# Evaluation of the Mid-Latitude Ionospheric trough Using GRACE Data
## 1 Introduction  
The mid-latitude ionospheric trough (MIT), also known as the main ionospheric trough, is defined as the depletion of plasma in the nighttime F region. This phenomenon has been studied since 1965 using various approaches [1]. However, results vary, and the MIT has not yet been represented in global empirical models to correct ionospheric propagation effects on GNSS signals, such as IRI [2], NeQuick-G [3, 4], NTCM-G [5], etc. The aim of this research is to detect and characterize the mid-latitude ionospheric trough using new satellite data and, enriching current knowledge of the MIT phenomena. There has been considerable interest in investigating the mid-latitude ionospheric trough, its dynamics, and morphology due to its impact on radio wave propagation. The MIT is associated with large-scale gradients in electron concentration. One of the mechanisms that may cause the mid-latitude trough is a stagnation of ionospheric plasma, which is cached between westward drift in the auroral zone and eastward (corotating) plasma drift [6]. The MIT is a V-shaped ionosphere structure, the poleward wall of which has a very steep gradient in the nighttime, and the equatorward boundary of which indicates a significant change from dusk to dawn [7]. Moreover, the poleward wall of the MIT is affiliated with the equatorward boundary of the auroral precipitation region [8], and its equatorward wall corresponds to the ionospheric footprint of the plasmapause [9, 10]. The latest study by [PERSON], [PERSON] et al. [11] on correlations between the MIT and plasmapause on a global scale confirms that they are directly related, although only at night. The plasmapause and MIT lie on the same field line and change at the same moment in time [11]. A literature study [12] showed that the MIT is mainly observed in darkness and at geomagnetic latitudes of 50-70\({}^{\circ}\). However, the actual location of the MIT changes depending on geomagnetic activity [13], magnetic local time, longitude [14], season, and solar activity [15]. The MIT is longitudinally elongated, and the width of the trough in the latitudinal direction is nearly 5-17\({}^{\circ}\). The MIT is mainly detected during nighttime, from18:00 until 06:00 magnetic local time [12]. In the afternoon hours, the MIT is located at higher latitudes, and it moves equatorward during later local time [6]. It is more prominent during the December solstice and equinoxes than during the June solstice [16]. In terms of morphology, the mid-latitude trough comprises of two slopes, namely the poleward and equatorward slope, between which lies the minimum point of electron concentration, i.e., the trough minimum. Furthermore, the MIT can be characterized by key parameters, such as the trough minimum position and trough width and depth [17]. Work in this field has focused primarily on the northern hemisphere trough. Recent evidence shows that the trough allocation is hemispherically asymmetrical [18]. However, during magnetic storms, the trough position evolves symmetrically in both hemispheres. The trough moves equatorward with increased geomagnetic activity [19]. The depth of the trough principally depends on solar activity conditions and deepens with increased F10.7 [17]. The width depends on the season, with a smaller width summer than in winter [16]. In recent decades, the trough has been studied using various data instruments and different approaches to trough detection. Investigations have been conducted using satellite data from the COSMOS-900, Intercosmos-19, DMSP (Defense Meteorological Satellite Program), CHAMP (CHAllenging Minisatellite Payload), and Swarm missions. To enrich current knowledge of MIT phenomena, we used GRACE (Gravity Recovery and Climate Experiment) satellite mission data for the period from 2002 to 2015. We statistically analyzed the trough minimum position, depth, and width according to varying geophysical conditions for the southern and northern hemispheres separately. In Section 2, we describe the database and method of trough identification.