[Note: This page describes my research as it was around the beginning of 1999. I have not updated the content of this page since then.]The main focus of my research is on radar backscatter from the auroral E-region ionosphere (radio or radar aurora). Recently I have been particularly concerned with auroral backscatter at large magnetic aspect angles, for which the radar beam makes an angle of more than 2º or 3º with the plane perpendicular to the earth's magnetic field. This is an interesting area of study, because the phenomenon is inherently non-linear, and theories which relate to these observations are just being developed.
- Large Aspect Angle Studies
- Small Aspect Angle Studies
- MIDAS-C Project
Backscatter power was found to be essentially independent of flow angle, in contrast to most previous small aspect angle studies [André, J. Geophys. Res., 88, 8043-8049, 1983; Mattin & Jones, J. Atmos. Terr, Phys., 49, 115-121, 1987], but in agreement with a recent UHF study at small aspect angle [Moorcroft, J. Geophys. Res., 101, 13,379-13,386, 1996 - see the section on the Homer Radar].
Aspect sensitivity was found to decrease with increasing backscatter power. This may help to account for some of the variations in estimates of aspect sensitivity [McDiarmid, Can. J. Phys., 50, 2557-2564, 1972; Moorcroft, Can. J. Phys., 63, 1005-1012, 1985; Foster et al., J. Geophys. Res., 97, 8601-8617, 1992], setting aside, of course, those VHF measurements which may be affected by refraction.
[Jackel, Moorcroft and Schlegel, 1996] , based on work which Brian did for his M.Sc. thesis. In this work we have done a near optimum job of determining the height profile of the backscatter; whenever a good estimate could be obtained, the layer was always singly-peaked, usually not far from Gaussian in shape, and at relatively low altitudes (around 104 km), compared with the height estimates of small aspect angle backscatter. We also made an extensive study of the skewness of the spectra (Previous estimates of skewness of EISCAT backscatter were based on only 5 seconds of data [Schlegel, Turunen & Moorcroft, J. Geophys. Res., 95, 19001-19009, 1990]), and found that for these large aspect angle observations skewness has an average close to +1 (for negative velocities) over a wide range of mean Doppler velocity.
The first work which has been done with this rejuvenated database has been a study of flow angle effects at small aspect angle [Moorcroft, 1996]. This appears to be the first small aspect angle flow angle study of backscattered power at UHF. The results differ in several ways from previous studies using 140 MHz STARE and SABRE data [André, J. Geophys. Res., 88, 8043-8049, 1983; Haldoupis & Nielsen, J. Geophys. Res., 89, 2305-2312, 1984; Mattin & Jones, J. Atmos. Terr, Phys., 49, 115-121, 1987]: backscatter power is virtually independent of flow angle; there is no clear distinction between type 1 and type 2 echoes; low velocity echoes vary with flow angle more rapidly than predicted by the 'cosine law'. These results, when considered in the light of mode coupling theory [Hamza & St-Maurice, J. Geophys. Res., 98, 11587-11599, 1993], suggest that there may be significant differences in the nature of the coupling processes at VHF and UHF.
Hamza and St-Maurice [J. Geophys. Res., 98, 11587-11599, 1993] have developed a non-linear theory from which they derive a theoretical relationship between mean velocity (first moment of spectrum) and spectral width (square root of second moment). They predict that the sum of the squares of these two quantities should be approximately constant, and equal to the square of the ion-acoustic speed. The Homer database is being re-analyzed to test this prediction.
Recently, Dr. Hamza (formerly at UWO, now at the University of New Brunswick) has been working on a theory of spectral skewness at small aspect angles. By making a suitable analysis, the Homer database can be used to test the predictions of that theory. Such a study is underway, and first indications are that the skewness may be rather less than what is found at large aspect angles at UHF (see above), and less than predicted by theory. The situation is complicated by the difficulty of getting an accurate measure of a higher moment such as skewness from experimental data.
Although some experiments have been made using a small antenna on the
roof of the Physics and Astronomy Building at the University of Western
Ontario, the interesting science for this experimental program makes use
of the 46m diameter antenna, formerly operated by the National Research
Council as the Algonquin Radio Observatory, and located within Algonquin
Provincial Park. Since this antenna is the same diameter as the Millstone
steerable antenna, they are well matched for bistatic experiments. The
magnetic geometry for the Algonquin-Millstone circuit makes it particularly
suitable for studies of backscatter from the auroral E-region. It is possible
to view a wide variety of pairs of different aspect angles from the same
scattering volume; in some directions the bistatic path is at such large
aspect angles that only incoherent scatter will be detected, while from
the same volume the Millstone Hill radar will be able to receive small
aspect angle coherent backscatter.
Although that was the theory, the practice has been somewhat more difficult. In April 1995, during a magnetically disturbed period, coherent backscatter was obtained at both Algonquin and Millstone, but, for reasons not yet understood, no incoherent backscatter was detected at Algonquin. Finally, in the fall of 1996, after successfully calibrating the system by means of moon-bounce experiments, weak incoherent backscatter was detected from the ionospheric F-region, but still considerably weaker than expected.
Brian Jackel worked on this project for his Ph.D. (completed November, 1997) and publications are in preparation.
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