Focusing on intensity:
* A furnace in the sky
* A fiery crucible
* A white-hot blaze
* An incandescent inferno
Focusing on impact:
* A scorching dragon's breath
* A searing brand
* A molten embrace
* A living flame
Focusing on the sun's nature:
* The eye of heaven (more metaphorical than literal)
* The golden orb
* The celestial fire
The best metaphor for you will depend on the specific context and the feeling you want to evoke.