On the factors controlling occurrence of F-region coherent echoes

• Published in: Annales Geophysicae
• Main Authors: Danskin, D. W., Koustov, A. V., Ogawa, T., Nishitani, N., Nozawa, S., Milan, S. E., Lester, M., Andre, D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: European Geosciences Union (EGU), Copernicus Publications 2017
• Subjects: Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere); Canada; Norway; Tromsø; EISCAT; Nordlysobservatoriet; University of Tromsø
• Online Access: http://www.ann-geophys.net/20/1385/2002/; http://hdl.handle.net/2381/39098; https://doi.org/10.5194/angeo-20-1385-2002

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. CUTLASS Finland radar is supported by PPARC, the Swedish Institute for Space Physics, Uppsala, and the Finnish Meteorological Institute. EISCAT is an international facility supported by Finland, France, Germany, Japan, Norway, Sweden and the UK. Data of the Sodankyla ionosonde and Finnish ¨ riometers are from the Sodankyla Geophysical Observatory. Ad- ¨ ditional riometer data originated from the Imaging Riometer for Ionospheric Studies (IRIS), operated by the Department of Communications Systems at Lancaster University (UK), funded by the Particle Physics and Astronomy Research Council (PPARC) in collaboration with the Sodankyla Geophysical Observatory. The ¨ geomagnetic data are from the Tromsø Geophysical Observatory (Nordlysobservatoriet), University of Tromsø, Norway. A.V.K. acknowledges the Solar-Terrestrial Environment Laboratory of the Nagoya University for funding during his stay in Japan. The work was also supported by an NSERC grant (Canada) to A.V.K Peer-reviewed Publisher Version