J. A. Koehler

Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

Ionospheric refraction effects in slant range profiles of auroral HF coherent echoes

The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.

Bars — A dual bistatic auroral radar system for the study of electric fields in the Canadian sector of the auroral zone

Abstract One instrument of the Canadian Auroral Network for the Open Program Unified Study (CANOPUS) is a pulsed dual bistatic auroral radar system (BARS) for the mapping of ionospheric electric fields, using the STARE technique originated by R.A. Greenwald [14]. The Canadian system is presently in the specification and design phases, with the objective of being operational by mid-1984. This paper describes the geometry of the BARS system, the design considerations, and the planned data and control network.

Aspect angle dependence of the radar aurora Doppler velocity

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50-MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5° and 4.0° and 0.6 to 0.7 between 3.2° and 5°. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for “ray spreading” arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken into account.

Localized and strongly unstable plasma regions in the auroral E region ionosphere and implications for radar experiments

The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter-scale E region irregularities. SAPPHIRE is a dual 50-MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross-beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial target grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna arrays showed a class of scatter characterized by a narrow power spectrum peaking at the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large-amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the “short discrete radar auroras” which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the large dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3-dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations.

Radio aurora magnetic and streaming aspect sensitivities on 6 simultaneous links at 50 MHz

Abstract During August 1981, a 50 MHz c.w. radar system was operated in central Canada to measure auroral scatter amplitudes and Doppler spectra from a scattering region centered near 66° magnetic invariant latitude (L ≈ 6.0). Narrow beams from 3 transmitters, differing in frequency by 1 KHz, were directed to cover a common volume of the ionosphere over a ground location at 56.3°N, 103.5°W. The scattered signals were received on narrow beam antennas at two receiving sites, and recorded in analog form on magnetic tape under the control of an AIM65 microcomputer. The analog tapes were digitized later and FFT-processed to obtain Doppler spectra and amplitudes. The 6 transmission paths were designed to provide several magnetic aspect angles varying by 1.5°-7° from perpendicularity with the earths field B and two streaming aspect angles differing by ∼38°. The objective was to employ controlled geometric factors to study the functional dependency of signal amplitudes and Doppler shifts on magnetic and streaming aspect angles. Several hundred hours of excellent data were obtained in continuous operation during the month of August 1981. Preliminary results will be reported.

Simultaneous observations of E region coherent radar echoes at 2‐m and 6‐m radio wavelengths at midlatitude

Two continuous wave Doppler radars, using scaled antenna arrays at the same receiving and transmitting sites in order to observe the same volume in the E region ionosphere, made simultaneous observations of coherent backscatter from the island of Crete, Greece. The radars operated at different frequencies, that is, 50 MHz and 144 MHz, which made it possible for the first time in midlatitudes to observe simultaneously aspect sensitive backscatter from about 3-m and 1-m irregularities, respectively. In this paper we introduce the experiment and present an overview of the observations. Both primary and secondary E region irregularities were observed at both radar frequencies, but the 144 MHz echoes were considerably weaker than those at 50 MHz. The observations demonstrated clearly the different character of E region type 1 and type 2 irregularities. The 144-MHz type 2 echoes were completely absent during times of weak to moderately strong 50-MHz backscatter activity and appeared only when the signal at 50 MHz became rather strong with relative intensities nearing and exceeding 20 dB above noise. The large differences in the observed signal levels between the 50- and 144-MHz radar type 2 echoes suggest a very steep wavenumber spectrum in the irregularity wavelength range from 3-m to 1-m. On the other hand, and in sharp contrast to type 2 echoes, there was one to one correspondence in the occurrence of 50-MHz and 144-MHz type 1 echoes, even when the signal at 50 MHz was only a few decibels above noise. Although at times the observed velocities of the simultaneous 3-m and 1-m type 1 irregularities were the same, on the average the evidence favored a slightly higher threshold for excitation at 144 MHz than at 50 MHz. The results are in general agreement with what is anticipated from basic theory of E region meter-scale plasma waves.

Auroral radar backscatter at off-perpendicular aspect angles due to enhanced ionospheric refraction

This paper studies the effect of ionospheric refraction upon auroral radar backscatter under conditions where the aspect angle appears far from ideal, i.e., when the unrefracted ray path trajectory is at least a few degrees from the perpendicular to the Earth’s magnetic field. It is found that wave trapping by curved electron density layers can cause ionospheric refraction as large as 20o, even at 150 MHz. This suggests that many so-called off-orthogonal VHF echoes are in reality due to backscattering at near-orthogonal aspect angles, the discrepancy arising from increased ionospheric refraction by curved or tilted layers.

Preferential phase velocities for type 4 irregularities in the auroral E region plasma

This paper presents additional evidence on the nature of 50-MHz type 4 auroral backscatter. By using a large number of events, recorded during several periods of continuous wave Doppler radar operation in the past few years, and a data sequence of long lasting type 4 echoes, the mean phase velocity of type 4 waves and its relation to type 1 phase velocity have been studied statistically. On the average, type 4 phase velocities range between 800 and 1200 m/s with the great majority grouping near 1000 m/s. The mean velocities of simultaneous type 4 and type 1 spectrum components are well separated in the Doppler shift range with an approximate factor of 2 in the type 4 to type 1 velocity ratio. Also, both spectrum components have the same Doppler polarity, which is indicative of a direct or indirect role for the electric field in type 4 wave generation. On the basis of the widely accepted conviction that type 1 waves have velocities saturating near the ion acoustic speed, it is concluded that type 4 waves propagate with velocities at about twice the acoustic speed in the plasma. The evidence presented here strongly suggests that type 4 waves propagate at preferential velocities near 1000 m/s, an experimental fact which must be explained in theoretical models.

A new look at type 4 echoes of radar aurora

Type 4 echoes represent a rare but distinct spectral signature of auroral electrojet backscatter during times of large electric fields and strong two-stream-generated ion acoustic turbulence. On the basis of many bistatic CW Doppler radar observations made over the past 15 years the great majority of type 4 echoes have a composite spectrum showing two main features, a broadened type 1 component peaking at elevated ion acoustic velocities and the type 4 narrow component at roughly double the ion acoustic speed. The most important property is the large phase velocities, mostly seen in the 900 to 1100 m/s range, associated with irregularities propagating in a narrow azimuthal cone along the destabilizing Hall current. In the present paper it is argued that the existing interpretation, in which it is hypothesized that the large Doppler velocities are simply from two stream waves originating in regions of highly elevated electron temperatures, cannot explain why the observed phase velocities of type 4 waves are mostly seen at preferential values near 1000 m/s and why these waves are highly directional. On the basis of the existing evidence we conclude that there is not yet satisfactory explanation for the type 4 echoes. The fact that type 4 waves have frequencies of about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest the possibility of a resonant three-wave coupling interaction process which might be in operation under some appropriate conditions in the plasma. At present this is only a preliminary proposition which would be investigated further in a future study.