Ionospheric disturbances associated with the solar flare of September 28, 1961

Abstract

Changes produced in the phase, frequency, and amplitude of stable HF waves during the solar flare of September 28, 1961, and the geomagnetic sudden commencement of September 30, 1961, are analyzed. The HF waves, of approximately 15 and 18 Mc/s, were transmitted over east-west paths 2420 and 5640 km long. Over the longer path, the phases of both signals were advanced by 1400 and 1200 cycles, respectively, whereas their respective frequencies were increased by 8.4 and 7 cps. Absorption commenced after the major portions of the phase and frequency changes had taken place. Over the shorter path, the frequencies of the 15 and 18 Mc/s signals were increased by 2.4 and 1.4 cps, respectively. Absorption data are not available over the short path. During the SC the frequency of the 18-Mc/s signal transmitted over the short path first increased by 1 cps and then decreased by 2.2 cps. Following the sudden frequency shift, the instantaneous received frequency oscillated with a period of about 30 sec. Since absorption commenced after the main frequency and phase advance, and since it disappeared before the phase recovery, it is suggested that the changes in the electron density were produced by UV radiation energy in the 140- to 200-km region. This hypothesis is strengthened by the circumstance that the maximum phase changes were—approximately—inversely proportional to the operating frequency. The appearance of solar X-rays (>20 kev) coincided with the onset of absorption of the signals and the onset of a second phase of peak activity in the phase and amplitude scintillation frequency of the signals. The ionization production, as obtained from the rate of change of the phase of the incoming HF signals, does not correspond exactly with the time history of the solar X rays in the >20-kev energy. Softer X rays must be responsible for the electron production that caused the phase changes.