Coordinated Studies of Magnetospheric/Ionospheric Coupling and Dynamics in the Diffuse Aurora

The results of the studies carried out under the grant are described in detail in the following publications: Atmospheric Response in Aurora experiment: Observations of E and F region neutral winds in a region of postmidnight diffuse aurora; Walterscheid, Modified geostrophy in the thermosphere, ARIA II neutral flywheel-driven field-aligned currents in the postmidnight sector of the auroral oval: A case study; Observations of unstable atmospheric shear layers in the lower E region in the post-midnight auroral oval; and Depletion of oxygen in aurora: Evidence for a local mechanism. The objective of the Atmospheric Response in Aurora (ARIA) experiment was to measure the response of the E-region neutral flow to substorm activity in the post-midnight sector of the auroral oval. To this end, neutral wind profiles were obtained from a series of four sets of rocket flights using the chemical release technique. The measurements covered conditions ranging from quiet to disturbed. A consistent feature of the observed winds in disturbed conditions is the presence of an E-region jet located between approximately 110 and 120 km altitude. The profiles presented here show that the neutral wind speeds near 110-120 km altitude increase and the hodographs become more elongated or linear in response to higher magnetic activity levels. The wind speeds decrease and the hodographs become more circular as the activity level decreases. The presence of a wind maximum implies shears both above and below. The observations consistently show larger shears below the peak and smaller shears above. In fact, the bottom-side shears during disturbed conditions have Richardson numbers close to or below the critical value of 0.25 suggesting that the flow is highly unstable in that height range. The instability is expected to produce waves, turbulence, and enhanced diffusivity.