Lightmine Field

• WWizards of the Coast2010-06-15
  Official Ruling
  Which creatures are dealt damage is based on which creatures were declared as attackers during the declare attackers step.
• WWizards of the Coast2010-06-15
  Official Ruling
  Lightmine Field’s ability triggers when any creatures attack, including your own.
• WWizards of the Coast2010-06-15
  Official Ruling
  For example: If a creature attacks but is removed from combat before Lightmine Field’s ability resolves (because it would be destroyed and regenerates, perhaps), that creature is dealt damage by Lightmine Field but isn’t counted when determining the amount of damage. On the other hand, if a creature is put onto the battlefield attacking (due to Preeminent Captain’s ability, perhaps), it isn’t dealt damage by Lightmine Field, but it is counted when determining the amount of damage.
• WWizards of the Coast2010-06-15
  Official Ruling
  How much damage each of those creatures is dealt is based how many creatures are attacking at the time the ability resolves.

Rulings via Scryfall