Finite Larmor radius effect ($k_{\perp }{\rho }_s$)

Modification of the plasma response because particles sample fields around a finite cyclotron orbit. Effects become order one at $k_{\perp }{\rho }_s=O(1)$; even at $k_{\perp }{\rho }_s\ll 1$, the leading polarization correction can be essential to a slow dispersion relation.