Coupled Thermosphere–Ionosphere Dynamics During the 17 January 2013 Storm: GITM–TIDAS Comparison With Millstone Hill ISR Measurements

Abstract

This article analyzes a weak geomagnetic storm on January 17–18, 2013, using a first‐principles model (Global Ionosphere Thermosphere Model (GITM) with an empirical assimilative ionospheric model (TIDAS), evaluated against observations in the form of Millstone Hill (MH) incoherent scatter radar (ISR) altitude profiles and global navigation satellite system (GNSS) derived keograms of ionospheric total electron content. Results show that the midlatitude ionosphere exhibits a clear two‐phase evolution. From about 16 to 20 UT, both systems show an enhancement that develops into an SED feature and migrates equatorward. In GITM, the spatial correspondence among equatorward meridional winds, upward ion motion, and elevated hmF2 ${\text{hmF}}_{2}$ indicates that meridional‐wind‐driven uplift is the primary mechanism associated with the initial enhancement. After about 22 UT, a depletion develops concurrently with a regional decrease and equatorward progression of O/ N2 ${\mathrm{N}}_{2}$ , indicating increasing composition control. Global Ionosphere Thermosphere Model and TIDAS agree on the large‐scale morphology, including storm enhanced density (SED) and mid‐latitude trough features, but differ in detail. Global Ionosphere Thermosphere Model tends to place features slightly west of TIDAS, overestimates wave amplitudes in the 265°to270°E $265{}^{\circ}\,\text{to}\,270{}^{\circ}\mathrm{E}$ sector while matching better in the 285°to290°E $285{}^{\circ}\,\text{to}\,290{}^{\circ}\mathrm{E}$ sector, and produces nightside electron densities that are too low due to missing plasmaspheric down‐welling. TIDAS reproduces storm morphology but smooths mesoscale structure. Millstone Hill ISR profiles of ionospheric plasma parameters confirm southward propagating wave‐like features between about 02 and 09 UT on January 18th. While GITM captures the timing and vertical reach of these features, they appear more muted when using TIDAS. These results reveal a wind‐drift‐composition physical pathway in which meridional‐wind‐driven uplift supports the initial SED enhancement, reduced O/ N2 ${\mathrm{N}}_{2}$ and downward ion motion contribute to the later broad depletion, and localized wind variations produce mesoscale structuring. This warrants two improvements for better upper atmospheric modeling: the inclusion of a plasmaspheric exchange in physics‐based models to correct nightside bias; and data assimilation settings that govern localized depletions and traveling ionospheric disturbances to better preserve composition‐ and neutral wind‐linked imprints.