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Abstract.A thin and highly dense sporadic E layer, which can occasionally block the upper ionospheric layers, is called blanketing sporadic E (E sb ). We present the statistical seasonal local time occurrence pattern of E sb at equatorial station Tirunelveli (8.7 • N, 77.8 • E, dip latitude 0.7 • N) during the extended minimum of solar cycle 24 (2007-2009). In spite of nearly the same average solar activity during both 2007 and 2009, considerable differences are noticed in the seasonal occurrence of E sb during this period. The percentage of E sb occurrence is found to be the highest during the summer solstice (≥ 50 %) for both 2007 and 2009, which is in general accordance with the earlier studies. The occurrences of E sb during the vernal equinox (∼ 33 %) and January-February (∼ 28 %) are substantial in 2009 as compared to those during the same seasons in 2007. We find that, during winter (January-February), ∼ 75 % of E sb occurred during or just after the period of sudden stratospheric warming (SSW). We suggest that enhanced E sb occurrence during winter (January-February) and the vernal equinox of 2009 could be associated with SSW-driven changes in the E region ambient conditions. Furthermore, the close association of E sb with counter equatorial electrojet (CEEJ) suggested by earlier studies is re-examined carefully using the scenario of E sb occurrence on non-CEEJ days. Such an exercise is crucial as we are unaware whether the physical mechanisms driving E sb and CEEJ are linked or not. We find that, of all the seasons, the association of E sb and CEEJ is strongest during winter (November-December).

Abstract. We have studied Forbush decrease (FD) event occurred on February 14, 1978 using 43 neutron monitor observatories to understand the global signature of FD. We have studied rigidity dependence of shock amplitude and total FD amplitude. We have found almost the same power law index for both shock phase amplitude and total FD amplitude.Local time variation of shock phase amplitude and maximum depression time of FD have been investigated which indicate possible effect of shock/CME orientation. We have analyzed rigidity dependence of time constants of two phase recovery. Time constants of slow component of recovery phase show rigidity dependence and implies possible effect of diffusion. Solar wind speed was observed to be well correlated with slow component of FD recovery phase. This indicates solar wind speed as possible driver of recovery phase. To investigate the contribution of interplanetary drivers, shock and CME in FD, we have used shock-only and CME-only models. We have applied these models separately to shock phase and main phase amplitudes respectively. This confirms present accepted physical scenario that the first step of FD is due to propagating shock barrier and second step is due to flux rope of CME/magnetic cloud.

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Generation of equatorial spread F (ESF) irregularities resulting from magnetic disturbance is known for past few decades. However, better prediction models for this phenomenon are still lacking. Magnetic storms also affects the F region plasma drifts. In this work we examined variability in (i) occurrence of such freshly generated ESF and (ii) low‐latitude F region zonal plasma drifts over Indian longitude. For this purpose simultaneous observations of amplitude scintillations on 251 MHz signal, recorded by a network of spaced receivers located at low‐latitude stations, are utilized. Observational stations are situated such that it longitudinally (latitudinally) covers an area of 5.6° (13°). Here effect of disturbance dynamo (DD) electric field at low‐latitude F region and its variability are studied for three magnetic storms occurring in 2011. These magnetic storms are having nearly similar type characteristics except their start time. We find that as time difference (i.e., ΔT) between local sunset and start of magnetic activity decreases, the DD effects seen at low‐latitude F region zonal irregularity drift around midnight becomes stronger. For a given magnetic storm the DD effect on F region zonal irregularity drifts is found to be only marginally stronger at dip equator in comparison to off‐equatorial stations. Although effect of DD on F region zonal irregularity drifts are felt simultaneously, generation of fresh ESF is variable within a smaller longitudinal belt of 5.6°. It is attributed to the presence of LSWS at the bottomside of F region, as initiation of ESF is highly likely (unlikely) in the vicinity of crest (trough) of such LSWS.