1. A measure of the attenuation caused by absorption of energy per unit of distance that occurs in an electromagnetic wave of given wavelength propagating in a material medium of given refractive index . Note: The value of the absorption index K ` is given by the relation
where K is the absorption coefficient , is the wavelength in vacuum, and n is the refractive index of the absorptive material medium. ( 188 ) [After 2196 ] 2. The functional relationship between the Sun angle--at any latitude and local time --and the ionospheric absorption . ( 188 )