The General Antiparticle Spectrometer (GAPS) is an Antarctic stratospheric balloon mission designed to provide unmatched sensitivity to low-energy (<0.25 GeV/n) cosmic-ray antiprotons, antideuterons, and antihelium nuclei as signatures of dark matter. The distinctive GAPS particle identification technique relies on measuring the energy loss along the track of an incoming antinucleus as it slows down and is captured into an exotic atom, and then detecting the de-excitation X-rays and the nuclear annihilation products. This measurement is realized using a Tracker composed of more than 1000 custom silicon strip detectors and a plastic scintillator time-of-flight (TOF) system instrumenting more than 40m^2. Together, these subsystems provide the velocity and energy resolution, stopping power, particle tracking, and X-ray identification necessary to distinguish rare antinucleus signals from the abundant positive-nucleus backgrounds, all within the constraints of a high-altitude mission. A multi-loop capillary heat pipe system has been developed to maintain the tracker operating temperature with significant mass and power savings over a conventional pump-based system. The first GAPS science payload flew for 25 days during the 2025/26 NASA Antarctic balloon campaign. We detail the design, integration, and commissioning of the payload prior to flight.

The accurate simulation of sub-GeV particle detectors is essential for interpreting experimental data and optimizing detector design. This work identifies and addresses several critical aspects in modeling such detectors, taking as a case study the High-Energy Particle Detector (HEPD-02), a space-borne instrument developed within the CSES-02 mission to measure electrons in the ∼3–100 MeV range, protons and light nuclei in the ∼30–200 MeV/n. The HEPD-02 instrument consists of a silicon tracker, plastic and LYSO scintillator calorimeters, and anticoincidence systems, making it a representative example of a complex low-energy particle detector operating in Low Earth Orbit. Key challenges arise from replicating intricate detector geometries derived from CAD models, selecting appropriate hadronic physics lists for low-energy interactions, and accurately describing the detector response—particularly quenching effects in scintillators and digitization in solid-state tracking planes. Particular attention is given to three critical aspects: the precise CAD-level geometry implementation, the impact of hadronic physics models on the detector response, and the parameterization of scintillation quenching. In this study, we present original solutions to these challenges and provide data–MC comparisons using data from HEPD-02 beam tests.

The China Seismo-Electromagnetic Satellite (CSES) mission is a joint China-Italy initiative focused on investigating Earth’s geophysical environment through non-imaging remote sensing from space. Its primary objective is to establish a constellation of satellites capable of continuously monitoring global electromagnetic fields, particle fluxes, and plasma parameters within the iono-magnetospheric system. Goals of this space program are the investigation of possible lithosphere-atmosphere-ionosphere-magnetosphere coupling mechanisms and their role in inducing perturbations in the upper ionosphere and the lower boundary of the radiation belts. Additionally, CSES contributes to space weather studies, including investigations of the magnetosphere, heliosphere, and galactic cosmic rays. Each satellite in the constellation carries multiple instruments to measure charged particles, electromagnetic fields, and plasma properties. CSES-01, launched in February 2018, remains operational. The second satellite, CSES-02, was launched on June 14, 2025, marking the transition to a multi-point observation capability. Italy contributed the High-Energy Particle Detector to both missions—HEPD-01 for CSES-01 and the upgraded HEPD-02 for CSES-02. These detectors are designed for precise measurements of electrons, protons, light nuclei, and transient gamma rays in the multi-MeV range. HEPD-02 significantly enhances energy resolution and extends the detection range when combined with the lower-energy instruments aboard CSES-02, enabling continuous coverage from 100 keV to 200 MeV. This article presents the scientific goals of the CSES program, with a focus on the role of HEPD-02 in energetic particle studies, both as a standalone detector and in synergy with the mission’s multi-instrument, multi-satellite framework.

This paper introduces TROPix, a parametric simulation tool designed to reproduce the output of silicon pixel detectors. Developed for the Direction Detector of the HEPD-02 detector on the second China Seismo-Electromagnetic Satellite (CSES-02), TROPix utilizes the GEANT4 toolkit for energy deposition and a parametric approach for charge generation and diffusion. The simulation process incorporates thermal noise and digitization, with all parameters fine-tuned using experimental beam-test data. Although TROPix is tuned for HEPD-02 data, it can also be adapted for use with other detectors employing pixel sensors, providing a versatile and computationally efficient solution for numerous applications. Results for Monolithic Active Pixel Sensors (MAPS) based on ALTAI chips are presented, highlighting TROPix’s ability to replicate the property of pixel clusters, defined as the number of pixels fired by the particle passage. These characteristics comprehend the dependencies on charged particle deposited energy and incident angle, as well as the typical shapes of clusters. Validation using beam test data acquired with ALTAI sensors demonstrates that TROPix reproduces the cluster size dependence on deposited energy, with agreement within 20%. For cluster shape occurrences, the simulated results align with the data within 25% of the total uncertainty.

