Solar Energetic Particle (SEP) events are interesting from a scientific perspective as they are the product of a broad set of physical processes from the corona out through the extent of the heliosphere, and provide insight into processes of particle acceleration and transport that are widely applicable in astrophysics. From the operations perspective, SEP events pose a radiation hazard for aviation, electronics in space, and human space exploration, in particular for missions outside of the Earth's protective magnetosphere including to the Moon and Mars. Thus, it is critical to improve the scientific understanding of SEP events and use this understanding to develop and improve SEP forecasting capabilities to support operations. Many SEP models exist or are in development using a wide variety of approaches and with differing goals. These include computationally intensive physics-based models, fast and light empirical models, machine learning-based models, and mixed-model approaches. The aim of this paper is to summarize all of the SEP models currently developed in the scientific community, including a description of model approach, inputs and outputs, free parameters, and any published validations or comparisons with data.

The original real-time analog heritage data from the Huancayo recording ionization chamber is differentiated to yield 15-60 s reso-lution of the first 15 min of the cosmic ray Ground-Level Event (GLE) of 23 February 1956. The initial High-Energy Impulsive (HEI) event of >12.5-20 GeV solar cosmic rays has a rise time of 2 min 15 s with an exponential rise time constant of 49 s, a fluctuating peak pulse of duration 5-7 min, followed by an abrupt decrease of <1 min duration to-33% of the peak flux. Detailed analysis of these data shows close consistency with the Carmichael-Sturrock-Hirayama-Kopp-Pneuman (CSHKP) standard flare model. The <49-second acceleration time to >12.5 GeV is explicable in terms of any one of three separate mechanisms: super-Dreier electric acceleration, sta-tistical (Fermi first order), and shock acceleration deep in the corona. The fluctuating solar cosmic ray flux, the abrupt cessation, and the strong cosmic ray anisotropy on arrival at Earth are consistent with the acceleration event consisting of a sequence of short-lived reconnection events within the parent active center. In addition, the HEI event had a duration and time domain profile similar to the gyro-synchrotron emission from electrons associated with the initial acceleration event in the CSHKP model, and the observation of a white light flare towards the end of the microwave pulse. The properties of the GLE-5 HEI event are similar to those accompanying ten other previously studied large GLEs to the west of 240 West on the solar disk. We conclude that the historic high-resolution data from the GLE on 23 February 1956 demonstrate previously unknown properties of the HEI event that are all consistent with cosmic ray acceleration to 25 GeV by the CSHKP and similar flare magnetic reconnection models. A slowly varying and smaller increase com-menced 6 min after the commencement of the HEI event and rose steadily to a maximum-40 min later. This is consistent with several models of acceleration in association with the coronal mass ejection generated by the flare. (c) 2023 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

A technique has been developed that will transfer the Tsyganenko 1989 modulation for cutoff rigidity values to different epochs of the International Geomagnetic Reference Field (IGRF). The technique separates the effect of the Tsyganenko external currents on cutoff rigidity values previously calculated at 450 km using the 1995 IGRF such that these effects can be applied to other models of the IGRF. The Tsyganenko modulation effects were then applied to the 2020 internal field model. Two improvements to the original set of calculations include a three-point longitudinal smoothing to reduce the unevenness of the penumbral effects and a replacement of the original -100 Dst values for Kp values > 5 to -50 Dst increments. These changes have extended the original library of 88 world grids to a total of 128 world grids of cutoff rigidity values that are applicable for all Kp values (0-9 + ) and Dst values to -500 nT. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

ABSTRACT In late 1940 March, at least five significant solar flares were reported. They likely launched interplanetary coronal mass ejections (ICMEs), and were associated with one of the largest storm sudden commencements (SSCs) since 1868, resulting in space weather hazards that would have significant societal impacts should it occur today. The initial solar activity is associated with a solar proton event. Afterwards, another flare was reported in the eastern solar quadrant (N12 E37-38) at 11:30–12:30 ut on March 23, with significant magnetic crochets (up to ≈ |80| nT at Eskdalemuir) during 11:07–11:40 ut. On their basis, we conservatively estimate the required energy flux of the source solar flare as X35 ± 1 in soft X-ray class. The resultant ICMEs caused enormous SSCs (up to >425 nT recorded at Tucson) and allowed us to estimate an extremely inward magnetopause position (estimated magnetopause stand-off position ≈3.4 RE). The time series of the resultant geomagnetic storm is reconstructed using a Dst estimate, which peaked at 20 ut on March 24 at ≈ −389 nT. Around the storm main phase, the equatorward boundary of the auroral oval extended ≤46.3° in invariant latitudes. This sequence also caused a solar proton event and Forbush decrease (≈3 per cent). These sequences indicate pile-up of multiple ICMEs, which even achieved a record value of inward magnetopause position. Our analyses of this historical pioneer event bring more insights into possible serious space weather hazards and provide a quantitative basis for future analyses and predictions.

