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The first ground-level enhancement of the current Solar Cycle 25 occurred on 28 October 2021. It was observed by several space-borne and ground-based instruments, specifically neutron monitors. A moderate count-rate increase over the background was observed by high-altitude polar stations on the South Pole and Dome C stations at the Antarctic plateau. Most of the neutron monitors registered only marginal count-rate increases. Using detrended records and employing a method verified by direct space-borne measurements, we derive the rigidity spectra and angular distributions of the incoming solar protons in the vicinity of Earth. For the analysis, we employed a newly computed and parameterized neutron-monitor yield function. The rigidity spectra and anisotropy of solar protons were obtained in their time evolution throughout the event. A comparison with the Solar and Heliospheric Observatory/Energetic and Relativistic Nuclei and Electron (SOHO/ENRE) experiment data is also performed. We briefly discuss the results derived from our analysis.

Fluxes of solar energetic particles (SEPs), produced and accelerated in and in the vicinity of the Sun, are an important part of cosmic ray induced terrestrial effects such as ionizing radiation on the Earth’s orbit, which affects the exposure to radiation in space as well as the atmospheric ionization. Calculation of these effects requires the knowledge of the integral fluences of SEPs. High-energy solar particles are subject of special interest since they can significantly contribute to the total radiation dose and/or ionization. The main instrument to study the high-energy SEP events is a network of ground-based neutron monitors (NMs), used over the years to register a specific class of SEP, which is called ground-level enhancement (GLE) events. Up today, we possess records from 72 GLE events. Reconstruction of SEP integral and differential fluxes for GLE events using NM data is not an easy task, requires a careful and precise calculation of particle transport in the magnetosphere, atmosphere, and detector itself. In this work, we compare two methods of fluence reconstruction, “fast” and “full”, for the last three registered GLE events and additionally verify one of them using PAMELA experimental data.

The atmospheric cutoff, similarly to the geomagnetic cutoff, is the lower energy limit for cosmic ray particles that can reach a given location on the ground and be registered by a detector there, e.g., by a neutron monitor. It is caused by the decreasing intensity of a cosmic-ray cascade in the lower atmosphere. Although the geomagnetic cutoff is higher than atmospheric over the most of the Earth’s surface, the latter is dominant and therefore defines the neutron monitor count rate in the polar regions. The atmospheric cutoff decreases with the altitude, and this provides additional sensitivity of high-altitude polar neutron monitors to low-energy particles, mainly solar energetic protons during the so-called ground-level enhancement events. In this work, we quantitatively estimated the atmospheric cutoff energies for 21 polar neutron monitor stations in conditions without a significant solar energetic particle event. The cutoff value can be as low as about 300 MeV for protons (VOSTOK and DOMC/DOMB stations), which is notably lower than about 430 MeV at sea level. In addition to that, we estimated the worst case scenario of the strongest-ever observed ground-level enhancement event GLE#05, occurred on the 23rd of February, 1956, and showed that the atmospheric cutoff becomes as low as about 100 MeV. In other words, some neutron monitor stations can register particles with energies of even about 100 MeV during an exceptionally strong solar particle event. It is explained by the highly intensive and soft spectrum of the event in its early delayed phase.

Cosmic rays, high-energy subatomic particles of extraterrestrial origin, are systematically measured by space-borne and ground-based instruments. A specific interest is paid to high-energy ions accelerated during solar eruptions, so-called solar energetic particles. In order to build a comprehensive picture of their nature, it is important to fill the gap and inter-calibrate ground-based and space-borne instruments. Here, we focus on ground-based detectors, specifically neutron monitors, which form a global network and provide continuous recording of cosmic ray intensity and its variability, used also to register relativistic solar energetic particles. The count rate of each neutron monitor is determined by the geomagnetic and atmospheric cut-offs, both being functions of the location. Here, on the basis of Monte Carlo simulations with the PLANETOCOSMICS code and by the employment of a new verified neutron monitor yield function, we assessed the atmospheric cut-off as a function of the altitude, as well as for specific stations located in the polar region. The assessed in this study altitude profile of the atmospheric cut-off for primary cosmic rays builds the basis for the joint analysis of strong solar proton events with different instruments and allows one to clarify recent definitions and related discussions about the new sub-class of events, so-called sub-ground-level enhancements (sub-GLEs).

