Using radio‐induced aurora to measure the horizontal structure of ion layers in the lower thermosphere

Abstract

Two‐dimensional images of sporadic E layers have been produced using a technique called radio‐induced aurora. This technique makes the ion layer regions glow when being stimulated by high‐power radio waves. Normally, the neutral regions near the ion layers do not radiate visible emissions. Experiments in January 1998 at Arecibo Observatory in Puerto Rico have shown that the ion layer regions at 120 km altitude can be made to glow at 557.7 nm and other wavelengths by illuminating them with radio waves at 3.175 MHz with effective radiated powers of 80 MW. The regions of the sporadic E layers that have electron densities greater than the critical density for reflection of the radio waves emit energetic electrons that collide with and excite atmospheric atomic oxygen and molecular nitrogen. Because the electron collisions increase at lower altitudes where the neutral densities are higher, major differences in excitation can occur between layers at 120 km altitude and at 100 km altitude. A charge‐coupled device imager located on the ground is used to capture images of the glowing E region structures. The camera exposure times were in the range of 15–45 s. The images obtained using this technique show a wide variety of structures in the sporadic E layers. Some layers cover the 15 × 30 km region illuminated by the radio wave beam. Other layers show strong modulation of the E region by instabilities driven by a shear in the neutral wind. Two‐dimensional computer simulations of the coupling between neutral wind turbulence and the ion layers replicate the structure shown in the images. The optical emissions are explained using one‐ and two‐dimensional simulations of radio propagation to produce localized regions of large electric fields. Electrons are accelerated out of these regions yielding the artificial aurora.