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

It is well known that the ionospheric plasma response to high-power HF radi o waves changes drastically as the heater frequency approaches harmonics of the electron gyrofrequency. These include changes in the spectrum of the s timulated electromagnetic emission, reduction in the anomalous absorption o f low-power diagnostic waves propagating through the heated volume, and red uction in the large scale F-region heating. Theoretical models as well as p revious experimental evidence point towards the absence of small-scale fiel d-aligned plasma density irregularities at pump frequencies close to electr on gyroharmonics as the main cause of these changes. Results presented in t his paper are the first direct observations of the reduced striations at th e 3rd gyroharmonic made by the CUTLASS radar. In addition, simultaneous EIS CAT 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 frequencies close to the 3rd electron gyroharmonics.