Millstone Hill Observations of Coherent Backscatter near Perpendicular Magnetic Aspect Angle

Presented at: Workshop on E-region Plasma Instabilities, MPI/Lindau, 1996.

John C. Foster (return to John Foster's Homepage

MIT Haystack Observatory, Westford, Massachusetts, USA

Introduction

The hilltop location of the Millstone Hill UHF radar permits the narrow-beamwidth, 46-meter antenna to observe coherent backscatter from the 110-km altitude region to the north of the facility at magnetic aspect angles (q = k B) ranging from -3° to +10° from perpendicularity. Backscatter intensity decreases at a rate -10 dB/deg at aspect angles > 3° [Foster et al., 1992] and the system sensitivity is such that the strongest coherent echoes exceed the incoherent scatter background by > 90 dB [Foster and Tetenbaum, 1990]. This unique combination of high sensitivity, narrow beamwidth, full antenna steerability, and good aspect angle coverage in the region of auroral electric field enhancement, has been exploited in experiments designed to examine details of the coherent backscatter characteristics near perpendicular aspect angle and at a variety of flow angles (f = k E. Ą B).

Experiment

The magnetic field geometry north of Millstone Hill is such that magnetic field perpendicularity at E-region heights is attained near L = 60°, at elevation angles between 4° and 14°, and at a range of 500 km - 800 km. For pre-midnight conditions with KBoulder > 3, ~E~ > 15 mV/m and the E Ą B direction is predominantly E - W. During greatly disturbed conditions (K > 6) the electric field strength across this region can greatly exceed the two-stream threshold and is rather uniform across the coherent-backscatter field of view of the radar. In such cases, experiments can be performed to investigate the aspect angle and altitude dependence of the backscatter spectra while holding the flow angle constant (^) by performing antenna elevation scans at the azimuth chosen to give k ^ E Ą B. A second experiment involves moving the antenna beam through a sequence of azimuth/elevation positions chosen to keep magnetic perpendicularity (q = 90°) while varying the flow angle (f). Such experiments were performed during an intense disturbance on June 5, 1991 when Kp > 8. Data were taken with a 5-pulse multipulse mode which provided 10-km range resolution. The antenna half-power beamwidth (1°) gives a comparable cross-beam weighting (10 km) at the ranges of interest. Because of the intense signal from the region near q = 90° and the beam shape of the Millstone Hill steerable antenna, presented in Foster and Tetenbaum [1990], there can be some weighting of the received signal from the region out of the main beam when the magnetic aspect angle is away from perpendicularity.The experiments described here were designed and performed with the assistance of D. Tetenbaum.

Observations

In performing the elevation scans at perpendicular flow angle, we found a marked altitude difference in the backscattered spectra near perpendicular aspect angle. Spectra above the peak of the layer at 113 km altitude (figure 1b) exhibit a single peak with fairly uniform negative Doppler (~200 m/s) as q was varied from -3° to +5°. In contrast, at 103 km altitude (figure 1a), below the peak of the layer, the Doppler was large and positive (> +500 m/s) for |q| > 2° and abruptly turned negative (~ -300 m/s) for |q| < 2°. Examination of the spectra at 104 km (figure 2) reveals a transition between two narrow spectra with opposite Doppler sign as a function of q near perpendicularity and a transition region where narrow spectra of both signs were recorded at the same time. Earlier studies at Millstone Hill have reported the occurrence of dual-peaked spectra [Hagfors, 1972; St. Maurice et al., 1989]. The latter study by St. Maurice et al. is noteworthy since the dual-peaked spectra occurred during an azimuth scan at the point at which the beam was approximately perpendicular to the flow angle (f). For the June, 1991 study, the variation of the spectra with altitude is shown in figure 3.

During the AZ/EL scans with constant perpendicular magnetic aspect, we observed two effects. First, there is little variation of the backscattered power with changing flow angle as seen in the top panels of figure 4a,b. Second, there is an abrupt change in sign from positive to negative Doppler as a function of flow angle at altitudes both above and below the layer peak, rather than a cosine-function variation as is seen when q is away from strict perpendicularity[del Pozo et al., 1993]. Also, the flow angle (azimuth) at which the abrupt Doppler transition takes place differs greatly for altitudes above (figure 4a) and below (figure 4b) the peak.

Conclusions

We find that the transition from positive to negative phase velocity (line of sight Doppler), as a function of flow angle, is abrupt when viewed near perpendicular aspect angle, but is smooth (sinusoidal) when viewed at aspect angles > 2°. There is a pronounced altitude dependence of the phase velocity when viewed near perpendicular aspect angle and at perpendicular flow angle. Narrow spectra with greatly different Doppler velocities are observed above and below the center of the irregularity layer (at altitudes of 103 km and 113 km). These observations are consistent with the existence of different irregularity processes at high and low altitudes under very strong electric field conditions. The narrow beamwidth of the Millstone Hill radar system permits the differentiation of populations narrowly separated magnetic aspect angles and altitudes.

References

del Pozo, C. F., J. C. Foster, and J.-P. St. Maurice, Dual-Mode E Region Plasma Wave observations from Millstone Hill, J. Geophys. Res., 98, 6013-6032, 1993.
Foster, J. C., and D. Tetenbaum, High Resolution Backscatter Power Observations of 440 MHz E Region Coherent Echoes at Millstone Hill, J. Geophys. Res., 96, 1251-1261, 1990.
Foster, J. C., D. Tetenbaum, C. F. del Pozo, J. -P. St. Maurice, and D. R. Moorcroft, Aspect Angle Variations in Intensity, Phase Velocity, and Altitude for High-Latitude 34-cm E Region Irregularities, J. Geophys. Res., 97, 8601-8617, 1992.
Hagfors, T., Some Properties of Radar Auroral Echoes as Observed at a Frequency of 1295 MHz, AGARD Conf. Proc., 97, paper # 9, 1992.
St. Maurice, J.-P., J. C. Foster, J. M. Holt, and C. del Pozo, First results on the Observation of 440 MHz High-latitude Coherent Echoes with the Millstone Hill Radar, J. Geophys. Res., 94, 6771-6798, 1989.

http://www.haystack.edu/~jcf/papers/MPI.htm -- Revised: June 11, 1996
E-mail: jcf@hyperion.haystack.edu