Spread in aspect angles of equatorial E region irregularities

Abstract

We revisit and extend the 50 MHz radar aspect angle study made in Peru by Kudeki and Farley in 1985, taking advantage of upgraded facilities at Jicamarca. We discuss here (1) type 1 and 2 echoes from the electrojet region, (2) early evening echoes from somewhat above the electrojet, and (3) daytime “150 km” echoes from the ∼145 to 170 km altitude region. Our electrojet data (1) confirm the earlier results for weak type 2 conditions, namely, that the rms aspect angle θrms decreases from about 0.3° at 99 km to slightly less than 0.2° at 108 km, and (2) provide much more information about the aspect angles of type 1 echoes and the apparent nonlinear interaction between type 1 waves and slower waves. Type 1 waves, with phase velocities near the ion‐acoustic velocity, have θrms decreasing from about 0.15° near 104 km to about 0.10° near 110 km. There is strong evidence that horizontally traveling type 1 waves in the upper midday electrojet, where upward electron density gradients are absent, can sometimes nonlinearly couple to slow, vertically traveling waves with aspect angles sometimes larger than 0.5°. In the early evening we often observe echoes in the 125–135 km region with θrms ≤ 0.05°. During daytime the relatively weak “150 km” echoes have even smaller θrms. At the end of the paper we discuss ideas concerning the nonlinear coupling of the plasma waves.