Can I play green Janna as a reaction for attacking with my unit on base and sent to battlefield?

Yes, you can play Janna, Savior as a Reaction during your attack — but she must be placed at a location you already control, not the battlefield you're attacking.

Here's how it works:

1. You move your unit from base to an occupied battlefield. That applies Contested and starts a Combat Showdown.
2. As the Attacker, you gain Focus in the Showdown Open state.
3. Janna has [Reaction] — she can be played at any time, including during a Showdown.

Placement restriction: When you move into an occupied battlefield, you do not control it (it's contested). Units must be played to a battlefield you control or your base. So Janna goes to your base or a different battlefield you already control — not the one you're attacking.

Why you might want to: If the enemy has a unit at that contested battlefield and you'd like to move it away, Janna's trigger can't reach it from a different location (her ability says "here"). But you could still play her to your base to set up for future turns or to use her healing on units elsewhere.

Refs: FAQ #11160; Combat reference §Attacker/Defender; Rule 457.2