Several rocket borne magnetometer experiments have been conducted from Thumba to measure the equatorial electrojet currents under different solar and geophysical conditions. The data from these experiments have been used to study the vertical structure of the electrojet, the relation between strength of the jet and the magnetic field variation at the ground and the causes of large day-to-day variability of the amplitude of H variation. The results show that the experimentally measured current density peaks at a significantly different altitude compared to the altitude of the peak of Cowling conductivity calculated using model values of Ionospheric parameters. The analysis indicates that given the magnetogram from a station under the electrojet, it is possible to predict the vertical distribution of current density in the electrojet on a quiet day. The mechanism of stabilisation of two stream instability suggested by the theoretical workers ro explain the back scatter radar echoes from type I irregularities on high jet days does not become operative atleast upto jet strengths equivalent to 4H<140nT.
The height integrated current density in the electrojet over Thumba has been measured between the years 1966 and 1976 on nine different occasions by rocketborne magnetometers. Using this data and the H variation data from a spread of ground magnetic stations in the region of the electrojet, the depth of the non-conducting layer has been evaluated. The results are compared with the results obtained from POGO and Kosmos 321 satellite experiments. It is shown that the depth of non-conducting layer in the region around Thumba is 230±20 km. Using the above data, the internal and the external contributions to the diurnal variation of geomagnetic field observed at the ground have been evaluated.
Investigations of the AC electric fields associated with plasma instabilities in the equatorial electrojet were carried out from Thumba using Langmuir double probes. AC electric fields, the electrojet current density and the electron density and its fluctuations were measured in near simultaneous launchings on two different occasions near local noon when the electrojet intensity was high. A third experiment was conducted in the early morning hours when the electrojet was not yet developed. The two noon time experiments detected AC electric fields associated with cross field instability, both during the ascent and the descent, between 85 and 105 km altitude. In the early morning experiment AC electric fields in the spectral band 10-100 Hz were observed between 115 and 130 km altitude.
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