Relativistic electron precipitation (REP) has been monitored by observing anomalous changes in phaseor amplitude of ionospheric radio waves which propagate through the ionospheric D region. Magnetospheric substorm activity appears to be a necessary though by no means sufficient condition for producing intense REP events. A general correlation is found between the onset of substorm activity and nocturnal REP events. In contrast, dayside REP events occur less frequently, are more restricted in local time, and are often delayed by up to several hours following the onset of substorm activity. The observational evidence supports the concept of parasitic strong diffusion scattering of trapped relativistic electrons by intense ion cyclotron turbulence which is generated following the injection of ring current ions into the outer radiation zone during the substorm expansive phase.

This article presents the results of a study of the energetic electron precipitation as well as important parameters of the D region over Ottawa (45°N, 76°W) during lightly to moderately disturbed conditions, using data for the period following the December 17, 1971, magnetic storm as well as data obtained on other selected days exhibiting anomalous D region absorption. Following the December 17, 1971, storm, significant fluxes of precipitating electrons of > 130 keV were observed near Ottawa, even during the poststorm period on December 20, when the geomagnetic activity had subsided (∑ Kp = 6). The excess ionization detected at D region altitudes above Ottawa can be explained as being due to ionization from a prolonged electron drizzle from the outer radiation belt. For the first time it has been proven that precipitating electrons from the radiation belt were the main cause of D region poststorm conditions of middle latitudes. The ground‐based radio probing measurements of the free electron concentration have been coordinated with simultaneous satellite observations of the quasi‐trapped and precipitating electron environment above the site. From the ground‐based measurements, by using the partial reflection technique at two frequencies (2.66 and 6.275 MHz), electron concentration profiles for altitudes between ≈ 60 and 90 km have been derived at times when the low‐altitude polar‐orbiting satellite 1971‐089A passed near Ottawa. On board this satellite, electron differential fluxes in 260 channels and at two pitch angles were measured at energies between ≈1 and ≈2800 keV. The ion pair production rate height profiles due to precipitating electrons were computed for D region heights by using nonisotropic energy‐dependent pitch angle distributions. Effective electron loss rates were deduced for heights between 63 and 91 km. These results indicate a significant variability in loss rates in the altitude range 75–85 km.

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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.

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