In June 1992 a NASA sponsored sounding rocket was flown through the Arecibo heater beam to study the structure of the heated volume. The rocket carried an instrument payload and traversed the 5.1-MHz reflection height at 268.5 kin. Data from the plasma density probe are presented in this paper. The rocket passed through several regions of disturbed plasma both above and below the reflection level. In these regions, over 180 deep filamentary density depletions were detected. Measured perpendicular to the magnetic field, these depletions or filaments have a mean width at half maximum of 7 m which is roughly equal to twice the ion gyroradius (0 +) and a mean depletion depth of 6%. The ratio of parallel to perpendicular scale for these structures exceeds 20,000, and the spacing between the filaments is around 15 m. A power spectrum of the rocket data clearly shows the spectral content of the filaments and also reveals peaks at longer wavelengths which we interpret as the spacing between the bunches and between sets of filaments within a given bunch. We believe that previous scintillation and satellite measurements emphasized these longer wavelengths. The power spectrum mea. sured by the rocket instrumentation falls off as k -4 for wavenumber k larger than 0.4/m and remains above the system noise for structure down to 1 m. It is clear that VHF backscatter from these structures can be explained by our data, as can many features of heater-related, field-aligned irregularities found in the literature. 17,367 17,374 KELLEY ET AL.' DENSITY DEPLETIONS AT THE 10-M SCALE Perkins, F. W., and M. V. Goldman, Self-focusing of radio waves in an underdense ionosphere, J. Geophys. Res., 86, 600, 1981. Perkins, F. W., and E. J. Valeo, Thermal self-focusing of electromagnetic waves in plasma, Phys. Rev. Lett., 3oe, 1234, 1974. Robinson, T. R., The heating of the high latitude ionosphere by high power radio waves, Phys. Rep., 179(2), 79, 1989. Rose, G., B. Grandal, E. Neske, W. Ott, K. Spenner, J. Holtet, K. M•eide, and J. Troim, Experimental results from the HERO project: In situ measurements of ionospheric modifications using sounding rockets, J. Geophys. Res., 90, 2851, 1985. Szuszczewicz, E. P., and J. C. Holmes, Observations of electron temperature gradients in mid-latitude E, layers, J. ) M. Sulzer, Arecibo Observatory, Arecibo, PR 00613. (email: sulzer@naic.edu) (

Optical measurements made at the Arecibo Observatory during the 1987 heating campaign showed large temporal and spatial variations in 630.0‐nm airglow enhancements during times of continuous power transmissions of high‐power radio waves. Photometric data displayed fluctuations of 60 R or more in the red‐line (630.0 nm) emission from atomic oxygen. These fluctuations were associated with heater‐induced cavities which drifted and evolved in the modified ionosphere. Data from the Arecibo incoherent scatter radar were used in conjunction with airglow images to provide a physical interpretation of the modification process. Electrons were accelerated by large amplitude Langmuir waves excited by parametric decay instabilities occurring near the wave reflection points inside the density cavities. Inelastic collisions with oxygen atoms produced excited states which yielded enhanced 630.0‐nm and 557.7‐nm emissions. A numerical model has been used to relate the enhanced airglow intensities to the energy spectrum of the accelerated electrons. The measured airglow could have been produced by an isotropic source at 340 km altitude that accelerated 0.01% of the ambient electrons into a suprathermal Maxwellian distribution with a temperature of 2.05 eV. Experimental and theoretical studies suggest that airglow clouds were directly coupled to plasma density cavities because (1) these cavities trapped the HF radio beam and (2) electrons accelerated out into regions of reduced plasma concentration were less effectively thermalized and, consequently, were more effective for collisional excitation of neutral species.

High-power electromagnetic waves beamed into the ionosphere from ground-based transmitters illuminate the night sky with enhanced airglow. The recent development of a new intensified, charge coupled-device imager made it possible to record optical emissions during ionospheric heating. Clouds of enhanced airglow are associated with large-scale plasma density cavities that are generated by the heater beam. Trapping and focusing of electromagnetic waves in these cavities produces accelerated electrons that collisionally excite oxygen atoms, which emit light at visible wavelengths. Convection of plasma across magnetic field lines is the primary source for horizontal motion of the cavities and the airglow enhancements. During ionospheric heating experiments, quasi-cyclic formation, convection, dissipation and reappearance of the cavites comprise a major source of long-term variability in plasma densities during ionospheric heating experiments.

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Self-focusing of high-frequency electromagnetic radiation is observed to produce largescale plasma striations in the ionosphere. Development of a new observational technique has allowed the first detailed study of the instability scale sizes and associated plasma movement. Experimental results are shown to support the theory of wave self-focusing through differential electron heatings Natural density fluctuations cause small variations in the index of refraction of a plasma, resulting in a slight focusing and defocusing of an electromagnetic wave as it propagates through the medium. The electric field intensity increases as the incident wave refracts into regions of comparatively underdense plasma. Ohmic heating 1 and the electric-field ponderomotive force 2 then drive plasma from these focused regions, amplifying the initial perturbation. This selffocusing instability continues until hydrodynamic equilibrium is reached, creating field-aligned striations within the plasma.The study of self-focusing waves in plasmas is motivated by its relevance to ionospheric modification research, 3 laser fusion-plasma heating, 4 and microwave-ionosphere interactions associated with solar-power satellite systems. 5 Development of a new diagnostic technique in conjunction with a recent ionospheric modification experiment has resulted in the first detailed observations of individual self-focused striations, as well as striation maps of the entire waveplasma interaction region. Measured striation scale sizes are in good agreement with the predictions of thermal self-focusing theory. Additional plasma effects can also be identified.Intense high-frequency (hf) electromagnetic radiation incident on an overdense ionospheric plasma is known to excite parametric instabilities, enhancing electron plasma oscillations observable by incoherent backscatter radar. 6 These instabilities continue to be the subject of intense experimental study. Of importance here is the fact that above instability threshold the strength of the enhanced plasma waves directly depends on the local power of the pump electric field. In addition, because of exact frequency and wavenumber matching conditions for both the parametric wave-plasma interaction and the radar incoherent backscatter process, these enhanced waves are detected at only one altitude. As a result, systematic scanning of the narrow radar beam across the interaction region of the enhanced plasma waves yields a two-dimensional cross-section characteristic of the local electric field intensity. These maps of electric field strength clearly show self-focusing striations and large-scale structuring of the illuminated plasma. Because the direction and rate of the radar scan are experimentally controlled, the cross-sectional dimensions of the individual striations are easily measured. Alternatively, if the radar is fixed, the irregularities follow a slow natural (EXB) drift through the beam, allowing a detailed study of the small-scale structure within individual striations. Once the irregularity size is d...