First direct observations of the reduced striations at pump frequencies close to the electron gyroharmonics

Abstract

It is well known that the ionospheric plasma response to high-power HF radio waves changes dras- tically as the heater frequency approaches harmonics of the electron gyrofrequency. These include changes in the spectrum of the stimulated electromagnetic emission, reduction in the anomalous absorption of low-power diagnostic waves propagating through the heated vol- ume, and reduction in the large scale F-region heating. Theoretical models as well as previous experimental evidence point towards the absence of small-scale field- aligned plasma density irregularities at pump frequen- cies close to electron gyroharmonics as the main cause of these changes. Results presented in this paper are the first direct observations of the reduced striations at the 3rd gyroharmonic made by the CUTLASS radar. In addition, simultaneous EISCAT observations have revealed that the ''enhanced ion-line'' usually present in the EISCAT ion-line spectrum during the first few seconds after heater switch on, persisted at varying strengths while the heater was transmitting at frequen- cies close to the 3rd electron gyroharmonics.