asymptotics of $ J_{iu} (ia)$ for a Bessel function

Question

Let $J_{iu}(ia)$ be the Bessel function of imaginary order. ($a$ is a real number (positive or negative) and $u$ is also real.)

In the limit $u \to \infty $ how does the function $J_{iu} (ia)$ behave?

How about $J_{iu}(a)$?

Thanks.