Solar Energetic Proton Events Research Articles

Solar Energetic-Particle-Associated Coronal Mass Ejections Observed by the Mauna Loa Solar Observatory Mk3 and Mk4 Coronameters

We report on the first comprehensive study of the coronal mass ejections (CMEs) associated with sim 25text{ MeV} solar energetic-proton (SEP) events in 1980 – 2013 observed in the low/inner corona by the Mauna Loa Solar Observatory (MLSO) Mk3 and Mk4 coronameters. Where possible, these observations are combined with space-based observations from the Solar Maximum Mission C/P, P78-1 SOLWIND, or SOHO/LASCO coronagraphs. The aim of the study is to understand directly measured (rather than inferred from proxies) CME motions in the low to midcorona and their association with SEP acceleration, and hence attempt to identify early signatures that are characteristic of SEP acceleration in ground-based CME observations that may be used to warn of impending SEP events. Although we find that SEP events are associated with CMEs that are on average faster and wider than typical CMEs observed by MLSO, a major challenge turns out to be determining reliable estimates of the CME dynamics in the low corona from the 3-min cadence Mk3/4 observations since different analysis techniques can produce inconsistent results. This complicates the assessment of what early information on a possible SEP event is available from these low-coronal observations.

Spatial Evolution of 20 MeV Solar Energetic Proton Events

The longitudinal extents of solar energetic (E > 10 MeV) particle (SEP) events in the heliosphere are a characteristic important for understanding SEP acceleration and transport as well as their space weather effects. SEP detectors on the STEREO A and B spacecraft launched in 2008, combined with those on Earth-orbiting spacecraft, have enabled recent studies of this characteristic for many events. Each SEP event distribution has been characterized by a single central longitude, width, and amplitude derived from Gaussian fits to peak intensities or fluences at each spacecraft. To capture dynamic changes of those parameters through SEP events, we apply Gaussian fits in solar-based Carrington longitude coordinates with 1 hr resolution to four selected large 20 MeV proton events. The limitations of single-Gaussian fits for very extended events is discussed. In all four examples the widths are increasing throughout the event, as expected, while the projected Gaussian centers at SEP onset start from 30° to 100° east of the associated flare, move westward, then remain stationary well east of the flare for several days before moving west as the event amplitudes decrease. Late decay phases can be characterized by eastward movements away from the flare longitudes. We introduce schematic Buffett plots to show successive snapshots of event longitudes and amplitudes.

Magnetic connectivity and solar energetic proton event intensity profiles at deka-MeV energy

We present an analysis of the time-intensity profiles of 25 solar energetic proton events at 18.2 MeV, modelled by fitting an analytical function form (a modified Weibull function) to the observed intensities. Additionally relying on previous work that characterized the magnetic connectivity between the event-related solar flare and the observer in these events with three angular parameters, we investigate the fit function parameters, the connectivity parameters, and the iron-to-carbon ratio of the events for dependencies and correlations. We find that the fit parameter controlling the basic shape of the profile (parameter a) is not clearly dependent on the connectivity parameters or the Fe/C ratio, suggesting that the profile shapes of neither well and weakly connected nor generally “impulsive” and “gradual” events differ systematically during the early stages of the event at 1 AU. In contrast, the time scaling of the fit function (parameter b) is at least moderately correlated with both the magnetic connectivity parameters and the Fe/C ratio, in that well-connected and iron-rich events are typically shorter in relative duration than weakly connected and nominal-abundance events; intensity rise times display a similar correlation with the connectivity parameters. We interpret the former result as following from the combined effect of various transport processes acting on the particles in interplanetary space, while the latter is essentially consistent with established knowledge regarding the observed dependence of the time-intensity profile shapes of solar energetic particle events on their magnetic connectivity and heavy ion abundances. The desirability of modelling the particle transport effects in detail and extending the analysis to cover higher energies is indicated.

Sunspot numbers and proton events in solar cycles 19 to 24

In this study we compare the mean annual sunspot numbers for the six most recent solar cycles (19 to 24) with the number of solar energetic proton events occurring in those years. Though most of the individual cycles exhibit quite strong correlations, when they are combined into a generic solar cycle the result is very significant (ρ = 0.98). For cycles 21 to 24 (when spacecraft observations have been regularly available), we also investigate the variation in the source location of the solar energetic proton events in relation to four peak flux thresholds in orders of magnitude from ≥10 to ≥10,000 pfu. For helio-latitudes within ±40° (the range within which active regions usually occur) there is negligible variation in the helio-longitude of the source regardless of the peak flux threshold. However, the effect on the helio-longitude of varying the peak flux threshold is very significant: the higher the threshold the closer is the median source longitude to 0°, the variation being almost uniform from ≥10 pfu (32°W) to ≥10,000 pfu (2°W). Finally, of the 10 events in cycles 19 to 24 with peak flux ≥10,000 pfu, all occurred between years 4 and 8 after solar minimum, all were the result of M- or X-class flares with associated halo CMEs and shock fronts, and the three most intense events (≥40,000 pfu) were all related to X-class flares which occurred in the longitude range 10–28°E. These results support and extend previous work by the cited authors, and have implications for solar-terrestrial relations and the effects of space weather within geospace.

