HF ray tracing at high latitudes using measured meridional electron density distributions

In many physical cases, small‐ and intermediate‐scale (1–100 m) electron density irregularities in the ionospheric E and F regions are elongated in the direction of the earth's magnetic field. As a result of this elongation or field alignment, radar studies directed toward the analysis of backscattered signals from ionospheric irregularities require that the radar wave vector propagate perpendicular to the magnetic field in the irregularity region. At low and middle latitudes, orthogonality may be achieved at any radar operating frequency, but at higher latitudes this geometry becomes increasingly difficult. Above 60° invariant latitude it is almost impossible to achieve normality with straight line propagation. In order to observe the wealth of ionospheric irregularities in the auroral zone and polar cap E and F regions, one must operate radars at HF frequencies where refraction affects wave propagation and aids one in achieving the orthogonality condition. However, refraction from large‐scale (1–100 km) ionospheric irregularity structure also increases the complexity in accurately determining the location of the scattering volume. In this paper we use a modified version of an existing HF ray tracing program to determine with high precision the HF propagation paths in a realistic ionosphere. In this analysis the ionosphere is simulated by a two‐dimensional array of electron density as a function of geomagnetic latitude and altitude. This analysis is then applied to real measurements obtained from a meridian scan of the Chatanika incoherent scatter radar, and we show how this electron density distribution affects propagation from a simulated HF radar located at Anchorage, Alaska. The variations of the propagation conditions are also studied as one moves the density pattern in the direction of the Anchorage radar.