MEASUREMENTS OF ARTIFICIAL PERIODIC INHOMOGENEITIES AT HIPAS OBSERVATORY

Results are presented from recent experiments that employ high-power, high-frequency (HF) radio waves to probe the mesosphere and lower ther mosphere. The measurements were made at the High-Power Auroral Stimula tion (HIPAS) Observatory located near Fairbanks, Alaska. One objective of the study was to determine the feasibility of using artificial ele ctron density perturbations created in the auroral environment to meas ure the properties of the background neutral gas between similar to 50 to similar to 120 km altitude. The observing technique relies on the production of so-called ''artificial periodic inhomogeneities'' (API) in the altitude region(s) of interest. These induced irregularities ar e believed to be horizontally stratified and conform to the standing w ave pattern produced by the reflection of the powerful HF wave in the ionosphere. In the D region above HIPAS, API decay curves are strictly exponential and the phase histories are strictly linear. On occasion, echoes are detected at very low altitudes (similar to 45 km) in the v icinity of the polar stratopause. The API backscatter at HIPAS is ofte n superimposed on regions of partial reflection, auroral E, and sporad ic E. Information about ambipolar diffusion rates and electron attachm ent to O-2 is obtained by measuring the relaxation time of the induced irregularities. In general, API phase velocities below similar to 95 km altitude appear to be related to vertical neutral motions. However, detailed validation studies throughout the mesosphere have not yet be en performed at HIPAS or any other API facility. At altitudes between similar to 45 and similar to 80 km, high-resolution observations at HI PAS reveal the presence of sharply defined bands of API scatter 15-20 km in altitude extent. The existence of such bands and their fluctuati on in altitude cannot be explained within the context of existing theo ry. Large variations in API backscatter power (10-20 dB) are typically observed in the D region over timescales of 30 s or less. Most likely this is caused by fading in the ionospherically reflected component o f the standing wave pattern. Finally, power stepping studies reveal a roughly linear relationship between D region backscatter power and HF power. Fundamental questions related to the horizontal dimension of th e API patch and its spatial structure remain to be addressed in future experiments.