Simultaneous rocket probe and radar measurements of equatorial spread <i>F</i>—Transitional and short wavelength results

Abstract

During the PLUMEX I rocket flight from Kwajalein Island, plasma density and electric field fluctuations were measured in situ, simultaneous with ground‐based radar backscatter measurements at 0.96‐m and 0.36‐m wavelengths. The rocket penetrated an extremely turbulent topside region which had associated intense backscatter. As measured by the radar the backscatter power was decaying with time during and after the flight. The intermediate wavelength (0.1–10 km) in situ electron density measurements are described in a companion paper, while here we report the transitional and short wavelength results (λ &lt; 100 m). These data include the first in situ equatorial spread F measurements of the electric field component of electrostatic fluctuations with wavelengths less than 1 m. At all altitudes above about 280 km, a repeatable form for the wave‐number spectrum was found for the electron density and electric field fluctuations at wavelengths less than about 100 m. The density spectrum varies approximately as k −5 and the electric field spectrum as k −3 . The steepness of the density spectrum corresponds to an absence of steep edges in the density waveform on the scale of 100 m and less. These two spectral forms are shown to be consistent with an explanation involving low‐frequency waves with finite wave numbers parallel to the magnetic field ( k ∥ ). Both theory and laboratory experiments show a power law density fluctuation spectrum for gradient‐driven drift waves with negative index in the range 4.5–6.0. Since such waves do have finite k ∥ , and since sharp gradients exist in the spread F environment, we conclude that at sufficiently high altitudes, drift waves act on the steep gradients caused by a primary longer‐wavelength instability to create the observed spectral form. These waves may then create an anomalous diffusion as discussed by Huba and Ossakow (1981 b ). At lower altitudes a shallower spectral index was observed in the tens of meters range, which may be related to a collisional damping regime. This suggests an altitude threshold for the drift waves that is probably related to ion neutral collisions. The power law spectra show no marked change near k ⊥ r i ≈ 1 where r i is the ion gyroradius. Since low‐frequency drift waves are linearly stable for k ⊥ r i ≳ 1, it seems that a wave‐wave interaction (cascade) operates to deposit energy in a range where waves are linearly damped. There is a slight suggestion of a spectral change (smaller negative index) for k ⊥ r e ≳ 0.2 which may be due to excitation of a lower‐hybrid drift wave and which may be related to the observed enhanced backscatter at wavelengths on the order of 1 m. A stability analysis shows that the plasma is near but on the stable side of the marginal stability boundary for the lower‐hybrid drift wave in the most intense region of backscatter. In regions devoid of drift waves, evidence is found for an exponential inner‐scale cutoff at a wavelength of 200 m.