During their propagation in the heliosphere, interplanetary coronal mass ejections (ICMEs) interact with galactic cosmic ray (GCR) particles, modifying their spectrum and driving anisotropies. We analyze the first large Forbush decrease (FD) of Solar Cycle 25 on 2021 November 3–5 by using multipoint in situ observations and neutron monitors to study the association between FD characteristics and ICME. We use the Grad–Shafranov reconstruction to infer the magnetic field configuration of the ICME. We model the neutron monitor response through primary spectrum and anisotropy. The primary spectrum is parameterized with the force-field approximation and the anisotropy is modeled through a spherical harmonic expansion. We optimize the model parameters during the FD by using ground-based observations provided by the worldwide neutron-monitor network. The model’s results are compared with space-based measurements of the differential proton flux measured by the HEPD-01 detector on board the CSES-01 satellite and of the integral counts of both the High-Energy Particle Detector (HEPD-01) and the High Energy Telescope on board the Solar Orbiter. Anisotropy develops during the ICME passage, within the magnetic flux rope (MFR) and is found to be bidirectional. The force-field parameterization of the primary GCR fluxes based on ground-based measurements is found to be in very good agreement with spacecraft observations in the sub-GeV range. The GCR anisotropy obtained by fitting the model to ground-based observations is consistent with interplanetary magnetic field observations. The results suggest that the local magnetic field has a substantial axial component that is aligned to the MFR axis, and determines the GCR anisotropy at the typical neutron monitor energies.

The High Energy Particle Detector HEPD-02 is primarily devoted to observe fluxes of cosmic-ray electrons, protons and light nuclei, with kinetic energies in the range from less than 10 MeV to few hundreds MeV, either incoming or trapped in the terrestrial magnetosphere. HEPD-02 will be hosted on board the China Seismo-Electromagnetic Satellite CSES-02. The CSES mission, coordinated by China National Space Administration (CNSA) and Italian Space Agency (ASI), aims at developing a series of satellites for the study of the near-Earth environment, by means of electromagnetic, ionospheric, magnetospheric and cosmic-ray observations. HEPD-02 is a state-of-the-art instrument for measurement of energy and arrival direction and for identification of incoming cosmic particles; it is formed by a tower of superimposed plastic and crystal scintillator layers, read-out by photo-multiplier tubes (PMTs), with a direction detector composed of monolithic active pixel sensors. This paper describes the design, structure and operation of the power supply component of HEPD-02, which comprises a low-voltage unit for power distribution to other sub-systems and a high-voltage unit for generation of PMT bias. The HEPD-02 power supply features specific and optimized design solutions matching the demanding requirements for employment on board the CSES-02 satellite: it is conceived to guarantee at least 6 years of continuous operation, delivering stable voltages with low susceptibility to electromagnetic interference, in a harsh environment (characterized by relatively intense ionizing radiation, wide temperature excursions, absence of heat dissipation by air convection), after sustaining strong mechanical stresses at launch, at the same time complying with strict limits in mass, dimensions and available power.

The South Atlantic Anomaly (SAA) is a region where the Earth's magnetic field is lower by a factor of 2-3 with respect to the mean field value, resulting in a remarkably higher density of trapped charged particles. Updated surveys of this unique region are of capital importance to track the evolution of the geomagnetic field and improve models of the Earth's ionosphere. The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite ( CSES-01 ), operational between the 24th and 25th Solar cycles, is one of the few instruments able to probe the radiation environment of the SAA at high energies. Sensitive to > 1 MeV electrons and > 10 MeV protons, HEPD-01 counters can reliably measure rates as large as several kHz/cm(2), overcoming SAA saturation issues that often affect spaceborne detectors. By mapping the total particle flux observed within the SAA, we report here on the geographical shift of its center during the 2018-2022 period and deliver HEPD-01's estimate of the associated integral particle flux, which validates NASA's AP9-AE9 model. The present assessment of the SAA drift, besides featuring a notable accuracy, is unique in its time coverage, altitude, and energy range. In addition, our flux estimate covers the proton energy range above similar to 200 MeV where the AP9 model relies on extrapolated data. The present results enrich the available observations of the radiation environment within the SAA, which are crucial for the investigation of the temporal evolution of the geomagnetic field, for models of geomagnetically trapped particles, and for the investigation of perturbations generated by space weather events. Knowledge of the SAA radiation environment is also relevant for spacecraft system design and to safeguard human crew health.

