light of the sun, to dry in the sun
晅 centers on sunlight and its warming, drying effect: the light of the sun, or the act of drying something in the sun.
晅 combines 日 (sun) with 亘, which likely contributes the sound and may suggest spreading or extending, fitting the idea of sunlight spreading to dry things.
The sun 日 spreads its light across 亘, like sunlight extending over a field to dry clothes or crops.
For ケン, imagine a ken (a traditional Japanese unit of length) of cloth laid out to dry under the sun: ken -> ケン, and the sun's light dries it.