We discuss an simple dynamic model of the magnetotail, in which the electric field in the current sheet evolves in response to a solar-wind-induced electric field a the magnetopause, and depends on the specific entropy of the plasma in the plasma sheet. The entropy varies due to nonadiabatic heating in a region where ULF waves are absorbed by resonant mode conversion. The feedback between temperature and entropy change leads to an evolution which exhibits chaos when the solar wind electric field is neither very small nor very large. The onset of chaos may be quite sudden or may proceed through a sequence of period doublings. ¿American Geophysical Union 1991