Distributions of wave normals at the duct exit of a whistler and the morphology of ionospheric exits of low-latitude whistlers observed by direction finding

Direction‐finding observations at low latitudes reveal that whistlers arrive from narrow regions of the zenith of tens of kilometers in diameter and that their ionospheric exits have a variety of shapes. The distribution of the wave normals of whistlers at the duct exit was studied using a field‐aligned duct model by ray tracing in order to examine the cause of the morphology of the ionospheric exits. The characteristics of the distribution and its relation to the trapping cone have been clarified. When the distribution is represented by plotting the wave normal dip angle against the escape position at the duct exit, two kinds of wave normal distributions are evident at the duct exit: One is a flat distribution of downward wave normals and has a region of almost vertically downward wave normals at the main area of the duct exit, and the other is a flat distribution of more upward wave normals. They can be explained, respectively, as the innermost direction and the outermost direction of a trapping cone. The region of vertical downward wave normals in the downward wave normal distribution occurs because of the vertical downward electron density gradient in the topside ionosphere and also because of the effects of the trapping cone shrinking on the whistler's passage through the ionospheric maximum electron density region and of the expansion of this cone in the lower ionosphere. This region of vertical downward wave normals is responsible for the whistler's penetration to the ground. Because the downgoing whistler's wave normals are oriented to the vertical downward direction uniformly at any point of the duct exit due to the downward wave normal distribution, whistler rays emitted from each point of the duct exit propagate in parallel and reach the bottom of the lower ionosphere. Thus the observed morphology of ionospheric exits can be interpreted as the result of the projection of the section of the duct exit onto the ionospheric lower boundary.