While plasma emission is the accepted mechanism, Type II bursts do not exhibit significant amounts of circular polarization as would be expected by standard plasma emission theory. The reason for this is unknown, but a leading hypothesis is that the polarization level is suppressed by dispersion effects related to having an inhomogeneous magnetic field near a magnetohydrodynamic shock. Type II bursts sometimes exhibit fine structures called herringbone bursts that emanate from the main burst, as it appears in a dynamic spectrum, and extend to lower frequencies. Herringbone structures are believed to result from shock-accelerated electrons that were able to escape far beyond the shock region to excite Langmuir waves in plasma of lower density than the primary burst region.