Abstract
Abstract. Part I of this history describes the motivations for developing radars in the high frequency (HF) band to study plasma density irregularities in the F region of the auroral zone and polar cap ionospheres. French and Swedish scientists were the first to use HF frequencies to study the Doppler velocities of HF radar backscatter from F-region plasma density irregularities over northern Sweden. These observations encouraged the author of this paper to pursue similar measurements over northeastern Alaska, and this eventually led to the construction of a large HF-phased-array radar at Goose Bay, Labrador, Canada. This radar utilized frequencies from 8–20 MHz and could be electronically steered over 16 beam directions, covering a 52∘ azimuth sector. Subsequently, similar radars were constructed at Schefferville, Quebec, and Halley Station, Antarctica. Observations with these radars showed that F-region backscatter often exhibited Doppler velocities that were significantly above and below the ion-acoustic velocity. This distinguished HF Doppler measurements from prior measurements of E-region irregularities that were obtained with radars operating at very high frequency (VHF) and ultra-high frequency (UHF). Results obtained with these early HF radars are also presented. They include comparisons of Doppler velocities observed with HF radars and incoherent scatter radars, comparisons of plasma convection patterns observed simultaneously in conjugate hemispheres, and the response of these patterns to changes in the interplanetary magnetic field, transient velocity enhancements in the dayside cusp, preferred frequencies for geomagnetic pulsations, and observations of medium-scale atmospheric gravity waves with HF radars.
Highlights
1.1 MotivationDuring most of the 20th century, radar backscatter investigations of electron-density irregularities in the Earth’s ionosphere were carried out predominantly with very high frequency (VHF) and ultra-high frequency (UHF) radars (e.g., Harang and Landmark, 1954; Leadabrand et al, 1965; Unwin, 1966; Ecklund et al, 1975; Greenwald et al, 1978)
A total of 2 years later, Ruohoniemi et al (1987) reported a similar comparison of HF radar Doppler velocities derived from refined autocorrelation functions (ACFs) that were associated with F-region ionospheric irregularities and UHF incoherent scatter observations of plasma Doppler velocities observed with the Sondrestrom radar
It is hoped that this post-1980 history of HF radar research in the USA, Canada, and Antarctica has provided the reader with some appreciation of the complexities that are associated with HF radar measurements in the ionosphere and, a greater appreciation of the potential that these measurements have for advancing our understanding of plasma processes in the Earth’s very high latitude ionosphere and outer magnetosphere
Summary
During most of the 20th century, radar backscatter investigations of electron-density irregularities in the Earth’s ionosphere were carried out predominantly with VHF (very high frequency) and UHF (ultra-high frequency) radars (e.g., Harang and Landmark, 1954; Leadabrand et al, 1965; Unwin, 1966; Ecklund et al, 1975; Greenwald et al, 1978). Thogonal to the steeply inclined magnetic field lines in the high-latitude auroral zone and polar cap If they encountered ionospheric irregularities while at these angles, backscattered signals would propagate along the reciprocal path and return to the radar site, while most of the energy in the transmission will continue to propagate upwards. If it passes through the peak of the F layer, it will enter the topside ionosphere and encounter a region of decreasing electron density, where the transmission will slowly return to its original propagation direction In the process, it must again pass through a range of angles where it is propagating within 2◦ of orthogonal to the geomagnetic field, and if ionospheric irregularities are again present, backscattered signals will be generated and follow the reciprocal path though the peak of the F layer and return to the radar site. I decided to pursue the HF backscatter path, because I felt that, if the effort were successful, the rewards would be far greater
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