Sudha Ravindran

Small-scale (∼3 m) E region irregularities at and off the magnetic equator

VHF backscatter radar observations at and off the magnetic equator (Trivandrum and Gadanki, respectively) have been analyzed to study the small-scale (∼3 m) irregularities at E region altitudes. The Doppler spectra observed at Gadanki invariably resemble type 2 (gradient drift instability) Doppler spectra in contrast to the spectra at Trivandrum where both type 1 and type 2 occur. The type 2 irregularities (at Gadanki) are attributed to the presence of steep (vertical) electron density gradients, which are necessary for the growth of gradient drift instabilities at the off magnetic equatorial latitude of Gadanki. The observed irregularity drifts at lower altitudes ( 101 km) to magnetic field line-linked F region electric fields at the magnetic equator.

Additional stratifications in the equatorial F region at dawn and dusk during geomagnetic storms: Role of electrodynamics

[1] The role of electrodynamics in producing additional stratifications in the equatorial F region (F 3 layer) at dawn and dusk during geomagnetic storms is discussed. Two cases of F 3 layer at dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case of F 3 layer at dusk (1600-1730 LT on 5 October 2000) are observed, for the first time, by the digital ionosonde at the equatorial station Trivandrum (8.5°N; 77°E; dip ∼ 0.5°N) in India. The unusual F 3 layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field B z , suggesting that eastward prompt penetration electric field could be the main cause of the F 3 layers. The dawn F 3 layer on 5 October is modeled using the Sheffield University Plasmasphere-Ionosphere Model by using the E x B drift estimated from the real height variation of the ionospheric peak during the morning period. The model qualitatively reproduces the dawn F 3 layer. While the existing F 2 layer rapidly drifts upward and forms the F 3 layer and topside ledge, a new layer forming at lower heights develops into the normal F 2 layer.

Observations of the atmospheric surface layer parameters over a semi arid region during the solar eclipse of August 11th, 1999

This paper discusses the observations of the Atmospheric Surface Layer (ASL) parameters during the solar eclipse of August 11th, 1999. Intensive surface layer experiments were conducted at Ahmedabad (23‡21′N, 72‡36′E), the western part of India, which was close to the totality path. This rare event provided by nature is utilised to document the surface layer effects during the eclipse period using measurements of high frequency fluctuations of temperature, tri-axial wind components as well as mean parameters such as temperature, humidity, wind speed and subsoil temperature. Analysis showed that during the eclipse period, the turbulence parameters were affected leading to the suppression of the turbulence process, the main dynamic process in the atmospheric boundary layer, while the mean parameters showed variations within the natural variability of the observational period. The spectra of the wind components and temperature indicated decrease in spectral power by one order in magnitude during the eclipse period. The rate of dissipation of turbulent kinetic energy is found to decrease by more than one order during the eclipse period. The stability parameter showed a change from unstable to stable condition during the period of eclipse and back to unstable condition by the end of eclipse

Variation of type I plasma wave phase velocity with electron drift velocity in the equatorial electrojet

Using a 54.95-MHz coherent backscatter radar at Thumba (8.5°N, 77°E; 0.5°N dip angle), the phase velocity variations of type I plasma waves generated by the modified two-stream instability process in the equatorial electrojet have been measured on several occasions of large and rapid electric field fluctuations associated with geomagnetic substorms and storms. The measurements show a clear linear variation of type I wave phase velocity (VpI) over a considerable range with the simultaneously measured phase velocity (VpII) of gradient drift instability-generated type II plasma waves. Since VpII is proportional to the drift velocity of electrons, the observed variations of VpI with VpII mean that VpI varies in step with the variations of the electron drift velocity Vey. The observed variation of VpI with Vey is consistent with the current theoretical understanding that the plasma wave-associated electric fields cause anomalous electron diffusion and heating which result in the stabilization of the type I wave phase velocity near the increased value of the ion-acoustic velocity. The enhanced electron temperatures estimated from the largest values of the observed VpI are found to be 2.0-2.4 times the ambient value in the absence of waves.

