Correlations between ULF wave power, solar wind speed, and relativistic electron flux in the magnetosphere: solar cycle dependence

Abstract

We present results from a rank order correlation coefficient (ROCC) analysis between ground-based 1– 10 mHz ultra-low frequency (ULF) wave power, upstream solar wind speed ( v SW), and MeV electron flux across the outer radiation belt. We use data spanning a complete solar cycle (1990–2001) from ground magnetometers from the SAMNET and IMAGE arrays, and MeV electron data from the Los Alamos spacecraft at geosynchronous orbit (GEO) and from a HEO spacecraft at L=5.5 and 4.5. We find a very high correlation between ULF wave power and v SW at all local times and on all L-shells between L=3.1 and 6.6. Very strong cross- L-shell ULF power coherence is maintained throughout the solar cycle, although the efficiency of v SW in driving a given ULF power response is solar cycle dependent. Peak ULF power and peak ROCC (∼0.75) are observed during the declining phase of the solar cycle around 1994–1995, with smaller localised peaks in these parameters occurring during slower average v SW conditions close to solar maximum in 2000. There is also a strong correlation of both v SW and ULF power with MeV electron flux, these correlations being more strongly dependent on solar cycle phase. The v SW and ULF power correlations with MeV electron flux peak during the late declining phase of the solar cycle, when the radiation belts are most intense. Our results confirm that radiation belt flux demonstrates extremely strong global cross- L-shell coherence throughout the solar cycle. The ROCC between v SW or ULF power and MeV electron flux demonstrate a clear and systematic time lag following the peak in either v SW or ULF power. Electron fluxes respond first at GEO (lag ∼2 days), before subsequently peaking at lower L-shells. Importantly, our results indicate that v SW-correlated MeV electron acceleration processes operate by inwards radial transport. The correlation between ULF power and MeV energy electron flux further suggests that ULF waves could be causally responsible via ULF wave enhanced radial diffusion.