How the magnetosphere-ionosphere/thermosphere-ground system is coupled by waves

We study the polar magnetospheric coupling to the ionosphere/thermosphere and ground system along the field by examining a spectrum of perturbations propagating from the magnetosphere downward. What distinguishes this study from conventional treatments of the magnetosphere-ionosphere/thermosphere system is that our treatment self-consistently includes the responses of the neutrals to the magnetic and plasma velocity perturbations. The thermosphere is coupled to the magnetosphere-ionosphere (M-I) system in a degree that depends on the time scale of the perturbations. There are three major processes that affect the perturbation propagation: damping that reduces the energy flux while producing heating, the neutral-inertia loading that reduces the propagation speed, and reflection which, associated with structures of the ionosphere and thermosphere, reduces the downward energy flux. The damping is stronger in higher frequencies, 10^-2~0 Hz for M-I coupling. As a result of reflection, significant energy fluxes of the magnetospheric perturbations cannot reach the lower ionosphere and hence the ground although some heating and energization may occur in the lower ionosphere resulting from the strong damping of high frequency fluctuations. However, the amplitude of the magnetic fluctuations of the transmitted flux into the lower ionosphere can be enhanced in lower frequencies because of the decrease in the propagation speed due to strong neutral-inertia loading. Combining the attenuation and amplitude enhancement effects, the net enhanced amplitudes occur in frequencies less than few Hertz, which may explain the ready observations of PC waves that are enhanced magnetic oscillations in periods from 0.5 sec to 30 min on the ground while little enhancement is observed below this period range. On the other hand, the smallness of the propagation velocity results in very small electric perturbations, forming a magneto-static condition for coupling from the lower ionosphere to the ground in low-frequencies, casting doubts on any ionosphere-ground coupling mechanisms based on static electric field in the lower frequencies.