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Earth's geomagnetic field is evolving rapidly (in geological time), and as a consequence the amount of geomagnetic shielding at a specific location is also changing. Geomagnetic cutoff rigidities derived from the International Geomagnetic Reference Fields (IGRF) are a basic quantity necessary to compute the cosmic radiation exposure at locations around the world. The most recent generation of the IGRF describes Earth's magnetic field to previously unattainable precision. The use of the Epoch 2015 geomagnetic field coefficients in the trajectory-tracing technique gives a more precise set of vertical geomagnetic cutoff rigidity values than previous internal field models. For example, an increase in cutoff rigidity values in the Western North Atlantic Ocean area and eastern North America is the result of the geomagnetic "westward drift". On a world-wide basis the cutoff rigidity values have decreased consistent with a decrease in the magnitude of the geomagnetic dipole term. While requiring an extremely stable neutron monitor over a long time period, neutron monitors in areas of the world with rapidly decreasing cutoff rigidity values should observe an increase in galactic cosmic radiation such as was observed at Huancayo, Peru. Examples of the change in vertical cutoff rigidity values for specific locations including cosmic ray stations are illustrated.

A re-examination of the first five Ground-Level Events (GLEs) has resulted in identification of characteristics that were not noted at the time of the initial publications. These five events, taken collectively, have not been reproduced by the sun since their occurrence primarily because of their intensity at energies >4 GeV as detected by ionization chambers. While we know that the GLE on 23 February 1956, the largest event to date, was recorded by equatorial cosmic ray detectors, examination of the Huancayo bi-hourly ionization data for the GLEs on 7 March 1942 and 19 November 1949 indicates that these events were also recorded near the equatorial region. In a study of the GLE on 23 February 1956 it was possible to obtain one minute time resolution of the cosmic ray intensity at Huancayo during this event. It should be possible to obtain fine resolution of the timing of the earlier events utilizing the original analog records.

The Nowcast of Atmospheric Ionizing Radiation for Aviation Safety climatological model and the Automated Radiation Measurements for Aerospace Safety (ARMAS) statistical database are presented as polynomial fit equations. Using equations based on altitude, L shell, and geomagnetic conditions an effective dose rate for any location from a galactic cosmic ray (GCR) environment can be calculated. A subset of the ARMAS database is represented by a second polynomial fit equation for the GCR plus probable relativistic energetic particle (REP; Van Allen belt REP) effective dose rates within a narrow band of L shells with altitudinal and geomagnetic dependency. Solar energetic particle events are not considered in this study since our databases do not contain these events. This work supports a suggestion that there may be a REP contribution having an effect at aviation altitudes. The ARMAS database is rich in Western Hemisphere observations for L shells between 1.5 and 5; there have been many cases of enhanced radiation events possibly related to effects from radiation belt particles. Our work identifies that the combined effects of an enhanced radiation environment in this L shell range are typically 15% higher than the GCR background. We also identify applications for the equations representing the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety and ARMAS databases. They include (i) effective dose rate climatology in comparison with measured weather variability and (ii) climatological and statistical weather nowcasting and forecasting. These databases may especially help predict the radiation environment for regional air traffic management, for airport overflight operations, and for air carrier route operations of individual aircraft.

Today the extreme space weather events of early August 1972 are discussed as benchmarks for Sun‐Earth transit times of solar ejecta (14.6 hr) and for solar energetic particle fluxes (10 MeV ion flux >70,000 cm−2·s−1·sr−1). Although the magnetic storm index, Dst, dipped to only −125 nT, the magnetopause was observed within 5.2 RE and the plasmapause within 2 RE. Widespread electric‐ and communication‐grid disturbances plagued North America late on 4 August. There was an additional effect, long buried in the Vietnam War archives that add credence to the severity of the storm impact: a nearly instantaneous, unintended detonation of dozens of sea mines south of Hai Phong, North Vietnam on 4 August 1972. The U.S. Navy attributed the dramatic event to magnetic perturbations of solar storms. Herein we discuss how such a finding is broadly consistent with terrestrial effects and technological impacts of the 4 August 1972 event and the propagation of major eruptive activity from the Sun to the Earth. We also provide insight into the solar, geophysical, and military circumstances of this extraordinary situation. In our view this storm deserves a scientific revisit as a grand challenge for the space weather community, as it provides space‐age terrestrial observations of what was likely a Carrington‐class storm.

We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope (LAT) having late-phase >100 MeV γ -ray emission (LPGRE), identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by >300 MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive >100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection (CME). The LPGRE start times range from CME onset to 2 hr later. Their durations range from ∼0.1 to 20 hr and appear to be correlated with durations of >100 MeV solar energetic particle (SEP) proton events. The power-law spectral indices of the >300 MeV protons producing LPGRE range from ∼2.5 to 6.5 and vary during some events. Combined γ -ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons >500 MeV is typically about 10× the number in the impulsive phase of the solar eruptive event and ranges in nine events from ∼0.01× to 0.5× the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to ∼10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics.

