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The VHF amplitude scintillation recorded during the period January 1991 to\udDecember 1993 in the declining phase of a solar cycle and April 1998 to December 1999 in\udthe ascending phase of the next solar cycle at Varanasi (geogr. lat.=25.3°,\udlong.=83.0°, dip=37°N) have been analyzed to\udstudy the behavior of ionospheric irregularities during active solar periods\udand magnetic storms. It is shown that irregularities occur at arbitrary\udtimes and may last for <30min. A rise in solar activity increases\udscintillations during winter (November-February) and near equinoxes\ud(March-April; September-October), whereas it depresses the scintillations\udduring the summer (May-July). In general, the role of magnetic activity is to\udsuppress scintillations in the pre-midnight period and to increase it in the\udpost-midnight period during equinox and winter seasons, whilst during summer\udmonths the effect is reversed. The pre-midnight scintillation is sometimes\udobserved when the main phase of <i>D<sub>st</sub></i> corresponds to the pre-midnight period.\udThe annual variation shows suppression of scintillations on disturbed days,\udboth during pre-midnight and post-midnight period, which becomes more\udeffective during years of high solar activity. It is observed that for\udmagnetic storms for which the recovery phase starts post-midnight, the\udprobability of occurrence of irregularities is enhanced during this time. If\udthe magnetic storm occurred during daytime, then the probability of\udoccurrence of scintillations during the night hours is decreased. The\udpenetration of magnetospheric electric fields to the magnetic equator\udaffects the evolution of low-latitude irregularities. A delayed disturbance\uddynamo electric field also affects the development of irregularities

Effect of solar and magnetic activity on VHF scintillations near the equatorial anomaly crest