Solar wind control of Pc5 pulsation power at geosynchronous orbit

Abstract

We statistically study the solar wind control of Pc5 magnetic pulsations at geosynchronous orbit. Solar wind conditions are quantified with the use of proton measurements at L1 from the ACE spacecraft that are propagated to the bow shock nose. In addition to the solar wind velocity, Vsw, density, Nsw, and dynamic pressure, Psw, we also consider the dynamic pressure variance in the Pc5 band, VarPsw. Pc5 pulsation parameters are derived from the GOES 8 magnetic field measurements at geosynchronous orbit at ∼10° magnetic latitude. The pulsations are quantified by three parameters: Sy, total power of the azimuthal magnetic field component in the 1.6‐ to 6.7‐mHz Pc5 band; Sz, total power of the field‐aligned (compressional) component in the same band; and Stor, the azimuthal power of narrow‐band Pc5 oscillations attributed to the fundamental toroidal standing Alfvén waves. All parameters are computed in non‐overlapping 1‐h time windows and then used in linear correlation analysis. We find that all pulsation parameters are positively correlated with Psw and VarPsw with a correlation coefficient of up to ∼0.7 on the dayside. While the correlation of pulsation parameters with the solar wind velocity and density is also positive, the correlation coefficients are smaller. We discuss three mechanisms relevant to the observed Psw and VarPsw control of pulsation power: (1) solar wind‐forcing of magnetic pulsations (breathing mode); (2) Psw control of the magnetopause‐satellite distance, which results in the Psw dependence of the amplitude of surface waves observed at the satellite; and (3) coupling of impulsively excited fast mode waves to toroidal standing Alfvén waves.