Diurnal variation of burst precipitation effects on subionospheric VLF/LF signal propagation near L = 2

Abstract

Burst precipitation effects on subionospheric VLF/LF signal propagation are studied using data recorded at Palmer Station, Antarctica (L ∼ 2.4) between March 3 and April 7, 1983. Relatively abrupt signal amplitude changes, known as “Trimpi” effects, were observed in conjunction with magnetospheric whistlers, which are inferred to induce electron precipitation that in turn causes enhanced ionization at the 80 to 90‐km altitude level. Earlier findings that Trimpi effects occur predominantly under nighttime ionospheric conditions were confirmed by comparing the sunrise‐sunset terminator position with perturbation activity on three VLF/LF signal paths for which the arrival bearings range from magnetic north to west. Events were found to occur on 70% of the observing nights. Signal paths making larger angles with the terminator were used to deduce that most events were caused by ionospheric effects occurring within ∼1000 km of Palmer Station. Some events were observed after sunrise over the observing station, apparently due to precipitation in dark regions as far as ∼1800 km away. Observed differences between two different paths in the occurrence of this postsunrise continuation of Trimpi events suggest that the Trimpi event activity rate decreases below L ∼2. This may be due to a reduction in whistler activity and/or lower particle scattering efficiency. The average hourly occurrence rate of transmitter NSS events on the 11 most active nights studied increased from sunset to near midnight and then remained at a roughly constant level until sunrise. The influence of the whistler rate and of whistler path locations on Trimpi effects is clearly important and requires further study.