Statistical Relationship Between Foreshock ULF Wave Power and Ground‐Based Pc3‐4 Wave Power

Abstract Magnetospheric ultralow frequency (ULF) waves are fundamental for coupling energy across the magnetosphere and ionosphere. Pc3‐4 waves (7–100 mHz) are associated with small interplanetary magnetic field (IMF) cone angles, suggesting that foreshock ULF waves are the source. To understand the relationship between foreshock and magnetospheric ULF waves, we perform a statistical study using the THEMIS spacecraft to observe foreshock ULF waves and ground magnetometers (GMAG) to infer magnetospheric ULF waves. We observe a power law correlation between foreshock and ground‐based ULF wave power, with correlation coefficients exceeding 0.5 around the typical period of foreshock “30s waves” in contrast to correlation coefficients ∼0.3 between solar wind and ground‐based ULF wave power. The power law parameters decrease with lower magnetic latitudes, indicating weaker power transport deeper in the magnetosphere. Additionally, the correlation coefficients decrease with larger magnetic local time (MLT) separation between the THEMIS spacecraft and GMAG. We also find that ground‐based Pc3‐4 wave power exhibits a stronger dependence on the local bow shock θ Bn (angle between the shock normal and the IMF) than on the IMF cone angle. These findings indicate that foreshock ULF waves are more likely to be transmitted from the local MLT than across the subsolar point. We also find that ground‐based Pc3‐4 waves show a stronger dependence on the solar wind speed than do foreshock ULF waves, suggesting a possible contribution from Kelvin‐Helmholtz instability and shock‐generated transients. Our results provide new insights into the connection between foreshock ULF waves and ground‐based Pc3‐4 waves.