  We review our understanding of ionized plasma and neutral gas coupling in the
weakly ionized, stratified, electromagnetically-permeated regions of the Sun's
chromosphere and Earth's ionosphere/thermosphere. Using representative models
for each environment we derive fundamental descriptions of the coupling of the
constituent parts to each other and to the electric and magnetic fields, and we
examine the variation in magnetization of the ionized component. Using these
descriptions we compare related phenomena in the two environments, and discuss
electric currents, energy transfer and dissipation. We present a coupled
theoretical and numerical study of plasma instabilities in the two environments
that serves as an example of how the chromospheric and ionospheric communities
can further collaborate. We also suggest future collaborative studies that will
help improve our understanding of these two different atmospheres which share
many similarities, but have large disparities in key quantities.
