P. B. Rao

Variations of ionospheric ionization and related solar fluxes during an intense solar cycle

Variations of ionospheric ionization (represented by ionospheric electron content (IEC)) and related solar fluxes with the 10.7-cm solar flux index (F10.7) are studied for the intense solar cycle 21 when F10.7 was as high as 367. The IEC data collected at several stations during 1980-1985, the solar EUV (50-1050 A) fluxes obtained from the EUV91 solar EUV flux model, and the measured values of Lyman α (1216 A) flux and He I (10,830 A) equivalent width (EW) are used for the study. It is shown that daily values of diurnal maximum IEC (IECmax) saturate (remain constant) when F10.7 (or its 81-day running average) exceeds a threshold (approximately 160-200) which depends slightly on season and latitude. Variations of the model values of the solar EUV fluxes reveal that when F10.7 exceeds the threshold: (1) the integrated solar EUV (50-1050 A) flux increases at a very low rate, and (2) the fluxes of the important (for thermospheric heating) chromospheric lines and intervals generally saturate (remain constant), while those of the coronal lines and intervals increase at a reduced rate. Lyman α flux and He I EW, which are used as input data in the solar EUV flux model, also increase at a very low rate when F10.7 exceeds the threshold. The saturation of ionospheric ionization, observed for high values of F10.7 during the last three solar cycles 19-21, is the result of the nonlinear variation of the solar EUV and Lyman α fluxes with F10.7. IECmax increases linearly with the integrated solar EUV flux, Lyman α flux and He I EW.

Dependence of ionospheric response on the local time of sudden commencement and the intensity of geomagnetic storms

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.

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.

HF Doppler observations of vector plasma drifts in the evening F-region at the magnetic equator

Abstract Using HF Doppler radar in a spaced-receiver configuration, observations conducted on the vector plasma drifts in the equatorial F-region at Trivandrum during the afternoon-midnight period of March–April 1988, are presented. The three components of the F-region plasma drift at the magnetic equator are such that the North-South component corresponds to the meridional neutral air wind while the other two components represent the electrodynamic drift. The vertical component of the plasma drift is characterized by a prereversal upward enhancement which fits the interpretation based on the F-region dynamo operating at sunset. Equatorial spread-F, which is linked to the prereversal enhancement, appears to be inhibited during the main phase of magnetic storms. The East-West component of the plasma drift is westward during the afternoon and eastward during the night with mean peak velocities of 30 and 110 ms−1, respectively. The large night-time drift is believed to be due to a large reduction in the degree of electromagnetic coupling between the E- and F-regions during the night. The North-South component, representing the meridional neutral air wind, is found to be northward during the afternoon and southward during the night with respective mean peak velocities of about 50 and 70 m s −1. The neutral air wind shows a tendency to reverse from southward to northward at around midnight and is thought to be caused by the midnight bulge in the neutral air temperature. The observations which correspond to the lower F-region are found to be consistent with those reported earlier using other experimental techniques.

Radar observations of updrafting and downdrafting plasma depletions associated with the equatorial spread F

Radar observations at VHF on equatorial spread F (ESF) made at Gadanki (13.5°N, 79.2°E; magnetic latitude 6.3°N) and Trivandrum (8.5°N, 77°E; magnetic laitude 0.3°N) are presented in the form of height-time maps of signal intensity and Doppler velocity. The peak signal intensities are found to be 30–40 dB above the noise level at Gadanki and, normalized to the same system sensitivity, about 6 dB higher at Trivandrum. The discrete plasma structures and the phase velocities of the 3m irregularities observed at Gadanki are well correlated to that observed at Trivandrum at the height linked by the same flux tube. The height-time-intensity maps show both updrafting and downdrafting of the plasma structures; the downdrafting observed on one occasion at Gadanki is somewhat unusual in that it extends down to the E region. The Doppler velocities observed at Gadanki show that the highest values are encountered in the rising plumes with the upward velocities ranging from 100 to 300 ms−1. The velocities are predominantly downward in the bottomside F region, particularly during the later phase of the ESF development. On occasion, the downdrafting is observed well into the topside, reaching as high as 550 km, which emphasizes the influence of electric field being extended to a greater height as pointed out by Anderson and Haerendel [1979] in their model based on flux tube integrated quantities. The downdrafting velocities range from 20 to 100 m s−1. These values being mostly well above the background plasma drift velocity, the downdrafting structures are to be regarded as plasma depletions.

