How is a delete of an aggregate root handled in DDD?

I have two an aggregate root referencing another aggregate root (first references the second via the identity of the second aggregate root).

A command from my application layer (via MVC asp.net) now deletes my second aggregate root.

At the point of deleting the root, do I send a Domain Event telling the first aggregate root to "NULL" the reference to the second aggregate which now does not exist?

JD