Effect of IMF Bᵧ on thermospheric composition at high and middle latitudes: 1. Numerical experiments

Magnetic storms and their effects on the thermosphere and ionosphere have been studied for many years, yet there are many aspects of the thermospheric and ionospheric responses that are not understood. The purpose of this paper is to show how the high-latitude composition depends on the sign of the IMF Bᵧ component, using controlled simulations with a global first principles model. Because the high-latitude convection and neutral wind systems are strongly controlled by the IMF Bᵧ component, it seems likely that the compositional response that is driven by high-latitude forcing should also be sensitive to the Bᵧ component. To date, no first-principles modeling has been performed to test the idea of IMF Bᵧ effects on composition. Numerical experiments using model simulations provide insight into this important scientific question, since the thermospheric compositional response to the convection patterns for different IMF BZ and Bᵧ can be studied in isolation in a model. In this paper we use a first-principles model to determine the effect of the IMF Bᵧ component on the compositional response of the high-latitude thermosphere. We show for the first time that a clockwise rotation of the potential pattern resulting from a change from Bᵧ-negative to Bᵧ-positive drives a corresponding rotation in the wind, neutral density, and composition distributions. Bᵧ control of thermospheric composition has been invoked in the literature to explain an apparent variability in the effectiveness of auroral activity in causing thermospheric storm effects at middle latitudes, as observed in global images of the far-ultraviolet (FUV) OI 130.4-nm emission from the DE-1 auroral imager. However, the effect in the simulations presented here is opposite from that suggested by earlier work based on DE data, indicating another explanation must be sought for the DE results. These simulations are highly relevant for interpreting data being provided by more modern UV imaging instruments on the DMSP, TIMED, and IMAGE satellites.