3549 20200904191428.0 10.1029/2019JA026743 DOI SCART-2018-0051 Katsavrias, Ch Highly relativistic electron flux enhancement during the weak geomagnetic storm of April–May 2017 2019 We report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with Sym − Hmin ≈ −50nT, resulted in a relativistic and ultra–relativistic electron enhancement of two orders of magnitude similar to the St.Patrick’s event of 2015, an extreme storm with Sym − Hmin ≈ −235nT. This enhancement appeared at energies up to ≈ 8 MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By analyzing radial profiles of PSD we show that the enhancement of relativistic and ultra–relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21–25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra–relativistic energies. STCE Long–lasting multi–MeV electron enhancement during a period of a relatively weak geomagnetic storm. Electron seed population was accelerated to relativistic energies by the enhanced chorus waves. Relativistic electrons were further accelerated up to 8 MeV by inward diffusion. Sandberg, I Li, W Podladchikova, 0 Daglis, I Papadimitriou, C Aminalragia-Giamini, S Journal Geophysical Research Volume 124 Issue 6 p 4402-4413 elena.podladchikova@observatoire.be https://doi.org/10.1029/2019JA026743 published in REFERD