Preface to measurement, specification and forecasting of the Solar Energetic Particle (SEP) environment and Ground Level Enhancements (GLEs)

The Sun emits energetic particles following eruptive events such as solar flares and Coronal Mass Ejections (CMEs). Solar Energetic Particles (SEPs) arrive in bursts known as Solar Particle Events (SPEs), which penetrate into the Earth’s magnetosphere. SEPs with large enough energy induce a complicated atmospheric cascade, which secondary particles lead to an enhancement of count rate of ground-based detectors e.g. Neutron Monitors (NMs). This class of SEPs is therefore referred as Ground Level Enhancements (GLEs). The characterisation of the high-energy SEPs environment with corresponding space weather effects is important for space flights, aviation, and satellite industry. In this topical issue recent developments, addressing important user needs in the space radiation environment domain are published. Some articles are relevant to the specification of the SEP environment whilst others focus on space weather prediction of SEP fluxes. Catalogues based on measurement and processing of SEPs including ground-based data, and modelling of aircrew radiation exposure during major events are also presented

Updated Model of the Solar Energetic Proton Environment in Space

The Solar Accumulated and Peak Proton and Heavy Ion Radiation Environment (SAPPHIRE) model provides environment specification outputs for all aspects of the Solar Energetic Particle (SEP) environment. The model is based upon a thoroughly cleaned and carefully processed data set. Herein the evolution of the solar proton model is discussed with comparisons to other models and data. This paper discusses the construction of the underlying data set, the modelling methodology, optimisation of fitted flux distributions and extrapolation of model outputs to cover a range of proton energies from 0.1 MeV to 1 GeV. The model provides outputs in terms of mission cumulative fluence, maximum event fluence and peak flux for both solar maximum and solar minimum periods. A new method for describing maximum event fluence and peak flux outputs in terms of 1-in-x-year SPEs is also described. SAPPHIRE proton model outputs are compared with previous models including CREME96, ESP-PSYCHIC and the JPL model. Low energy outputs are compared to SEP data from ACE/EPAM whilst high energy outputs are compared to a new model based on GLEs detected by Neutron Monitors (NMs).Comment: 37 pages, 17 figure

Cutoff Latitudes of Solar Proton Events Measured by GPS Satellites

Solar energetic particles (SEPs), one of the main causes of particle radiation in interplanetary space, can disrupt radio communication, induce spacecraft failures and change the heating and cooling rates in the atmosphere among others. To investigate the impact of SEPs and more specifically solar proton events (SPEs), we established a cutoff latitude database based on energetic particle data from Combined X-ray Dosimeters (CXDs) on board the Global Positioning System (GPS) spacecraft. Introducing a novel normalization method involving proton fluxes from the Geostationary Operational Environmental Satellites enabled us to include the CXD data from its introduction (2001) onwards. The database contains 5714 cutoff latitudes divided over six energies between 18 and 115 MeV which occur during 58 SPEs from 2001 to 2015. Based on the database, a cutoff latitude parameterization as a function of solar wind dynamic pressure and geomagnetic indices Kp and Dst is created for each energy. Moreover, comparisons to previous studies on energetic particle data from the Polar Orbiting Environmental Satellites have been performed to put the GPS data into perspective. A 1–2° poleward offset is found for the GPS based cutoff latitude models, for which several causes are discussed. Furthermore, the limitation of GPS data to geomagnetic latitudes above 60° should be considered. All in all, the usage of the long time span of GPS data in this study combined with its recent release (2016) opens up a new range of studies involving GPS energetic particle data such as investigating long-term trends with respect to our solar cycle or magnetospheric trends.publishedVersio

On the semi-annual variation of relativistic electrons in the outer radiation belt

The nature of the semi-annual variation in the relativistic electron fluxes in the Earth's outer radiation belt is investigated using Van Allen Probes (MagEIS and REPT) and Geostationary Operational Environmental Satellite Energetic Particle Sensor (GOES/EPS) data during solar cycle 24. We perform wavelet and cross-wavelet analysis in a broad energy and spatial range of electron fluxes and examine their phase relationship with the axial, equinoctial and Russell–McPherron mechanisms. It is found that the semi-annual variation in the relativistic electron fluxes exhibits pronounced power in the 0.3–4.2 MeV energy range at L shells higher than 3.5, and, moreover, it exhibits an in-phase relationship with the Russell–McPherron effect, indicating the former is primarily driven by the latter. Furthermore, the analysis of the past three solar cycles with GOES/EPS indicates that the semi-annual variation at geosynchronous orbit is evident during the descending phases and coincides with periods of a higher (lower) high-speed stream (HSS) (interplanetary coronal mass ejection, ICME) occurrence

