Zhao, X. P., Hoeksema, J. T., and Scherrer, P. H., JGR 104, 9,735 (1999)


Page 9735:

1. "Coronal holes... are observationally defined 
as features devoid of coronal emission in X ray 
and extreme-ultraviolet images of the Sun." 

Page 9750:

2. "Coronal holes are open field regions..."