The Contribution of the Russell‐McPherron Effect to the Semiannual Variation in Thermospheric Density

A number of external sources and internal circulation mechanisms have been proposed to explain the well-known semiannual variation in global mean thermospheric densities. We use the polarity of the Y-component (dawn to dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) frame to identify the influence, via geomagnetic currents, of the Russell-McPherron (R-M) effect on solar wind-to-magnetosphere energy coupling. Using data from 1996-2025, our best estimate is that for an IMF $|[B_Y]_{GSEQ}|$ component of magnitude 4 nT or greater, more than half of the semiannual variation in global averages of thermospheric density at altitudes between 460 km and 530 km is externally driven by the R-M effect and the associated geomagnetic activity and that other effects that do not depend on the IMF Y-component, such as the ”spoon” mechanism (ultimately driven by the annual variation in the Sun-Earth distance) or upward energy transport from below by waves and tides, are responsible for the remainder. For all data, including quieter geomagnetic conditions, our best estimate is that the R-M mechanism contributes 20 of the semiannual variation of thermospheric densities.