50 MHz Backscatter Observations in the Polar Cap Ionospheric E Region
Results of VHF Doppler measurements made at an invariant geomagnetic latitude near 77° are presented. The experiment was set up so that high resolution spectral measurements could be made from a common E region volume along two bistatic CW radio links at magnetic aspect angles of 9.5° and 13.5° from perpendicularity to the earth’s magnetic field. For the events considered in this study the evidence, based on K p index and meridional magnetograms, shows the backscatter region to be located mostly poleward of the auroral electrojet boundary inside the polar cap. The most striking feature of the data is the temporal morphology of the polar cap echo occurrence which differs from that in the auroral zone. In particular there is a complete absence of echoes at midnight and early morning hours, unlike the auroral zone where strong westward electrojet echoes are often present. The strongest and most frequently occurring echoes were observed for a few hours prior to midnight and were always associated with southward Hall drift motions. Comparison with ionosonde data shows a close connection between echo occurrence and the presence of a strong sporadic E s layer. Magnetometer and riometer data suggest that, at times, other factors, than electrojet currents and absorption, control the echo occurrence at these high latitudes. On the other hand, the Doppler spectral signatures of the echoes suggest the same instability mechanisms to be responsible for the generation of short scale irregularities in the polar cap as in the auroral zone. Finally, the results show the magnetic aspect control is not strong at 50 MHz. thus VHF coherent radars can be useful diagnostic tools in probing the lower ionosphere at polar cap regions.
KeywordsInterplanetary Magnetic Field Doppler Spectrum Auroral Zone Auroral Oval Aspect Angle
Unable to display preview. Download preview PDF.
- Acasofu, S.-I. and Chapman, S. (1972) Solar-Terrestrial Physics, University Press, Oxford.Google Scholar
- Greenwald, R. A., Ecklund, W. L. and Balsley, B. B. (1975) ‘Radar observations of auroral electro jet currents’, 80, 3635–3641.Google Scholar
- Iialdoupis, C. (1989) ‘A review on radio studies of auroral E region ionospheric irregularities’, Ann. Geophysicae, 7.3.Google Scholar
- McNamara, A. G. (1972) ‘The occurence of radio aurora at high latitudes: The IGY period, 1957–1959’, Geophys. Publ., 29,135–149.Google Scholar
- Piggot, W. R. and Rawer, K. (1975) URSI handbook of ionogram interpretation and reduction, Elsvier Publ., Amsterdam.Google Scholar
- Sandholt, P. E., Egeland, A., Holtet, J. A., Lybekk, B., Svenes, K. and Asheim, S. (1985) ‘Large- and small- scale dynamics in the polar cusp’, 90, 4407–4414.Google Scholar
- Unwin, R. S. (1972) ‘The radio aurora’, Ann. Geophys., 28(1), 111–127.Google Scholar