The High-energy Particle Detector (HEPD) on board the China Seismo-Electromagnetic Satellite (CSES-01)—launched on 2018 February 2—is a light and compact payload suitable for measuring electrons (3–100 MeV), protons (30–250 MeV), and light nuclei (up to a few hundreds of MeV). The Sun-synchronous orbit and large acceptance allow HEPD to measure cosmic-ray particles near the ±65° latitude limit for a fair amount of time per day. In this work, three semiannual galactic hydrogen energy spectra between ∼40 and 250 MeV are presented, including a comparison with theoretical spectra from HelMod, a 2D Monte Carlo model developed to simulate the solar modulation of cosmic rays throughout the heliosphere. To our knowledge, these are the first hydrogen energy spectra below 250 MeV measured at 1 au between 2018 and 2020.

On 25 August 2018, a G3-class geomagnetic storm reached the Earth’s magnetosphere, causing a transient rearrangement of the charged particle environment around the planet, which was detected by the High-Energy Particle Detector (HEPD) on board the China Seismo-Electromagnetic Satellite (CSES-01). We found that the count rates of electrons in the MeV range were characterized by a depletion during the storm’s main phase and a clear enhancement during the recovery caused by large substorm activity, with the key role played by auroral processes mapped into the outer belt. A post-storm rate increase was localized at L-shells immediately above ∼3 and mostly driven by non-adiabatic local acceleration caused by possible resonant interaction with low-frequency magnetospheric waves.

The High-Energy Particle Detector (HEPD) on board the China Seismo-Electromagnetic Satellite (CSES-01) was launched in February 2018, with a foreseen mission lifetime of over 5 years. It is providing crucial new insight in the physical dynamics of the radiation belts in the Earth’s magnetosphere, in particular in the South Atlantic Anomaly (SAA). In this work, proton data from HEPD in the 40 MeV–250 MeV energy range, collected inside the SAA during the period between August 2018 and December 2020, are presented and compared with the up-to-date AP9 model by NASA. These are the first results on SAA protons at Low-Earth Orbit during the minimum activity phase between the 24th and the 25th solar cycles below 250 MeV. They enable an extensive testing and validation of current theoretical and empirical models aimed at predictions of temporal changes in this critical region of space. HEPD is advancing the observations collected by the PAMELA space experiment and NASA Van Allen Probe during the last 15 years through the 23rd and 24th solar cycles.

Time-dependent energy spectra of galactic cosmic rays (GCRs) carry crucial information regarding their origin and propagation throughout the interstellar environment. When observed at the Earth, after traversing the interplanetary medium, such spectra are heavily affected by the solar wind and the embedded solar magnetic field permeating the inner sectors of the heliosphere. The activity of the Sun changes significantly over an 11 yr solar cycle—and so does the effect on cosmic particles; this translates into a phenomenon called solar modulation. Moreover, GCR spectra during different epochs of solar activity provide invaluable information for a complete understanding of the plethora of mechanisms taking place in various layers of the Sun’s atmosphere and how they evolve over time. The High-Energy Particle Detector (HEPD-01) has been continuously collecting data since 2018 August, during the quiet phase between solar cycles 24 and 25; the activity of the Sun is slowly but steadily rising and is expected to peak around 2025/2026. In this paper, we present the first spectra for ∼50–250 MeV galactic protons measured by the HEPD-01 instrument—placed on board the CSES-01 satellite—from 2018 August to 2022 March over a one-Carrington-rotation time basis. Such data are compared to the ones from other spaceborne experiments, present (e.g., EPHIN, Parker Solar Probe) and past (PAMELA), and to a state-of-the-art three-dimensional model describing the GCRs propagation through the heliosphere.

On 12 May 2021 the interplanetary doppelgänger of the 9 May 2021 coronal mass ejection impacted the Earth's magnetosphere, giving rise to a strong geomagnetic storm. This paper discusses the evolution of the various events linking the solar activity to the Earth's ionosphere with special focus on the effects observed in the circumterrestrial environment. We investigate the propagation of the interplanetary coronal mass ejection and its interaction with the magnetosphere—ionosphere system in terms of both magnetospheric current systems and particle redistribution, by jointly analyzing data from interplanetary, magnetospheric, and low Earth orbiting satellites. The principal magnetospheric current system activated during the different phases of the geomagnetic storm was correctly identified through the direct comparison between geosynchronous orbit observations and model predictions. From the particle point of view, we have found that the primary impact of the storm development is a net and rapid loss of relativistic electrons from the entire outer radiation belt. Our analysis shows no evidence for any short‐term recovery to pre‐storm levels during the days following the main phase. Storm effects also included a small Forbush decrease driven by the interplay between the interplanetary shock and subsequent magnetic cloud arrival.