Measurements of artificial periodic inhomogeneities at HIPAS Observatory


  • F. T. Djuth,

  • K. M. Groves,

  • J. H. Elder,

  • E. R. Shinn,

  • J. M. Quinn,

  • J. Villasenor,

  • A. Y. Wong


Results are presented from recent experiments that employ high-power, high-frequency (HF) radio waves to probe the mesosphere and lower thermosphere. The measurements were made at the High-Power Auroral Stimulation (HIPAS) Observatory located near Fairbanks, Alaska. One objective of the study was to determine the feasibility of using artificial electron density perturbations created in the auroral environment to measure the properties of the background neutral gas between ∼50 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 are believed to be horizontally stratified and conform to the standing wave 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 (∼45 km) in the vicinity of the polar stratopause. The API backscatter at HIPAS is often superimposed on regions of partial reflection, auroral E, and sporadic E. Information about ambipolar diffusion rates and electron attachment to O2 is obtained by measuring the relaxation time of the induced irregularities. In general, API phase velocities below ∼95 km altitude appear to be related to vertical neutral motions. However, detailed validation studies throughout the mesosphere have not yet been performed at HIPAS or any other API facility. At altitudes between ∼45 and ∼80 km, high-resolution observations at HIPAS reveal the presence of sharply defined bands of API scatter 15–20 km in altitude extent. The existence of such bands and their fluctuation in altitude cannot be explained within the context of existing theory. 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 of 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 the API patch and its spatial structure remain to be addressed in future experiments.