Rule 702.117a

Surge is a static ability that functions while the spell with surge is on the stack. “Surge [cost]” means “You may pay [cost] rather than pay this spell’s mana cost as you cast this spell if you or one of your teammates has cast another spell this turn.” Casting a spell for its surge cost follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.