Simultaneous VHF and UHF radar observations of the mesosphere at Arecibo during a solar flare: A check on the gradient‐mixing hypothesis

Simultaneous UHF and VHF radar observations of the 60–90 km mesosphere at Arecibo were made during the occurrence of a January 1981 type 4 solar X ray flare. The observations involved use of the UHF incoherent scatter technique at 430 MHz and the VHF turbulent scatter technique at 46.8 MHz. These observations provided a unique opportunity to test the basic premises of the turbulent gradient mixing hypothesis. UHF measurements show that enhanced electron concentration gradients were established with the flare onset. Two turbulent layers were observed with the 46.8 MHz radar before, during and after the flare. Enhanced scattering from both layers peaked within 3–6 min of the beginning of the flare. Only a slight variability in Doppler width of VHF returns from both layers was observed over the pre‐ to post flare period indicating that the power dissipation associated with turbulence remained essentially constant. We find that almost all the enhancement of VHF signals can be attributed to the flare induced increase in electron concentration gradients. We also conclude that the observed delay of 3–6 min between flare onset and the peak of VHF returns is related to an eddy overturning time required for mixing‐in of gradients to the Bragg scale.