Undulations in MeV solar energetic particle fluxes in Earth's magnetosphere associated with substorm magnetic field reconfigurations

Abstract

East‐west anisotropies in GOES solar energetic particle (SEP) flux observations provide evidence of a time‐varying radial gradient between L ≈ 4 and L ≈ 7. During elevated auroral activity, the gradient, represented by the ratio of inner (eastward‐observed) to outer (westward‐observed) fluxes, exhibits undulations lasting 1–3 h. A superposed epoch analysis based on 85 geosynchronous field dipolarizations observed during 19 SEP events between 1998 and 2006 shows that >4 MeV inner proton fluxes increase as the geosynchronous field stretches during substorm growth phase, then decrease gradually following field dipolarization at substorm onset. This behavior is distinct from the sharp increase of outer belt electrons and protons at the time of the dipolarization. The undulations indicate that substorm‐related field changes in the near‐Earth magnetotail enhance access of SEPs to the inner magnetosphere (down to L ≈ 4). The observation of undulations at geosynchronous orbit is suppressed during high solar wind dynamic pressure (Pdyn > 5 nPa) or strong geomagnetic storm conditions (Dst∗ < −100 nT) because the radial gradient is greatly reduced under these conditions and the interplanetary flux level provides an upper limit to the SEP fluxes in the magnetosphere. A sequence of 10 SEP undulations was observed during and following the 18 April 2002 sawtooth event. Analysis of this sequence shows that undulations observed at all local times are associated with substorm‐related field changes on the nightside. GOES eastward SEP observations at all local times can thus serve as global trace particle sensing of substorm‐related field reconfigurations in the near‐Earth magnetotail.