Strong Polar Ionospheric Electron Density Depletion Under Prolonged Northward IMF B z Condition

Previous studies have primarily focused on high‐latitude ionospheric‐thermospheric (I‐T) variabilities under southward interplanetary magnetic field (IMF) conditions, while rare studies have investigated the I‐T responses to due northward B z (N B z ), especially in the presence of strong IMF B y conditions. This study utilizes multi‐instrument observations and the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations to examine the ionospheric and thermospheric variabilities during prolonged N B z (∼25 nT, lasting 3–4 hr) and varying B y from ∼−31 to 37 nT condition. Multi‐instrument observations showed the occurrence of obvious particle precipitation around the auroral oval, two additional field‐aligned currents around noon, a three‐cell ionospheric convection pattern, and ∼50% polar ionospheric total electron content depletion at high latitude in the Northern Hemisphere. To separate B y and B z effects on the high‐latitude ionosphere‐thermosphere system, TIEGCM simulations indicated that a dawnside Joule heating increases up to 1 × 10 6 erg/g/s (∼83%) above 70°N magnetic latitude (MLAT) due to positive B y effect, and a dayside Joule heating increases up to 5.5 × 10 5 erg/g/s (∼46%) above 75°N MLAT due to N B z effect. This produced upward neutral winds and thus reduction in ∑O/N 2 in the polar cap. Positive B y and N B z induce westward and eastward winds, extending high‐latitude reduced ∑O/N 2 and electron density depletion toward midnight and dayside low latitudes, respectively.