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Schematic illustrating the equinoctial effect. Part (a) is the same as in Figure 2, but includes the magnetic axis at selected UTs and the angle Ψ that it makes with the XGSE axis. Parts (b), (c) and (d) are views of Earth from the dusk side (i.e., in the −YGSE direction) and show the XGSE and ZGSE axes and the projection of the angle Ψ into this plane, called the dipole tilt angle ϕ that varies between a maximum of δa + δd = +33.1° in Northern hemisphere summer at 4.72 UT (part b) and a minimum of −(δa + δd) = −33.1° in Southern hemisphere summer at 16.72 UT (part c). There are a variety of UT and F combinations that give ϕ = 0, and one is illustrated in part d. The equinoctial effect applies to anything that varies with ϕ or cos(ϕ) and gives the characteristic pattern F-UT pattern and F variation of daily means shown in part e. The peaks of the equinoctial pattern are currently on 21 March (F = 0.219) and 19 September (F = 0.719).
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