## 1 Introduction  
Space weather events such as the May 2024 Mother's Day Superstorm, caused by a series of interplanetary coronal mass ejections (ICMEs) and high-speed streams, induce hazardous perturbations in the coupled thermosphere-ionosphere-magnetosphere system. Analyzing such unique events offers valuable insight into the evolution of extreme geospace dynamics.  
The Mother's Day storm event originated from the solar active region classified as AR13664 [1], which produced at least four M-class flares and one X1.0 flare on 8 May 2024, followed by further strong solar flares (including six X-class flares) between 9 and 11 May, launching a series of fast ICMEs. For a detailed analysis of this storm, including a list of the ICMEs and their associated flares, see [PERSON] et al. [1]. Another study by [PERSON] et al. [2] examines the role of storm-time electric fields in the formation of the equatorial ionization anomaly (EIA) during this storm. Additionally, [PERSON] et al. [3] investigate the expansion and merging of the EIA in the American and Atlantic longitude sectors through multi-instrumental data analysis. Further aspects of this event have been studied by [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15].  
When an ICME collides with Earth's magnetic field, it triggers a geomagnetic storm. The resulting disturbance in the ionosphere is referred to as an ionospheric storm, which is traditionally classified into two types: positive and negative ionospheric storms, depending on whether the storm causes an increase or decrease in the electron density in the ionospheric layers, respectively [16; 17]. Their effects can last from 1 to 10 days. There are several comprehensive reviews and studies on the potential physical and chemical mechanisms that may arise during ionospheric storms (see, e.g., [16; 17; 18; 19; 20; 21; 22; 23] and references therein).  
The ionosphere interacts with the plasmosphere through plasma diffusion and transport processes. Magnetospheric phenomena, such as field-aligned currents (FACs), particle precipitation, plasma drifts, and enhanced electric fields near the plasmopause (PP), are closely linked to ionosphere dynamics. For example, the ionospheric footprint of the nightside PP was found to be aligned with the midlatitude ionospheric trough (MIT) minimum [24] and the equatorward boundary of small-scale FACs ([25; 26]). These boundaries are highly sensitive to geomagnetic activity.  
This article examines the ionospheric F-layer, which splits into F1 and F2 sublayers during daylight. The F1-layer's electron density is primarily induced by solar radiation, while the F2-layer is more affected by transport processes driven by neutral winds or electric fields [27]. The F2-layer's peak density (NmF2) usually surpasses that of the F1-layer (NmF1). However, under special conditions (called G-conditions), such as intense geomagnetic storms, the F1-layer can blanket the F2-layer due to the latter's electron density dropping below the former's [28; 29], with an occurrence rate of \(\sim\)0.34% [30]. Auroral reflections, another unique feature, appear on ionograms at low- and mid-latitudes only during the strongest geomagnetic storms [29; 31].  
By employing ground-based Digisonde as well as Swarm satellite data, this study aims to identify the primary mechanisms responsible for the observed mid-latitude ionospheric perturbations arising due to the Mother's Day superstorm. Extreme geomagnetic storms, like the Mother's Day Superstorm, occur on average once in a solar cycle, providing a unique opportunity for studying the ionospheric response to such extraordinary conditions. Good quality Digisonde data in Europe and Swarm data were not available when the last same magnitude superstorm occurred in October/November 2003.  
The rest of the paper is organized as follows: Section 2 introduces the data utilized for this study, Section 3 presents the observations, and Section 4 provides the discussion. Finally, Section 5 summarizes the results and concludes the paper.