Simultaneous generation of large‐scale density irregularities and geomagnetic pulsations via filamentation instability

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Results of experiment conducted at Gakona, Alaska, using intensity‐modulated HF heating waves of 3.2 MHz to generate geomagnetic pulsations and large‐scale field‐aligned density irregularities (FAI), are reported. The echo traces of o‐ and x‐mode sounding pulses from 3.53 to 4 MHz were recorded during heater on/off periods. The ionograms showed that only x‐mode echo traces were significantly affected by the HF heater. The returns from 3.53 to ∼3.8 MHz disappeared during heater on period and reappeared after heater was turned off. Ray tracings are performed to explore the effect of FAI on the backscattering of o‐ and x‐mode sounding pulses. The drastic difference between the FAI effects on o‐ and x‐mode backscattering trajectories provides a theoretical interpretation of the observation. Geomagnetic pulsations as large as 2.5 nT were also recorded by a Fluxgate Magnetometer. Thermal filamentation of the HF heater leads to the simultaneous generation of FAI and geomagnetic pulsations.