CLIMATIC CHANGES IN THE UPPER IONOSPHERE: LONG-TERM VARIATIONS AND TRENDS IN THE CRITICAL FREQUENCY OF THE F2 LAYER IN THE KAZAKHSTAN REGION. Journal of open systems evolution problems, [S. l.], v. 27, n. 1, p. 35–48, 2025. DOI: 10.26577/JPEOS20252715. Disponível em: https://peos.kaznu.kz/index.php/peos/article/view/342. Acesso em: 5 jul. 2026.