Modeled IMF By effects on the polar ionosphere and thermosphere coupling

There is still an inadequate understanding of how the interplanetary magnetic field (IMF) east-west component (B-y) affects thermospheric composition, and other ionospheric and thermospheric fields in a systematic way. Utilizing the state-of-art first-principles Coupled Magnetosphere Ionosphere Thermosphere (CMIT) modeling and TIMED/Global Ultraviolet Imager (GUVI)-observed Sigma O/N-2 covering an entire solar cycle (year 2002-2016), as well as a neutral parcel trajectory tracing technique, we emphasize that not only the direction of B-y, but also its strength relative to the IMF north-south component (B-z) that has important effects on high latitude convection, Joule heating, electron density, neutral winds, and neutral composition patterns in the upper thermosphere. The Northern Hemisphere convection pattern becomes more twisted for positive B-y cases than negative cases: the dusk cell becomes more rounded compared with the dawn cell. Consequently, equatorward neutral winds are stronger during postmidnight hours in negative B-y cases than in positive B-y cases, creating a favorable condition for neutral composition disturbances (characterized by low Sigma O/N-2) to expand to lower latitudes. This may lead to a more elongated Sigma O/N-2 depletion area along the morning-premidnight direction for negative B-y conditions compared with the positive B-y conditions. Backward neutral parcel trajectories indicate that a lower Sigma O/N-2 parcel in negative B-y cases comes from lower altitudes, as compared with that for positive B-y cases, leading to larger enhancements of N-2 in the former case.