Variability of the F-region parameter h0.5

Abstract

High resolution electron density profiles, measured with the Arecibo incoherent scatter radar have been used to derive h0.5, the height of the half density point. More than 2400 profiles, for the period August 1974 – May 1977, have been used to study the behaviour of this parameter. Our analysis shows that during the night, h0.5 varies mostly between 200 and 350 km, but during the day the excursion is larger and values lower than 150 km are also seen. These lower values are often coincident with the presence of a layer (F1) between h0.5 and hmF2. We have therefore, divided the daytime data into two classes, class ‘A’ where h0.5 is more than hmF1 and class ‘B’ where h0.5 is less than hmF1. We find that in both the classes, most of the variability in h0.5 is due to the variability in hmF2 and a linear relationship between h0.5 and hmF2 is seen, although the dispersion in class ‘B’ is larger. Within the same class, no difference is seen in the relationship between day and night. We also compare the parameter Y0.5 hmF2 obtained from the Arecibo measurements with that calculated from the IRI-90 model, based upon Gulyaeva's (1987) formula. In class ‘A’ the median values of this parameter vary between 0.15 and 2.0 with no major seasonal differences. The IRI values, however, are significantly larger. On the other hand, class ‘B’ shows larger seasonal differences, particularly between summer and winter, but the discrepancy with the IRI is somewhat smaller.