Energetic electron precipitation and vlf phase disturbances at middle latitudes following the magnetic storm of December 16, 1971

Enhanced fluxes of electrons precipitating over middle latitudes (L ∼ 3–4) were detected by the polar-orbiting satellite 1971-089A following a period of magnetic activity starting on December 16, 1971. The electron fluxes measured in 256 differential channels between 130 and 2800 keV have been coordinated with phase observations of VLF radio waves propagating in the earth-ionosphere wave guide. The VLF paths in question, NLK (near Seattle, Washington) and GBR (at Rugby, England) to APL (near Washington, D. C.), cover ≈120° in longitude and range from L ∼ 2.5 to L ∼ 4.0 in invariant latitude. These paths showed marked daytime and nighttime phase advances from 1650 UT on December 17 (in excess of 10 µs during maximum disturbance). The phase values did not return to prestorm levels before December 22–23. The unusual presence of these daytime VLF disturbances is offered as evidence for the widespread precipitation at low L shell values of nearly relativistic electrons (Ee ≳ 200 keV) which would be required to penetrate below ∼70-km altitude to affect the daytime VLF transmissions. Wave guide mode calculations using D region electron density profiles deduced from the satellite particle data predict phase advances which agree reasonably well with the observed values. It is concluded that the observed long-lived VLF phase disturbances can be explained by excess D region ionization caused by energetic electrons precipitating from the earth's radiation belt following their injection deep into the magnetosphere during the magnetic storm.