Radar Echoes from the Sun

The study of the sun by radar which was begun less than five years ago should become a valuable supplement to the study, by other methods, of the sun and interplanetary space. High powered transmitters and large antennas are required to detect a solar echo. Frequencies less than 50 Mc should be optimum, primarily because of increasing coronal absorption with increasing frequency. Routine observations were begun by the Lincoln Laboaratory of the Massachusetts Institute of Technology in April, 1961, at a site near El Campo, Texas. Observations since that time have been made on about 200 days per year. The transmitter has an average power output of 500 kw and operates at a frequency of 38.2 Mc. The system includes two cross-polarized antennas consisting of large arrays of dipoles. These antennas have maximum gains of 33 and 36 db. The received solar echo is usually 20 to 30 db below the solar noise and signal integration is required to detect the echo. The average measured solar radar cross section is approximately equal to that of the projected area of the photosphere although there are large fluctuations about the mean. Some possible reasons for these variations in cross section are discussed. The Doppler spreading of the solar echoes varies between 20 and 70 kc and is apparently due to mass motions on the sun. These indicated mass motions are large enough to affect the coronal temperature measurements made by the emission-line broadening method.