Geomagnetic disturbance effects on the occurrence of range and frequency spread at Huancayo are studied for the period 1957–74. The occurrence of frequency spread is decreased on disturbed days forD-months andE-months while duringJ-months an increase is noted in the post-midnight period. The occurrence of range spread is decreased on disturbed days in the pre-midnight hours duringD andE-months. Post-midnight hours ofD andE-months and all hours of night duringJ-months show an increase of range spread on disturbed days, which is most prominent during low sunspot years. The mean occurrence of frequency spread (2300–0100 LT) and range spread (2000–2300 LT) decrease with increasing ΣKp. On the other hand, occurrence of range spread (0300–0500 LT) increases with increasing ΣKp, and is suggested as the consequence of the changes of the horizontal electric field in theF-region associated with the geomagnetic storms.

Daily variations of the E-W and N-S components of ionospheric drifts, measured using closely spaced receiver method over Tiruchirapalli near magnetic equator, are described for the periods 1973–75 forE region and 1974–75 forF region. N-S component is not observed except for a few occasions during summer months for theE region. E-W component is generally eastward during night hours and westward during day hours. There is a large day-to-day variability in the magnitude as well as in the evening reversal time. Daily variations primarily consist of the diurnal component with average amplitude of about 90 m/sec forF region and of about 80 m/sec forE region. The drift speeds are found to decrease with magnetic activity. It is concluded that the abnormal electrojet region extends upto north of Tiruchirapalli in the Indian zone and drifts here can be used as a monitor of electric field.

A number of spaced fading records obtained at Buckland Park, South Australia, using partial reflection technique (at 80–100 km) have been subjected to different methods of analysis, viz., time delay methods, the full correlation method and the cross-spectrum method. True drift velocities derived from the full correlation method appear to be the most correct. The variation of velocity with fading frequency obtained from cross-spectrum method is most likely to be due to the variation of velocity with time within the record itself.

The nature of theD-region irregularities responsible for partial reflections is studied using amplitude, phase and range information taken at Buckland Park, South Australia, utilising a 178-dipole receiving array. Stratifications with distinct layer type structures below 80 km are noticed. A major part of the echo amplitude consists of specularly reflected signals.

Faraday rotation measurements made at a chain of stations and group delay measurements made at Ootacamund from ATS-6 are examined for the partial solar eclipse event of 29 April 1976. There is no evidence of eclipse-induced gravity waves in these measurements extending from Ootacamund near dip equator to Patiala (dip 45° N). Eclipse-induced fluctuations were however reported at Trivandrum, about 300 km south of Ootacamund.

Ionospheric drift records obtained by the closely spaced receiver technique at Udaipur are subjected to low and high pass filtering with varying cutoff frequencies and the effects of filtering on various drift and anisotropy parameters as determined by full correlation analysis are studied. Limits in which filtering could be employed are suggested on the basis of results obtained.

Two meteor events which were sighted in the Gujarat skies of India, were accompained by the visibility of sporadicE ionization on the ionograms recorded at Ahmedabad (Geog. Lat. 23·2°N, long. 72·30°E). The first event was the Dhajala fireball which flashed into the geoatmosphere along an E-N to W-S trail at about 20·40 h IST on 28 January 1976; the closest distance of the ground projection of meteor trail from Ahmedabad was 50km. The other event was a possible meteor group sighted over Ahmedabad on 28 May 1978, at about 21·10 h IST. This work describes the nature of the sporadicE ionization observed on Ahmedabad ionograms during the two events. Features of theEs echo during the Dhajala event which indicate that it could be of meteoric origin are discussed. Meteor theory is used to relate the observed ionization with the physical dimensions of the Dhajala meteorite as obtained by other workers.

The geomagnetic storm of November 1998 is a unique event where IMF-Bz remained southward withvalues exceeding –15 nT for more than a day. The SYM/H index decreased from about 07 hr on 13November 1998 reaching a minimum of about –120 nT around midnight of 13–14 November 1998.Features of the equatorial electrojet in the Indian region are studied during the geomagnetic storm eventof 13–14 November 1998, based on the geomagnetic data from the chain of observatories in India. Suddennorthward turning of IMF-Bz for a very short duration around 08 hr on 13 November 1998 resultedin a small and very short duration counter electrojet. A strong (–50 nT) and a long duration counterelectrojet, right from 08 to 13 hr on 14 November 1998 was observed resulting in the absence of equatorialEs at Thumba. Absence of the equatorial ionization anomaly was also observed as seen from theionograms over Thumba and ionspheric data from Ahmedabad. The delayed effect on 14 November 1998is due to the disturbance dynamo effect.

The effect of solar flare, sudden commencement of magnetic storm and of the disturbances ring current on the equatorial electrojet in the Eastern Brazil region, where the ground magnetic declination is as large as 20^∘W is studied based on geomagnetic data with one minute resolution from Bacabal during November–December 1990. It is shown that the mean diurnal vector of the horizontal field was aligned along 2^∘E of north at Huancayo and 30^∘W of north at Bacabal during the month of December 1990. Number of solar flares that occurred on 30 December 1990 indicated the direction of solar flare related $\Delta H$ vector to be aligned along 5^∘E of north at Huancayo and 28^∘W of north at Bacabal. This is expected as the solar flare effects are due to the enhanced conductivity in the ionosphere. The SC at 2230 UT on 26 November 1990 produced a positive impulse in $\Delta X$ and negative impulse in $\Delta Y$ at Bacabal with $\Delta H$ vector aligned along 27^∘W of north. At Huancayo the $\Delta H$ vector associated with SC is aligned along 8^∘E of north, few degrees east to the alignment of the diurnal vector of H. The magnetic storm that followed the SC had a minimum Dst index of –150 nT. The corresponding storm time disturbance in $\Delta X$ at Huancayo as well as at Bacabal were about –250 nT but $\Delta Y$ at Bacabal was about +70 nT and very small at Huancayo, that give the alignment of the H vector due to ring current about 16∘W of north at Bacabal and almost along N–S at Huancayo. Thus alignment of the $\Delta H$ vector due to ring current at Bacabal is 14^∘E of the mean direction of $\Delta H$ vector during December 1990. This is consistent with the direction of ring current dependent on the dipole declination at the ring current altitude which is about 5^∘W of north over Bacabal and the deviation of declination due to the ring current during disturbed period given by the angle $(\psi-D)$.