Statistical Properties of Solar Wind Parameters Driving Relativistic Electron Precipitation

We analyze the solar wind (SW) properties associated with relativistic electron precipitation (REP) observed at low-Earth orbit (LEO). A statistical analysis is performed on the SW associated with ~7,000 REP events likely driven by wave-particle interactions. We analyze OMNI data to quantify the SW properties prior to REP and reveal temporal patterns that are favorable for electron precipitation. Compared to typical values, SW associated with REP typically exhibits a stronger North-South IMF (interplanetary magnetic field) Bzcomponent and higher plasma density, indicating dayside reconnection and compression. REP events observed at low L shell (L≲4), particularly from noon to post-midnight, are triggered by enhanced Bz and density. Dayside REP is associated with slightly faster SW, while dawnside REP coincides with high plasma density without strong Bz. A typical SW trend leading to REP exhibits enhanced density, magnetic field increase, and a Bz minimum before the REP observation. Through k-means clustering, we further identify three distinct SW temporal profiles. Dayside REP at high L shells is associated with ~500 km/s speeds, duskside REP is associated with enhanced SW density prior to REP, and REP over noon-to-dusk at low L shells is triggered by strong dayside reconnection. Furthermore, we found that REP events are more frequent during the declining phase of the solar cycle, with a 6-month periodicity in occurrence rate. The findings in this study are critical to establishing a relationship between SW and REP, enabling future modeling efforts to predict REP from SW observations.