Spectra of the ac electric fields in the post-sunset F-region at the magnetic equator

Power spectra of the fluctuations in the East-West electric field in the post-sunset F-region at the magnetic equator (determined from vertical plasma drift measurements) are presented. The spectra reveal that the fluctuations consist of several components with periods varying from a few minutes to several tens of minutes. The dominant fluctuations have periods of the order of a few tens of minutes with the most frequent being about 30 min; the periods show no dependence on the level of magnetic activity. The spectra also indicate that one component of the fluctuations, usually the component with period about 30 min, is amplified during the period of the upward drift enhancement and attains maximum amplitude at around the time of the evening downward reversal. In general, the fluctuations have minimum amplitude at around 22:00 local standard time (L.S.T.). The characteristics of the spectra suggest that the frequently occurring medium scale gravity waves could be a source for the observed fluctuations in the East-West electric fields.

Radar observations of 2.8 m equatorial spread-F irregularities

Abstract VHF radar observations made of two intense equatorial spread-F events at Gadanki (geographic 13.5 °N, 79.2 °E; geomagnetic 6.3 °N) are presented in the form of Doppler spectra and height-time variations of signal intensity, Doppler velocity and spectral width. The Doppler spectra are found to be discrete and narrow during the early phase of instability evolution and fairly broad at a later stage of evolution, involving rising plasma bubbles and turbulent irregularities. The most intense backscattered signals are associated with the plume structures with peak power values at 42 dB above the noise level. The Doppler velocities, presented only for the bottomside in view of the restricted Doppler window, are found to be in accord with the slopes observed of the scattering structures in the height-time-intensity plots. The drifts are predominantly downward with a peak value of about 70 m s−1 which is significantly above that of the background plasma. The downdrafting structures in both the events are found to descend all the way to the E region, an unusual feature not seen in the RTI plots observed over the magnetic equator. The spectral widths are of the order of 10–20 m s−1 during the initial phase and exceed 100 m s−1 at a later stage of the spread-F evolution.

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

Abstract HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.

VHF and HF radar measurements of E and F region plasma drifts at the magnetic equator

Simultaneous observations of E region horizontal irregularity drifts by VHF backscatter radar and of F region vertical plasma drifts by HF Doppler radar conducted during daytime on a few magnetically quiet days at Trivandrum (dip 0.2°N) are presented. A comparative study of the two measurements indicates broadly (1) a resemblance in the daytime changes of the E-W component between the electric fields and (2) evidence of quasi-periodic electric field variations with periods ranging mostly from 1 to 2 hours. The electric fields derived from HF Doppler radar observations are somewhat lower than those deduced by VHF radar observations. The correlation coefficient for the variations of the electric fields measured by the two experimental techniques is found to be in the range of about 0.5 to 0.9. The observed difference in the E and F region electric fields at the magnetic equator is discussed in terms of the measurement uncertainties and the limitations involved in deriving E-W electric fields. The observations are suggestive of a latitudinal variation in the E-W component of the electric field in the equatorial ionosphere.

A study of equatorial wave characteristics using rockets, balloons, lidar and radar

Abstract A co-ordmated experimental campaign was conducted for 40 consecutive days from 21 February to 01 April 2000 using RH-200 rockets, balloons, Rayleigh lidar and MST radar, with the objective of delineating the equatorial waves and estimating momentum fluxes associated with them. Winds and temperatures in the troposphere, stratosphere and mesosphere over two low latitude stations Gadanki (13.5°N, 79.2°E) and SHAR (13.7°N, 80.2°E) were measured and were used for the study of equatorial waves and their interactions with the background mean flow in various atmospheric regions. The study shows the occurrence of a strong stratospheric cooling (∼25 K) anomaly along with a zonal wind anomaly and this low-latitude event appears to be linked to high-latitude stratospheric warming event and followed by subsequent generation of short period (∼5 days) oscillations lasting for a few cycles in the stratosphere. Slow and fast Kelvin waves and RG wave (∼-17-day and ∼7.2-day and ∼4.2-day periods respectively) have been identified. The mean flow acceleration produced by the divergence of the momentum flux due to the observed Kelvin waves in the 35–60 km height region were compared with the zonal flow accelerations computed from the observed zonal winds. Contribution by the slow and fast Kelvin waves was found to be only ∼25 % of the observed acceleration during the evolution of the westerly phase of the semi-annual oscillation.