This article describes the innovative system performing the trigger and the readout of the photomultiplier tubes (PMTs) for the high-energy particle detector (HEPD) onboard the second satellite of the China Seismo Electromagnetic Satellite (CSES) mission. The second HEPD is designed to measure cosmic rays covering an energy spectrum ranging from a few megaelectronvolts to several hundreds of megaelectronvolts. This high-precision detector consists of different subsystems: a solid-state tracker, a segmented trigger, a calorimeter composed of a tower of plastic scintillators, and two layers of lutetium-yttrium oxyorthosilicate (LYSO) crystals, along with a containment detector. The data acquisition (DAQ) process for the trigger, calorimeter, and containment detector is carried out by a single electronic board, relying on two Weeroc CITIROC application-specific integrated circuits (ASICs), which are being utilized in space for the first time. This board also generates and manages trigger signals for the entire detector. It effectively captures signals with distinct timing characteristics from plastic scintillators and inorganic crystals. Given the wide range of particle fluxes encountered during CSES's orbit, adaptability of the trigger generation system becomes crucial, and DAQ is optimized to ensure consistent energy spectra measurement with a substantial duty cycle. The trigger system of the second HEPD implements concurrent trigger patterns and the ability to select DAQ strategies based on orbital zones and the presence of impulsive events. Each trigger configuration is designed to meet scientific demands concerning the field of view and the characteristics of the particles reaching the detector, with prescaling settings fine-tuned accordingly. In addition to monitoring particle bursts, trigger configurations specific to gamma rays will be tracked in 5-ms intervals to measure photon fluxes in the energy range from 2 to 20 MeV and provide sensitivity for impulsive events, such as gamma-ray bursts (GRBs). This article provides a comprehensive account of the design criteria, the architectural choices specifically adapted for space applications, and the original trigger management strategy. Additionally, this article presents an in-depth analysis of the performance of the trigger system and comprehensive results from laboratory and beam tests conducted on the qualification and flight models of second HEPD.

The High-Energy Particle Detector-02 (HEPD-02), onboard the second China Seismo-Electromagnetic Satellite (CSES-02), introduces-for the first time in a space application-a tracker designed entirely using monolithic active pixel sensors (MAPS). The MAPS selected for the HEPD-02 tracker were developed as part of CERN's R&D efforts for the realization of the ALICE Inner Tracking System (ITS). The readout system has been optimized, significantly reducing power consumption. The performance of the MAPS has been validated for HEPD-02's scientific objectives through beam tests using particle and ion beams in the tens to hundreds of MeV energy range. This article details the design principles and structure of the tracker. The integration procedure is outlined step by step, with a discussion of the challenges encountered at each stage and the strategies adopted to address them. The description concludes with an analysis of production yields and the criteria used for selecting components for the flight and qualification models. In addition, an overview of the space qualification activities is provided. The launch of CSES-02 is scheduled for mid-2025. Following an initial in-orbit commissioning phase, the technology will achieve technology readiness level 9, making it available for future space missions.

The High-Energy Particle Detector (HEPD-02) is one of the scientific payloads of the China Seismo-Electromagnetic Satellite (CSES-02). The HEPD-02’s main purpose is to characterize the particle environment in the Earth’s vicinity, identifying sudden changes in the fluxes and correlating them with solar and terrestrial phenomena. Additionally, HEPD-02 also has capabilities in detecting Gamma-Ray Bursts. At the core of HEPD-02, a tower of scintillation counters made of plastic and LYSO crystals is able to recognize electrons in the range between 3 and 100 MeV, protons and nuclei between 30 and 200 MeV/n. Plastic scintillators covering the calorimeter on five sides allow to reject particles entering from the top and not completely absorbed within its volume. In this work, the design of the HEPD-02 is reviewed in comparison to its predecessor, HEPD-01, highlighting the innovations of the new design. The design of each scintillation counter type has been fully validated through a campaign of prototype realization, testing, and characterization. The production of the scintillation counters, including the PMT selection process, is also discussed. Finally, the performance of the counters is compared with simulations, showing an agreement of within 20% with the expected performance, thereby meeting expectations.

