We study the dynamics of a twisted tilted disc under the influence of an external radiation field .
Assuming the effect of absorption and reemission/scattering is that a pressure is applied to the disc surface where the local optical depth is of order unity , we determine the response of the vertical structure and the influence it has on the possibility of instability to warping .

We derive a pair of equations describing the evolution of a small tilt as a function of radius in the small amplitude regime that applies to both the diffusive and bending wave regimes .
We also study the non linear vertical response of the disc numerically using an analogous one dimensional slab model .
For global warps , we find that in order for the disc vertical structure to respond as a quasi uniform shift or tilt , as has been assumed in previous work , the product of the ratio of the external radiation momentum flux to the local disc mid plane pressure , where it is absorbed , with the disc aspect ratio should be significantly less than unity .
Namely , this quantity should be of the order of or smaller than the ratio of the disc gas density corresponding to the layer intercepting radiation to the mid plane density , \lambda \ll 1 .

When this condition is not satisfied the disc surface tends to adjust so that the local normal becomes perpendicular to the radiation propagation direction .
In this case dynamical quantities determined by the disc twist and warp tend to oscillate with a large characteristic period T _ { * } \sim \lambda ^ { -1 } T _ { K } , where T _ { K } is some ’ typical ’ orbital period of a gas element in the disc .
The possibility of warping instability then becomes significantly reduced .

In addition , when the vertical response is non uniform , the possible production of shocks may lead to an important dissipation mechanism .