In situ measurement of nighttime plasma density irregularities over an equatorial station Trivandrum

[1] A rocket-borne Langmuir probe was flown from an equatorial station Trivandrum (8.5°N, 76.9°E) during nighttime to study the nature of electron density irregularities present in the E region. Irregularities were observed over a large-scale size range (few kilometers to 1 m) in regions of both positive and negative electron density gradients. Amplitudes of irregularities were in the range of 0.01% to 15%. In addition to these, irregularities with scale sizes as small as 13 cm were also detected in 97–104 km altitude region. Spectral indices of irregularities in large-scale (few kilometers > λ > 50 m), medium-scale (50 m > λ > 10 m), and small-scale (10 m > λ > 1 m) size ranges were −1.7 ± 0.1, −2.2 ± 0.4, and −2.7 ± 0.5, respectively. A wind-driven gradient drift instability (GDI) is invoked as the mechanism for irregularities observed in positive gradient regions. The power spectrum of irregularities showed an unusual behavior in 102 ± 1.4 km altitude region, where an almost flat spectrum was observed for the scale size range of 15–120 m. The only other nighttime rocket measurement of irregularities over an equatorial station reported so far in the literature highlighted the presence of irregularities in regions of negative electron density gradients and ascribed these to the electric field driven GDI but did not discuss the mechanism of generation of irregularities with comparable strengths in the positive gradient regions observed in the same flight. Also the present electron density profile did not show sharp layers as reported earlier.