Current Problems in Polar-Cap Absorption

The night and day relationships between polar cap riometer absorption and solar protons

“…At night, however, the O concentration decreases very sharply below about 75 km, while above that altitude it is similar to its day level [Hunt, 1966;Swider et al, 1971; see also profiles in Narcisi, 1972]. This then introduces a diurnal variation in electron density below 75 km that is believed to be responsible for the observed diurnal variation in absorption [LeLevier and Branscomb, 1968;Reid, 1969Reid, , 1970. It is important to note, however, that above 75 km this detachment mechanism is still operative at night, and here En+ << D as in daytime conditions [Stroscio and Sellers, 1974].…”

“…Hence such a study was not made. Earlier, Reid [1969Reid [ , 1970 had investigated the correlation of absorption with E• for both day and night, although values of re(E,) were not derived. In one of these studies [Reid, 1970], a general survey of polar cap absorption (PCA) event problems, it was concluded that "... during the night the riometer is apparently more sensitive to protons with energies between 3 and 10 MeV than to the fluxes above 20 MeV that cause the bulk of the daytime absorption."…”

“…Earlier, Reid [1969Reid [ , 1970 had investigated the correlation of absorption with E• for both day and night, although values of re(E,) were not derived. In one of these studies [Reid, 1970], a general survey of polar cap absorption (PCA) event problems, it was concluded that "... during the night the riometer is apparently more sensitive to protons with energies between 3 and 10 MeV than to the fluxes above 20 MeV that cause the bulk of the daytime absorption." The other study was associated specifically with the 2 September 1966 SPE [Reid, 1969], where the conclusion was: "These results demonstrate that daytime PCA measurements appear to give a good indication of time variations in the flux of protons with energy greater than about 5 MeV, whereas nighttime measurements tend to emphasize variations in the flux at somewhat lower energies, in the neighborhood of 3 MeV."…”

The Relationship Between Polar Cap Riometer Absorption and Solar Particles.

“…[Reid,1 9b 5 , p 2 2 7 ; F i cht e l et al, 1 96 3 ; V a n A l l e n et al . , In one of these studies [Reid, 1970], a g e n e r a l s u r v e y of pola r cap absorption (PCA) event problems , it w as conc luded " . .…”

ELF sferic occurrences in the Antarctic during a solar proton event: Case study of occurrences at Byrd Station during the event of June 9, 1968

“…The mos t genera l reason follo ws from the observa tion of SPE-rela ted chan ges at frequencies above the ELF range: it is well-known that electromagnetic waves with frequencies above the E~F range are susce ptib le to a w id e variety of SPE effects when propagating either directly through the polar ionosphere or through a polar region in the earth-ionosphere waveguide. These effects include intense absorption , now known as polar -cap absorption (PCA), for waves with frequencies in the HF MHz) an d lower VHF (30-300 MHz) bands , and phase and ampl i tude changes for waves with frequencies in the VLF (3-30 kHz) and LF (30-300 kHz) bands ( References 17 ,18 ,19 ,and 20). Because of their proximity in frequency to the ELF range , and their stability of propagation (which has led to the use of these frequencies for coninunication , navigation , and precise time and frequency comparisons over global distances [Reference 20]), the SPE-induced changes in the propagation characteristics of the VLF waves are of par ti cular s i gn i f i cance .…”