The intricate behavior of particle acceleration and transport mechanisms complicates the overall efforts in formulating a comprehensive understanding of solar energetic particle (SEP) events; these efforts include observations of low-energy particles (from tens of keV to hundreds of MeV) by space-borne instruments and measurements by the ground-based neutron monitors of the secondary particles generated in the Earth atmosphere by SEPs in the GeV range. Numerous space-borne missions provided good data on the nature/characteristics of these solar particles in past solar cycles, but more recently-concurrently with the rise toward the maximum of solar cycle 25-the High-Energy Particle Detector (HEPD-01) proved to be well suited for the study of solar physics and space weather. Its nominal 30-300 MeV energy range for protons can enlarge the detection capabilities of solar particles at low Earth orbit, closer to the injection limit of many SEP events. In this work, we characterize three SEP events within the first six months of 2022 through spectral and velocity dispersion analysis, assessing the response of HEPD-01 to >M1 events.

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 ∼40–∼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 ∼500 km. Employing a dedicated trajectory-tracing simulation routine, the protons collected by HEPD-01 are classified into quasi-trapped (QT), long lifetime (≳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 (UTS) and pseudo-trapped long lifetime (UTL) 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.

The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite, located on a Sun-synchronous orbit at 500 km of altitude with an inclination of 97 degrees, features a dedicated logic counting low-energy event rates, which proved sensitive to intense Gamma-Ray Burst (GRB). The present work reports a comprehensive analysis of signals induced by GRBs in the event-rate data collected between 2018 August and 2022 June. After accurately modeling the background rate as observed in different passages of the satellite over the same geographical area, we detected significant deviations to be compared with observations of GRB candidates from other observatories. The analysis revealed 12 statistically significant excesses, that have been associated with GRB 181222B, GRB 190114C, GRB 190129B, GRB 190305A, GRB 190928A, GRB 200412B, GRB 200422A, GRB 200826B, GRB 201009A, GRB 210702A, GRB 211211A, and GRB 220624A. We report light curves for 0.3-50 MeV photons, comparing them with findings from other space telescopes. The catalog of observations is published, complete of GRB observation time, duration, integrated counts, and fluence.

In this paper we report the detection of five strong gamma-ray bursts (GRBs) by the High-Energy Particle Detector (HEPD-01) mounted on board the China Seismo-Electromagnetic Satellite, operational since 2018 on a Sun-synchronous polar orbit at a ∼507 km altitude and 97° inclination. HEPD-01 was designed to detect high-energy electrons in the energy range 3–100 MeV, protons in the range 30–300 MeV, and light nuclei in the range 30–300 MeV n −1 . Nonetheless, Monte Carlo simulations have shown HEPD-01 is sensitive to gamma-ray photons in the energy range 300 keV–50 MeV, even if with a moderate effective area above ∼5 MeV. A dedicated time correlation analysis between GRBs reported in literature and signals from a set of HEPD-01 trigger configuration masks has confirmed the anticipated detector sensitivity to high-energy photons. A comparison between the simultaneous time profiles of HEPD-01 electron fluxes and photons from GRB190114C, GRB190305A, GRB190928A, GRB200826B, and GRB211211A has shown a remarkable similarity, in spite of the different energy ranges. The high-energy response, with peak sensitivity at about 2 MeV, and moderate effective area of the detector in the actual flight configuration explain why these five GRBs, characterized by a fluence above ∼3 × 10 −5 erg cm −2 in the energy interval 300 keV–50 MeV, have been detected.

Earth’s magnetosphere is part of a dynamic, interconnected system which responds to solar, planetary, and interstellar conditions. It encloses two belts of magnetically trapped, energetic charged particles, which constitute a well-known hazard to spacecraft systems and crews, significantly constraining human activities in space. Despite notable improvements made in the latest decades, the modeling of the trapped environment is still incomplete, with largest uncertainties affecting the high-energy fluxes (> 50 MeV) in the inner regions and in the South Atlantic Anomaly (SAA). Additionally, space weather events increase the spatial and composition variability of the magnetospheric radiation belts, e.g. during geomagnetic storms. The Italian High-Energy Particle Detector (HEPD-01), on a low-Earth orbit since February 2018, is providing crucial new insight in the physical dynamics of the radiation belts, thus enabling an extensive testing and validation of current theoretical and empirical models of the near-Earth environment (e.g. AP9 and AE9). In this contribution, a review of magnetospheric and space weather observations by HEPD-01 is presented, including the study of some major geomagnetic storms, such as the G3-class ones of August 2018 and May 2021, the re-entrant lepton spectrum between 20 and 100 MeV and the proton fluxes inside the SAA in the 40-250 MeV energy range.

