[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

Prince Albert Radar data

I have re-analyzed data taken with the Prince Albert Radar in 1968 [Moorcroft, 1996]. This is a unique database of large aspect angle UHF observations; although old, there is no prospect of such an extensive set of observations being made in the foreseeable future, at least at UHF. A statistical study of these data has provided the first large aspect angle UHF flow angle study of backscatter power, as well as new results on aspect sensitivity at UHF, as shown in the figure below.


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.

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EISCAT UHF Study

Brian Jackel (Ph.D student, graduated Nov. 1997) and I have studied large aspect angle observations using the European EISCAT incoherent scatter radar [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.

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VHF Windprofiler radar at Resolute Bay

This 50 MHz radar, designed and constructed by Dr. W.K. Hocking, has recently started operation in the Canadian arctic. It is primarily designed for lower atmospheric dynamical studies, but also includes provision for ionospheric measurements. A second set of antennas have been constructed which look south towards Cambridge Bay, approximately duplicating part of the experimental arrangement of the NRC International Geophysical Year (1957) radar [McDiarmid and McNamara, Can. J. Phys., 45, 3009-3027, 1967]. Those very puzzling results included backscatter at geometric magnetic aspect angles of up to 20º, far larger than any other observations which have ever been recorded. They continue to be a topic of active consideration [McDiarmid D. R., Watermann J., McNamara A. G., J. Geophys. Res., 95, 17261-17266, 1990]. A likely possibility is that total or partial oblique reflection from a sporadic E layer was involved. The new Resolute Bay radar, together with auxiliary observations using the modern digital ionosonde at Cambridge Bay (operated by Dr. J. W. MacDougall of the University of Western Ontario) should lead to an explanation of these unusual observations. Because of the expected sensitivity of this radar, these backscatter observations may also serve as a monitor of strong electric fields in the cleft and polar cap regions. As of March 1998, only a few hours of observations have been obtained, all during quiet magnetic conditions.

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Small Aspect Angle Studies

Homer, Alaska radar

In the 1970's I made several field trips to the 398 MHz phased-array radar at Homer, Alaska, operated by SRI International. That resulted in a large database (several million records) which has been maintained over the years, so that new questions could be asked of it as they arose. During the past year, the entire database has been converted from binary to ascii format and stored on several CD-ROMs. The files of processed data have also been converted and stored on CD-ROM. This new compact format makes possible a number of studies which were difficult or nearly impossible when the data was stored on dozens of magnetic tapes.
   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.

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Small aspect angle VHF experiments at London, Ontario - CLOVAR Radar

Most of the large databases of observations available in the world at frequencies around 50 MHz have been taken at geometrically large aspect angles. Because these observations are significantly affected by refraction, it is often difficult to know which observations are truly large aspect angle, and which are due to backscatter along ray paths that have been refracted to small aspect angle. Thus, there is a need for a statistical study of small aspect angle backscatter at these frequencies as a basis of comparison with the existing 'large aspect angle' observations. It is very easy to observe near zero aspect angle over a very large region by locating a monostatic radar near London, Ontario. We are, therefore, carrying out such experiments using antennas connected to the 40 MHz windprofiler radar (the CLOVAR radar) operating by Dr. W. K. Hocking at a field site near London. As of August 1998, preliminary observations have been made, but no auroral echoes have as yet been detected.

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MIDAS-C project

The purpose of this research program is to add a bistatic capability to the Millstone Hill incoherent scatter radar through the construction of a portable radar receiving system. It was initiated in 1992 with the support of the Canadian Network for Space Research. The heart of the project is the MIDAS-C (Millstone Data Analysis System - Canada) radar receiving and data analysis system, which was constructed by the Atmospheric Sciences Group at Haystack Observatory. This system fits into a single equipment rack, and is easily transportable in a small van.

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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