Amplitude Scintillation of Radio Star at Ultra-High Frequency

Observations of radio star scintillation at a frequency of 915 Mc over a 12-month period (October, 1957-September, 1958) are described. By daily observations of Cygnus A (IAU 19N4A) over a wide range of elevation angles, both the amplitude and rate of scintillation were studied. It was found that the amplitude scintillation is strongest near the horizon and decreases rapidly as the altitude increases. The amount of the intensity fluctuation ranges from less than 3 per cent to over 50 per cent of the undisturbed intensity, while the rate of fluctuation varies from ¿ to 8 peaks per minute. A fluctuation amplitude greater than 10 per cent was observed at low-altitude angles for 71 per cent of the total observations. The frequency of occurrence of scintillation follows a seasonal variation having two maxima; one in the winter and the other in the summer. During these two maximum periods, the mean fluctuation amplitude was found to be higher than during other periods. It was also found that the mean fluctuation rate in the winter is about three times faster than that in the summer. It was also observed that the scintillation characteristics are markedly affected by the presence of auroras and/or geomagnetic disturbances. During the winter months, the fluctuation rate is roughly proportional to the three-hour geomagnetic K index.