RESEARCH ON SOLAR-TERRESTRIAL INTERACTIONS AND REGIONAL GEOMAGNETIC STORM VARIATIONS

<jats:p>This study presents a newly compiled long-term catalog of geomagnetic storms observed over the Mongolian region during the period 1995-2026 and investigates their statistical characteristics, with particular emphasis on the intense geomagnetic storm that occurred on 5-6 December 2020. Data obtained from the space-based OMNI database and ground-based INTERMAGNET and MAGDAS magnetometer networks were employed to conduct a comprehensive temporal and spatial analysis of geomagnetic field variations, focusing primarily on the horizontal (H) component. The catalog comprises 356 geomagnetic storm events and reveals a strong relationship between storm occurrence, intensity, and the approximately 11-year solar activity cycle. Analysis of the selected storm event indicates that a sudden enhancement in the dynamic pressure associated with a coronal mass ejection (CME) significantly compressed the terrestrial magnetopause to approximately 7 Earth radii (RE), producing a pronounced geomagnetic sudden impulse (SI) detected by ground-based observatories. The sustained southward orientation of the interplanetary magnetic field (IMF) Bz component facilitated magnetic reconnection at the dayside magnetopause, providing an efficient mechanism for solar wind energy transfer into the magnetosphere. The propagation time of the solar wind dynamic pressure pulse from the magnetopause to the Earth's surface was estimated to be approximately 57 minutes. Furthermore, the substantial depressions observed in the H-component at regional observatories exhibit strong consistency with global satellite measurements, indicating close agreement in the physical processes governing geomagnetic storm development and magnetospheric response.</jats:p>