Ada. Space Sea. Vol.5, No.4, pp.205—208, 1985 0273—1177/85 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © COSPAR ELECTRIC FIELDS AND THE ELECTRON DENSITY IRREGULARITIES IN THE EQUATORIAL ELECTROJET S. Prakash and S. Pal Physical Research Laboratory, Ahmedabad, India INTRODUCTION Studies of irregularities in the electron density and electric field in the equatorial electrojet were carried out from Thumba (8°31’N, 00 14.7t5 dip), India,using the rocket borne Langmuir probe /1/ and double probes /2/ onboard three centaure rockets. The first rocket (C-76) Was launched on February ~ 1981 at 1~30hrs 1ST as a test flight, the secor~. (C-77) on February 12, 1981 at 1051 hrs 1ST and the third (C-73) on January 30, 1982 at 1129 hrs 1ST. All the three rockets were launched when type I irregularities were observed with the Vt~’ baekscatter radar at Thumba. In the present system, a pair of sensors A & 3 was mounted on a boom extended along the spin axis of the rocket (ref- erred to as central boom) to measure the electric field component along th1~ axis. Another pair of sensors C & D was deployed in the direction perpendi- cular to the soin axis, referred to as side sensors. These two sensors were used to measure the electric field component perpendicular to the spin axis of the rocket. For electron density measurement, a spherical Langmuir probe sensor was mounted at the tip of the central boom. This arrangement of s~isor~ when used in the equatorial region, has many advantages. As the floating potential of a body kept in plasma depends on the flow of ions and electrons to it, any obstruction to their flow would modify its potential and therefore this would not correspond to the floating potential In the undisturbed plasma. As the velocity of the rocket is supersonic, the relative flow of ions~wards the rocket Is along Its velocity vector, and hence each body on the rocket and the rocket itself will leave behind a wake. This wake not only has a reduced ion density but also a disturbed potential distribution. A~ the ele- ctrons are constrained tp move along the geomagnetic field lines and as their velocity is 100 km sec~, which is much larger than the rocket velocity, their free flow is obstructed only if another body happens to be on or fairly close to the geomagnetic field lines passing through the sensor. In other words the sensor should not be close or get connected to another body via the geomagnetic lines passing through it. At the geomagnetic equator, the geom- agnetic field lines are horizontal, the sensors on the central boom In the present arrangement will not interact with each other and with the central boom via the geomagnetic field lines. As the orientation of the spin axis of the rocket changes slowly, this arrangement enables continuous measurement of electric field along the spin axis of the rocket. The interaction between the side sensors and the lower central sensor can however, take place if the spin axis of the rocket has a large inclination in the north south direction. As the diameter of the side boom Is small~20mm, this interaction would take place only for a small fraction of the spin period. HORIZONTAL ELECTRIC FiELD (E~) In a spinning rocket the potential difference between the side sensors is sinusoidal in shape. The amplitude of the sine wave, obtained by making best sine wave fit, Was used to calculate E,~. The profile of E~for the flight C-73 is given in Fig.1. Since the Vx~ term is not subtra~ted from this dat~ the E~%profile shows some what monotonous decrease as the rocket velocity decrettees with altitude. In this figure, large structures in E,,!~ can be seen between 90 and 108.~km region. The largest structures were ob~ervedin 100.~ to 105.~km region with peak to peak amplitude of about 20 mV/rn and scale size about ~ km. In the 90 km region, the scale size of the structures was smaller and their peak to peak amplitude was about 10 mV/rn. Above 109 km there are no large visible structures. The large scale structures seen abo~ve 12~km are mainly due to the precession of the rocket. 205