SA43A-1880
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.