The worldwide network of neutron monitors (NMs) includes two stations, South Pole (SOPO/B) and Dome C (DOMC/B), which are exceptionally sensitive to solar energetic particles. Their locations on the high Antarctic plateau make them favorable for detection of low-energy particles because of the low geomagnetic rigidity cutoff and the thin atmosphere above. This pair of cosmic ray stations is able to register relatively weak solar energetic particle events, which would not have been detected by the NM network otherwise. Since DOMC/B station is in operation only since 2015, now the NM network is more sensitive to a SEP event than ever before. Considering such weak events as the "official" ground-level enhancements (GLEs) may break the homogeneity of the GLE definition and cause an observational bias in studies based on the GLE occurrence rate over decades. In order to keep the "official" GLE list homogeneous, we propose to slightly modify the conventional definition of a GLE, which refers to a statistically significant enhancement of the count rate of at least two differently located neutron monitors over the background. The proposed change is as follows: "... at least two neutron monitors, one of which is located near the sea level". We also propose an introduction of a new class of SEP events called sub-GLE to identify the events registered by high-elevation polar NMs but not satisfying the revised definition of a GLE.

The conventional definition of ground-level enhancement (GLE) events requires a detection of solar energetic particles (SEP) by at least two differently located neutron monitors. Some places are exceptionally well suitable for ground-based detection of SEP – high-elevation polar regions with negligible geomagnetic and reduced atmospheric energy/rigidity cutoffs. At present, there are two neutron-monitor stations in such locations on the Antarctic plateau: SOPO/SOPB (at Amundsen–Scott station, 2835 m elevation), and DOMC/DOMB (at Concordia station, 3233 m elevation). Since 2015, when the DOMC/DOMB station started continuous operation, a relatively weak SEP event that was not detected by sea-level neutron-monitor stations was registered by both SOPO/SOPB and DOMC/DOMB, and it was accordingly classified as a GLE. This would lead to a distortion of the homogeneity of the historic GLE list and the corresponding statistics. To address this issue, we propose to modify the GLE definition so that it maintains the homogeneity: A GLE event is registered when there are near-time coincident and statistically significant enhancements of the count rates of at least two differently located neutron monitors, including at least one neutron monitor near sea level and a corresponding enhancement in the proton flux measured by a space-borne instrument(s). Relatively weak SEP events registered only by high-altitude polar neutron monitors, but with no response from cosmic-ray stations at sea level, can be classified as sub-GLEs.

Although listed as one of the most significant events of the last 80 years, the space weather storm of late May 1967 has been of mostly fading academic interest. The storm made its initial mark with a colossal solar radio burst causing radio interference at frequencies between 0.01 and 9.0 GHz and near‐simultaneous disruptions of dayside radio communication by intense fluxes of ionizing solar X‐rays. Aspects of military control and communication were immediately challenged. Within hours a solar energetic particle event disrupted high‐frequency communication in the polar cap. Subsequently, record‐setting geomagnetic and ionospheric storms compounded the disruptions. We explain how the May 1967 storm was nearly one with ultimate societal impact, were it not for the nascent efforts of the United States Air Force in expanding its terrestrial weather monitoring‐analysis‐warning‐prediction efforts into the realm of space weather forecasting. An important and long‐lasting outcome of this storm was more formal Department of Defense‐support for current‐day space weather forecasting. This story develops during the rapid rise of solar cycle 20 and the intense Cold War in the latter half of the twentieth century. We detail the events of late May 1967 in the intersecting categories of solar‐terrestrial interactions and the political‐military backdrop of the Cold War. This was one of the “Great Storms” of the twentieth century, despite the apparent lack of large geomagnetically induced currents. Radio disruptions like those discussed here warrant the attention of today's radio‐reliant, cellular‐phone and satellite‐navigation enabled world.

The Automated Radiation Measurements for Aerospace Safety (ARMAS) program has successfully deployed a fleet of six instruments measuring the ambient radiation environment at commercial aircraft altitudes. ARMAS transmits real-time data to the ground and provides quality, tissue-relevant ambient dose equivalent rates with 5min latency for dose rates on 213 flights up to 17.3km (56,700ft). We show five cases from different aircraft; the source particles are dominated by galactic cosmic rays but include particle fluxes for minor radiation periods and geomagnetically disturbed conditions. The measurements from 2013 to 2016 do not cover a period of time to quantify galactic cosmic rays' dependence on solar cycle variation and their effect on aviation radiation. However, we report on small radiation clouds in specific magnetic latitude regions and note that active geomagnetic, variable space weather conditions may sufficiently modify the magnetospheric magnetic field that can enhance the radiation environment, particularly at high altitudes and middle to high latitudes. When there is no significant space weather, high-latitude flights produce a dose rate analogous to a chest X-ray every 12.5h, every 25h for midlatitudes, and every 100h for equatorial latitudes at typical commercial flight altitudes of 37,000ft (similar to 11 km). The dose rate doubles every 2km altitude increase, suggesting a radiation event management strategy for pilots or air traffic control; i.e., where event-driven radiation regions can be identified, they can be treated like volcanic ash clouds to achieve radiation safety goals with slightly lower flight altitudes or more equatorial flight paths.

