Rocket Measurements of Ionization Irregularities in the Equatorial Ionosphere at Thumba & Identification of Plasma Instabilities

Abstract

A Langmuir probe system capable of high frequency response has been designed and developed at the Physical Research Laboratory, Ahmedabad, and used at Thumba to study ionization irregularities in the equatorial lower ionosphere in the scale size ranges 1-15 and 30-300 m. Studies have been conducted at different times of the day as well as during night. It is found that during evening and night-time periods, many of the observed features of the irregularities can be explained in terms of a drift dissipative instability mechanism connected with background electron density gradients and the presence of electric fields in the medium. The mechanism is sometimes referred to as cross-field instability mechanism’ in literature. Similar to evening and night-time results and could probably be due to a similar cause. Above 95 km, at times when the electrojet currents are strong, the irregularities are found to occur continuously and are present in all scale sizes in the 1-15 m range. The irregularity amplitudes show a correlation with the electrojet current strength, but cannot be explained by the presently available linear theories of two-stream instability which require a threshold. Pending proper identification, they are attributed to some form of current-induced. Intensity measurements agree with the results of the spaced receiver technique.