Characteristics of the equatorial ionization anomaly in relation to the day-to-day variability of ionospheric irregularities around the postsunset period

The equatorial ionosphere is characterized by the equatorial ionization anomaly (EIA) over a major part of the day, and ionospheric F region irregularities causing amplitude and phase scintillations on transionospheric satellite links during the postsunset period. Scintillations of transionospheric signals constitute one of the most intense Space Weather related propagation effects and exhibit extreme variability in space and time. The EIA exhibits an asymmetry in terms of the extents of the crests and peak ion densities on either side of the magnetic equator. It has been suggested that transequatorial winds cause significant redistribution of ionization with respect to the magnetic equator resulting in this asymmetry. The asymmetry in EIA between the two hemispheres has been suggested to influence growth of plasma instability and hence occurrence of scintillations. This paper presents some measures of EIA asymmetry derived from the latitudinal distribution of topside F region ionization density over the magnetic equator in the Indian longitude sector obtained from DMSP satellites and investigates their correspondence with the day-to-day variability of geostationary L band scintillations observed from Calcutta (latitude: 22.58°N; longitude: 88.38°E geographic; magnetic dip: 32°N) situated near the northern crest of the EIA during the high sunspot number years 2000–2001. A parameter combining the strength as well as the asymmetry of the EIA has been derived which shows good correlation with the occurrence of postsunset geostationary L band scintillations. These quantitative estimates may be used for forecasting occurrences of scintillations by system operators.