We have determined eight-second averaged geomagnetic transmissions of 36–80MeV protons for the large Solar Energetic Particle (SEP) events and geomagnetic activity level variations of October 1989 using measurements from the NOAA-10 and GOES-7 satellites. We have compared the geomagnetic transmission measurements with model calculations employing trajectory tracings through the combined International Geomagnetic Reference Field (IGRF) and Kp/Dst modified 1989 Tsyganenko model. We present threshold geomagnetic transmission geographic latitudes and magnetic latitudes, as well as (a) differences between the measured and calculated threshold geographic latitudes and magnetic latitudes and (b) differences between measured and calculated polar pass durations. We find that for less disturbed geomagnetic activity levels, the measured threshold geomagnetic transmission geographic and magnetic latitudes are typically about 1–1.5° equatorward of the calculated geographic and magnetic latitudes, while for larger geomagnetic activity levels, the measured geographic and magnetic latitudes can be about 1.5° poleward of the calculated geographic and magnetic latitudes. For the eight Kp bins, we also compare the mean measured magnetic latitudes as a function of mean Dst with the mean calculated magnetic latitudes, interpolated to the mean measured Dst values. These comparisons of mean magnetic latitudes illustrate the improvement in the accuracy of the model calculations resulting from employing the actual mean measured Dst values.

Ice cores are archives of climate change and possibly large solar proton events (SPEs). Wolff et al. (2012) used a single event, a nitrate peak in the GISP2-H core, which McCracken et al. (2001a) time associated with the poorly quantified 1859 Carrington event, to discredit SPE-produced, impulsive nitrate deposition in polar ice. This is not the ideal test case. We critique the Wolff et al. analysis and demonstrate that the data they used cannot detect impulsive nitrate events because of resolution limitations. We suggest re-examination of the top of the Greenland ice sheet at key intervals over the last two millennia with attention to fine resolution and replicate sampling of multiple species. This will allow further insight into polar depositional processes on a sub-seasonal scale, including atmospheric sources, transport mechanisms to the ice sheet, post-depositional interactions, and a potential SPE association.

Manifestations of the 11-year solar cycle and longer time-scale variability in the heliosphere and cosmic rays are considered. We briefly review the cyclic variability of such heliospheric parameters as solar wind speed and density and heliospheric magnetic field, open magnetic flux and latitude variations of the heliospheric current sheet. It is discussed whether the local in-situ observation near Earth can represent the global 3D heliospheric pattern. Variability of cosmic rays near Earth provides an indirect useful tool to study the heliosphere. We discuss details of the heliospheric modulation of galactic cosmic rays, as recorded at and near Earth, and their relation to the heliospheric conditions in the outer heliosphere. On the other hand, solar energetic particles can serve as probes for explosive phenomena on the Sun and conditions in the corona and inner heliosphere. The occurrence of major solar proton events depicts an overall tendency to follow the solar cycle but individual events may appear at different phases of the solar cycle, as defined by various factors. The solar cycle in the heliosphere and cosmic rays depicts a complex pattern which includes different processes and cannot be described by a simple correlation with sunspot number.

Solar proton events can adversely affect space and ground-based systems. Ground-level events are a subset of solar proton events that have a harder spectrum than average solar proton events and are detectable on Earth's surface by cosmic radiation ionization chambers, muon detectors, and neutron monitors. This paper summarizes the space weather effects associated with ground-level solar proton events during the 23rd solar cycle. These effects include communication and navigation systems, spacecraft electronics and operations, space power systems, manned space missions, and commercial aircraft operations. The major effect of ground-level events that affect manned spacecraft operations is increased radiation exposure. The primary effect on commercial aircraft operations is the loss of high frequency communication and, at extreme polar latitudes, an increase in the radiation exposure above that experienced from the background galactic cosmic radiation. Calculations of the maximum potential aircraft polar route exposure for each ground-level event of the 23rd solar cycle are presented. The space weather effects in October and November 2003 are highlighted together with on-going efforts to utilize cosmic ray neutron monitors to predict high energy solar proton events, thus providing an alert so that system operators can possibly make adjustments to vulnerable spacecraft operations and polar aircraft routes.