Dear Jyoti,
Arunachala - is the correct spelling. Arunachala is Siva with concorporate
Sakti. Because, this is where Uma got her half-portion of Siva and Siva
became Ardhnari- half woman / half man. Chanting Arunachla is chanting
for both Siva and Sakti.
Arunachala Siva.