, that the degree of irregularity of the ionosphere always increases during magnetic disturbances in medium latitudes, and that the negative values of the correlation coefficient between scintillation index and magnetic K-index which are sometimes obtained are due to a secondary effect related to the finite size of the source. The results of the present paper explain the inconclusive and conflicting results of earlier workers.