Statistics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa32a..02j&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA32A-02
Statistics
[2403] Ionosphere / Active Experiments, [2407] Ionosphere / Auroral Ionosphere, [2487] Ionosphere / Wave Propagation
Scientific paper
Techniques for using modulated HF heating of the D-region ionosphere near the auroral electrojet to generate extremely low frequency (ELF) waves in the kilohertz range have been refined over many decades. Even with the high power of the newest ionospheric heater at the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, practical use of modulated heating as a means of ELF generation has been hindered by the variability of ELF generation with changes in natural conditions. Magnetometers, which measure the intensity of the overhead electrojet currents, are often used as a predictor of ELF generation. We assemble statistics using the HAARP magnetometer and ELF amplitude measured at Chistochina, Alaska 37 km from HAARP. While correlation between the two is often good, we show that the relative change in ELF amplitude per unit change in magnetometer measurements decreases during strong electrojets. Dramatic increases in electrojet strength do not result in proportional increases in ELF generation. Data also show ELF generation to be surprisingly robust; ELF signals were always detectable at Chistochina even when magnetometers showed no discernible electrojet. Measurements from the HAARP riometer, which can measure increases in ionospheric absorption in the lower ionosphere, suggest that strong electrojets associated with increased absorption are not as beneficial for ELF generation. Statistical models using neural networks and multiple regression to fit ELF amplitude to magnetometer, riometer and other ionospheric diagnostics show that ELF amplitude and magnetometer are nearly linearly proportional, with the proportionality constant increasing as absorption decreases. We also present numerical simulations using a model for HF heating and a full-wave model for ELF propagation. These also show that denser electron density profiles increase the ambient conductivity more than they improve ELF generation. Certain profiles increase ambient conductivity while decreasing ELF generation, which may explain some observed cases of negative correlation between ELF amplitude and magnetometer data.
Cohen Martin
Inan Umran S.
Jin Guanhao
Lehtinen Nikolai G.
Spasojevic Maria
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