Satellite model of foF2 in winter high-latitude ionosphere describing the trough structure

Abstract

An empirical foF2 model was constructed for the winter high-latitude ionosphere taking into account the structure of the ionization trough. This model describes the trough position and its shape in terms of longitudinal-latitudinal variations in foF2 in geographic latitude intervals of 40–85°N and 40–85°S. The model is valid for November to February in the Northern hemisphere and May-August in the Southern hemisphere. The simulation describes quiet geomagnetic conditions (Kp from 0 to 3+ for all local time (LT) and universal time (UT) hours at any solar activity level in the interval F_10.7 = 70–200 s.f.u. The model is based on the Cosmos-900, Interkosmos-19 and CHAMP satellite data. The radio occultation observation data obtained from the GRACE, CHAMP and COSMIC/FORMOSAT experiments were partially used. The International Reference Ionosphere (IRI) was also used; thus, the ground-based station data were indirectly considered. Within the model framework, the variations in the trough minimum position with longitude and local time were identified and studied in detail. The latitudinal and longitudinal variations in foF2 were also revealed and investigated. The constructed model more adequately reproduces variations in foF2 than the IRI-2016 model. The new model is available on the IZMIRAN website: http://www.izmiran.ru/ionosphere/sm-mit/.

Introduction

The ionospheric trough was discovered by Muldrew (1965) from Alouette satellite data. Trough is the main feature of the winter and the nighttime equinox high-latitude ionosphere, as it is regularly observed in these conditions (Karpachev and Afonin, 1998). The main morphological characteristics and causes of trough formation have been described in reviews (Ahmed et␣al., 1979, Moffett and Quegan, 1983, Rodger et␣al., 1992, Karpachev, 2003). The trough is primarily a night phenomenon; however, it is quite regularly observed in the winter daytime ionosphere under low solar activity (Tulunay and Grebowsky, 1978, Ahmed et␣al., 1979, Oksman, 1982, Evans et␣al., 1983, Whalen, 1989, Sojka et␣al., 1990, Werner and Prolss, 1997, Karpachev et␣al., 1998). The dynamics of the trough largely determines the ionosphere behavior; hence, it is impossible to construct an accurate model of the high-latitude (subauroral) ionosphere that does not consider the trough structure. This also applies to the IRI model, which does not describe the trough.

Attempts to build a trough model have been made repeatedly; however, they only consider the nighttime trough and are limited (Halcrow and Nisbet, 1977, Feichter and Leitinger, 2002, Wielgosz et␣al., 2004, Pryse et␣al., 2006, Le et␣al., 2017, Parker et␣al., 2018). This is because the trough is created by several competing mechanisms so that it is a very variable structure [Moffett and Quegan, 1983]. The trough structure (position and shape) strongly depends on local time, season, hemisphere, geomagnetic and solar activity (Ahmed et␣al., 1979, Moffett and Quegan, 1983, Rodger et␣al., 1992, Karpachev and Afonin, 1998, Karpachev, 2003, Karpachev, 2019a). The trough characteristics are also highly variable with longitude (Tulunay, 1973, Karpachev et␣al., 1996, 2016, 2018, Karpachev and Afonin, 1998); therefore, the complete model can be constructed only based on satellite observations, which allow us to obtain a global pattern of the ionosphere.

To construct a trough model, it is necessary to know its position as accurately as possible, since at large gradients on its equatorward and poleward walls, the errors in the foF2 determination can be very large. In this study, the trough is understood to be the main ionospheric trough (MIT). According to the definition, the MIT is a subauroral structure since it is usually located 2-5° equatorward of the auroral oval [Ahmed et␣al., 1979]. To accurately determine the MIT position, it must be separated from other troughs: lower-latitude and higher-latitude troughs. This complicated problem was discussed in several papers [Werner and Prolss, 1997, Karpachev and Afonin, 1998, Karpachev, 2019a, Karpachev, 2021]. The conclusions drawn from these studies are used below in creating the MIT position model.

The occurrence probability of a daytime trough decreases with the increase in the illumination of the high-latitude ionosphere. Thus, strictly speaking, the daytime trough model must be probabilistic. In this case, it is more correct to speak not about the trough model but the model of the F2 layer critical frequency foF2 in the high-latitude ionosphere.

All of the above problems have been solved for many years, but the nighttime winter trough model for both hemispheres was built quite recently in [Karpachev et␣al., 2016] and the daytime winter trough model in [Karpachev, 2019b]. The Kosmos-900, Interkosmos-19, and CHAMP satellite data were used in these models. In the present study, a large-scale task of significant modification, expansion, and refinement of the previously created models of the winter ionospheric trough for quiet geomagnetic conditions was set. The radio occultation data were additionally used for the simulation and indirectly, through the IRI model, data from ground-based ionospheric stations. As a result, the foF2 model of the high-latitude winter ionosphere taking into account both nighttime and daytime troughs for quiet conditions in both hemispheres, for all levels of solar activity and for all local time hours was built.