This wondrous word, describing a complex and ethereal effect of light, is mercifully preserved for posterity by Williamson. It doesn’t yet appear in the 600,000 word-hoard of the OED.

May you enjoy many magical Ammil mornings.

‘When the sun, like an immense dandelion, looked over the light-smitten height of Cosdon Beacon, Tarka was returning along a lynch, or rough trackway, to the river.  The grasses, the heather, the lichens, the whortleberry bushes, the mosses, the boulders – everything in front of the otter vanished as though drowned or dissolved in a luminous strange sea.  The icy casings of leaves and grasses and blades and sprigs were glowing and hid in a mist of sun-fire.  Moor-folk call this morning glory the Ammil.’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 128-29