Regional Differences in Ionospheric Response Over East Asia During the 1–2 October 2002 Geomagnetic Storm

Abstract This study analyzes extensive ground‐based and space‐borne ionospheric data over East Asia during the geomagnetic storm of 1–2 October 2002, focusing on regional differences during the main and recovery phases. During the main phase, strong negative disturbances propagated from high to mid‐latitudes, affecting Northeast Asia near sunset, while prolonged southward interplanetary magnetic field Bz drove sustained prompt penetration electric fields (PPEFs) into mid‐ and low latitudes, producing local time (LT)‐dependent positive storm effects over China and Japan. However, PPEF did not consistently uplift hmF2 at magnetic latitudes 15–25°N. Rapid universal time (UT)‐and latitude‐dependent changes in downward diffusion led to complex responses: hmF2 at 15–20°N rose slightly and then declined nearly synchronously before recovering, whereas uplift occurred near 25°N. During the recovery phase, mid‐latitude Japan exhibited a strong negative storm effect, in contrast to a weak positive response at comparable latitudes in central and western China. ROCSAT‐1 observations show that the disturbance dynamo electric field displayed distinct UT–LT evolution, with a stronger westward component over the Japanese sector and an eastward component over eastern China. Simulations using the Thermosphere Ionosphere Electrodynamics General Circulation Model reproduced the longitudinal gradient in electron density across East Asia. Diagnostic analysis indicates that the pronounced zonal gradient in total electron content is mainly controlled by longitudinal variations in the electric field, while chemical processes, neutral wind transport, and ambipolar diffusion play secondary roles. These results demonstrate marked regional variability in the East Asian ionosphere even under non‐extreme geomagnetic activity.