You could change your sun textures, but before you do, try this. Browse to your main FS9 folder, and find "suneffect.cfg" and BACK IT UP.
Then open it and look for this:
[general]
SunSize=xxxx
(Right at the top)
Now change the size to something smaller (I use 800 myself). You may need to play around with it until you find a setting you like, but it isn't hard at all. You also may find you'll need to adjust the size and intensity parameters of the first couple [LensFlare.X] entries too, but don't despair, you'll be rewarded with a completely custom sun effect that's all your own!

**EDIT**
The only [LensFlare.x] entries you should need to be concerned with, are those that DO NOT use textures that have "flare" in the name.