On the power‐velocity relationship for 12‐ and 50‐MHz auroral coherent echoes

Abstract

Nearly simultaneous observations of 12‐ and 50‐MHz coherent echoes at the Antarctic Syowa station are considered for studying the power‐Doppler velocity relationship at these significantly different radar frequencies. We concentrate on postmidnight measurements when both 12‐ and 50‐MHz echoes were seen as narrow bands (∼150–200 km) of strong backscatter at short distances of <500 km. First, we examine the slant range profiles for the power and Doppler velocity by averaging data for nine separate events in March 1997. At 50 MHz the power was found to increase with velocity. At 12 MHz a power increase was found only in the velocity range of 100–350 m s−1. For larger velocities a saturation of 12‐MHz echo power and even some power decrease were seen. To further explore the effect, we restricted the database to only almost simultaneous records at 12 and 50 MHz. We show that overall measured 12‐ and 50‐MHz velocity values are similar, with significant deviations at some moments. We again found a power increase with velocity for 50‐MHz echoes. We also confirm a 12‐MHz power increase for relatively low velocities, in between 50 and 450 m s−1. For larger 12‐MHz velocities, data indicate clear saturation of the power and power decrease, though the latter effect is very subtle. We argue that the 12‐MHz power saturation and decrease at large velocities are not due to smaller electrojet irregularity intensity at these velocities but, rather, to irregularity intensity decrease with height and 12‐MHz radio wave focusing on the top of the electrojet layer.