Proceedings with Extended Abstracts (single PDF file) - Radio ...

100 m/syrFigure 4: Geometry of the RI system sensitive toBragg scale irregularities confined to a very narrowlayer centered about altitude z.Figure 4 illustrates the geometry pertinent for theRI system responding to Bragg scale irregularitiesconfined to a very narrow (sub-resolution) altitudelayer centered about some z. If Bragg scale irregularitieswithin the layer are strictly field alignedthen backscattering will only be possible in directionθ corresponding to a magnetic “aspect angle”of α = 0 o zand a unique target rangecos θ . Otherwise,i.e., if irregularities with non-zero α can exist withinthe same layer, backscattered echoes will arrive fromzranges between r min =cos(θ−α) and r zmax =cos(θ+α) .From earlier studies of magnetic aspect sensitivity ofmid-latitude E-region echoes reported in Huang etal.[1995] we know that rms spread of aspect angleα can be as large as 0.2 o at about ∼101 km altitudeand even larger at lower heights. By using z= 93 km and α = 0.8 o , as well as θ = 49 o , we findthat ∆r ≡ r max − r min ≈ 4.5 km, or, equivalently,∆z = ∆r cos θ ≈ 3 km, which is close to the observedheight extent of LQP structures in Figures2b and 3a. Thus we find ourselves in a position toexplain the LQP radar data of Figures 2 and 3 asfollows: Due to physical mechanisms to be speculatedabout in the next section, regions of enhancedand nearly field-aligned Bragg scale (3 m) waves wereformed and traveled within a periodic structure witha ∼7 km horizontal wavelength and ∼70 m/s velocityin ∼southwest direction (or more precisely, a6.25 km wave with 62.5 m/s velocity along a bearing51 o south of west if we are more careful withthe data). Backscattering from these regions wasdetectible (above the noise floor) up to a magneticaspect angle of ±0.8 o as they transited the northlooking radar beam at a fixed altitude of ∼93 km.αθzWestward component of the motion of scattering regionswithin each gated radar scattering volume accountsfor the quasi-periodic phase streaks of Figures3b and c. Southward displacements of localized scatteringregions register as descending power structuresof Figures 2b and 3a given the height/range ambiguityinherent in power maps. In summary, some of theoutcomes of this explanation are:1. Velocities and scales inferred from power mapsshown in Figures 2b and 3a are related to meridionaldynamics of localized scattering regions,2. Bragg scale irregularities responsible for coherentradar backscatter are observed over a finiterange of aspect angles and the aspect sensitivitydecreases with decreasing altitude,3. Large scale density perturbation that organizesthe Bragg scale regions is characterized by anaspect angle of about 30 o , indicating that thedensity filaments of Figure 2a are not at all fieldaligned.4 ReferencesHuang, C. M., E. Kudeki, S. J. Franke, C. H. Liu andJ. Rottger, Brightness distribution of mid-latitude E-region echoes detected at the Chung-Li VHF radar,Journ. Geophys. Res., 100, 14,703, 1995.Larsen, M. F., A shear instability mechanism forquasi-periodic radar echoes, J. Geophys. Res., 11,41–44, 1999.Pan, C. J., and P. B. Rao, Low altitude quasi-periodicradar echoes observed by the Gadanki VHF radarGeophys. Res. Lett., 29, 25-1, 2002.Patra, A. K., S. Sripathi, V. S. Kumar, and P. B.Rao, Evidence of kilometer-scale waves in the lowere region from high resolution vhf radar observationsover Gadanki, Geophys. Res. Lett., 29, 13,340, 2002.Rao, P. B., M. Yamamoto, A. Uchida, I. Hassenpflug,and S. Fukao, Mu radar observations of kilometerscalewaves in the midlatitude lower e-region, Geophys.Res. Lett., 27, 3667–3670, 2000.Urbina, J., E. Kudeki, S. J. Franke, S. A. Gonzalez,Q. Zhou, and S. Collins, 50 mhz radar observationsof mid-latitude e-region irregularities at camp santiago,puerto rico, Geophys. Res. Lett., 27, 2853–2856,2000.Yamamoto, M., S. Fukao, R. F. Woodman, T. Ogawa,T. Tsuda, and S. Kato, Mid-latitude e region fieldalignedirregularities observed with the mu radar, J.Geophys. Res., 96, 15,943–15,949, 1991.93