The solar flare radiations responsible for sudden frequency deviations

The solar-flare radiations responsible for a type of sudden ionospheric disturbance called ‘sudden frequency deviation’ (SFD) are investigated. It is shown that X-ray bursts at wavelengths less than 0.6 A and Lyman α enhancements do not contribute significantly to SFD's. X-ray bursts at 0.6–10 A are of minor importance to SFD's. X-ray bursts at 10–100 A do contribute significantly to SFD's, but flashes of radiation at wavelengths greater than 100 A are required to explain most SFD's.