Parameters characterising the centre-to-limb variation in the O v 630 Å line during the last two solar minima. Diamonds: USUN; squares: SYNOP (2007–2009), circles: SYNOP (1996–1997). The average values are indicated by horizontal lines: dotted (USUN), solid (SYNOP, 2007–2009), and dashed (SYNOP, 1996–1997). In the upper-left panel, the centre-disk intensity, I(1), from the power-law fit are in black, while results from quadratic fits are in grey. The correction factors fcl, whose average values are listed also in Table 1, are shown in the right-hand panels; the value of centre-to-limb function at ρ = 0.85 (i.e. μ ~ 0.52), derived from the quadratic fit of Eq. (2), is shown in the lower-left panel.
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