VLF phase perturbations produced by solar protons in the event of February 5, 1965

Solar protons in the event of February 5, 1965, caused day and nighttime phase disturbances in the 18.6-khz VLF transmission between station NLK in Jim Creek, Washington and Thule, Greenland. The phase anomalies persisted for three days with a maximum deviation of 20 μsec, representing 67% of the total normal diurnal variation. A quantitative connection is made between the observed VLF phase variations and the solar protons measured by satellites in the near-earth environment by means of specific ionosphere and VLF propagation models. The lower ionosphere is described by effective recombination coefficients, αeff's, appropriate to day and nighttime ionization conditions. The effective VLF reflecting height of a sharply bounded ionosphere is located at the altitude where the ionospheric conductivity parameter ωr equals 2.5 × 105 sec−1. The VLF variations are also explicable by an exponential ionosphere with a reference height equal to the effective reflecting height of the sharply bounded model. An integral flux of 150 protons/cm² sec with E ≥ 20 Mev produced a daytime lowering of the VLF reflecting height by 10 km. The ionization model of the lower ionosphere derived in relation to the VLF is used to compute daytime riometer absorption that fits well to the absorption measured at McMurdo Sound, Antarctica, at 30 Mhz, but not as well at 50 Mhz.