Viscous transport response exponent ($q=\partial \log (\bar{\nu }\Sigma )/\partial \log \Sigma$)

The response to a small surface density perturbation is $\delta (\bar{\nu }\Sigma )=q\bar{\nu }\delta \Sigma$. For an instantaneous law $\bar{\nu }\propto r^a{\Sigma }^b$, $q=b+1$. This exponent is not the orbital shear exponent or a satellite-to-star mass ratio, which also often use the letter $q$.