Search

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). Plain Language Summary: Early theoretical studies have pointed out that there are several instabilities for generating plasma density irregularities on both the equatorward and poleward sides of the midlatitude trough. However, few observational studies were performed to focus on the irregularities in the trough region, possibly due to the relatively low‐resolution in situ plasma density measurements of previous missions. The high‐resolution plasma and magnetic measurements from the Swarm satellites provide an excellent data set for investigating the irregularities in the midlatitude trough region. Our results confirm the existence of plasma irregularities frequently observed in the walls of the MIT. Statistical analysis indicate that the dayside MIT region irregularities occur much more frequently than that on the nightside, which is mainly due to the dayside MIT overlapping with the auroral active area, while the nightside MIT is located outside the auroral oval. Study on the diurnal variation of the MIT region irregularities suggests that the dayside irregularities are mainly caused by the auroral particle precipitations, while the nightside irregularities are related to the temperature gradient drift instability proposed by Hudson and Kelley (1976, https://doi.org/10.1029/JA081i022p03913). Key Points: The Swarm in situ Ne and Te measurements are used for the first time to investigate the plasma irregularities inside the MITThe auroral particle precipitations dominate the dayside MIT region irregularitiesThe temperature gradient drift instability contributes mostly to the nightside MIT region irregularities [ABSTRACT FROM AUTHOR]

The dynamics and climatology of midlatitude trough minimum have been widely investigated. However, the detailed magnetic local time (MLT) evolution of the trough walls (both equatorward and poleward) has not been well addressed. In this study, we used nearly 10‐year Planar Langmuir Probe (PLP) data from the CHAMP satellite to investigate how the location and shape of midlatitude trough evolve during night hours. We find that (1) the trough equatorward wall experiences an obvious equatorward extension in the premidnight sector, with steepness rapidly decreasing during the same local time sector; (2) the trough equatorward wall is as steep as poleward wall in the dusk sector; and (3) the trough poleward wall shows no remarkable change during the whole nighttime, except for a weakly reduced width and a slightly enhanced steepness near midnight. We suggest that the rapid extension of the equatorward wall in the premidnight sector is closely related to the plasma equatorward movement caused by the neutral winds. The rapid decrease of the steepness of the equatorward wall in the premidnight sector is not only influenced by its own width expansion but possibly also related to the upward plasma drift caused by neutral winds. The trough minimum and equatorward/poleward boundaries are found located at slightly higher magnetic latitude during equinoxes than that during solstice seasons. Key Points: The CHAMP Ne measurements are used to reveal the evolution of nighttime midlatitude trough shape under geomagnetic quiet conditionThe trough equatorward wall experiences an obvious equatorward extension at premidnight, and it is as steep as poleward wall at duskThe morphology evolution of midlatitude trough before midnight is suggested to be mainly controlled by the horizontal neutral winds [ABSTRACT FROM AUTHOR]

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]