Interpretation of increased energetic particle flux measurements by SEPT aboard the STEREO spacecraft and contamination

摘要

Context. Interplanetary (IP) shocks are known to be accelerators of energeticcharged particles observed in-situ in the heliosphere. However, theacceleration of near-relativistic electrons by shocks in the interplanetarymedium is often questioned. On 9 August 2011 a Corotating Interaction Region(CIR) passed STEREO B (STB) that resulted in a flux increase in the electronand ion channels of the Solar Electron and Proton Telescope (SEPT). Becauseelectron measurements in the few keV to several 100 keV range rely on theso-called magnet foil technique, which is utilized by SEPT, ions can contributeto the electron channels. Aims. We aim to investigate whether the flux increasein the electron channels of SEPT during the CIR event on 9 August 2011 iscaused by ion contamination only. Methods. We compute the SEPT responsefunctions for protons and helium utilizing an updated GEANT4 model of SEPT. TheCIR energetic particle ion spectra for protons and helium are assumed to followa Band function in energy per nucleon with a constant helium to proton ratio.Results. Our analysis leads to a helium to proton ratio of 16.9% and a protonflux following a Band function with the parameters $I_0 = 1.24 \cdot 10^4$ /(cm2 s sr MeV/nuc.), $E_c = 79$ keV/nuc. and spectral indices of $\gamma_1 =-0.94$ and $\gamma_2 = -3.80$ which are in good agreement with measurements bythe Suprathermal Ion Telescope (SIT) aboard STB. Conclusions. Since our resultsexplain the SEPT measurements, we conclude that no significant amount ofelectrons were accelerated between $55$ keV and $425$ keV by the CIR.