Some Considerations in the Operational Classification of Sudden Ionospheric Disturbances and Computation of Peak Increased Absorption on HF Circuits

摘要

The report reviews various approaches to the application of sudden phase anomaly (SPA) data derived at very low frequency (VLF) to the operational estimation of increased high frequency (HF) absorption resulting from solar flares. The means of categorizing 'weak' and 'strong' SPA events originally devised by Chilton is described. Within the 'strong' category, five further classifications relative to the ratio of D-layer ionization densities before and at the peak of a flare event are proposed because this ratio is related to the geographic coverage of the event and the maximum downward shift of the D-layer effective reflecting height. The relationship of HF absorption to associated SPA data is concluded to depend upon the electron density and collision frequency profiles produced by a flare. Calculation of enhanced HF absorption for a postulated flare event and HF path is demonstrated by the use of an empirical relationship. (Author)