The first High-Energy Particle Detector (HEPD-01) covered the highest-energy region of sensitivity among the scientific payloads onboard the CSES-01 satellite, measuring electrons (3-100 MeV), protons and light nuclei (30-300 MeV/n) up to Oxygen.
During its continuous data-taking, HEPD-01 proved itself capable of studying a plethora of natural phenomena. The instrument measured the spectrum of cosmic-ray protons between 40 and 250 MeV (Figure 1), following their time evolution up to 2022 (Figure 2). It was able to gain valuable information from the October 28, 2021 Ground Level Enhancement, regarding both the spectrum (Figure 3) and the time of arrival of protons up to a few hundreds MeV (Figure 4). Furthermore, HEPD-01 reported good observations of rearrangements/variations of low-energy populations inside the Earth’s magnetosphere during the geomagnetic storms of August 2018 (Figure 5) and May 2021 (Figure 6).
Figure 1: Large panels: galactic proton spectra as a function of energy measured by HEPD-01 in three consecutive periods between August 2018 and January 2020. Narrow panels: ratios between HEPD-01 data and HelMod model, as a function of energy.
Figure 2: Evolution of HEPD-01 galactic cosmic-ray protons as a function of energy and time between 2018 and 2022.
Figure 3: The pure, energy-extended event-integrated solar proton spectrum during the October 28, 2021 Ground Level Enhancement, as obtained by combining data from HEPD-01, ACE, SOHO/EPHIN, and SOHO/ERNE.
Figure 4: Onset times of the October 28, 2021 solar event versus 1/velocity, using HEPD-01, EPHIN and ERNE proton data.
Figure 5: Top three panels: trigger rates for three different HEPD-01 configurations during the August 26, 2018 geomagnetic storm. Bottom panel: time evolution of the Dst index.
Figure 6. Top two panels: time profiles of the solar wind dynamic pressure and position of the magnetopause standoff distance during the May 12, 2021 geomagnetic storm. Bottom panel: HEPD-01 rates of > 4.5 MeV particles over the same period.
The physics of the Earth’s inner radiation belt was investigated and an estimation of the trapped proton populations was given as a function of energy, pitch angle and L-shell in the 2018-2021 period (Figure 7), showing good agreement with the NASA Ap9 model. HEPD-01 analyzed the trajectories of < 250 MeV re-entrant albedo protons at Low-Earth Orbit (Figure 8), assessing the stability of these populations over time and validating the results obtained in the past by PAMELA and AMS-01 (Figure 9). More recently, by mapping the total particle flux measured within the South Atlantic Anomaly, HEPD-01 observed a geographical shift of its center during the 2018–2022 period, as shown in Figure 10.
HEPD-01 was also capable of detecting some powerful Gamma-Ray Bursts as an excess of low-energy electrons (by product of the photon interaction with the upper layers of the detector) seconds after the occurrence of these astrophysical events (Figure 11).
Finally, from a more technical point of view, HEPD-01 explored the application of deep learning algorithms to accompany classical event reconstruction techniques.
Figure 7. South Atlantic Anomaly proton fluxes as a function of energy (top panels), local pitch angle (middle panels) and L-shell (bottom panels) obtained by HEPD-01 between August 2018 and December 2020.
Figure 8. Generation (left panels) and absorption (right panels) points for quasi-trapped (upper panels, blue), untrapped short (middle panels, red) and untrapped long protons (bottom panels, green).
Figure 9. Comparison between total albedo proton flux measured outside the South Atlantic Anomaly by HEPD-01 (blue), PAMELA (red) and AMS-01 (black) in two regions of magnetic latitude.
Figure 10. South Atlantic Anomaly ellipse center coordinates for different time periods, semiorbit conditions, and energy thresholds within the analyzed epoch 2018-2022.
Figure 11. Time-resolved and detection profiles for the 12 Gamma-Ray Bursts observed by HEPD-01.
