C.A. Reddy

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Abstract Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E y in the EEJ can be deduced from the experimental height profiles of V p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated. It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E -region.

Global scale electrodynamic coupling of the auroral and equatorial dynamo regions

Abstract The VHP backscatter radar observations on the equatorial electrojet at Thumba near Trivandrum (dip ~1°S) show that most of the bay-type disturbances in the surface geomagnetic field, with typical durations of 20–50 min, result from corresponding fluctuations of the overhead currents in the ionospheric dynamo region. The analyses of such events, using the VHF radar observations at Thumba and the magnetograms at different locations, have shown that the bay-type disturbances in the equatorial electrojet have their origin in corresponding disturbances in the auroral or polar dynamo region. Even some minor disturbances in the dynamo region electric fields at the auroral and polar latitudes are found to result in observable perturbations in the equatorial electrojet. The results of the detailed analyses of two disturbance events are presented in this paper. The simultaneity and the relative strengths of the estimated electric field perturbations during the disturbances at the auroral and equatorial electrojet latitudes indicate that the auroral and equatorial dynamo regions are coupled much more strongly than what has been suspected hitherto. Moreover, it is shown that the global scale current system associated with a bay-type disturbance undergoes large changes even within the duration of the disturbance. The implications of the above results in relation to the prevailing notions about the substorm-associated current systems are discussed.

Backscatter radar measurements of storm-time electric field changes in the equatorial electrojet

Abstract The mean Doppler frequency ( f D ) of the VHP backscatter radar signals from type II plasma irregularities in the equatorial electrojet is a measure of the driving electric field in the electrojet. The measured Doppler frequency values are used to delineate the complex time variations of the electric field and current in the equatorial electrojet during the two geomagnetic storms of 25 April and 18 September in 1979. Using a recently evolved empirical method, the ionospheric and the magnetospheric current contributions to the storm-time surface magnetic field perturbations at Thumba have been estimated; and the characteristic time variations during the initial, main and recovery phases of the magnetic storms are presented. The most significant conclusions of the present study are: (a) the daytime SSC-related increases in ΔH at electrojet latitudes have a large contribution from an impulsive change of electrojet electric field during the SSC; (b) during the initial phase, both the magnetospheric current and electrojet current exhibit rapid fluctuations which are in antiphase; (c) the Dst, values seem to underestimate, by nearly a factor of two, the asymmetric or partial ring current contribution around the time when the main phase decrease in AH reaches its peak level; and (d) the ionosphere and magnetosphere seem to respond as a single electrodynamic system to the electrical disturbances of all scales which originate in the interaction of solar wind with the outer magnetosphere.

The equatorial electrojet: a review of the ionospheric and geomagnetic aspects

Abstract In this review, the recent additional knowledge about the physical processes in two broad areas of the equatorial electrojet are emphasized 1. (i) the local interactions of the neutral winds with the electrojet plasma which result in diverse but physically inter-related effects on the altitude structure of electric fields and currents, ionization convergences and the formation of blanketing E s layers and F -region ionization dynamics 2. (ii) the very sensitive responses of the equatorial electrojet to the complex auroral ionosphere-magnetosphere interaction processes. On both the aspects, a synthesis of the observational results and theoretical models and ideas is attempted. The factors which contribute to apparent contradictions in the morphological results on the equatorial ‘counter-electrojet’ are discussed along with the possible physical processes causing the ‘counter-electrojet’.

Evidence of a meridional circulation cell in the lower thermosphere during a magnetic storm

Abstract The incoherent scatter radar observations on ionization drifts at St. Santin (France) have been analyzed to yield the meridional neutral winds in the lower thermosphere during a magnetically disturbed period. The results suggest that a meridional circulation cell is established in the lower thermosphere following the onset of a magnetic disturbance. For the event studied, the excess meridional wind generated during the disturbed period is found to be poleward at 100 km and equatorward above 120 km altitude.

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (Vp) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (Ey) are deduced by matching experimentally observed Vp profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental Ey values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ Ap ⩽ 30); (iv) a significant decrease of the average Ey on disturbed days compared to that on quiet days during 0900–1200 h L.T.

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations-III. Comparison of observed ΔH values with those estimated from measured electric fields

Abstract From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field ( E y ) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D st ) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

Abstract A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s −1 (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s −1 also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere. The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.

Annual and semiannual oscillations in the equatorial middle atmospheric winds

Abstract A quantitative analysis is made of the amplitudes and phases of the annual and semiannual oscillations in the zonal wind in the equatorial stratosphere and lower mesosphere, using the rocket data in the 20–80 km region at three stations, Thumba, Kwajalein, and Ascension Island which are around ±8° latitude, but at different longitudes. The cycle-to-cycle variations in the amplitudes of annual and semiannual oscillations at the above three stations are studied for a 12-year period. It is observed that the annual and semiannual oscillation amplitudes have large cycle-to-cycle variation around the stratopause and also significant longitudinal asymmetries. A comparison with QBO shows that the variations of the AO and SAO amplitudes seem to be correlated with the phase of the quasi-biennial oscillation at 28 km, with a tendency of larger amplitudes occuring during the westerly phase of the QBO.

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

Abstract The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field EW(z), or from the radar-measured phase velocity VpII(z) of the type II plasma waves. The method is found to be satisfactory when Ew ⪢ Ep, where Ep is the vertical polarization electric field generated by the global scale east-west electric field, EY, and Ey