POSSIBLE OSCILLATION MODES OF MAGNETIC WHITE-DWARFS

Observed field strengths of magnetic white dwarfs (MWDs) range from similar to 0.1 to 1000 MG. In the degenerate interior of an MWD, the sound speed is typically much higher than the Alfven speed. Assuming a simple dipolar magnetic field geometry threading throughout an MWD covered with an extremely thin non-degenerate surface layer atmosphere in radiative equilibrium, we describe basic properties of globally trapped oscillations of Alfven waves, magnetohydrodynamic (MHD) slow waves, magneto-acoustic waves, and magneto-gravity waves. Similar to the excitation of g modes of DA and DB variable white dwarfs, these four types of MHD modes could be self-excited via the kappa-mechanism for an isolated MWD. In cataclysmic variables such as AM Hers, these MHD modes of the primary MWD could be driven to excitation by magnetically channelled accretion flow from the low-mass companion on to the polar regions of the primary MWD. In particular, the excited magneto-acoustic modes with time-scales of similar to 0.1-10 s and magneto-gravity modes with time-scales of similar to 100-1000 s could be responsible for the quasi-periodic oscillations observed in optical and X-ray emissions from AM Hers.