  The role of turbulence in low-$\beta$, guide-field reconnection exhausts is
explored in 2D reconnection and 2D and 3D Riemann simulations. The structure of
the exhaust and associated turbulence is controlled by a pair of rotational
discontinuities (RDs) at the exhaust boundary and a pair of slow shocks (SSs)
that are generated by counterstreaming ions beams. In 2D the exhaust develops
large-amplitude striations at the ion Larmor radius scale that are produced by
electron-beam-driven ion cyclotron waves. The electron beams driving the
instability are injected into the exhaust from one of the RDs. However, in 3D
Riemann simulations, the additional dimension (in the out-of-plane direction)
results in strong Buneman and electron-electron streaming instabilities at the
RD which suppress electron beam formation and therefore the striations in the
exhaust. The strength of the streaming instabilities at the RD are controlled
by the ratio of the electron thermal speed to Alfv\'en speed, lower thermal
speed being more unstable. In the 3D simulations an ion-ion streaming
instability acts to partially thermalize the counterstreaming ion beams at the
SSs. This instability is controlled by the ratio of the sound speed to Alfv\'en
speed and is expected to be stable in the low $\beta$ solar corona. The results
suggest that in a guide field reconnection exhaust with $1 \gg \beta >
m_e/m_i$, the kinetic-scale turbulence that develops will be too weak to play a
significant role in energy conversion and particle acceleration. Therefore, the
energy conversion will be mostly controlled by laminar physics or multi-x-line
reconnection.
