(a) Scatter plots of the percentage deviation of observed f_oF₂ from the fitted values, δf_oF₂, versus <ap>_24 h at Petropavlovsk in March equinox season; the red line is the linear fitting of the dots for the geomagnetic activity level of <ap>_24 h<30. (b) Latitude variations of the fitting slope of δf_oF₂ versus <ap>_24 h; the red, blue, black, and green lines correspond to the seasonal conditions of March equinox, June Solstice, September equinox, and December solstice, respectively.