  The azimuthal anisotropy of high $p_{T}$ particle production in heavy ion
collisions is a sensitive probe of the jet quenching mechanism. Recent PHENIX
measurements for Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV show a $\pi^{0}$
$v_{2}$ signal that exceeds the pQCD energy loss calculations up to $p_{T} \sim
10$ GeV/c, challenging the traditional perturbative picture of the energy loss
process. Here, we present an update and details of that measurement, as well as
new high $p_T$ measurements at $\sqrt{s_{NN}}=62$ and 39 GeV. These
measurements not only provide an important constraint for understanding the
path-length dependence of jet energy loss and the role of initial collision
geometry, but also allow a search for the onset of jet quenching as
$\sqrt{s_{NN}}$ is varied.
