The International Solar Alliance (ISA) is the alliance of countries most of which lie partially or completely:

Most members of the International Solar Alliance lie fully or partly between the two tropics.

This sunny belt around the equator receives abundant, near-vertical solar radiation.

Lying above the Tropic of Cancer would place countries too far north of that rich solar zone.

Lying below the Tropic of Capricorn would place them too far south of it.

Restricting the alliance to the Southern Hemisphere would leave out its many northern-tropic members.

The alliance was launched by India and France in 2015 to promote solar power among these nations.