Survivor Bias in Longitudinal Asymmetry of Impulsive Solar Energetic Particle Events

Abstract

We follow H.-Q. He and W. Wan in studying the distribution of impulsive solar energetic particle (SEP) events from the updated observations. We confirm their findings of the longitudinal asymmetry of the distribution of SEPs with respect to the east side and west side of the magnetic footpoint, which is indicated as the “east–west azimuthal asymmetry.” With a qualitative analysis, H.-Q. He and W. Wan suggested that this asymmetry is caused by perpendicular diffusion and heliospheric magnetic field structure. We use a simple probability-based statistical method to provide the distribution of SEP events as an analytical function of relative longitude. We find that the impulsive SEP events not correlated to observed flares were indeed flare-induced but went unobserved due to the flares occurring on the far side of the Sun. Furthermore, the observed east–west azimuthal asymmetry of SEPs is caused by the survivor bias.

Keywords

• Solar flares
• Solar energetic particles