In Nils Henrik Asheim's organ solo improvisations, the music is based on exploration of the instrument's sounds and possibilities. The musical language is related to contemporary classical music as well as to the free improvised genre (as in avantgarde jazz and sound art). A good example is the album release of Nils Henrik Asheim's solo concert entitled 19 March 2004, Oslo Cathedral, which received the Spellemannsprisen (Norwegian Grammy award).
More examples from improvised solo concerts are found here:
If you are curious about how an improvisation concert is created,
read: Aarhus 031210, Inside an improvised organ concert