Investigation of the negative ionospheric response of the 8 September 2017 geomagnetic storm over the European sector

Highlights

• Negative ionospheric response due to mid-latitude trough displacement over Europe.

• LSTIDs were detected during the 8 September storms accompanied with Spread F.

• TEC, foF2, hmF2, Spread-F were more prominent over northern mid-latitude regions.

Abstract

In this study, we investigate the negative ionospheric response over the European sector during two storms that took place on 8 September 2017, primarily, by exploiting observations over ten European locations. The spatial and temporal variations of TEC, foF2 and hmF2 ionospheric characteristics are examined with the aim to explain the physical mechanisms underlying the strong negative ionospheric response. We detected very sharp electron density (in terms of foF2 and TEC) decrease during the main phases of the two storms and we attributed this phenomenon to the large displacement of the Midlatitude Ionospheric Trough (MIT). Our study also revealed that the two storms show different features caused by different processes. In addition, Large Scale Traveling Ionospheric Disturbances (LSTIDs) were observed during both storms, followed by enhanced Spread F conditions over Digisonde stations. The regional dependence of ionospheric storm effects was demonstrated, as the behavior of ionospheric effects over the northern part of Europe differed from that over the southern part.

Introduction

The geomagnetic storm of 8 September 2017 has been extensively studied on a global and regional scale from diverse perspectives, including the ionospheric and thermospheric response and their underlying causal mechanisms, the electrodynamic coupling of the solar wind–magnetosphere–ionosphere system and the severe impact on various technological systems. Specifically, Habarulema et al. (2020) investigated the ionospheric response at conjugate locations over the European-African longitude sector and found that the increase of Total Electron Content (TEC) induced by the storm event on 8 September in the Southern Hemisphere was at least twice than that in the Northern Hemisphere midlatitude areas. On the same day, over the American sector, Aa et al. (2019) observed post-sunset Equatorial Plasma Bubbles (EPBs) triggered by the Prompt Penetration Electric Field (PPEF) to merge with midlatitude Traveling Ionospheric Disturbances (TIDs) creating elongated TEC depletion formations. Moreover, they detected a poleward portion of TEC depletion moving westward, reaching the equatorward boundary of the Midlatitude Ionospheric Trough (MIT), which is most probably due to a strong convection flow near the Subauroral Polarization Stream (SAPs) region. The ionospheric response over China was studied by Wen and Mei (2020), who concluded that pronounced TEC enhancements during the main storm phase were primarily linked to neutral wind variations and to the eastward PPEF.

The response of the thermosphere on the 8 September event has also been extensively investigated. Liangliang et al. (2019) reported symmetric mass density perturbations between the two hemispheres and a slightly stronger enhancement in the Southern Hemisphere by utilising Swarm observations. Furthermore, the existence of three low-thermospheric density cells was reported by Yuan and Jin (2021) at the altitude of 350 km by employing GRACE observations, approximately one hour after the reversal of Interplanetary Magnetic Field (IMF) By.

Lukianova et al. (2020) focused on the effect of the enhanced solar-wind–magnetosphere interaction on the intensity of Field Aligned Currents (FACs) during the 8 September geomagnetic event. The peculiarity of this event is that it consists of two consecutive storms of equivalent intensities that occurred 13 h apart and were not of one main storm with a two-step main phase, driven by different physical mechanisms (Blagoveshchensky et al. 2019).

With respect to effects on GNSS, Alfonsi et al. (2021) investigated the scintillation variability and the Equatorial Electrojet (EEJ) modification by applying a new method to detect the occurrence of EPBs over the Indian sector with the aid of L-band TEC observations. D’Angelo et al. (2021) discussed the scintillation generation mechanisms observed over polar and high latitude regions and reported high levels of ionization and a strong variability of plasma dynamics associated with scintillation during the geomagnetic event. By conducting a multi-instrument analysis, Blagoveshchensky and Sergeeva (2019) underlined the rare occurrence of two different storms within a 13-hour interval, and its complicated impact on HF radio propagation. Zhang et al., 2019a, Zhang et al., 2019b studied the degradation of Swarm satellites orbit during the storm main phase time and found a precision degradation exceeding 10 cm. In particular, the root‐mean‐square (RMS) of 5‐hr overlap differences for Swarm B reached 8 cm, while for Swarm A and C exceeded 10 cm. Their results also suggested that during this event, the increased thermospheric neutral density comprises a more important error source than the increased ionospheric plasma density for Swarm satellite orbit determination.

Despite the fact that the 8 September 2017 geomagnetic storms have been exhaustively investigated, only a few studies were devoted to the ionospheric response over the European sector. In particular, Mosna et al. (2019) reported the occurrence of extraordinary stratification of ionospheric layers, as well as of Large Scale Traveling Ionospheric Disturbances (LSTIDs) over one mid-latitude European station (Pruhonice). Ferreira et al. (2020) also evaluated the applicability of various ionospheric indices as precursors for LSTIDs during the September 2017 storms over the European sector. Nevertheless, as far as we are aware of, a comprehensive study of the negative ionospheric response over the whole European area and its causal physical mechanisms has not yet been reported. In this respect, the present study aims at investigating the spatio-temporal ionospheric variability characteristics over the European sector and provide insight on the underlying dynamics, physical processes, by combining GNSS TEC and Digisonde observations from multiple European stations. Particular emphasis is also placed on the displacement of the MIT over Europe during the two consecutive severe storms.