Imaging HF‐induced large‐scale irregularities above HAARP

The University of Massachusetts‐Lowell digisonde is used with the HAARP high‐frequency (HF), ionospheric modification facility to obtain radio images of artificially‐produced, large‐scale, geomagnetic field‐aligned irregularities. F region irregularities generated with the HAARP beam pointed in the vertical and geomagnetic field‐aligned directions are examined in a smooth background plasma. It is found that limited large‐scale irregularity production takes place with vertical transmissions, whereas there is a dramatic increase in the number of source irregularities with the beam pointed parallel to the geomagnetic field. Strong irregularity production appears to be confined to within ∼5° of the geomagnetic zenith and does not fill the volume occupied by the HF beam. A similar effect is observed in optical images of artificial airglow.