J. Hanumath Sastri

Transient composite electric field disturbances near dip equator associated with auroral substorms

Ionosonde data of Kodaikanal (Geog.Long. 77° 29′E, dip 3.0°N) and Huancayo (Geog.Long. 75°18′W, dip 2.0°N) are used to show the simultaneous occurrence of a transient disturbance in F region height of composite polarity in day and night sectors near the dip equator during the auroral substorm activity on 20 August 1979. At Kodaikanal which is on the nightside at the time of the substorm activity, h′F first underwent an abrupt and rapid decrease (80km in 1 hr) followed by a much larger increase (120km in 1 hr). Perturbation in hpF2 of exactly opposite polarity was simultaneously seen at Huancayo which is on the dayside. The decrease in h′F at Kodaikanal (increase in hpF2 at Huancayo) occurred in association with an increase in polar cap potential drop, o (estimated from IMF parameters), and the subsequent increase (decrease at Huancayo) with a decrease in polar cap potential. The F-region height disturbance is interpreted as the manifestation of a global transient composite disturbance in equatorial zonal electric field caused by the prompt penetration of substorm-related high latitude electric fields into the equatorial ionosphere. The polarity pattern of the electric field disturbance is consistent with the global convection models which predict westward (eastward) electric fields at night (by day) near the geomagnetic equator in response to an increase in polar cap potential drop, and fields of opposite signs for a decrease in polar cap potential.

Longitudinal dependence of equatorial F region vertical plasma drifts in the dusk sector

The effect of solar activity on the postsunset peak of F region vertical plasma drift at Kodaikanal (77°28′E, dip 3°N) is determined as a function of season and for two levels of magnetic activity using extensive HF phase path observations and compared with that at Jicamarca (75° W, dip 2°N) reported in the literature. The postsunset peak vertical velocity Vzp at Kodaikanal increases linearly with 10.7-cm solar flux (F10.7) irrespective of season and level of magnetic activity. The sensitivity of Vzp to solar flux changes is more at Jicamarca than at Kodaikanal during equinoxes and December solstice. This feature is virtually absent in June solstice. The seasonally averaged values of dusktime vertical velocity at Kodaikanal for moderate to high solar flux conditions and quiet magnetic conditions(Kp ≤ 3) are in reasonable agreement with those reported very recently for the Indian sector (80°E) from AE-E satellite data [Fejer et al., 1995]. In June solstice, the evening upward vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160°–200°E), while the postreversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230°–310°E) where there they are higher. The present study supports the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June solstice.

Equatorial Ionosphere‐Thermosphere System During Geomagnetic Storms

This paper presents the recent progress in understanding the behavior of the structure and dynamics of the equatorial ionosphere-thermosphere system (EITS) under disturbed geomagnetic conditions. The storm-time modifications in the key parameters and major phenomena of the ionized and neutral domains of the system, which prevail with distinctive time delays with reference to the onset of enhanced geomagnetic activity and excess energy deposition at high latitudes, are traced broadly to fluid dynamical and electrodynamical couplings of the high latitude-low latitude upper atmospheres. Some of the major unsettled issues concerning the storm-time EITS that merit continued and concerted investigations for a comprehensive and quantitative empirical and theoretical understanding are highlighted.

Height gradient of F region vertical drift in the evening equatorial ionosphere

Simultaneous measurements of F-region vertical drift are made in the evening hours (1700-2100 IST) at Trivandrum (dip 0.6°N) and Kodaikanal (dip 4°N) on fifteen days during December 1993-January 1994 using the HF phase path technique on two different probing frequencies. The data are used to study the height dependence of vertical plasma drift in the bottomside F-region in the dusk sector after correcting the drifts (at Kodaikanal) for meridional wind effects and chemical loss. It is found that growth and decay of a positive height gradient in vertical drift occurs fairly regularly in the dusk period. On the average the vertical velocity gradient is positive in in the interval 1815-1925 IST and is preceded by negative values. The positive height gradient of vertical plasma drift below the F layer peak is interpreted in terms of altitude dependence of the relative contributions of E and F region dynamos to the electric fields responsible for plasma drifts (vertical and zonal) of the dusktime equatorial F-region. These results are for winter solstice solar minimum conditions.

Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

At dip equatorial stations in the Indian zone. spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N: 77.5°E: dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum The implications of this finding are discussed.

On the nature of substorm-related transient electric field disturbances in the equatorial ionosphere

Abstract Data derived from 5-min interval ionograms of Kodaikanal (10°14′N, 77°29′E, geomag. lat. 0.6°N) on the night of 29–30 August 1957 showed the presence of a marked perturbation in F-region height (h′F) in the midnight-to-morning period. The perturbation is characterized by a sudden decrease in h′F (65 km in 1 hr, with h′F reaching a low value of 200 km) followed by a prominent increase (200 km in 75 min) over a 3-h period. Changes in h′F of essentially the same nature were also noticed at Calcutta (geomag. lat. 12.25°N) and Ahmedabad (geomag. lat. 14°N) in the same longitude sector simultaneous with those at Kodaikanal. Examination of the auroral electrojet (AE) index and magnetograms of high latitude stations widely separated in longitude revealed the prevalence of an isolated substorm of moderate strength at the time of the perturbations in equatorial F-region height. The decrease in h′F is found to occur around the onset of the substorm and the subsequent increase during the substorm recovery phase. The observed F-region height disturbance is interpreted as the signature of a transient composite disturbance in the equatorial east-west (E-W) electric field caused by the prompt penetration of substormrelated perturbations in high latitude electric fields. The nature of the electric field disturbance is in good agreement with the recent modelling results which predict a westward peturbation in E-W field at equatorial latitudes in response to an increase in polar cap potential (around the onset of a substorm) and an eastward one with a decrease in polar cap potential (during the recovery phase) in the midnight-dawn sector.