Toward prediction of L band scintillations in the equatorial ionization anomaly region

[1] The first observations of the duration and spread of equatorial spread F (ESF) at the magnetic equator and their relationship with the L band scintillations in the equatorial ionization anomaly (EIA) region have been presented here. The analysis is done for the equinoctial months of low solar activity period 2005–2006 and the moderate solar activity year 2004. Ionosonde and CRABEX data from Trivandrum and GPS data from four stations in the EIA region centered around 77°E meridian have been used for the study. The results show that the maximum scintillation index (s4) in the EIA region is linearly dependent on the spread of ESF traces for both the equinoxes. The corresponding duration of L band scintillations is also found to be linearly dependent on the duration of ESF at the magnetic equator. Further, the study for the first time reveals the plausible use of the ESF prediction parameter during 1600–1845 IST period for predicting L band scintillations and its inverse relationship with F10.7 cm flux.

Effect of small‐scale plasma turbulence on altitude profiles of electron drift velocity in the equatorial electrojet: An experimental study

The authors report recent observations of the effect of turbulence on the altitude behavior of the equatorial electrojet. Their results show that with an increase in observed turbulence that the altitude of maximum in the electron drift velocity shifts to higher altitudes. This is consistent with recent theoretical work which shows that small scale turbulence can produce large-scale changes in the dynamics of the equatorial electrojet.

Occurrence of type I plasma waves in the equatorial electrojet during morning and evening hours

Type I waves are generally observed under strong electrojet (EEJ) conditions because of the threshold condition on the electron drift velocity (with respect to ions) for their generation. In this paper we present observation of type I waves during morning and evening hours under weak (as revealed from the low values of mean drift velocity of type II waves and also from low ground magnetic field values) EEJ conditions. In these conditions, type I waves are observed when the plasma turbulence in the EEJ as revealed by the width of the Doppler spectra is high even though the electron drift velocity as given by the mean Doppler frequency is much smaller than the ion acoustic velocity. This suggests that there are regions within the volume probed by the radar wherein the electron drift velocity exceeds the ion acoustic velocity Cs (even though the mean electron drift velocity is very much less than Cs) due to the irregular electric fields of the primary irregularities.

The seasonal and solar cycle variations of electron density gradient scale length, vertical drift and layer height during magnetically quiet days: Implications for Spread F over Trivandrum, India

A study has been carried out on the behaviour of electron density gradient scale length, L, vertical drift and layer height, around post sunset hours, during the magnetically quiet days of summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995), using ionosonde data of Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) in the Indian longitude sector. The results indicate a clear seasonal and solar cycle variation in all the three parameters. Further, the seasonal variation of equatorial Spread F (ESF) during the above period is examined in terms of the relative roles of L, the vertical drift and layer height (of the F layer) in the triggering of the collisional Rayleigh-Taylor instability. The results, show for the first time, that L also plays an important role, in controlling the quiet time seasonal and solar cycle variability of ESF; whereas in earlier studies this parameter had been taken to be constant. The detailed results are presented and discussed.

The seasonal and solar cycle variations of electron density gradient scale length during magnetically disturbed days: implications for Spread F

The behaviour of electron density gradient scale length, L, around post-sunset hours during the magnetically disturbed days of the summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995) has been studied, using ionosonde data of Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) in the Indian longitude sector. The results indicate a clear seasonal and solar cycle variation in L. Seasonal variations of the maximum vertical drift of the F layer were also examined on these days. In particular, the seasonal variation of the Equatorial Spread F (ESF) during this period is examined in terms of the relative roles of L and the vertical drift of the F layer in the triggering of the collisional Rayleigh-Taylor instability. Our results on the clear-cut seasonal and solar cycle variation in L for disturbed days and its control of ESF occurrence are presented and discussed.