Flow angle dependence of 1‐m ionospheric plasma wave turbulence for near‐threshold radar echo electric fields

Abstract

Coordinated STARE–EISCAT data from the E‐region Rocket and Radar Instability Study (ERRRIS) campaign are used to study the flow angle distributions of threshold (signal‐to‐noise ratio [SNR] ≤ 1 dB) ionospheric parameters controlling the STARE radar echo appearance for either radar above Tromsø. Altogether, there are 64 measurements for the Finnish radar and 128 for the Norwegian radar. For the Finnish radar, the threshold E‐field strength is drift‐aligned with minimum‐to‐maximum ratio of the electron drift velocities of about 3. The strengths tend to decrease when going from positive to negative flow angles. For the Norwegian radar, the threshold electric fields are practically independent of flow angle. For the Finnish radar, the STARE line‐of‐sight Doppler velocities are exclusively positive, large, and well correlated with the corresponding EISCAT plasma velocity components. The Norwegian radar Doppler velocities are randomly distributed around zero and are practically uncorrelated. For either radar, the N(h) profiles have permanent upward vertical density gradients within the echo layers. The jet averaged threshold E‐fields are lower in the westjet than within the eastjet, but the averaged threshold electron densities are higher in the westjet than in the eastjet. For the Norwegian radar, the jet averaged turbulence level is about two times higher within the eastjet. The flow angle distributions of the plasma wave turbulence level are different. The westjet distribution is of the equilibrium type with a maximum at small flow angles and a minimum at large angles. The eastjet distribution is consistent with a flat one and can be kept stationary only if there is a damping of the turbulence for small flow angles and an enhancement for large angles. It is then conjectured that Finnish radar threshold echoes are generated by the Farley–Buneman instability, but the Norwegian echoes by a nonlinear gradient drift or/and wind‐driven mechanism. The gradient drift instability, due to the permanent vertical gradients in the lower part of the N(h) profile and the meridional component of the ionospheric E‐field, is suggested to be the mechanism responsible for the turbulence level enhancement in the eastjet and damping in the westjet.