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Figure 4

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Schematic illustration of the axial effect. Part (a) shows the ecliptic plane as in Figure 2, but added is the Sun’s equatorial plane which is inclined at an angle α = 7.25° to the ecliptic, as shown by the blue circle that is shaded pale blue. This has two effects: firstly, Earth has an annual variation in its heliographic latitude which reaches maximum values in the solar southern and northern hemisphere on dates that happen to be close to the equinoxes, namely 5 March (F = 0.175) and 8 September (F = 0.689). Secondly there is an annual variation in the relationship between the heliocentric RTN frame, on which the inner heliosphere is structured, and the GSE frame. Parts (b)–(d) show Earth and the heliographic equator looking from the Sun at four times of year (i.e., viewing in the −XGSE direction). The axial pattern, shown in part f, depends on F but has no UT variation. The GSEQ frame is also defined in the figure: the XGSEQ axis points from the Earth to the Sun and so is the same as XGSE a XGSM axes. Whereas the YGSE axis lies in the ecliptic plane, YGSEQ is parallel to the Sun’s equatorial plane which is inclined to the ecliptic. ZGSEQ makes up the right hand set and has a component that is positive in a northward direction. Note that since XGSEQ lies is in the ecliptic plane it is not generally in the Sun’s equatorial plane, and ZGSEQ will not generally be parallel to the Sun’s axis of rotation. In term of RTN heliographic coordinates, XGSEQ = −R, YGSEQ = −T and ZGSEQ = N.

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