Rocket measurements of high-altitude spread F irregularities at the magnetic dip equator

We present the first rocket measurements of equatorial spread F irregularities at altitudes significantly above 600 km, in what is believed to be the inertially dominated regime of spread F. On October 14, 1994, at 1955 LT (2255 UT) a Black Brant X rocket instrumented to measure plasma density and electric fields was launched from Alcântara, Brazil (2° 18.75'S, 44°22.3'W, dip angle 1.5° at 300 km altitude), into active topside spread F plumes. On upleg the payload remained continuously in these plumes up to an altitude of 822 km. Power spectra of electron density fluctuations show characteristic dual‐power law behavior with spectral indices of −1.7 at frequencies below 60 Hz and −5 at frequencies above 60 Hz. Shocklike coherent structures characterized the density waveforms. The electric field fluctuations had spectral indices ranging from −3 to −4. We observe no evidence of altitude variation in the spectral indices or in the density fluctuation waveforms. Over the entire altitude range of the observations, 350–822 km, the data remain in agreement with previous measurements of collisional spread F below 600 km.