With the recent electronics upgrade of Antarctic neutron monitors (NMs) DOMC and DOMB in 2019, the Oulu cosmic ray station (Sodankylä Geophysical Observatory, Finland) receives a significantly larger amount of data than before. This has led to a need for an important upgrade of the configuration of servers working at the station. The new configuration has three types of servers: a web-server, a datamaster server and data acquisition machines. The web-server provides a user interface for services of the station: the main website, the GLE database and other services. The datamaster is the main server, which stores all data in raw files and a database. Data acquisition machines are computers that directly receive data from the instruments and send the files farther to the datamaster server. This work describes technical details of the cosmic ray station setup providing reliable and secure data acquisition, handling and publication.

DOMC and DOMB neutron monitors (NM) operate at the Concordia research station (Dome C on the Antarctic plateau, 75°06’S, 123°23’E, 3233 m a.s.l.) since 2015. Their high elevation and proximity to the geomagnetic pole provide low atmospheric and geomagnetic cutoffs and, therefore, the exceptionally high sensitivity to lowenergy cosmic rays. The instruments are the so-called mini neutron monitors with BF3-filled counter tubes. DOMC has the standard design with a lead neutron multiplier and DOMB is a so-called “bare” (lead-free) unit. We report on a recent upgrade of the electronics heads of these instruments. The new heads have a modular architecture, built upon a single-board computer Raspberry Pi. The upgrade increases the capabilities of the instruments in two aspects: (1) measurements, particularly, of cosmic ray multiplicity; (2) remote control and monitoring. The new electronic heads register each pulse from a detector, giving a timestamp with microsecond precision, which is crucial for multiplicity measurements. Many important parameters (e.g., high voltage, pulse detection thresholds) can be controlled and adjusted remotely with the new design. High computing power allows performing data processing on the fly. The upgrade increases the capability of DOMC and DOMB in cosmic ray measurements and improves control of the operation of the neutron monitors.

A pair of neutron monitors (NMs) is installed on the high Central Antarctic plateau, at the Concordia station (3200 m altitude) and measures the nucleonic component of nucleonic-muon-electromagnetic cascades induced by high-energy cosmic rays in the atmosphere. The installation includes two NMs: DOMC, a standard mini-NM, and a bare (lead-free) DOMB NM. The newly installed data acquisition (DAQ) system records individual pulses corresponding to mostly neutrons in the detector’s counting tube. Here we analyze different types of pulses and study the distribution of the waiting times between individual pulses as well as the pulse height, recorded by the DOMC NM during a quiet period of January 2020. The distribution appears doublepeaked with peaks corresponding to the frequency of individual atmospheric cascades and the intra-cascade variability, respectively. We discuss also the nature of different components contributing to the pulses and separation of the signal from noise. It is shown that the waiting-time distribution has distinguished timescales, >30 ms defined by the cosmic-ray induced atmospheric cascades, and <10 ms reflecting the intra-cascade variability. The new DAQ system allows one to study the development of the atmospheric cascade.

During Solar Cycle 23 16 ground-level enhancement events were registered by the global neutron monitor network. In this work we focus on the period with increased solar activity during late October – early November 2003 producing a sequence of three events, specifically on ground-level enhancement GLE 67 on 2 November 2003. On the basis of an analysis of neutron monitor and space-borne data we derived the spectra and pitch-angle distribution of high-energy solar particles with their dynamical evolution throughout the event. According to our analysis, the best fit of the spectral and angular properties of solar particles was obtained by a modified power-law rigidity spectrum and a double Gaussian, respectively. The derived angular distribution is consistent with the observations where an early count rate increase at Oulu neutron monitor with asymptotic viewing direction in the anti-Sun direction was registered. The quality of the fit and model constraints were assessed by a forward modeling. The event integrated particle fluence was derived using two different methods. The derived results are briefly discussed.

