First in situ measurement of electric field fluctuations during strong spread F in the Indian zone

An RH-560 rocket flight was conducted from Sriharikota rocket range (SHAR) (14 degrees N, 80 degrees E, dip 14 degrees N) along with other experiments , as a part of equatorial spread F (ESF) campaign, to study the nature of i rregularities in electric field and electron density. The rocket was launch ed at 2130 local time (LT) and it attained an apogee of 348 km. Results of vertical and horizontal electric field fluctuations are presented here. Sca le sizes of electric field fluctuations were measured in the vertical direc tion only. Strong ESF irregularities were observed in three regions, viz., 160-190 km, 210-257 km and 290-330 km. Some of the valley region vertical e lectric field irregularities (at 165 km and 168 km), in the intermediate-sc ale size range, observed during this flight, show spectral peak at kilomete r scales and can be interpreted in terms of the image striation theory sugg ested by Vickrey et al. The irregularities at 176 km do not exhibit any pea k at kilometer scales and appear to be of a new type. Scale sizes of vertic al electric field fluctuations showed a decrease with increasing altitude. The most prominent scales were of the order of a few kilometers around 170 km and a few hundred meters around 310 km. Spectra of intermediate-scale ve rtical electric field fluctuations below the base of the F region (210-257 km) showed a tendency to become slightly flatter (spectral index n = -2.1 /- 0.7) as compared to the valley region (n = -3.6 +/- 0.8) and the region below the F peak (n = -2.8 +/- 0.5). Correlation analysis of the electron d ensity and vertical electric field fluctuations suggests the presence of a sheared flow of current in 160-330 km region.