Controlled study of mid-latitude spread F by the Arecibo heater

Summary form only given. The authors´ recent experiments at the Arecibo Observatory have indicated a possible link between instability-produced spread-F and subsequent maintenance of the resulting ionospheric plasma density structures by HF heating. Specifically, in two heating experiments during August 1992 spread-F was observed via Digisonde profiles to appear after tens of minutes of heating. The large-scale structures causing this spreading persisted until the heating was suspended, whereupon it rapidly disappeared. The effect could not be reproduced on either night by reapplication of the RF despite hours of heating. It is proposed in this work that HF heating may amplify the effects of the Perkins instability (F. Perkins, J. Geophys. Res., 78, 218, 1973) enough for detection by ionosonde. Preliminary analyses have indicated that the heater pump wave influences the instability growth rate and may allow the instability process to grow to proportions it could not attain naturally. Interaction of the instability with passing gravity waves as described in Huang, et al., (Radio Sci., 29, 395, 1994) might amplify the effects further, and the heating then maintain the resulting structures until it was removed. No electric field data exists to verify the collect ambient conditions for operation of the Perkins instability during the authors´ 1992 heating campaign. Their new heating experiments at the Arecibo Observatory are scheduled for late summer 1996 and will attempt to observe the electric field configuration, the presence of gravity waves, and the formation and amplification of spread F structures (Lee et al., 1995, scheduled Arecibo experiments in 1996).