A two-component (hot and cold) magnetospheric plasma, with spatially varying densities and temperatures, is considered for the case where the equilibrium magnetic field is two-dimensional: B = ?zBz(x, z) +?xBx(x, z). Such a slab geometry includes the effects of field line bending, to first order. A linear theory of low-frequency electromagnetic perturbations is developed, for long perpendicular wavelength modes (‖∇lnn‖-1~&lgr;y≲&lgr;x). We consider modes that are driven unstable by internal sources of free energy: the spatial gradients in density, temperature, and B field. The electrons of the cold component determine the polarization of the perturbed field (Bly<Blx,Blz). The unperturbed orbits of the hot plasma particles consist of the fast cyclotron gyration coupled with the cross-field drifts and with a slow bounce motion along the principal (i.e., z) direction of the unperturbed B field. A fluidlike instability of the drift-compressional mode is found to occur, for both the ion-drift and electron-drift branches of the mode. A critical value of the field curvature scale length is found, below which this mode is stable. Analytic expressions for the eigenfrequency and eigenvector are given. The perturbed fields are found to be strongly localized in the x direction.