On the asymptotics of Bessel functions in the Fresnel regime

We introduce a version of the asymptotic expansions for Bessel functions Jν(z)Jν(z), Yν(z)Yν(z) that are valid whenever |z|>ν|z|>ν (which is deep in the Fresnel regime), as opposed to the standard expansions that are applicable only in the Fraunhofer regime (i.e. when |z|>ν2|z|>ν2). As expected, in the Fraunhofer regime our asymptotics reduce to the classical ones. The approach is based on the observation that Bessel's equation admits a non-oscillatory phase function, and uses classical formulae to obtain an asymptotic expansion for this function; this in turn leads to both an analytical tool and a numerical scheme for the efficient evaluation of Jν(z)Jν(z), Yν(z)Yν(z), as well as various related quantities. The effectiveness of the technique is demonstrated via several numerical examples. We also observe that the procedure admits far-reaching generalizations to wide classes of second order differential equations, to be reported at a later date.