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We have taken a key step in evaluating the importance of ionospheric outflows relative to electrodynamic coupling in the thermosphere’s impact on geospace dynamics. We isolated the thermosphere’s material influence and suppressed electrodynamic feedback in whole geospace simulations by imposing a time‐constant ionospheric conductance in the ionospheric Ohm’s law in a coupled model that combines the multifluid Lyon‐Fedder‐Mobarry magnetosphere model with the Thermosphere Ionosphere Electrodynamic General Circulation Model and the Ionosphere Polar Wind Model that includes both polar wind and transversely accelerated ion species. Numerical experiments were conducted for different thermospheric states parameterized by F10.7 for interplanetary driving representative of the stream interaction region that swept past Earth on March 27, 2003. We demonstrate that thermosphere through its regulation of ionospheric outflows influences magnetosphere‐ionosphere (MI) convection and the ion composition, symmetries, x‐line perimeter and magnetic merging of the magnetosphere. Feedback to the ionosphere‐thermosphere from evolving MI convection, and Alfvénic Poynting fluxes and soft (∼few 100 eV) electron precipitation originating in the magnetosphere, in turn, modify the evolving O⁺outflow properties. The simulation results identify a variety of observed magnetospheric features that are attributable directly to the thermosphere’s material influence: Asymmetries in O⁺outflow fluxes and velocities in the pre/postnoon low‐altitude magnetosphere, dawn/duskside lobes and pre/postmidnight plasmasheet; O⁺distribution of the plasmasheet; magnetic x‐line location and reconnection rate along it. O⁺outflows during solar maximum conditions (high F10.7) tend to counteract the plasmasheet’s pre/postmidnight asymmetries caused by the night‐to‐day gradient in ionospheric Hall conductance.

Thermospheric mass density perturbations are commonly observed during geomagnetic storms and fundamental to upper atmosphere dynamics, but the sources of these perturbations are not well understood. Large neutral density perturbations during storms greatly affect the drag experienced by low Earth orbit. We investigated the thermospheric density perturbations at all latitudes observed along the CHAMP and GRACE satellite trajectories during the August 24–25, 2005 geomagnetic storm. Observations show that large neutral density enhancements occurred not only at high latitudes, but also globally. Large density perturbations were seen in the equatorial regions away from the high‐latitude, magnetospheric energy sources. We used the high‐resolution Multiscale Atmosphere Geospace Environment (MAGE) model to simulate consecutive neutral density changes observed by satellites during the storm. The MAGE simulation, which resolved mesoscale high‐latitude convection electric fields and field‐aligned currents, and included physics‐based specification of auroral precipitation, was contrasted with a standalone ionosphere‐thermosphere simulation driven by a high‐latitude electrodynamics empirical model. The comparison demonstrates that first‐principles representations of highly dynamic and localized Joule heating events in a fully coupled whole geospace model is critical to accurately capture both generation and propagation of traveling atmospheric disturbances (TADs) that produce neutral density perturbations globally. The MAGE simulation shows that larger density peaks in the equatorial region observed by CHAMP and GRACE are the result of TADs generated at high‐latitudes in both hemispheres, and intersect at low‐latitudes. This study reveals the importance of investigating thermospheric density variations at all latitudes in a fully coupled geospace model with sufficiently high resolving power.