Investigation of the Drivers of Long-Duration Positive Ionospheric Storms During the Geomagnetic Storm on February 26-27, 2023

A typical long-duration positive ionospheric storm (LDPS) developed in the midlatitude ionosphere in the European sector in response to a strong geomagnetic storm of February 26-27, 2023 (Kp = 7-, minimum SYM-H = -161 nT). To advance the current understanding of storm-time midlatitude ionosphere, we investigated the drivers of this LDPS using combination of multi-instrument observations and modeling, with focus on magnetically conjugate locations. Simulations with the field line interhemispheric plasma (FLIP) model constrained by the observed F2-layer peak height (h_mF₂) and density (N_mF₂) data at Kharkiv (50^oN, 36^oE) and Grahamstown (33.3^oS, 26.5^oE) were validated with the O/N₂ ratio data from the Global Ultraviolet Imager (GUVI). Our results indicate that neither the F2-layer peak uplift nor the O/N₂ ratio increase can be considered exclusive drivers of an LDPS. Each driver can be dominant depending on conditions. An LDPS can develop even when the h_mF₂ decreases and sometimes, a small h_mF₂ increase of ~10-20 km can cause a strong LDPS. Similarly, an O/N₂ increase is not a primary or necessary condition for an LDPS to develop but a small O/N₂ increase of ~20-30% can cause a prominent LDPS. Finally, the formation of a positive or negative storm can be inhibited if the raising/lowering of h_mF₂ is counterbalanced by a decrease/increase in the O/N₂ ratio.