Ionospheric absorption investigations at Hawaii and Johnston Island

Abstract

Measurements of ionospheric absorption by the cosmic-noise monitoring method show that, at certain tropical latitudes, an irregular component of absorption is often present in the evening hours. If the variable absorption is present at one observing site (Johnston Island), then variations are also likely to be present at another station about 1325 km away (Hawaii). The individual variations of absorption as a function of time are not correlated at the two stations, however, suggesting that the scale of the patches causing the absorption must be less than about 1000 km. An attempt is made to find a correspondence between hourly averages of the apparent absorption with other ionospheric parameters. Both a nighttime and daytime absorption are observed. A greater nighttime component appears at Johnston Island than at Hawaii, implying the existence of a latitude dependence. Correlation with spread F or with sporadic E on the basis of ionosonde data from Maui was not found, although a correlation is apparent between cosmic-noise absorption and ionosonde minimum reflection frequency during geomagnetically quiet periods. There is good correlation between average hourly values of the absorption and F2 critical frequency ƒoF2. This latter observation is explainable by the shielding effect of the F region.