Mid-latitude thermospheric plasma physics and electrodynamics: A review

Although the mid-latitude thermosphere contains both neutral and plasma fluids, most studies of wave dynamics and instabilities in this region of the atmosphere had focused, until recently, on either neutral gravity waves or plasma instabilities to the exclusion of the other. As a result, there was little to draw attention to coupled mid-latitude physics when compared to the auroral and equatorial zones. In the past decade, however, an increasing array of remote sensing and in situ techniques have revealed new phenomena in the mid-latitude ionosphere which are dramatically challenging our current understanding. These experiments have revealed a growing awareness of the interconnections between gravity waves, traveling ionospheric disturbances, elongated bands of raised ionosphere, density depletions, and the recently discovered turbulent upwellings of mid-latitude plasma which, phenomenologically at least, are quite similar to their equatorial spread F cousins. At the same time, a renewed interest in theoretical and numerical investigations of mid-latitude physics is demonstrating the unique interaction of atmospheric dynamics and ionospheric electrodynamics which drives mid-latitude disturbances. In this paper, we describe the basic plasma physics of the mid-latitude ionosphere and its connections to both old and new observations; we end with some suggestions for the near term future of mid-latitude plasma research.