The control of auroral zone dynamics and thermodynamics by the interplanetary magnetic field dawn‐dusk (Y) component

It is well known that ion drag momentum forcing is one of the primary drivers of the thermospheric wind at high latitudes. Previous theoretical and experimental studies have shown that the dawn‐dusk component of the interplanetary magnetic field (IMF B y ) expands the classical symmetric two‐cell convection pattern toward dusk ( B y negative) or toward dawn ( B y positive) in the northern hemisphere, altering the ion drag forcing on the neutral atmosphere. Measurements of the neutral dynamics associated with these convection patterns have been presented primarily at magnetic latitudes greater than 70° in the polar cap. In this study, nights with coincident IMF measurements have been selected from the extensive four‐year auroral zone thermospheric wind and temperature data set derived from Fabry‐Perot spectrometer measurements of the Doppler shifts and widths of the O(¹ D ) 15,867 cm −1 (630.0 nm) emission from College, Alaska. These nights were required to have (1) B y values that do not change sign through the night, being either positive or negative, (2) B x opposite the sign of B y , (3) B z less than or equal to 0, and (4) Ap less than 40 (case 1). The 13 nights which fit the above criteria have been averaged into B y positive and B y negative groups. Averages from 112 nights of measurements from College were also computed using a selection criterion that depended on the previous 2 hours of IMF measurements (case 2). This procedure yielded averages that differed at times from case 1. The wind and temperature averages for both cases show large variations with B y in the auroral zone. The B y negative (positive) winds exhibit stronger sunward zonal flow in the evening (morning) with corresponding neutral temperature enhancements in these regions. Furthermore, the zonal wind changes from westward to eastward earlier for B y positive than for B y negative, consistent with the Harang discontinuity shifting to earlier magnetic local times in the northern hemisphere as B y becomes more positive. The wind averages for B y negative and positive are compared with National Center for Atmospheric Research thermospheric general circulation model predictions that use a B y ‐dependent model of ionospheric convection. The results for B y negative compare favorably with the averages, but there are significant differences between model calculations and averages for the B y positive case. Possible causes for these discrepancies are discussed.