3819 20190123122023.0 CTALK-2019-0027 Daglis, I. A. Long-lived relativistic electron flux enhancement during a weak geomagnetic storm 2019 2019-04-08 The relatively weak magnetic storm in April 2017, with SymH reaching only -50nT, resulted in a two orders of magnitude enhancement of relativistic and ultra relativistic electrons. This impressive enhancement is comparable to the St. Patrick’s event in 2015, an extreme storm with SymH reaching -235nT. The April 2017 enhancement appeared at energies up to 10 MeV, and was long-lived, lasting for more than 20 days. Nevertheless, the enhancement of super-relativistic electrons (6 MeV) was not recorded by geosynchronous spacecraft, which collect most of our space weather alert data. By analyzing radial profiles of PSD we show that the enhancement of relativistic and ultra–relativistic electrons is caused by different subsequent mechanisms: first, a series of intense substorms provided seed electrons and drove the growth of whistler mode chorus waves; second, the chorus waves accelerated the seed electron population to relativistic energies; third, the relativistic electrons were further accelerated to ultra–relativistic energies through inward diffusion driven by Pc5 ULF waves, which persisted throughout the magnetic storm duration. STCE Katsavrias, Ch. Sandberg, I. Podladchikova, O. Li, W. 3 , C. Papadimitriou3 , C. Tsironis2,7and S. Aminalragia-Giamini3 European Gephysical Union General Assembly, Vienna, Austria elena.podladchikova@observatoire.be Contributed CTALKCONT