  When a wave packet composed of short wavelength lower hybrid modes traveling
in an homogeneous plasma region encounters an inhomogeneity, it can resonantly
excite long wavelength whistler waves via a linear mechanism known as mode
conversion. An enhancement of lower hybrid/whistler activity has been often
observed by sounding rockets and satellites in the presence of density
depletions (striations) in the upper ionosphere. We address here the process of
linear mode conversion of lower hybrid to whistler waves, mediated by a density
striation, using a scalar-field formalism (in the limit of cold plasma linear
theory) which we solve numerically. We show that the mode conversion can
effectively transfer a large amount of energy from the short to the long
wavelength modes. We also study how the efficiency scales by changing the
properties (width and amplitude) of the density striation. We present a general
criterion for the width of the striation that, if fulfilled, maximizes the
conversion efficiency. Such a criterion could provide an interpretation of
recent laboratory experiments carried out on the Large Plasma Device at UCLA.
