Solar wind influence on Pc4 and Pc5 ULF wave activity in the inner magnetosphere

Abstract

Abundant evidence has shown that ULF wave activity measured at geosynchronous orbit is well correlated with solar wind parameters, such as the solar wind velocity and dynamic pressure. However, many of the past studies were based on magnetic field measurements near the equatorial plane and thus could not unambiguously describe ULF waves, as magnetic field oscillations in the fundamental toroidal mode have a node at the magnetic equator. In this study, we use, for the first time, simultaneous electric and magnetic field measurements by Time History of Events and Macroscale Interactions during Substorms satellites throughout the inner magnetosphere to statistically examine the correlation between ULF wave activity in the inner magnetosphere and the solar wind parameters: velocity, dynamic pressure, and variation in dynamic pressure. On the basis of electric field observations from August 2007 to May 2009, we found that, among the three parameters, the solar wind velocity has the strongest correlation with the daily averaged Pc4 and 5 wave magnitude in 4 ∼ 9 RE. For example, the correlation coefficient of δEr (the square root of the integrated power spectral density of the radial component of the electric field) in the Pc5 frequency range with the solar wind dynamic pressure is 0.35; with the dynamic pressure variation, it is 0.42; and with the solar wind velocity, it is 0.55. However, using only magnetic field observations, the variation in dynamic pressure is best correlated, with correlation coefficients of δBr (the square root of the integrated power spectral density of the radial component of the magnetic field) with the dynamic pressure being 0.59, with the dynamic pressure variation being 0.63 and with the solar wind velocity being 0.53. We suggest that this difference arises because toroidal and poloidal mode pulsations are not detected with the same effectiveness in the electric field and the magnetic field near the magnetic equator and/or because of the presence of broadband ULF noise. We further suggest that either directly measured or flow‐derived electric field measurements are better suited in the study of the relationship between wave power of fundamental field line resonance (i.e., in Pc5 frequency range) and solar wind conditions.