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Ann. Geophys., 20, 1385-1397, 2002
www.ann-geophys.net/20/1385/2002/
© European Geosciences Union 2002
On the factors controlling occurrence of F-region coherent echoes
D. W. Danskin1, A. V. Koustov1,2, T. Ogawa2, N. Nishitani2, S. Nozawa2, S. E. Milan3, M. Lester3, and D. Andre1
1Institute of Space and Atmospheric Studies, University of Saskatchewan, 116 Science Place, Saskatoon, S7N 5E2 Canada
2Solar-Terrestrial Environment Laboratory, Nagoya University, 3–13 Honohara, Toyokawa, Aichi 442, Japan
3Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
Correspondence to: D. W. Danskin
(danskin@dansas.usask.ca)
Abstract. Several factors are known
to control the HF echo occurrence rate, including electron density distribution
in the ionosphere (affecting the propagation path of the radar wave), D-region
radio wave absorption, and ionospheric irregularity intensity. In this study,
we consider 4 days of CUTLASS Finland radar observations over an area where the
EISCAT incoherent scatter radar has continuously monitored ionospheric
parameters. We illustrate that for the event under consideration, the D-region
absorption was not the major factor affecting the echo appearance. We show that
the electron density distribution and the radar frequency selection were much
more significant factors. The electron density magnitude affects the echo
occurrence in two different ways. For small F-region densities, a minimum value
of 1 × 1011 m-3 is required to have sufficient radio
wave refraction so that the orthogonality (with the magnetic field lines)
condition is met. For too large densities, radio wave strong
"over-refraction" leads to the ionospheric echo disappearance. We
estimate that the over-refraction is important for densities greater than 4 ×
1011 m-3. We also investigated the backscatter power and
the electric field magnitude relationship and found no obvious relationship
contrary to the expectation that the gradient-drift plasma instability would
lead to stronger irregularity intensity/echo power for larger electric fields.
Key words. Ionosphere (ionospheric
irregularities; plasma waves and instabilities; auroral ionosphere)
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