IONOSPHERIC EFFECTS OF THE SOLAR ECLIPSE OF SEPTEMBER 23, 1987, AROUND THE EQUATORIAL ANOMALY CREST REGION

The ionospheric responses to the solar eclipse of September 23, 1987, in the equatorial anomaly crest region have been investigated by using ionospheric vertical sounding, VLF propagation delay time, and differential Doppler shift data observed at Chungli, which is located near the northern equatorial anomaly crest region. It has been found that temporal variations of the F1 layer and D region are mainly controlled by local solar radiation. Quantitative analysis of the variations of the F1 layer critical frequency, f(o)F1, shows that electrons are removed from the F1 layer through ionic recombination. However, the temporal variations of f(o)F2 and electron density above 200 km show that the variations of the F2 layer around the equatorial anomaly region are controlled not by local solar radiation but by solar radiation at the equator. The fountain effect plays an important role even during the solar eclipse. The VLF propagation time delay is controlled by the variations of average path obscuration. Atmospheric gravity waves produced by the moving bow wave front of the solar eclipse are found with a period around 17-23 min and wavelength about 293 km.