On the factors controlling occurrence of F-region coherent echoes

• Main Authors: Danskin, D. W., Koustov, A. V., Ogawa, T., Nishitani, N., Nozawa, S., Milan, S. E., Lester, M., Andre, D.
• Other Authors: Institute of Space and Atmospheric Studies Saskatoon (ISAS), Department of Physics and Engineering Physics Saskatoon, University of Saskatchewan Saskatoon (U of S)-University of Saskatchewan Saskatoon (U of S), Solar-Terrestrial Environment Laboratory Nagoya (STEL), Nagoya University, Department of Physics and Astronomy Leicester, University of Leicester
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2002
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; EISCAT
• Online Access: https://hal.science/hal-00317139; https://hal.science/hal-00317139/document; https://hal.science/hal-00317139/file/angeo-20-1385-2002.pdf

International audience 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 × 10 11 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 × 10 11 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.