A study of tomographically reconstructed ionospheric images during a solar eclipse

The low-latitude ionospheric tomography network (LITN) consists of a chain of six Naval Navigation Satellite System (NNSS) receiving stations established along 121 degrees E longitude from a geographic latitude of 14.6 degrees N to 31 degrees N. ft is specifically designed to observe large-scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques. Recently, the network LITN was applied to observations of the October 24, 1995, solar eclipse. Two-dimensional images of ionospheric electron density during the eclipse period were reconstructed. These images and the corresponding results from a nearby ionosonde were compared with those for a reference day. It is shown that during the eclipse day the ionosphere experienced some large-scale changes. In particular, four episodes of electron density enhancement or depression have been identified. (1) The maximum enhancement occurred before the maximum phase of the solar eclipse at approximately 7 degrees- 10 degrees N geomagnetic latitude at the 275-300 km ionospheric height. (2) The second enhancement appeared roughly 3 1/2 hours after the maximum obscuration at 15 degrees-22 degrees N geomagnetic latitude and 300-325 km ionospheric height. (3) The largest electron density depression occurred roughly 2 hours after the maximum obscuration at approximately 9 degrees-15 degrees N geomagnetic latitude and on both the bottom and topside ionosphere. (4) The-second depression occurred about 4 hours after the maximum obscuration at approximately 5.5 degrees N geomagnetic latitude and mainly on the topside ionosphere. More detailed study suggests that the two enhancements have their origins in the ionospheric day-to-day variations, the first depression is related to the combined photochemical and the equatorial fountain effects, and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres. These observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region.