HF auroral backscatter from the E and F regions

In this thesis, several aspects of HF coherent backscatter from the high-latitude E and F regions are studied with the focus on the relationship between the echo characteristics and the parameters of the ionosphere. The Hankasalmi CUTLASS/SuperDARN radar is the primary instrument for the undertaken studies. The starting point in the research is that coherent echo characteristics are affected by two factors: the plasma physics of magnetic field-aligned irregularity formation and the propagation conditions in that the HF radio waves need to be close to the normal of the Earth’s magnetic field to detect the irregularities. Since the mechanisms of irregularity production are believed to be different at various heights, observations in the E and F regions are considered separately. For the F-region backscatter, we first investigate the ionospheric conditions necessary for backscatter to be detected at specific latitudes and in specific time sectors. To achieve this goal, two approaches are employed. First, a long-term statistical study of diurnal, seasonal and solar cycle effects on echo occurrence is done to assess the relative importance of changes in plasma instability conditions and radio wave propagation. Next, echo occurrence is studied for an area in which ionospheric parameters are measured by EISCAT and other instruments. Both approaches indicate that F-region echoes occur if the electric field is enhanced (above 5-10 mV/m). We show that, once the electric field is above the threshold, the echo power is only slightly dependent on it. We demonstrate that the strongest echoes are received when the F-region electron density is optimal for the selected range and altitude. This optimal value is found to be about 2x1011 m-3 for the Hankasalmi radar. The role of the conducting E region on irregularity excitation and HF radio wave absorption are discussed. The next problem considered with respect to the F-region echoes is the relationship between the velocity of the F-region echoes and plasma convection. We give additional evidence that the observed HF line-of-sight velocity is the projection of the convection velocity on the radar beam and that the Map Potential technique (currently in use for building the global-scale convection maps) compares well with the local EISCAT convection measurements. With respect to the E-region backscatter, two major features are studied. First, a more detailed (as compared to the standard SuperDARN approach) analysis of the spectra is performed. By employing the Burg spectrum analysis method, we show that the E-region echoes are double-peaked in ~35% of observations. Variations of the peak separation with the range and azimuth of observations are investigated. The occurrence of double-peak echoes is associated with scatter from two different heights within the E region. HF ray tracing indicates that for typical ionospheric conditions, scatter from the top and the bottom of the E region is possible at certain slant ranges. In the upper layer the plasma waves move with the velocity close to the ExB convection component. For the lower layer, the plasma wave velocity is reduced due to enhanced ion and electron collision frequencies. A second issue is how do the velocities of HF and VHF E-region echoes compare for observations along the same direction. We concluded that the velocity of E-region echoes at HF can be comparable to or below the VHF velocity and well below the ExB convection component, implying that the scatter can often come from the bottom of the electrojet layer. Other aspects of VHF velocities are also discussed