The probabilistic solar particle event forecasting (PROSPER) model

The Probabilistic Solar Particle Event foRecasting (PROSPER) model predicts the probability of occurrence and the expected peak flux of solar energetic particle (SEP) events. Predictions are derived for a set of integral proton energies (i.e., E > 10, > 30, and > 100 MeV) from characteristics of solar flares (longitude, magnitude), coronal mass ejections (width, speed), and combinations of both. Herein the PROSPER model methodology for deriving the SEP event forecasts is described, and the validation of the model, based on archived data, is presented for a set of case studies. The PROSPER model has been incorporated into the new operational advanced solar particle event casting system (ASPECS) tool to provide nowcasting (short term forecasting) of SEP events as part of ESA's future SEP advanced warning system (SAWS). ASPECS also provides the capability to interrogate PROSPER for historical cases via a run-on-demand functionality

Space Weather Prediction System providing forecasts and alerts on solar flares and SEP events

A web-based prototype system for predicting Solar Flares and Solar Energetic Particle (SEP) events for its use by space launcher operators or any interested user has been implemented. The main goal of this system, called SEPsFLAREs, is to provide warnings/predictions with forecast horizons from 48 hours before to a few hours before to the SEP peak flux, and duration predictions. The module responsible for predicting solar flares, the SF_PMod, is based on the well-known ASAP flare predictor [T. Colak & R. Qahwaji, Automated solar activity prediction: A hybrid computer platform using machine learning and solar imaging for automated prediction of solar flares, Space Weather, 7 (S06001), 2009], which learns rules by using machine learning techniques on SDO/SOHO solar images to automatically detect sunspots, classify them based on the McIntosh classification system, and predict C-, M-, and X-class flares with forecast horizon from 6 h to 48 h. Regarding the performance of the flare predictor, the 24-hour forecast horizon was found to provide the best performance: the Probability of Detection (POD), False Alarm Ratio (FAR) and True Skill Statistics estimations were 63.8%, 99.0% and 0.5 respectively for predicting X-class flares; and 88.7%, 87.0% and 0.59 respectively, for predicting M-class flares. The module responsible for predicting the SEP onset and occurrence, the SEP_OO_PMod, is based on the well-known UMASEP predictor [M. Núñez, Predicting solar energetic proton events (E > 10 MeV), Space Weather, 9 (S07003), 2011], which performs X-ray and proton flux correlations to find the first symptoms of future well- and poorly-connected SEP events. The SEP_OO_PMod also provides a Warning Tool which is able to warn about potential proton enhancements (including SEP events) from flare predictions. Regarding the performance of the SEP_OO_PMod, it was validated taking into account all 129 SEP events from January 1994 to June 2014 and obtained a POD of 86.82%, a FAR of 25.83%, and an Average Warning Time (AWT) of 3.93 h. Regarding the evaluation of the Warning Tool, the best performance, obtained with a set of user-defined parameters, were a POD of 58.3%, FAR of 90.1%, and AWT of 23.1 h. The module responsible for predicting SEP peak and duration, the SEP_FID_PMod, identifies the parent solar flare associated to an observed/predicted SEP, simulates the radial propagation of the predicted shock on a representative IMF structure (i.e. a static Parker Spiral), and predicts the SEP peak and duration. The SEP_FID_PMod, validated taking into account all 129 SEP events from January 1994 to June 2014, obtained a Mean Absolute Error (MAE) of SEP peak time predictions of 11.3 h, a MAE of peak intensity predictions of 0.53 in log10 units of pfu, and a MAE of SEP end time predictions of 28.8 h. The SEPsFLAREs system also acquires data for solar flares nowcasting (including GSFLAD proxy and SISTED detector from MONITOR’s ESA-funded project; [Hernández-Pajares, M., A. García-Rigo, J.M. Juan, J. Sanz, E. Monte and A. Aragón-Ángel (2012), GNSS measurement of EUV photons flux rate during strong and mid solar flares. Space Weather, Volume 10, Issue 12, doi:10.1029/2012SW000826] and [García-Rigo, A. (2012), Contributions to ionospheric determination with Global Positioning System: solar flare detection and prediction of global maps of Total Electron Content, Ph.D. dissertation. Doctoral Program in Aerospace Science & Technology, Technical University of Catalonia, Barcelona, Spain]).Postprint (published version

