Latitudinal and longitudinal variation in ionospheric response to geomagnetic storm of May 2017: multi-instrument observations

Abstract

This study investigates the response of ionosphere to the 26–31 May 2017 geomagnetic storm, during which the Dst index reached a minimum value of approximately −125 nT on 28 May 2017, over the American, European-African, and Asian-Australian sectors using space-based (Swarm, GUVI/TIMED) and ground-based (GPS, Madrigal, magnetometer) observations. Quiet mean total electron content (TEC) and plasma density (Ne) values were used as a baseline to quantify storm-time perturbations.The ionospheric response is classified as significant when the the magnitude of the relative TEC perturbation ($\left|\Delta \mathrm{TEC}\right|$) exceeds 45%. Magnetometer observations were analyzed to characterize storm-time ionospheric current systems, with a high-pass and band-pass filtering techniques were applied to D_iono to extract Dp₂ and D_dyn currents. During the storm main phase, a pronounced daytime positive ionospheric storm was observed at low and mid-latitudes in the Asian-Australian sector, while positive responses emerged in the American and European-African sectors predominantly during the recovery phase. The positive storm phases were frequent and intense than negative phases. Space-based Ne and ground-based TEC variations during the storm event exhibited consistent response. Hemispheric asymmetries in ionospheric response were also identified. The observed positive ionospheric responses are mainly attributed to prompt penetration electric fields (PPEF) and storm-time thermospheric winds, whereas the negative phase that developed over the Asian-Australian sector during the late recovery phase is likely associated with storm-induced reductions in the thermospheric [O]/[N₂] ratio. Analysis of magnetic field variations revealed the presence of a diurnal component, characterized by a reversal in the typical diurnal pattern of the geomagnetic field. The largest D_dyn amplitudes occurred at HUA in the American sector, followed by MBO in the West African sector. While diurnal oscillations persisted over longer durations, the semi-diurnal components attained comparatively higher amplitudes.