High-energy, long gamma-ray bursts (GRBs) can be generated by the core collapse of massive stars at the end of their lives. When they happen in the close-by universe they can be exceptionally bright, as seen from the Earth in the case of the recent, giant, long-lasting GRB221009A. GRB221009A was produced by a collapsing star with a redshift of 0.152: this event was observed by many gamma-ray space experiments, which also detected an extraordinary long gamma-ray afterglow. The exceptionally large fluence of the prompt emission of about 0.013 erg cm −2 illuminated a large geographical region centered on India and including Europe and Asia. We report in this paper the observation of sudden electron flux changes correlated with GRB221009A and measured by the HEPP-L charged particle detector on board the China Seismo-Electromagnetic Satellite, which was orbiting over Europe at the time of the GRB event. The time structure of the observed electron flux closely matches the very distinctive time dependence of the photon flux associated with the main part of the emission at around 13:20 UTC on 2022 October 9. To test the origin of these signals, we set up a simplified simulation of one HEPP-L subdetector: the results of this analysis suggest that the signals observed are mostly due to electrons created within the aluminum collimator surrounding the silicon detector, providing real-time monitoring of the very intense photon fluxes. We discuss the implications of this observation for existing and forthcoming particle detectors on low Earth orbits.

Abstract In this work we present the High‐Energy Particle Detector (HEPD‐01) observations of proton fluxes from space during the 28 October 2021 solar energetic particle event, which produced a ground‐level enhancement on Earth. The event was associated with the major, long‐duration X1‐class flare and the concomitant coronal mass ejection (CME) that erupted from the Active Region 12887. This is the first direct measurement from space of solar particles emitted during the current solar cycle, recorded by a single instrument in the energy range from ∼50 MeV/n up to ∼250 MeV/n. We have performed a Weibull‐modeled spectral analysis of the energy spectrum in the wide energy range 300 keV–250 MeV, obtained from combination of HEPD‐01 proton measurements with the ones from ACE/ULEIS, SOHO/EPHIN, and SOHO/ERNE. The good agreement between data and model, also corroborated by a comparison with other spectral shapes commonly used in these studies, suggests that particles could have possibly been accelerated out from the ambient corona through the contribution of stochastic acceleration at the CME‐driven shock, even if the presence of seed populations influencing spectral shape could not be excluded. Finally, a Solar Proton Release time of 16:01 UTC ± 13 min and a magnetic path‐length of L = 1.32 ± 0.24 AU have been obtained, in agreement with previous results for this event. We remark that new and precise data on protons in the tens/hundreds MeV energy range—like the one provided by HEPD‐01—could shed more light on particle acceleration as well as provide a reliable parametrization of solar energetic particle spectra for Space Weather purposes.

In this study, a state-of-the-art three-dimensional (3D) drift model is used to distinguish the significant role played by particle drift in modulating galactic protons in the past two successive unusually quiet solar minima. This is done by comparing the model computations to available observations from both PAMELA and HEPD01, respectively taken in the A < 0 and A > 0 magnetic field cycles. For this abridged report, it is illustrated to what extent particle drifts occurred during the two minima. Because of these drift effects, the proton flux at lower energy is found as predicted to be even higher during the A > 0 solar minimum period of 2020 than during the A < 0 solar minimum of 2009. As such, the record of the highest ever recorded GCRs at Earth set by PAMELA has now been surpassed

Earth’s atmosphere, whose ionization stability plays a fundamental role for the evolution and endurance of life, is exposed to the effect of cosmic explosions producing high energy Gamma-ray-bursts. Being able to abruptly increase the atmospheric ionization, they might deplete stratospheric ozone on a global scale. During the last decades, an average of more than one Gamma-ray-burst per day were recorded. Nevertheless, measurable effects on the ionosphere were rarely observed, in any case on its bottom-side (from about 60 km up to about 350 km of altitude). Here, we report evidence of an intense top-side (about 500 km) ionospheric perturbation induced by significant sudden ionospheric disturbance, and a large variation of the ionospheric electric field at 500 km, which are both correlated with the October 9, 2022 Gamma-ray-burst (GRB221009A). Gamma-ray bursts (GRBs) are known to have impact on Earth’s lower ionosphere, but GRB impacts on the upper ionosphere was not observed before. Here, the authors show strong electric field variation at 500 km in the ionosphere caused by GRB221009A.