The newly published spectra of protons and helium over time directly measured in space by the Alpha Magnetic Spectrometer (AMS‐02) experiment for the period 2011–2017 provide a unique opportunity to calibrate ground‐based neutron monitors (NMs). Here, calibration of several stable sea level NMs (Inuvik, Apatity, Oulu, Newark, Moscow, Hermanus, and Athens) was performed using these spectra. Four modern NM yield functions were verified: Mi13 (Mishev et al., 2013, https://doi.org/10.1002/jgra.50325), Ma16 (Mangeard et al., 2016, https://doi.org/10.1002/2016JA023515), CM12 (Caballero‐Lopez & Moraal, 2012, https://doi.org/10.1029/2012JA017794), and CD00 (Clem & Dorman, 2000, https://doi.org/10.1023/A:1026508915269), on the basis of the cosmic ray spectra measured by AMS‐02. The Mi13 yield function was found to realistically represent the NM response to galactic cosmic rays. CM12 yield function leads to a small skew in the solar cycle dependence of the scaling factor. In contrast, Ma16 and CD00 yield functions tend to overestimate the NM sensitivity to low‐rigidity (<10 GV) cosmic rays. This effect may be important for an analysis of ground level enhancements, leading to a potential underestimate of fluxes of solar energetic particles as based on NM data. The Mi13 yield function is recommended for quantitative analyses of NM data, especially for ground level enhancements. The validity of the force field approximation was studied, and it was found that it fits well the directly measured proton spectra, within a few percent for periods of low to moderate activity and up to ≈10% for active periods. The results of this work strengthen and validate the method of the cosmic ray variability analysis based on the NM data and yield function formalism and improve its accuracy.

As a result of intense solar activity during the first 10 days of September, a ground level enhancement occurred on 10 September 2017. Here we computed the effective dose rates in the polar region at several altitudes during the event using the derived rigidity spectra of the energetic solar protons. The contribution of different populations of energetic particles, namely, galactic cosmic rays and solar protons, to the exposure is explicitly considered and compared. We also assessed the exposure of a crew members/passengers to radiation at different locations and at several cruise flight altitudes and calculated the received doses for two typical intercontinental flights. The estimated received dose during a high latitude, 40 kft, ∼10‐hr flight is ∼100 μSv.

The method of assessment of galactic cosmic rays (GCR) variability over different timescales, using energy-integrating ground-based detectors such as a neutron monitor and cosmogenic isotopes ¹⁰Be and ¹⁴C stored in natural archives is revisited here. The effective yield functions for cosmogenic ¹⁴C (globally mixed in the atmosphere) and ¹⁰Be (realistically deposited in the polar region) are calculated and provided, in a tabulated form, in the supporting information. The effective energy of a detector is redefined so that the variability of the flux of GCR particles at this energy is equal to that of the detector’s count rate. The effective energy is found as 11–12 GeV/nucleon for the standard polar neutron monitor, and 6–7 GeV/nucleon and 5.5–6 GeV/nucleon for ¹⁴C and ¹⁰Be, respectively. New “calibration” relations between the force-field modulation potentials, based on different models of local interstellar spectra (LIS) are provided. While such relations are typically based on refitting the modeled cosmic ray spectra with a prescribed LIS model, the method introduced here straightforwardly accounts for the exact type of the detector used to assess the spectrum. The relations are given separately for ground-based neutron monitors and cosmogenic isotopes. This work allows for harmonization of different works related to variability of galactic cosmic ray flux in the vicinity of Earth, on long-term scale.

On the basis of neutron monitor data, we estimate the energy spectrum, anisotropy axis direction, and pitch-angle distribution of solar energetic particles during two major ground-level enhancements (GLE 59 on 14 July 2000 and GLE 70 on 13 December 2006). For the analysis we used a newly computed neutron monitor yield function. The method consists of several consecutive steps: definition of the asymptotic viewing cones of neutron monitor stations considered for the data analysis by computing the cosmic ray particle propagation in a model magnetosphere with the MAGNETOCOSMICS code, computing the neutron monitor model responses, and deriving the solar energetic particle characteristics on the basis of inverse problem solution. The pitch-angle distribution and rigidity spectrum of high-energy protons are obtained as a function of time in the course of ground-level enhancements. A comparison with previously reported results is performed and reasonable agreement is achieved. A discussion of the obtained results is included. An erratum to this article can be found at http://dx.doi.org/10.1007/s11207-016-0937-7

This report describes investigated results on the concept of neutron monitors applicable to Japanese crews in the spacecraft such as JEM (Japanese Experiment Module) during spaceflight, in order to evaluate the dose equivalents or effective dose by neutron irradiation. The spectra of neutrons in the JEM were evaluated by use of the three dimensional Monte Carlo code system HERMES with the data of trapped proton model and the shielding model. The proton spectra were also calculated by use of the same method. The validity of the calculated spectra were introduced from the agreements of the evaluated spectra and those of measured in spacecrafts and satellites. Investigation was also performed on the neutron spectrometer available on the ground facilities, for the effectivity in the mixed field of neutrons and protons. The concept of neutron monitors applicable to the dose evaluation of space crews were presented on the basis of these investigated results., 日本人搭乗員の中性子被曝線量を精度よく把握することを目的として、捕捉陽子モデルとJEM(日本宇宙ステーション実験モジュール)などの宇宙船で想定される遮蔽モデルから発生が予想される船内の中性子スペクトルと、遮蔽により減衰する陽子のスペクトルを3次元モンテカルロコードシステムHERMESを用いて計算評価した。この計算値は調査で得られた実測値と比較して計算結果の妥当性を検証した。また、地上で使用実績のある中性子検出装置の調査検討を行い、それらが宇宙空間における陽子、中性子混在場でも使用可能な中性子スペクトル測定技術を用いていることを確認した。これらの検討結果に基づいて、搭乗員の中性子による被曝線量評価を目的とした中性子モニターに関する概念をまとめた。

