Answers are grounded in this paper and cite their sources.
During two periods in May and July, I959, following strong solar flares, the signal strength of receptions at Kiruna of VLF transmissions from Rugby (16 kc) showed no or only slight diurnal variation. It is proposed that the change of the diurnal variation is due to the production by solar protons of an ionized layer very deep in the atmosphere, the electron density of which is sufficient for reflection of very long waves but too low to cause measurable nondeviative absorption in the HF band at geomagnetic latitudes lower than approximately60.
No takes yet. Share an insight, caveat, or question.
Ortner et al. (1960) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: