It is demonstrated that the local absorption of an electromagnetic wave incident normally on a plasma can be obtained accurately by the geometrical optics approximation even in a region including the critical density provided that the electron density variation is moderate. The deviation from the WKB result with increasing density gradient is mapped out numerically for a linear profile in both the (isothermal well-ionized) plasma and the (constant collision frequency) ionosphere models; implications for a more general distribution are discussed.
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