Properties of Regions of ELF Radiation Induced by HF Ionospheric Heating
Denys Piddyachiy1, Timothy F. Bell1, Umran S. Inan1,2, Forrest Foust1, Nikolai G. Lehtinen1, Michel Parrot3
1. Electrical Engineering, Stanford University, Stanford, CA, USA, 2. Koc University, Istanbul, Turkey, 3. LPC2E, CNRS, Orleans, France

ELF wave (30 - 3000 Hz) generation and propagation is an important topic of research affecting many areas of space physics. For example, ELF waves generated by lightning discharges can effectively interact with particles in the Earth’s radiation belts. Also, ELF waves can penetrate effectively under water to allow wireless communication with submersible crafts. However, it is difficult to generate ELF waves artificially because of their long wavelengths. In this work, the High Frequency Active Auroral Research Program (HAARP) transmitter array (3.6 MW, 2.75 - 10 MHz) is used to generate ELF waves in a controlled manner through periodic heating of the ionospheric D-layer and subsequent modulation of the conductivity of the auroral electrojet. The low-earth-orbit DEMETER satellite is used to study ELF power distribution as a function of the distance from the source. The spatial power distribution depends on many factors. Some of them can be controlled: the ELF and HF frequencies, direction, and modulation techniques. Other parameters are natural and cannot be directly affected: strength of the electrojet current, plasma density, and so on. Initial studies were conducted on a case by case basis, but now they are complemented by a statistical study of multiple experiments over four years. Three regions of ELF radiation are seen in case studies and in an averaged pattern. The most important feature is a column of radiation into space about the size of the heated region (~50 km) and average field strength of 100-150 uV/m. Total ELF power in the column is estimated to be about 1 W. It is found that the column is displaced by 50 - 100 km to the South from the field line of the source. A full-wave model predicts a column of about the same size, but displaced to the North from the field line by 50 km. In addition, the model enables the identification of different physical mechanisms of wave propagation to the three regions of radiation. In brief, in region 1 (the column) and region 2 (up to 300 km from the source in the horizontal distance) waves reach the satellite directly; while in region 3 waves at first are propagating in the Earth-ionosphere waveguide and then leak to the satellite through the ionosphere. The sizes of regions in observations and modeling are comparable. Raytracing is used to interpret the difference between the position of the column in the observations and full-wave modeling. Full-wave modeling assumes vertically stratified ionospheric density, while ray tracing can be used with more realistic models of plasma density including horizontal gradients. It is shown that a horizontal gradient indeed can explain the bending of the column in observations. Employing simple model with linear horizontal gradient of Log(Ne), it was deduced that density should change by an order magnitude over about 5 degrees in latitude in order for ray trajectories to match observations.