ESA Nanosatellites for D3S (Distributed Space Weather Sensor System)

In early 2021, SSTL was selected to be the prime contractor for an ongoing 18 month ESA-funded Phase 0/A study titled “SSA P3-SWE-LIII Nanosatellites for D3S”. The objective of the study is to assess the feasibility of using nanosatellites for future operational space weather monitoring missions as part of ESA's Distributed Space Weather Sensor System (D3S). The Phase 0 study initially involved an analysis of science measurement requirements and space weather instruments as well as an analysis of recent relevant nanosatellite missions and nanosatellite technologies which could be used on future ESA D3S Nanosatellites. This was followed by an initial trade-off of a range of high-level mission architecture concepts, eventually converging down to two mission architecture concepts proposed for further analysis during the remainder of the Phase 0 study. The aim of the first mission architecture concept is to provide near-real time measurements of radiation, thermal plasma and Ionospheric neutrals/plasma, via a constellation of 20x SSTL-21 satellites. The objective of the second mission architecture concept is to provide near-real time measurements of radiation, the Ionosphere and the Thermosphere, via a constellation of 6x 16U SSTL-Cube satellites. ESA selected the second mission architecture concept to take through into the Phase A study. This paper will mainly describe the details of the Phase 0 study, as well as touching on the current status of the Phase A study

Prediction of solar proton event fluence spectra from their peak flux spectra

Solar Proton Events (SPEs) are of great importance and significance for the study of Space Weather and Heliophysics. These populations of protons are accelerated at high energies ranging from a few MeVs to hundreds of MeVs and can pose a significant hazard both to equipment on board spacecrafts as well as astronauts as they are ionizing radiation. The ongoing study of SPEs can help to understand their characteristics, relative underlying physical mechanisms, and help in the design of forecasting and nowcasting systems which provide warnings and predictions. In this work, we present a study on the relationships between the Peak Flux and Fluence spectra of SPEs. This study builds upon existing work and provides further insights into the characteristics and the relationships of SPE Peak flux and Fluence spectra. Moreover it is shown how these relationships can be quantified in a sound manner and exploited in a simple methodology with which the Fluence spectrum of an SPE can be well predicted from its given Peak spectrum across two orders of magnitude of proton energies, from 5 MeV to 200 MeV. Finally it is discussed how the methodology in this work can be easily applied to forecasting and nowcasting systems

Very high energy proton peak flux model

Solar energetic particles (SEPs) pose a serious radiation hazard to spacecraft and astronauts. The highest energy SEPs are a significant threat even in heavily shielded applications. We present a new probabilistic model of very high energy differential peak proton fluxes. The model is based on GOES/HEPAD observations between 1986 and 2018, i.e., covering very nearly three complete solar cycles. The SEP event list for the model was defined using a statistical criterion derived by setting the possibility of false detection of an event to 1%. The peak flux distributions were calculated for the interpolated energies 405 MeV, 500 MeV and 620 MeV, and modelled with exponentially cut off power law functions. The HEPAD data were cleaned and corrected using a "bow-tie" method which is based on the response functions of the HEPAD channels P8-P10 found in the instrument calibration reports. The results of the model are available to the Space Weather community as a web-based tool at the ESA's Space Situational Awareness Programme Space Weather Service Network

Two solar proton fluence models based on ground level enhancement observations

Solar energetic particles (SEPs) constitute an important component of the radiation environment in interplanetary space. Accurate modeling of SEP events is crucial for the mitigation of radiation hazards in spacecraft design. In this study we present two new statistical models of high energy solar proton fluences based on ground level enhancement (GLE) observations during solar cycles 19–24. As the basis of our modeling, we utilize a four parameter double power law function (known as the Band function) fits to integral GLE fluence spectra in rigidity. In the first model, the integral and differential fluences for protons with energies between 10 MeV and 1 GeV are calculated using the fits, and the distributions of the fluences at certain energies are modeled with an exponentially cut-off power law function. In the second model, we use a more advanced methodology: by investigating the distributions and relationships of the spectral fit parameters we find that they can be modeled as two independent and two dependent variables. Therefore, instead of modeling the fluences separately at different energies, we can model the shape of the fluence spectrum. We present examples of modeling results and show that the two methodologies agree well except for a short mission duration (1 year) at low confidence level. We also show that there is a reasonable agreement between our models and three well-known solar proton models (JPL, ESP and SEPEM), despite the differences in both the modeling methodologies and the data used to construct the models.</p