A new method for deducing the effective collision frequency profile in the D-region

A method based on linear regularization is presented for the deduction of the effective collision frequency profile in the D‐region using an incoherent scatter radar and a colocated riometer. The deduced collision frequency above EISCAT, Tromsø, for the period 09 November 2004 to 13 November 2004 matches very well with that calculated using neutral densities and temperatures from the NRLMSISe‐00 atmospheric model between 68 km and 95 km during active (ap > 50) and quiet (ap < 50) conditions. We also confirm the validity of using the Appleton‐Hartree equation with effective collision frequencies when calculating the ionospheric absorption at high frequencies.