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

Nearly simultaneous observations of 12- and 50-MHz coherent echoes at the A ntarctic Syowa station are considered for studying the power-Doppler veloci ty relationship at these significantly different radar frequencies. We conc entrate on postmidnight measurements when both 12- and 50-MHz echoes were s een as narrow bands (similar to 150-200 km) of strong backscatter at short distances of < 500 km. First, we examine the slant range profiles for the p ower and Doppler velocity by averaging data for nine separate events in Mar ch 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 datab ase to only almost simultaneous records at 12 and 50 MHz. We show that over all measured 12- and 50-MHz velocity values are similar, with significant d eviations at some moments. We again found a power increase with velocity fo r 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, d ata indicate clear saturation of the power and power decrease, though the l atter 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 decre ase with height and 12-MHz radio wave focusing on the top of the electrojet layer.