Irregularities in the Equatorial E Region Over Thumba

A rocket‐borne Langmuir probe and plasma‐noise probe have been used to study the ionization irregularities of scale sizes 20 meters to 0.4 meter in the lower equatorial ionosphere over Thumba during the evening twilight of February 2, 1968. lonization irregularities were observed mostly in the height range 97–106 km, 142–155 km and around 170 km. The spectral index n was calculated for a power law of the type E(k) = Ak n . In the 100‐km region, irregularities with different spectral indices were found to coexist, indicating the superposition of at least two different spectra. The irregularities with spectral index of −3.0 to −3.5 are probably produced through neutral turbulence, whereas irregularities with spectral index of −1.6 to −2.0 are probably produced by electric fields. The irregularities observed near the rocket apogee are produced by the rocket itself and have a spectral index of +3.0. The actual values of these indices are likely to be more (including sign) than given by the present calculations since the detection efficiency of a Langmuir probe for the irregularities may be size‐dependent.