In this paper, the observations of additional F layer stratification are presented in a region which is situated at the outer edge of equatorial ionization anomaly (EIA) zone. The digital ionosonde data observed over Delhi(28.6° N, 77.2° E, dip 42.44° N)during March-April, 2001 has been used to carry out the present work. The observational period was geomagnetically disturbed and represents the high solar activity phase of 23rd solar cycle. The additional transient cusp is frequently observed in the considered months before the noon hours; however, in this paper only five prominent cases are presented. From the analysis of ionograms, it is observed that the transient additional cusp is formed between the pre-existing F_1 and F_2 layer; hence, named as the cusp of F_1.5. Study reveals that the Traveling Atmospheric Disturbances(TADs)along with the vertical expansion of F layer provides the necessary condition for the existence of this transient feature. The intensification of F_1 layer along with increased altitude immediately after the disappearance of additional stratification remains one of the fascinating features of the present results. The present investigation demonstrates that the daytime F_1 and F_2 region over low-midlatitude station is strongly modulated by the passage of TIDs, originating at high latitudes or by atmospheric disturbances of local origin during the high solar activity period. The concurrent presence of TADs and the associated disturbance composition appears to be plausible reason behind the present observations.
展开▼
