G. Manju

Gravity wave signatures in the dip equatorial ionosphere‐thermosphere system during the annular solar eclipse of 15 January 2010

The present work pertains to the eclipse-induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde-derived foF1 and foF2 parameters are used to analyze the characteristics of the eclipse-induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30–100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave-induced winds in generating blanketing Es over the equator is also examined.

On the solar activity variations of nocturnal F region vertical drifts covering two solar cycles in the Indian longitude sector

A comprehensive analysis of the seasonal and solar cycle variabilities of nighttime vertical drift over the Indian longitude sector is accomplished using ionosonde data located at the magnetic equatorial location, Trivandrum (8.5°N, 76.5°E). The analysis extends over a span of two decades (1988–2008). The representative seasonal variations based on the extensive data of nocturnal vertical drift during three different solar activity epochs is arrived at, for the first time. Seasonally, it is seen that maximum post sunset Vd is obtained in vernal equinox (VE), followed by autumnal equinox (AE), winter solstice (WS), and summer solstice (SS) for high and moderate solar epochs, while for low solar epoch, maximum Vd occurs in WS followed by VE, AE, and SS. Further, the role of sunset times at the magnetic conjugate points in modulating the time and magnitude of peak drifts during different solar epochs is ascertained. The equinoctial asymmetry in peak Vd during high and moderate solar epochs is another significant outcome of this study. The solar activity dependence of vertical drift for a wide range of solar fluxes has been quantified for all the seasons. In the present era of GPS-based communication and navigation, these are important results that give a better handle in understanding essential factors that impact equatorial ionospheric phenomena.

Ionospheric response to a geomagnetic storm during November 8-10, 2004

This paper investigates the response of the equatorial, and near equatorial, ionosphere to geomagnetic disturbances during the period November 8–10, 2004. Ionosonde data from Trivandrum (8.5°N 77°E and dip 0.5°N) and SHAR (13.5°N, 80.2°E, dip ~5.5°N), magnetic field data from Tirunelveli (8.7°N, 76.9°E, dip latitude 0.5°S) and Alibag (18.64°N, 72.87°E), and GUVI O/N2 data in the Indian longitude sector, are used for the study. The behavior of interplanetary parameters is also examined in conjunction with the ionospheric data. On 8 November, the EIA around noontime is not fully inhibited even though the electrojet strength an indicates inhibition of EIA due to a disturbance dynamo electric field effect. It is the enhanced O/N2 over TRV and SHAR, with a larger increase over SHAR, which results in a larger (than expected) value of the EIA proxy parameter. On 9 November, the comparable values of foF2 at TRV and SHAR around noon time is due to the combined effect of a weakened anomaly in the presence disturbance dynamo electric field effects leading to the EIA crest being near SHAR, and increased O/N2 values at TRV and SHAR with a larger increase at TRV. On 10 November, the very strong values of the EIA proxy-SHAR parameter is attributed to the combined effects of prompt penetration electric field related modulations of EIA, and significant O/N2 changes at the equatorial, and near equatorial, latitude. Thus, the study reveals the important role of storm-induced O/N2 changes, along with prompt penetration electric fields and disturbance dynamo electric fields in modulating the ionization distribution in the equatorial ionization anomaly (EIA) region during this period.

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.

Gravity wave control on ESF day‐to‐day variability: An empirical approach

The gravity wave control on the daily variation in night time ionization irregularity occurrence is studied using ionosonde data for the period 2002 - 2007 at magnetic equatorial location Trivandrum. Recent studies during low solar activity period have revealed that, the seed perturbations should have the threshold amplitude required to trigger Equatorial Spread F (ESF), at a particular altitude and that this threshold amplitude undergoes seasonal and solar cycle changes. In the present study, the altitude variation of the threshold seed perturbations is examined for autumnal equinox of different years. Thereafter, a unique empirical model, incorporating the electro dynamical effects and the gravity wave modulation is developed. Using the model the threshold curve for autumnal equinox (ae) season of any year may be delineated if the solar flux index (F10.7) is known. The empirical model is validated using the data for high, moderate, and low solar epochs in 2001, 2004 and 1995 respectively. This model has the potential to be developed further, to forecast ESF incidence, if the base height of ionosphere is in the altitude region where electrodynamics controls the occurrence of ESF. ESF irregularities are harmful for communication and navigation systems and therefore research is ongoing globally to predict them. In this context, this study is crucial for evolving a methodology to predict communication as well as navigation outages.

On the unique divergent response of the equatorial electrojet vertical polarization electric field to different solar flare events

The response of ionospheric E region to different flare events is investigated using coherent HF backscatter radar to bring out divergent observations. The study reveals the following aspects: (i) increase of absolute mean Doppler frequency by ~25–52% during the initial phase of the morning time flares of 9 September 2005 with concurrent fall in backscattered power and (ii) decrease of absolute mean Doppler frequency by ~11.6–16.2% during the initial phase of the flare of 20 February 2002 with concurrent fall in backscattered power. The Doppler frequency is directly related to vertical polarization electric field. Therefore, these observations are unique and in contrast to earlier works which have, in general, reported only a decrease in equatorial electrojet (EEJ) vertical polarization electric field during the initial phase of flare events. The present study also brings out the possible important role played by the height-integrated conductivities in producing the divergent response of the EEJ. The large reduction in backscattered power for weak M class flare events is also observed in this study. In view of the importance of the ionosphere as a major source of error in GPS-based navigation, the present result assumes significance.

The Response Time of Equatorial Ionization Anomaly Crest: A Unique Precursor to the Time of Equatorial Spread F Initiation

The time delay with which the magnitude and location of the Total Electron Content (TEC) at the Equatorial Ionization Anomaly (EIA) crest responds to the mean height of F - layer (hmF2) is examined seasonally using the ionosonde data at Trivandrum and the GPS data at different stations in the EIA region for low and high solar activity years. The study brings out the fact that for the low solar activity year, the crest TEC responds fastest in winter solstice (ws) compared to other seasons. Further, the response time of the crest is found to decrease with solar activity. The seasonal variation in the EIA response time is attributed to the modulation by meridional neutral winds whereas the solar activity variation seems to be basically controlled by the diffusion times and the background ionization gradients in the respective epochs. The derived relationship between EIA crest location/ magnitude and hmF2 for any season can be used for the prediction purposes. Furthermore, this study for the first time, establishes the relation of EIA response time to Equatorial Spread - F (ESF) start time, for days when the F layer is in the neutral dynamically controlled domain. The deleterious effects of ESF irregularities on communication and navigation systems are well-known and the above result is significant in this context. The present study has the potential to be extended into a model to predict the ESF start time, from the EIA response time, earlier in the day.