Detection asymmetry in solar energetic particle events

Context. Solar energetic particles (SEPs) are detected in interplanetary space in association with solar flares and coronal mass ejections (CMEs). The magnetic connection between the observing spacecraft and the solar active region (AR) source of the event is a key parameter in determining whether SEPs are observed and the particle event's properties. Aims. We investigate whether an east-west asymmetry in the detection of SEP events is present in observations and discuss its possible link to corotation of magnetic flux tubes with the Sun. Methods. We used a published dataset of 239 CMEs recorded between 2006 and 2017 and having source regions both on the Sun's front and far sides as seen from Earth. We produced distributions of occurrence of in-situ SEP intensity enhancements associated with the CME events, versus \Delta\phi, the longitudinal separation between source active region and spacecraft magnetic footpoint based on the nominal Parker spiral. We focused on protons of energy >10 MeV measured by STEREO A, STEREO B and GOES at 1 au. We also considered occurrences of 71-112 keV electron events detected by MESSENGER between 0.31 and 0.47 au. Results. We find an east-west asymmetry with respect to the best magnetic connection (\Delta\phi=0) in the detection of >10 MeV proton events and of 71-112 keV electron events. For protons, observers for which the source AR is on the east side of the spacecraft footpoint and not well connected (-180<\Delta\phi<-40) are 93% more likely to detect an SEP event compared to observers with +40<\Delta\phi<+180. The asymmetry may be a signature of corotation of magnetic flux tubes with the Sun, since for events with \Delta\phi<0 corotation sweeps particle-filled flux tubes towards the observing spacecraft, while for \Delta\phi>0 it takes them away from it. Alternatively it may be related to asymmetric acceleration or propagation effects.