Investigating the Solar Wind Dynamic Pressure Control on the Magnetosphere-Ionosphere-Thermosphere System Using Global Numerical Models

[摘要] The geospace system, consisting of the intrinsically coupled magnetosphere, ionosphere and thermosphere (M-I-T), is in pressure balance with the solar wind. During sudden changes in the solar wind dynamic pressure, the magnetosphere undergoes rapid compression or expansion processes which significantly perturb the magnetospheric flow profiles and the global current systems. These sudden global changes are called sudden impulses (SIs). Based on the low-latitude magnetic field perturbation measurements by the ground magnetometers, the SIs are traditionally defined as positive SIs (SI+s); indicating magnetospheric compression or negative SIs (SI^-s), indicating magnetospheric decompression. The magnetospheric and ionospheric responses to the SI+s and SI-s under different IMF and solar wind drivers are not well established mainly due to the sparsity of observations. Therefore, the modelling approach was adopted to understand the geospace system response. The University of Michigan Block Adaptive Tree Solarwind Roe Upwind Scheme (BATS-R-US) global magnetohydrodynamic (MHD) code was employed to study the generation and propagation of the perturbations associated with the compression and decompression of the magnetosphere system. The high-resolution electric potential and auroral power output from this coupled model were then used to drive the Global Ionosphere Thermosphere Model (GITM) to investigate the I-T system responses to the solar wind dynamic pressure variations.In this study, we investigated the SI+ and SI- processes and their effects on the geospace system. Through idealized simulations, we showed that a two-step response existed in the magnetosphere and the ionosphere. Both in the SI+s and SI-s cases, the initial response included magnetopause boundary deformation and forming vortex-like structures in the boundary. The second response was the formation of magnetospheric flow vortices with opposite senses of rotation on the dawn and dusk sectors. These perturbed magnetospheric flows were associated with Field-Aligned Currents (FACs) during both stages that mapped to the ionosphere. Moreover, the ionospheric convection response due to these perturbation FACs preserved the two-step behavior, since the transient currents reversed directions between stages. The dawn-dusk asymmetry seen in the magnetospheric flows were also maintained in the ionospheric convection patterns. We also established the role of the IMF By on the geospace response during SI+ events, through idealized simulations. We showed that even though the magnetospheric and ionospheric perturbations that formed during SI+ were very similar, the superposition of these perturbation currents with the BY controlled NBZ (Northward Bz) current system resulted in different FAC profiles. Therefore, the simulated magnetic field perturbations on the ground showed significant variability with the IMF By. Furthermore, we performed two case studies of an SI+ and an SI-. The simulations showed that the two-step behaviour was conveyed to the thermosphere, through the ion-neutral coupling. For the SI+ case, both simulation and observation results showed enhanced ion and electron temperatures, and decreased electron density. The SI- case study showed observational evidence for the simulated magnetospheric flow profiles. Within this study, the following scientific questions have been addressed: (i) the role of IMF By on the ground magnetometer response to the solar wind dynamic pressure enhancements, (ii) the magnetospheric and ionospheric responses, such as field-aligned currents and convection, (iii) the role of pressure variation in determining the geospace system response and (iv) the ionosphere-thermosphere coupled responses to the sudden changes in the solar wind dynamic pressure.

[发布机构] University of Michigan

[效力级别] Magnetosphere-Ionosphere-Thermosphere

[关键词] Sudden impulse;Magnetosphere-Ionosphere-Thermosphere;Geospace modeling;Atmospheric;Oceanic and Space Sciences;Science;Atmospheric, Oceanic & Space Science