Effects of solar flares on the absorption of 18.3 Mc/s cosmic noise

Two effects of solar flares on the received intensity of 18.3 Mc/s cosmic noise are discussed. It is shown that observations of the variations in 18.3 Mc/s cosmic noise intensity provide a very sensitive method of detecting sudden ionospheric disturbances (SIDs). The characteristics of the increases in absorption observed during SIDs are discussed and comparisons made with other SID observations, particularly very long wave sudden phase anomalies. It is shown that an increase of 1 db in the absorption of 18.3 Mc/s radiation passing once through the ionosphere is probably associated with a sudden phase anomaly of size about 200 degrees at 16 kc/s. Such a relationship is incompatible with a D-region of the simple Chapman type or one of the "exhaustion region" type. About 30 hours after certain solar flares of Class 3 the absorption of cosmic noise increased, the increase lasting for periods of the order of 10 hours. Magnetic storms did not follow until some time later and at the time of the abnormally high attenuation there were no outstanding peculiarities in magnetic records. It is uncertain whether the excess attenuation occurs within the ionosphere or well away from the earth