D3 spicules and the lower chromosphere

High resolution filtergrams of the solar limb in D₃ and off-band H have been used to investigate the spatial structure of the D₃ chromosphere. It was found that spicules provide the major contribution to the intensity of the D₃ emission band observed above the limb, with the remainder of the emission coming from a semi-homogeneous background component at low heights.The observations can be understood on the basis of the photoionization model, whereby it is found that helium is only slightly ionized at the height of peak intensity in the D₃ emission band, and that spicules are at least 3 times denser than their surroundings at this height.In coronal holes, the D₃ emission is confined to isolated emission patches, and these patches contain a fine structure resembling normal chromospheric spicules.