Dynamics of the correlation between polar cap radio absorption and solar energetic proton fluxes in the interplanetary medium

Abstract

During solar energetic particle (SEP) events, large fluxes of energetic particles spreading throughout the interplanetary medium (IPM) have access to the upper polar atmosphere where they play important roles in physical and chemical processes. We examine the dynamics of the relation between solar energetic proton (SEP) fluxes detected outside the magnetosphere by SOHO, at geosynchronous orbit by GOES 8, and in low earth orbit by NOAA 15 (POES) satellites on one hand, and ionospheric absorption measured by a NORSTAR riometer in the polar cap on the other. We attempt to isolate SEP‐related signals by rejecting periods influenced by electron precipitation and solar radio bursts. Under these conditions we find R2of up to 0.9 between the logarithms of SEP flux and absorption for all satellites, but with the additional restriction to periods beginning 10 h after event onsets and within tens of hours following times of maximum flux. We find that the flux‐absorption correlation is poor during the first few hours of an event, which we attribute to the fact that SEP angular distributions are often highly peaked along the interplanetary magnetic field during the early stages of an event. Such highly anisotropic distributions map to small regions inside the polar caps, making it unlikely that the corresponding absorption signal will be detected by a single riometer. These observations suggest that riometers in combination with a suitable numerical tool can be used as a diagnostic of properties of energetic proton populations in the interplanetary medium.