  Let $X$ be an ordinary smooth curve defined over an algebraically closed
field of characteristic 2. The absolute Frobenius induces a rational map $F$ on
the moduli space $M_X$ of rank 2 vector bundles with fixed trivial determinant.
If the genus of $X$ is 2, the moduli space $M_X$ is isomorphic to projective
space of dimension 3 (as over the complex numbers). In this case we explicitly
give the equations of $F$, which enables us to determine, for example, its base
locus (one point) and its image (different from $M_X$).
