Ultralow frequency waves with frequencies of 1.3, 1.9, 2.6, and 3.4 mHz are often observed and associated with field line resonances (FLRs) in the inner magnetosphere of the Earth. Waveguide modes have been proposed as a mechanism for generating FLRs. Using a compressible two-dimensional MHD simulation, we will explore the nonlinear effects on the excitation of waveguide modes and the frequencies of the modes. We will discuss how changes in key parameters affect waveguide modes. ¿ 2001 American Geophysical Union