The Transmission of Pc 3 Waves From the Foreshock Into the Earth's Magnetosphere: 3D Global Hybrid Simulation.

Although initially it was presumed that foreshock waves would propagate directly into the dayside magnetosphere, observational evidence for sinusoidal Pc3 waves in the downstream of quasi‐parallel shocks is scarce. The transmission of these waves from the foreshock into the magnetosphere remains uncertain. In this paper, we employ a 3D global hybrid simulation at a realistic scale to explore the generation and transmission of the dayside ULF waves under a radial interplanetary magnetic field. Our findings demonstrate that the Pc3 waves are self‐consistently generated in the foreshock region and then transmitted into the magnetosheath and magnetosphere. In the foreshock, the waves are excited at approximately 25 mHz and exhibit right‐handed helicity in the plasma frame, characterizing them as quasi‐parallel fast magnetosonic waves. In the magnetosphere, the fluctuating magnetic field is mainly parallel to the background magnetic field, which indicates the dominant wave modes are compressional. Fluctuations in the magnetosheath show a broader spectrum (10–100 mHz) compared to those in the magnetosphere and foreshock, potentially explaining the little observation of sinusoidal Pc3 waves in the magnetosheath. Additionally, only lower frequency compressional waves (below 30 mHz) are effectively transmitted into the dayside magnetosphere. Our simulation provides critical insights into the interactions between the solar wind and Earth's magnetosphere. Plain Language Summary: Ultralow frequency waves in the Pc3 range, with periods of 10–45 s, are frequently detected in the Earth's magnetosphere. These waves are thought to originate from ion foreshock regions upstream of Earth's quasi‐parallel bow shock. They arise from ion beam instabilities triggered by the interaction between shock‐reflected suprathermal ions and the incoming solar wind. While it was assumed that these Pc3 waves would directly enter the dayside magnetosphere, in‐situ observations of sinusoidal Pc3 waves in the magnetosheath remain rare. This scarcity of evidence has left the mechanisms of their propagation into the magnetosphere unclear. This paper employs a 3D global hybrid simulation at a realistic scale to investigate how the Pc3 waves are transmited through the bow shock, magnetosheath, and into the magnetosphere under a radial interplanetary magnetic field. Our results indicate that the Pc3 waves are self‐consistently generated in the foreshock and transmitted into the magnetosheath and magnetosphere, with only lower frequency compressional waves effectively propagating into the magnetosphere. Fluctuations in the magnetosheath exhibit a broader spectrum compared to those in the magnetosphere and foreshock. Key Points: A 3D global hybrid simulation at a realistic scale has been used to investigate the dayside ULF waves under a radial interplanetary magnetic fieldPc 3 waves are self‐consistently generated at the foreshock region and transmitted into the magnetosheath and magnetosphereFluctuations in the magnetosheath exhibit a broad spectrum, and only lower frequency compressional waves are effectively transmitted into the magnetosphere [ABSTRACT FROM AUTHOR]