A 52-year-old man presents to the emergency department with acute onset chest pain radiating to the back, associated with severe hypertension (BP 180/110 mmHg) and a blood pressure differential of 20 mmHg between the two arms. Chest X-ray shows a widened mediastinum. CT angiography with contrast confirms a type A aortic dissection extending from the ascending aorta into the descending thoracic aorta. During surgical planning, the cardiothoracic surgeon needs to identify the mediastinal compartment primarily affected. Which mediastinal division contains the ascending aorta and the site of origin of this dissection?