No abstract

Detailed investigations have been carried out on the occurrence of bottom side Equatorial Spread F (ESF) and the thermospheric meridional wind characteristics just before the former's initiation using ground based ionospheric data corresponding to the equinoctial periods of 1993–1998, from Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) and Sriharikota (13.7°N, 80.2°E, dip ∼ 10°N) in the Indian longitudes. Critical analysis of the base height of the F‐region h′F at the time of triggering of ESF and the polarity of the meridional winds revealed that, if the h′F is above a certain level ESF occurred under both equatorward and poleward wind conditions. Below that level, ESF occurred only when equatorward winds were present implying that the equatorward winds must somehow be able to offset the reduced growth rate of the plasma instability responsible for ESF. A plausible explanation linking Equatorial Ionization Anomaly (EIA) and the consequent Equatorial Temperature and Wind Anomaly (ETWA) and the consequent neutral dynamics effectively enabling the instability even at lower height has been offered. The threshold height (h′F)c gleaned out on the basis of the polarity of the meridional winds has been shown to bear a linear relation to the solar activity and sheds light on the enigmatic short and long term variability of ESF.

Abstract. The effects on the electrodynamics of the equatorial E-and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5 • N; 77 • E; dip lat. 0.5 • N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E-and Fregions in terms of (i) the sudden intensification of a weak blanketing E S -layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing E S , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the Eregion loading of the F-region dynamo, resulting in a larger post sunset F-region height (h F ) rise. These aspects of Eand F-region behaviour on the eclipse day are discussed in relation to those observed on the control day.

Simultaneous observations of E and F region irregularities made using the Gadanki MST radar are presented. The observations show that the E region echoes weaken or disappear during the growth phase of the topside F region irregularities. Unlike Jicamarca observations, no valley region echoes are observed during this phase. It is shown that the weakening or disappearance of E region signals are not directly coupled with the F region irregularities just overhead, but linked with the instability processes over the magnetic equator through the magnetic field lines. It is proposed that the fringe fields present in the valley region in association with the equatorial F region plasma bubbles, in the presence of appropriate background electric field conditions, are responsible candidates. It is shown that these fringe fields and the electric fields associated with the irregularities in the valley region can map to the low latitude E region and thereby inhibit the growth of the E region instability processes as revealed by the Gadanki radar observations.

[1] Simultaneous observations of equatorial spread F irregularities made with an 18 MHz radar from Trivandrum, located at the geomagnetic equator, and a 53 MHz radar from Gadanki, located at a magnetic latitude of 6.5°N, corresponding to nearly the same longitude zone, are presented. The observations correspond to 8.3 and 2.8 m irregularities, respectively. The spread F irregularities at both the locations are found to occur nearly at the same time but are observed for longer duration at Gadanki than at Trivandrum. The spread F structures as observed in the intensity maps corresponding to Gadanki are characterized by multiple periodic plumes in contrast to a limited number of plumes observed over Trivandrum. The Doppler velocities associated with these irregularities corresponding to Trivandrum are in the range of À100-150 m s À1, whereas they are in the range of À100-250 m s À1 at Gadanki. Further, the fluctuating velocity fields are much stronger in the Gadanki observations than in the Trivandrum observations. Remarkably, the spectral widths are <100 m s À1 in Trivandrum observations in contrast to those observed at Gadanki with values as high as 300 m s À1 . The observations are compared with those made elsewhere and are discussed in the light of current understanding of the meter-scale irregularities responsible for the radar backscatter.

[1] This paper presents a study based on Gadanki radar observations of 150 km echoes made during the solar eclipse of 15 January 2010 that occurred during 1122-1515 IST. Radar echoes were observed only during 1150-1215 IST and 1344-1356 IST linked with the entry and recovery phases of the eclipse, respectively. The most striking observation found is the unusual ascending and descending features of the echoing regions observed during these two time periods. Although these echoes occurred at higher altitudes than those of control days, SNR and spectral width of these echoes are similar to those of the lower echoing region observed on the control days. Further, Doppler velocities suggest the presence of westward electric field unlike those of the control days. Concurrent ionosonde observations showed ascent and descent of the F 1 layer very similar to those observed in the 150 km echoes. These observations and related analysis suggest that the observed echoes were due to the combined action of the electron density gradients and the reduced recombination rate linked with the solar eclipse effect. These observations are first of its kind and elucidate the role of density gradient and recombination rate in the 150 km echoing process.

Abstract. In this paper we present a case study of the annular solar eclipse effects on the ionization of E and F regions of equatorial ionosphere over Tirunelveli [77.8 • E, 8.7 • N, dip 0.4 • N] by means of digital ionosonde on 15 January 2010. The maximum obscuration of the eclipse at this station was 84 % and it occurred in the afternoon. The E and F1 layers of the ionosphere showed very clear decrease in their electron concentrations, whereas the F2 layer did not show appreciable changes. A reduction of 30 % was observed in the foF1 during the maximum phase of the eclipse. During the beginning phase of the eclipse, an enhancement of 0.97 MHz was observed in the foF2 as compared to that of the control days. But the foF2 decreased gradually as the eclipse progressed and a decrease of 0.59 MHz was observed towards the end phase of the eclipse. Observed variations in the h F2 and hmF2 showed lower values than the control days, although hmF2 was found to increase a bit during the eclipse. Observed variability in the E, F1 and F2 layer ionospheric parameters on the eclipse day and their departure from the control days are discussed as the combined effect of annular eclipse and presence of counter equatorial electrojet (CEEJ).

[1] The characteristics of different types of Sporadic E (E S ) layers and the associated plasma density irregularities over the magnetic equator have been studied in a campaign mode using VHF backscatter radar, digital ionosonde, and ground magnetometer data from Trivandrum (dip latitude 0.5°N, geographic latitude 8.5°N, geographic longitude 77°E), India. The presence of blanketing type E S (E Sb ) in the ionograms with varying intensity and duration were observed in association with afternoon Counter Equatorial Electrojet (CEEJ) events. E Sb was associated with intense backscatter returns and with either very low zonal electric field and/or with distortions present in the altitude profile of the drift velocity of the type II irregularities. The results of the coordinated study indicate the possible role of vertical electron density gradients in E Sb layers in addition to providing evidence for the local winds to be responsible for the vertical gradients themselves.