A Pi2 wave event was observed by the CRRES satellite on day 48 (17 February) of 1991, 1220--1250 UT <Takahashi et al., 2001>. Using the electron density profile measured by CRRES on its outward pass (during which the wave event was observed), and assuming that the ions are all H+, we calculated the theoretical plasmaspheric cavity resonance frequencies using a two-dimensional MHD simulation in dipole geometry (azimuthal symmetry). Our theoretical frequencies are in good agreement with the observed frequencies, and this supports the cavity resonance mechanism as an explanation for the observations. The observed and theoretical harmonic frequency ratios f2/f1 and f3/f1 were in rough agreement with earlier studies. Comparison of the outbound and inbound legs of the CRRES orbit on 17 February 1991 shows that the density on the outbound leg at magnetic local time (MLT) = 20--23.4 was different than that on the inbound leg at MLT = 23.4--4.5. On the outbound leg an enhanced density region beyond the region of the nominal plasmapause (which may have been part of a plasmaspheric tail <Sandel et al., 2001>) played an important role in determining the cavity resonance frequencies. Observations by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft have revealed that there is often a great deal of azimuthal structure in the plasmasphere (including such tails). Such structure may play an important role in determining observed Pi2 frequencies.