What should you communicate to an aircraft declaring an emergency that is not radar identified?

When an aircraft declares an emergency and is not radar identified, it is crucial to provide guidance that ensures the pilot can communicate effectively and take necessary actions for their safety. Advising the aircraft to "Squawk mayday on 7700" is appropriate because 7700 is the dedicated transponder code for emergency situations. This allows air traffic control to identify the aircraft as being in distress and prioritize their situation.

Using this code also enhances communication among all air traffic services and surrounding aircraft. It alerts them of the emergency, allowing for better coordination and support. The declaration of an emergency serves to focus attention and resources on the aircraft's situation, and squawking 7700 is a universally recognized step that indicates the urgency of their distress.

The other options do not address the critical need for immediate identification of the emergency status. Returning to an original frequency, maintaining the current squawk, or switching to an emergency frequency may not provide the same level of urgency or identification needed in an emergency situation. Hence, the instruction to squawk 7700 directly communicates the severity of the aircraft's condition while aiding in swift and effective response from air traffic control and nearby aircraft.