This project investigates the possible association of high trace-velocity signals, observed by infrasound arrays, with the aurora and seeks to establish the sources of the signals. The origin of the proposed study is a 2004 discovery of high trace velocity infrasound signals that appeared to be caused by pulsating aurora. A relatively large data of "high trace-velocity events" and it now seems clear that pulsating aurora may not be the sole source of such signals: while events are observed during pulsating aurora, there are many nights with extensive displays of pulsating aurora but show no signs of the infrasound signals. However, there does seem to be a connection with the level of geomagnetic activity, since many events occurred during times of high activity as recorded by local ground-based magnetometers. No physical process has yet been identified for the cause of these signals and there is not good understanding of the acoustic wave propagation from ionospheric sources. The project will analyze the infrasound data set in conjunction with auroral optical and magnetic data to establish and quantify any relationships between the observed parameters and to identify possible ionospheric sources for the signals. A large infrasonic data set is currently available, which will be complemented by additional events observed by infrasonic arrays; that data acquisition and analysis are funded through a separate program. An important new data element will be provided by the Advanced Modular Incoherent Scatter Radar (AMISR) currently located at Poker Flat Research Range, Alaska. AMISR can provide detailed spatial and temporal ionospheric information which may help in the identification of the source or sources of the infrasound signals. The project will also provide funding for the continued operation of the auroral video camera at the University of Alaska's Geophysical Institute which will provide useful information on auroral dynamics. In addition to the analysis and interpretation of the infrasound signals, the project will undertake a complementary study on the mechanisms by which energy can be transferred into atmospheric acoustic waves and under what conditions the energy transfer occurs. The broader impacts of the project includes the support of a graduate student who will receive training in data acquisition and hardware, atmospheric and ionospheric dynamics and processes, and manipulation, analysis, and interpretation of large data sets. The project will enable improved understanding of ionospheric processes and acoustic wave propagation in realistic atmospheres.
This project has been aimed at identifying and understanding infrasonic signals associated with aurora activity. Infrasonic signals are atmospheric pressure waves with frequencies well below 1 Hz, which is outside the range of audible sound. We note that there are plenty anecdotal claims that the aurora can at times produce audible sound, which would be at significantly higher frequencies, but such claims have not, to our knowledge, been verified in the scientific literature. The infrasonic data used in our study were recorded on the ground with an array of 8 highly sensitive microphones distributed over a 2 km circular area. The array is located on the University of Alaska Fairbanks campus and the related auroral observations use instrumentation located at the Geophysical Institute operated Poker Flat Research Range 30 miles north-east of campus. It has long been known that a bright, fast moving auroral arc, e.g. a westward travelling surge, can produce infrasonic signals. The work done with the support from this project has shown that auroral infrasonic signals are actually fairly common, and we have developed a number of computer based analysis procedures to identify end extract the signals. The physical process generating the pressure signals in fast moving arcs appears to be Lorenz force coupling between the electrojet currents associated with the arc and the ambient atmosphere. Pulsating aurora can also generate infrasonic signals, but generally with lower pressure amplitudes. We have speculated that the cause may be heating by the energetic electron responsible for the optical pulsation, but modeling indicate that while a signal can be produced it appears to be somewhat weaker than observed, and we are now investigating if Joule heating from ionospheric currents associated with the pulsationg aurora can provide additional energy input. The research results have been published in a number of peer-reviewed publications. In addition to the scientific research the project has supported the thesis research by 2 graduate students. Dr. D.C. Lee graduated in 2010 with a thesis "Neural Network Approach to Classification of Infrasound Signals". Dr Lee is now applying his PhD thesis work to biomedical research in Alberta, Canada. The second graduate student, Mr. J. Oldham, is in the final year of his PhD program. His research is concentrating on identifying and understanding infrasonic signals associated with auroral absorption events. These events are caused by the precipitation of energetic (>10 keV) electrons which will enhance ionization in the 80-100 km altitude region leading to absorption of electromagnetic waves in the HF frequency range. The level of absorption can be measured by an instrument called a riometer. The instrument basically records the signal level of extra-terrestrial electromagnetic radiation. For this research he is using data from a 256 element imaging riometer which provides both spatial and temporal information of the precipitating energetic electrons. The infrasonics array on the University of Alaska Fairbanks campus is as part of a larger worldwide network of similar instrumentation maintained and operated in support of the nuclear Comprehensive Test Ban Treaty (CTBT). An important part of this effort is the description and identification of the many natural sources of infrasonic signals, the aurora being one. The research conducted in this project has provided significant data on auroral sources of infrasonic signals, and as such the project has broader impact than just its contribution to auroral physics.
