[1] A patchy sporadic E ionization layer was observed using the 430 MHz incoherent scatter radar at the Arecibo Radio Observatory and a new, 30 MHz coherent scatter radar imager located on St. Croix. The former measured state parameters of the layer (plasma density, temperature, and composition) while the latter observed small-scale plasma waves and the associated quasiperiodic (QP) echoes. Regularization was used to estimate horizontal vector ion drifts from Arecibo line-of-sight data. Assuming the ion temperatures and drifts to be representative of the neutral atmospheric temperatures and drifts in the mesosphere lower-thermosphere (MLT) region, we evaluated the Richardson number criterion for neutral dynamic instability in regions occupied by the layers. We find that the condition for instability was generally satisfied and thus that dynamic instability is consistent with the layer breakup. The electrodynamics of the resulting patchy layers could then give rise to the QP echoes.Citation: Hysell, D. L., E. Nossa, M. F. Larsen, J. Munro, M. P. Sulzer, and S. A. González (2009), Sporadic E layer observations over Arecibo using coherent and incoherent scatter radar: Assessing dynamic stability in the lower thermosphere,
[1] Simultaneous observations of an irregular sporadic E layer from the Arecibo incoherent scatter radar and a coherent scatter radar located on St. Croix are presented. The layers exhibit periodic structuring which is attributed to shear instability in the neutral flow. Estimates of the time-varying vector neutral wind profiles in which the layer was embedded are analyzed and shown to be shear unstable in the Richardson number sense. In addition to the calculation of the Richardson number values, we present an eigenvalue analysis of the model of Miles (1961) and Howard (1961) for the observed wind profiles. The calculated eigenmodes have dominant Kelvin-Helmholtz modes for the estimated flow that are propagating to the southwest with phase speeds near 50 m/s and horizontal wavelengths between 10-15 km. The growth times for the waves would have been as little as about 1 min. These features are in reasonable agreement with the observed of E s -layer structure. The Miles-Howard model has been analyzed extensively in the past using both analytic and numerical techniques, but calculations of eigenmodes for the equations in a case with background winds that have turning and speed shear have not been carried out previously, as far as we know. The difficulties associated with the calculation are related to identifying the fastest growing modes among the large number of modes that satisfy the equations. The technique and the relationship of the solutions to the observed sporadic E layer wave structure are described.
The Naval Research Laboratory Sami3 is Also a Model of the Ionosphere (SAMI3) ionosphere/plasmasphere code is used to examine the physics of metallic layers at altitudes from 80 to 160 km. Results are presented near the simulated location of the Arecibo observatory (18°N, 66°W). We find that simulations, using winds from the empirical horizontal wind model, produce layers consistent with those observed at Arecibo. Specifically, we find upper semidiurnal and lower diurnal traces similar to those identified in previous observational surveys. While metallic layers are shaped by meridional winds, zonal winds, and electric fields, much of the observed structure is produced if only meridional wind forces are included in the model. Stratification below 110 km, where the ions are very weakly magnetized, is supported mainly by meridional wind shear.
[1] Ionospheric modification experiments have been carried out using the HAARP facility along with a 30 MHz coherent scatter radar imager in Alaska to examine properties of artificial E region field-aligned plasma density irregularities (FAIs). In one set of experiments, the RF emission power was varied gradually in order to determine the threshold electric field for irregularity generation. A threshold O mode peak electric field amplitude of 170-195 mV/m at an altitude of 99 km and a heating frequency of 2.7 MHz was identified based on the full-wave formalism of Thidé and Lundborg (1986). In another, the pump frequency was varied gradually to investigate the suppression of the FAIs at frequencies near the second electron gyroharmonic frequency (2W e ). Coherent echoes were found to be suppressed for pump frequencies in an asymmetric band 40-50 kHz wide around 2W e but only for irregularities driven marginally above threshold. Theoretical context for these results is provided.Citation: Hysell, D. L., E. Nossa, and M. McCarrick (2010), Excitation threshold and gyroharmonic suppression of artificial E region field-aligned plasma density irregularities, Radio Sci., 45, RS6003,
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