Avaushymni means "Opening Hymn" and is for reed organ solo. It is the first piece of a series of 11 forming the Ritual Music op. 113, written for the Helsinki Masonic Lodge in 1927. Sibelius was a member of this Masonic Lodge.
You will find more information on the background of this music and the other 10 pieces on this interesting website:
Thank you again to Ulrich Bartels for the score, which I have attached.