Measurements of low-energy, re-entrant albedo protons by the HEPD-01 space-borne detector

Cosmic rays' interactions with the residual atmosphere surrounding the Earth produce a variety of particles, like electrons, positrons, protons, anti -protons, and Helium nuclei that can be observed below the local geomagnetic cutoff. In this work, we present new measurements of downward -going, albedo proton fluxes with kinetic energy in the range similar to 40- similar to 250 MeV, performed by the High -Energy Particle Detector (HEPD-01) on board of the China Seismo-Electromagnetic Satellite - CSES-01 - at an altitude of similar to 500 km. Employing a dedicated trajectory -tracing simulation routine, the protons collected by HEPD-01 are classified into quasi -trapped (QT), long lifetime ( & Agrave;10 s) particles concentrating in the equatorial region of the Earth, and un-trapped (UT), distributed at all latitudes; the latter includes both precipitating short lifetime particles (UT S ) and pseudotrapped long lifetime (UT L ) populations, abundant in the so-called penumbra regions. The temporal trend of re-entrant protons between 2018 and 2022 is also reported, assessing the stability of such population during the data -taking period of HEPD-01; this highlights their independence from the long-term modulating effect of the solar activity.