This effective collision frequency is very variable, ranging from about a kHz up to about 100 MHz. Fortunately, if the total attenuation through the ionosphere at a given frequency is known, then the differential absorption between LHCP and RHCP components propagating longitudinally along magnetic field lines is relatively simple to calculate. Details of the calculation, copied from a Mathcad worksheet, are given here. The results are summarised in the table below. Under extreme conditions at 100 MHz day-time absorptions up to about 10 dB have been reported.
Absorption decreases as the square of frequency.