Radar measurements of very small aspect angles in the equatorial ionosphere

Equatorial spread F (ESF) irregularities are very highly aligned with; the geomagnetic held. In order to understand the physics of the irregularities, that is, the unstable plasma waves, it is important to measure this degree of alignment as accurately as possible. Measuring the angles with a radar is no easy task, however, since the angles in question, perhaps a few times 10(-2) degrees or less, are much smaller than the radar beam widths available at any equatorial radar, and there are numerous potential sources of error that can make the angles appear larger than they really are. Nevertheless, the measurement can be done. At Jicamarca we have determined an upper limit on the rms deviations from perfect alignment that is often as small as 0.015 degrees. This paper escribes the measurement technique and presents a few illustrative examples of the data. The following companion paper [Hysell and Farley, this issue] discusses the theoretical implications of these measurements.