For the study of space environment measurement apparatus, and the evaluation and analysis of its data, followings were done: (1) as a space neutron monitor, a study was made of the reduction of optical cross talk between scintillators, the anti-vibration measure for anode photomultiplier tube, and the development of neutron monitor for shuttle/Mir mission onboard; (2) gamma ray irradiation test, study was made of MFD (Manipulator Flight Demonstration) onboard ESEM and MDS-1 (Mission Demonstration test Saatellite-1) onboard sensor in order to acquire basic characteristics of micro Radiation Sensitive Field Effect Transistor (RADFET); (3) studies were conducted of adoption of BGO (Bi4GE3O12) as the materials for scintillator and downsizing of digital processing circuit for trial manufacturing of small size radiation measurement system; (4) studies were made of the measurement of Linear Energy Transfer (LET) distribution, observation of oxygen and nitrogen ions trapped in magnetic field, measurements of high energy molecule distribution, single event, charge potential etc. for evaluation and analysis of data by ADEOS (Advanced Earth Observing Satellite) onboard Technical Data Acquisition Equipment (TEDA); and (5) studies were conducted of space radiation model, incorporation of radiation belt particle extinction effect by direct collision with atmospheric particle into the model built., 新規宇宙環境計測装置の研究および宇宙環境計測データの評価・解析に関して、以下の項目を実施した。(1)宇宙用中性子モニタの研究では、シンチレータ間の光学的クロストークの低減化、マルチアノードフォトマルの耐振動対策の検討、シャトル/ミールミッション搭載用中性子モニタの開発を行った。(2)超小型放射線総吸収線量モニタ(RADFET)の基本特性の取得では、ガンマ線による照射試験、MFD(マニピュレータ飛行実証実験)搭載ESEM、MDS-1(民生部品・コンポーネント実証衛星)搭載用センサの検討を行った。(3)小型放射線計測装置の試作では、シンチレータの材料にBGO(Bi4GE3O12)の採用、デジタル処理回路の小型化などを検討した。(4)ADEOS(地球観測プラットフォーム技術衛星)搭載技術データ取得装置(TEDA)データの評価・解析では、線エネルギー付与(LET)分布の実測、磁場に捕捉されている酸素イオンと窒素イオンの観測、高エネルギー分子の分布、シングルイベントの実測値、帯電電位の実測値などを得た。(5)宇宙放射モデルの検討では、構築したモデルに大気粒子と直接衝突による放射線帯粒子消滅効果を盛りこんだ。

This report presents the results on 'The development of experimental model on neutron monitors in space, part 1' carried out under the contract with National Space Development Agency of Japan. Considering from the studies and results obtained before, development items and elements with high priority are selected as follows: (1) experimental performance confirmation of the sensors in preestimated neutron environments; (2) studies and experimental confirmation of small and light-weighted sensors and circuits; and (3) reexamination of developed and on-board model after the study and confirmation works. Based on the above discussions, studies and results on development of neutron monitor system and design and manufacturing of on-board model, aiming to board on Japanese Experimental Module (JEM), are reported., 本成果報告書は、宇宙開発事業団の委託業務計画書「宇宙用中性子モニタ研究試作モデルの開発 その1」に基づいて実施した作業の結果について報告するものである。これまでの検討結果から、優先度の高い開発項目、開発要素を(1)予測中性子環境でのセンサの実験による動作確認、(2)小型軽量化をめざした、センサおよび回路要素の検討と実験による確認、(3)検討確認作業を反映した、開発、搭載モデルの再検討、と設定した。これによって日本宇宙ステーション実験モジュール(JEM)に搭載することを目的とした、中性子モニタ装置の開発、搭載モデルの設計製作の検討結果を報告した。