Asa is a very old raga, once popular in the Punjab but seldom heard in concerts today. In the Ragmala (final Bani in the SGGS) this is a ragini of raga Megha. However, today it is assigned to the Bilaval thata. Asa is a devotional raga for the cold season and is performed in the early morning just before sunrise. However, it is also known as a twilight melody with a calm mystical mood. Asa was used by Guru Nanak, Guru Angad Dev, Guru Amar Das, Guru Ram Das, Guru Arjan and Guru Teg Bahadur.
Aroh : Sa Re Ma Pa Dha Sa
Avroh : Sa Ni Dha Pa Ma Ga Re Sa
Vadi : Ma
Samvadi : Sa
Asa is a crooked (vakra) raga in that approaches to certain notes have to be made from a set position. Its variants as given in the Holy Book are Kafi and Asavari, both of which have many features in common with Asa. This raga may have originated in Maharashtra about the time of the major Muslim invasions. Its pleasing sound made it suitable bhajans by the Hindu devotees.
Excerpts taken from Encyclopedia of Sikhism by Harbans Singh. Published by Punjabi University, Patiala With Compliments to AllAboutSikhs.com