Solar-Terrestrial Physics Solar-Terrestrial Physics Solar-Terrestrial Physics 2500-0535 109303 10.12737/stp-114202505 Results of current research Results of current research Results of current research Trends in the F2-layer critical frequency from data obtained at the ionospheric station Yakutsk during the period from 1956 to 2017 Trends in the F2-layer critical frequency from data obtained at the ionospheric station Yakutsk during the period from 1956 to 2017 Кобякова Саргылана Егоровна Kobyakova Sargylana Egorovna s.e.kobyakova@ikfia.ysn.ru https://orcid.org/0000-0003-2184-8089 Гололобов Артем Юрьевич Gololobov Artem Yuryevich golart87@gmail.com

кандидат физико-математических наук;

candidate of physical and mathematical sciences;

 Каримов Рустам Рамильевич Karimov Rustam Ramilyevich karimov@ikfia.ysn.ru

кандидат физико-математических наук;

candidate of physical and mathematical sciences;

 Степанов Александр Егорович Stepanov Aleksandr Egorovich a_e_stepanov@ikfia.ysn.ru Институт космофизических исследований и аэрономии им. Ю.Г. Шафера СО РАН Якутск Россия Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy SB RAS Yakutsk Russian Federation Институт космофизических исследований и аэрономии им. Ю.Г. Шафера СО РАН Якутск Россия Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy SB RAS Yakutsk Russian Federation Северо-Восточный федеральный университет им. М.К. Аммосова Якутск Россия North-Eastern Federal University Yakutsk Russian Federation Институт космофизических исследований и аэрономии им. Ю.Г. Шафера СО РАН Якутск Россия Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy of SB RAS Yakutsk Russian Federation Институт космофизических исследований и аэрономии им. Ю.Г. Шафера СО РАН Якутск Россия Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy SB RAS Yakutsk Russian Federation 10 12 2025 10 12 2025 11 4 49 56 25 03 2025 20 08 2025 https://naukaru.ru/en/nauka/article/109303/view

A statistical analysis of vertical ionospheric sounding data from the Yakutsk station (62.01° N, 129.43° E, 57.12° MLAT) for the period from 1956 to 2017 encompassing six solar cycles has been carried out to identify long-term changes in the F2 layer of the subauroral ionosphere and their relationship with solar and geomagnetic activity. We examined variations in one of the main parameters of the ionospheric F2 layer, the critical frequency. A high correlation was found between the F2-layer critical frequency and the solar activity index F10.7. It is shown that during six solar cycles (cycles 19–24) there were negative trends in annual average F2-layer critical frequencies both at midday and at midnight. It has been revealed that foF2 trends depend on the season and time of day. Absolute values of the trends are higher in equinoctial and summer seasons. Peak negative trends are observed at midday during equinoctial months, reaching approximately –11 kHz/year.

A statistical analysis of vertical ionospheric sounding data from the Yakutsk station (62.01° N, 129.43° E, 57.12° MLAT) for the period from 1956 to 2017 encompassing six solar cycles has been carried out to identify long-term changes in the F2 layer of the subauroral ionosphere and their relationship with solar and geomagnetic activity. We examined variations in one of the main parameters of the ionospheric F2 layer, the critical frequency. A high correlation was found between the F2-layer critical frequency and the solar activity index F10.7. It is shown that during six solar cycles (cycles 19–24) there were negative trends in annual average F2-layer critical frequencies both at midday and at midnight. It has been revealed that foF2 trends depend on the season and time of day. Absolute values of the trends are higher in equinoctial and summer seasons. Peak negative trends are observed at midday during equinoctial months, reaching approximately –11 kHz/year.

 long-term trends solar activity subauroral ionosphere F2-layer critical frequencies long-term trends solar activity subauroral ionosphere F2-layer critical frequencies The work was financially supported by the Russian Science Foundation (Project No. 25-17-20002) The work was financially supported by the Russian Science Foundation (Project No. 25-17-20002)

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