On the development of abnormally large postsunset upward drift of equatorial F region under quiet geomagnetic conditions

The regular postsunset enhancement of upward drift of the equatorial F region is observed to be abnormally large on certain quiet days (Ap ≤ 5) as manifested by an anomalous dusktime increase of F region height. We explored the origin of this extreme form of the quiet-time variability of postsunset vertical drift/height through case studies using data from the ionosonde and magnetometer networks in the Indian equatorial region. It is found that on the days with an unsually large postsunset increase of F region height near the magnetic equator, the diurnal profile of the equatorial electrojet (EEJ) strength is severly distorted (with a shift, in some cases, of Sq(H) phase from the usual time interval, charactersitic of the abnormal quiet days) with enhanced EEJ conditions in the postnoon period (1300–1600 LT). This is accompanied, near the magnetic equator, by higher values of F layer peak height (hpF2) and lower values of peak electron density (ƒ0F2) in the early evening period (1600–1800 LT), compared with the monthly median/quiet day mean values. These changes in EEJ and hpF2/ƒ0F2 are consistently seen in all eight cases studied. It is suggested that the perturbations in plasma density distribution of equatorial F region increase the therinospheric zonal wind and its local time gradient and the ratio of flux-tube-integrated Pedersen conductivity of the F to E region. These modifications just prior to sunset of the properties of the equatorial thermosphere-ionosphere prompt an efficient F region dynamo action, resulting in the observed abnormally large dusktime increase of F region height.

Response of equatorial-low latitude ionosphere to sudden expansion of magnetosphere

The response of the equatorial-low latitude nightside ionosphere to the geomagnetic negative sudden impulse(si−) on March 15, 1993 is studied using recordings of Doppler velocity of ionospheric echoes at vertical incidence at Kodaikanal(10.2°N) and Doppler shift of standard HF signals on oblique paths at Lunping(25.0°N) and Kure(34.25°N). The si− at 1541 UT is characterised by a simple decrease of H-field at low latitude stations widely distributed in longitude, and by a double-pulse structure at mid and high latitude stations on the dayside. The usual downward drift of F region plasma during the premidnight hours over Kodaikanal near the dip equator abruptly increased for ≈ 2.5 min coincident with the first pulse of the si− and immediately reversed direction to upward. Recordings at Lunping and Kure also showed short-lived Doppler frequency deviations simulataneous with those at Kodaikanal and of the same polarity and sequence. These observations constitute the first and direct experimental evidence for vertical plasma motions due to si− associated electric fields in the nighttime equatorial-low latitude ionosphere. The case study supports the view that si− can be explained by the physical model of sc/si+ with a reversal in the direction of the global current systems responsible for the groundlevel magnetic field variations.

Optical interferometer measurements of thermospheric temperature at Kavalur (12.5°N, 78.5°E), India

Abstract First results on the behaviour of thermospheric temperature over Kavalur (12.5°N, 78.5°E geographic; 2.8°N geomagnetic latitude) located close to the geomagnetic equator in the Indian zone are presented. The results are based on measurements of the Doppler width of O( 1 D) night airglow emission at 630 nm made with a pressure-scanned Fabry-Perot interferometer (FPI) on 16 nights during March April 1992. The average nighttime (2130-0430 IST) thermospheric temperature is found to be consistently higher than the MSIS-86 predictions on all but one of the nights. The mean difference between the observed nightly temperatures and model values is 269 K with a standard error of 91 K. On one of the nights (9/10 April 1992, Ap = 6) the temperature is found to increase by ~250 K around 2330 IST and is accompanied by a ‘midnight collapse’ of the F -region over Ahmedabad (23°N, 72°E, dip 26.3°N). This relationship between the temperature increase at Kavalur and F -region height decrease at Ahmedabad is also seen in the average behaviour of the two parameters. The temperature enhancement at Kavalur is interpreted as the signature of the equatorial midnight temperature maximum (MTM) and the descent of the F -region over Ahmedabad as the effect of the poleward neutral winds associated with the MTM.

Doppler frequency fluctuations of lower thermospheric reflections in the equatorial electrojet region

Abstract A common and characteristic feature of lower F-region (~ 200 km) reflections at vertical incidence over Kodaikanal (dip 3.0°N) during the daytime is the presence of quasi-sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift, Δƒ ) with periodicities of 30–600 s. We have studied here the dependence of the lower-period segment (30–300 s) of the Doppler frequency variations on the ambient electrojet strength and associated ionospheric conditions. The level of wave activity in P in the period range 30–300 s is quantified by computing the variance of the P fluctuations synthesized through reverse FFT, in the chosen frequency band, of the FFT of original data. The variance of P (computed at ~1 h intervals) is found to bear a significant linear relationship to the ambient electrojet strength. P fluctuations in the range 30–120 s are found to be most sensitive to changes in electrojet strength compared to the other two sub-bands 120–210 s and 210–300 s, and tend to dominate, in general, the spectral content of P fluctuations in the range 30–300 s. The Doppler fluctuations in band 30–300 s are consistently seen to practically cease at times of disappearances of Esq on bottomside ionograms (i.e. during partial/complete counter-electrojet conditions). The short-period fluctuations in P are interpreted in terms of phase path changes imposed on lower F-region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (types I and II) in the daytime equatorial electrojet.