V.V. Somayajulu

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.

Ionization hole campaign-a coordinated rocket and ground-based study at the onset of equatorial spread-F: first results

Abstract A comprehensive multi-technique campaign involving the launch of two high altitude RH-560 rockets was carried out from Sriharikota (SHAR), India, a near-equatorial rocket launching station at the onset of equatorial spread-F, along with a host of ground-based complementary experiments at other locations spread over the country. The main objectives were to obtain the background ionospheric and thermospheric conditions at the onset of equatorial spread-F, and to be able to evaluate the relative importance of the various agencies, the neutral dynamics in particular. Multiple barium cloud releases, in situ plasma diagnostic measurements, along with complementary optical and radio probing experiments were carried out as a part of this campaign. The presence of large scale gradients in the ambient electric fields at ~185 km altitude and also of vertical winds of significant magnitudes at higher altitudes were found. The presence of large scale irregularities in the ion densities at heights above 250 km in a region of negative background density gradient is one of the significant new results. The details of the Ionization hole campaign along with the first results are presented and discussed in the context of the present knowledge of the phenomenon of equatorial spread-F.

Response of night-time equatorial F-region to magnetic disturbances

Abstract The response of the equatorial night-time F -region to magnetic stormtime disturbances has been examined using mainly ionograms recorded at Trivandrum and magnetograms recorded at high, middle and low latitudes during the magnetic storm of 23–26 November 1986. The analysis revealed a close coupling between the equatorial F -region and high latitude magnetic field disturbances originating in solar wind-magnetosphere interactions. The presence of spread- F on ionograms during this period is found to be consistent with the Rayleigh-Taylor instability mechanism for the growth of the irregularities.

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.

VHF RADAR OBSERVATIONS OF POSSIBLE GRAVITY WAVE-GENERATED ELECTRIC FIELDS IN THE EQUATORIAL ELECTROJET

Small amplitude oscillations of 4 − 5 minute period and larger amplitude quasi-periodic fluctuations of 20 − 25 minutes period have been observed in the mean Doppler frequency of the VHF radar signals backscattered from the equatorial electrojet irregularities at Thumba. The Doppler frequency fluctuations are attributed to the electric field fluctuations generated by propagating acoustic-gravity waves in the electrojet. Such an interpretation is supported by two other observational evidences: (a) the change of phase of the fluctuations with height as observed with the backscatter radar and (b) the occurrence of blanketing ES layers (as indicated by ionograms) along with the oscillations of the electric fields.

Role of periodic fluctuations in solar wind-magnetosphere coupling

In this paper, the behaviour of the fluctuating component in the solar wind parameters (V, Bz), the auroral electrojet indices (AU, AL), the ring current index (Dst) and the interplanetary electric field (V × B2) during 10 magnetic storms is analysed to understand the solar wind-magnetosphere coupling. It is found that during the moderate storms (Dst > −100 nT), the fluctuating component of 3–4 h periodicity is clearly discernible in all the parameters, and during the intense storms (Dst < −100 nT) the periodic fluctuations are not well defined.

ELECTRODYNAMIC COUPLING OF AURORAL AND EQUATORIAL DYNAMO REGIONS - II. QUIET DAYS

Using the VHF backscatter radar observations on the equatorial electrojet at Thumba, the possibility of the short period (20–40 mins.) fluctuations of the electrojet on quiet days being a part of the global scale electric field fluctuations has been examined. The observations on days with Ap ≤ 6 indicate that the equatorial electrojet fluctuations even on quiet days are likely to be caused by global scale electric field fluctuations which originate in the auroral-polar dynamo region disturbances.

THE LUNAR PHASE AND THE EQUATORIAL ELECTROJET

Simultaneous observations with a VHF backscatter radar and a ground based magnetometer at Thumba show that on some occasions, the magnetogram does not show the afternoon depression, which is characteristic of a ‘partial counter electrojet event’, even while the time variations of backscatter radar signal strength and the mean Doppler frequency clearly exhibit the characteristic depression. An explanation is offered for the above observed feature. The implication of the above feature in relation to the lunar phase dependence of the counter electrojet events is discussed.

IONOSPHERIC AND MAGNETOSPHERIC CONTRIBUTIONS TO LOW LATITUDE GEOMAGNETIC FIELD VARIATIONS DURING A MAGNETIC STORM

Based on a novel approach, it is demonstrated that VHF Backscatter Radar observations on the equatorial electrojet can be used to estimate the ionospheric and magnetospheric contributions to the low latitude geomagnetic field perturbations during magnetic storms.