Relationship Between Magnetosheath ULF Waves and Ground‐Based Pc3‐4 Waves: A Statistical Study

Foreshock ultralow frequency (ULF) waves are a major contributor to magnetospheric Pc3–4 waves (7–100 mHz), but their transmission through the magnetosheath is not well understood. Using 109 THEMIS traversals from the bow shock to the magnetopause, in conjunction with ground magnetometer (GMAG) measurements, we performed a statistical analysis of magnetosheath ULF waves and their relationship to ground‐based Pc3–4 waves. Our findings reveal that in quasi‐parallel regions, magnetic and dynamic pressure wave power are correlated with ground‐based magnetic wave power at periods of ∼30s, with correlation coefficients reaching up to ∼0.7. The correlation depends on the THEMIS spacecraft's position in the magnetosheath as well as the magnetic local time and latitude of the ground stations. In contrast, in quasi‐perpendicular regions, the correlation is weaker (up to ∼0.3) and increases with decreasing frequency. Additionally, in quasi‐perpendicular regions, wave power increases from the bow shock to the magnetopause, consistent with local excitation, whereas waves in quasi‐parallel regions are less compressive with power relatively stable, consistent with a foreshock origin. Our results suggest that foreshock‐originated waves in quasi‐parallel regions contribute more effectively to magnetospheric ULF waves than those in quasi‐perpendicular regions, through both magnetic field and dynamic pressure oscillations, with a preferential propagation from the dawnside magnetosheath to both sides of the magnetosphere.