Estimation of the Ionospheric Effects of Geomagnetic Disturbances Based on Data from Wakkanai Station
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mation of the Ionospheric Effects of Geomagnetic Disturbances Based on Data from Wakkanai Station A. D. Legen’kaa, * and V. V. Khegaia, ** a
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Moscow, Troitsk, 108840 Russia *e-mail: [email protected] **e-mail: [email protected] Received November 26, 2019; revised February 5, 2020; accepted May 21, 2020
Abstract—Ionosonde measurements of the critical frequency of the ionospheric F2 layer (foF2) at Wakkanai station (Japan) are analyzed with the method of superposed epochs. The station is situated in a region of high seismicity. Winter periods with low sunspot and geomagnetic activity and high seismic activity are considered. The goals of this work were the quantitative characterization and comparison of ionospheric disturbances induced by the development of geomagnetic storms/geomagnetic disturbances. Geomagnetic disturbances refer to manifestations of geomagnetic activity with an intensity that is higher than “quiet” geomagnetic background but not higher than the threshold characteristic of weak geomagnetic storms. It is ascertained that weak geomagnetic storms and geomagnetic disturbances, from the time of the main trough in the Dst index after the onset of a geomagnetic storm/geomagnetic disturbance to the end of the third day, result in only positive foF2 deviations. The deviations exceed 6% of the background; the maximal relative deviations from the background (δfoF2max) are ~17% and ~14%, respectively. For geomagnetic disturbances, the most significant response of the ionosphere is observed ~7 h after the main trough in the Dst index. The resulting assessments can be used to filter out false ionospheric earthquake precursors with analysis of the ionospheric data from Wakkanai station. DOI: 10.1134/S0016793220050126
1. INTRODUCTION Ionospheric storms/substorms (ISs/ISSs) in the F region associated with the development of geomagnetic storms/substorms (GSs/GSSs) and the physical causes of them have long been studied and are well described (Matsushita, 1959; Rishbeth, 1991; Prölss, 1993, 1995; Mikhailov, 2000; Danilov and Laštovička, 2001; Deminov, 2015). The main features of ISs in the ionospheric F region in terms of changes in the critical frequency of the ionospheric F2 layer (foF2) recorded at a ground-based ionospheric vertical sounding station (IVSS) can be briefly summarized as follows (Rishbeth, 1991; Prölss, 1995; Danilov and Laštovička, 2001). (1) The foF2 increases soon after the onset of an IS. This is the so-called positive IS phase, which lasts several hours. (2) The “main or negative phase” of the storm begins a few hours later. It is characterized by a strong foF2 decrease and typically lasts 24–36 h. (3) The foF2 sometimes increases during the main phase of the storm, especially in the winter, and exceeds its values on quiet days. This mainly occurs in low latitudes.
(4) The foF2 values return to their undisturbed levels for 3 days on average after the storm onset. (5) Significant complex v
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