Scatter plots of Ap_c as a function of normalised Pα values (for the best fit α of 0.46) based on 1-hour resolution interplanetary data for 1964–2016. Po is the average of Pα for all the available data from 1964–2016, and Ap_c are hourly Ap values (linearly interpolated from the observed 3-hourly Ap values) that are coincident with a valid hourly Pα value. The blue dots are for daily means (τ = 1 day) and the red dots for annual means (τ = 1 year). The black line is the linear regression fit to the τ = 1 day data and gives the correlation C in Table 2, the red line is the linear regression fit to the τ = 1 year data and gives the correlation D in Table 2.