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The present study describes a spatial and temporal analysis of the ionospheric variability over the European sector during a major storm event from 23 to 24 March 2023. The analysis is mainly based on ionospheric characteristics such as foF2, hmF2 and Vertical Total Electron Content (VTEC) over ten Digisondes and co-located GNSS receivers. The event started at 12:00UT on 23 March as a positive ionospheric storm followed by a negative ionospheric storm to 04:00 UT on the next day SYM-H<-183nT. A notable latitude dependence of the ionospheric response is more prominent over the northern mid-latitude region attributed to the equatorward displacement of the Midlatitude Ionospheric Trough (MIT). To verify this hypothesis, Swarm latitudinal electron density profiles from in-situ Langmuir probe measurements over Europe were considered. These profiles demonstrate a clear MIT displacement from ∼65°N on 22 March to ∼55°N on 23 March with a subsequent recovery around ∼65°N on 25 March. [ABSTRACT FROM AUTHOR]

A multi-instrumental analysis of the meridional ionospheric response is presented over Europe during the two largest ICME-driven geomagnetic storms of solar cycle #24 maximum. Data from 5 European digisonde stations, ground-based Global Navigation Satellite System, Total Electron Content (GNSS TEC), the ratio of the TEC difference (rTEC), as well as Swarm and Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) satellite observations have been used for the investigation of selected intervals (11–17 November, 2012, and 16–25 March, 2015). The storm evolution is monitored by digisonde foF2 critical frequency (related to the maximum electron density of F2-layer) and GNSS TEC data. Moreover, Global Ultraviolet Imager (GUVI) measurements from the TIMED satellite are used to investigate the changes in the thermospheric O/N2 ratio. Our main focus was on the main phase of the geomagnetic storms, when during the nighttime hours extremely depleted plasma was detected. The extreme depletion is observed in foF2, TEC and rTEC, which is found to be directly connected to the equatorward motion of the midlatitude ionospheric trough (MIT) on the nightside. We demonstrate a method (beside the existing ones) which allows the monitoring of the storm-time evolution of the disturbances (e.g., MIT, SAPS, SED) in the thermosphere-ionosphere-plasmasphere system by the combined analysis of the worldwide digisonde system data (with the drift measurements and the ionospheric layer parameters with 5–15 min cadence), with rTEC and GNSS TEC data, and with the satellite data like Swarm, TIMED/GUVI.

Abstract The midlatitude ionospheric trough (MIT) is a well‐known feature in the topside ionosphere. Previous MIT related studies mainly focused on the MIT structure itself, while few studies focused on irregularities inside the MIT region. In this study, we provided for the first time the magnetic latitude and magnetic local time distribution of irregularities within the MIT, by using the Swarm in situ plasma measurements. Orbital cases show that both the MIT equatorward and poleward walls are embedded with small‐scale irregularities. Further statistical analysis reveals that the particle precipitation is the dominant mechanism for the dayside MIT region irregularities, while nightside MIT region irregularities are strongly related with the temperature gradient drift instability. The Swarm observations show that the temperature gradient drift instability occur not only in MIT equatorward wall but also in MIT polarward wall, which is different to the theoretical prediction of Hudson and Kelley (1976, https://doi.org/10.1029/JA081i022p03913).

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28° inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation. [ABSTRACT FROM AUTHOR]

Abstract: Temporal and spatial variations of the midlatitude ionospheric trough during a geomagnetic storm on 4 April 2017 have been investigated using Global Navigation Satellite System total electron content data together with Arase satellite observations. After the geomagnetic storm commencement, the trough minimum location moves equatorward from 60 to 48° in geomagnetic latitude within 4 hr. The trough minimum location identified from the Global Navigation Satellite System total electron content data is located near the footprint of an abrupt drop of electron density detected by the Arase High‐Frequency Analyzer instrument. The longitudinal variation of the trough minimum location shows a significant variation with a scale of 1,000–2,500 km during both storm and quiet times. This phenomenon has not yet been reported by previous studies. After the onset of the storm recovery phase, the trough minimum location rapidly moves poleward back to the quiet time location within 4 hr. [ABSTRACT FROM AUTHOR]