Effects of Energetic Solar Proton Events of Solar Cycle 23 on Intensity of the Stratospheric Polar Vortex

Effects of Energetic Solar Proton Events of Solar Cycle 23 on Intensity of the Stratospheric Polar Vortex

Ionization effect in the Earth’s atmosphere during the sequence of October–November 2003 Halloween GLE events

The effect of precipitating high-energy particles on atmospheric physics and chemistry is extensively studied over the last decade. In majority of the existing models, the precipitating particles induced ionization plays an essential role. For such effects, it is necessary to possess enhanced increase in ion production, specifically during the winter period. In this study, we focus on highly penetrating particles — cosmic rays. The galactic cosmic rays are the main source of ionization in the Earth’s stratosphere and troposphere. On the other hand, the atmospheric ionization may be significantly enhanced during strong solar energetic particle events, mainly over the polar caps. A specific interest is paid to the most energetic solar proton events leading to counting rate enhancement of ground-based detectors, namely the so-called ground level enhancements (GLEs). During solar cycle 23, several strong ground level enhancements were observed. A sequence of three GLEs was observed in October–November 2003, the Halloween events. Here, on the basis of 3-D Monte Carlo model, we computed the energetic particles induced atmospheric ionization, explicitly considering the contribution of cosmic rays with galactic and solar origin. The ion production rates were computed as a function of the altitude above sea level using reconstructed solar energetic particles spectra. The 24 h and event averaged ionization effects relative to the average due to galactic cosmic rays were also computed.

Global Prompt Proton Sensor Network: Monitoring Solar Energetic Protons Based on GPS Satellite Constellation

AbstractEnergetic particle instruments on board Global Positioning System (GPS) satellites form a powerful global prompt proton sensor network (GPPSn) that provides an unprecedented opportunity to monitor and characterize solar energetic protons targeting the Earth. The medium‐Earth orbits of the GPS constellation have the unique advantage of allowing solar energetic protons to be simultaneously measured from multiple points in both open‐ and closed‐field line regions. Examining two example intervals of solar proton events, we showcase in this study how GPS proton data are prepared, calibrated, and utilized to reveal important features of solar protons, including their source, acceleration/scattering by interplanetary shocks, the relative position of Earth when impinged by these shocks, the shape of solar particle fronts, the access of solar protons inside the dynamic geomagnetic field, and temporally varying proton distributions in both energy and space. By comparing to Van Allen Probes data, GPS proton observations are further demonstrated not only to be useful for qualitatively monitoring the dynamics of solar protons but also for quantitative scientific research including determining cutoff L‐shells. Our results establish that this GPPSn can join forces with other existing solar proton monitors and contribute to observing, warning, understanding, and ultimately forecasting the incoming solar energetic proton events.

The effect of drifts on the decay phase of SEP events

Aims. We study the effect of the magnetic gradient and curvature drifts on the pitch-angle dependent transport of solar energetic particles (SEPs) in the heliosphere, focussing on ∼3–36 MeV protons. By considering observers located at different positions in the heliosphere, we investigate how drifts may alter the measured intensity-time profiles and energy spectra. We focus on the decay phase of solar energetic proton events in which a temporal invariant spectrum and disappearing spatial intensity gradients are often observed; a phenomenon known as the “reservoir effect” or the “SEP flood”. We study the effects of drifts by propagating particles both in nominal and non-nominal solar wind conditions. Methods. We used a three-dimensional (3D) particle transport model, solving the focused transport equation extended with the effect of particle drifts in the spatial term. Nominal Parker solar wind configurations of different speeds and a magnetohydrodynamic (MHD) generated solar wind containing a corotating interaction region (CIR) were considered. The latter configuration gives rise to a magnetic bottle structure, with one bottleneck at the Sun and the other at the CIR. We inject protons from a fixed source at 0.1 AU, the inner boundary of the MHD model. Results. When the drift induced particle net-flux is zero, the modelled intensity-time profiles obtained at different radial distances along an IMF line show the same intensity fall-off after the prompt phase of the particle event, which is in accordance with the SEP flood phenomenon. However, observers magnetically connected close to the edges of the particle injection site can experience, as a result of drifts, a sudden drop in the intensities occurring at different times for different energies such that no SEP flood phenomenon is established. In the magnetic bottle structure, this effect is enhanced due to the presence of magnetic field gradients strengthening the nominal particle drifts. Moreover, anisotropies can be large for observers that only receive particles through drifts, illustrating the importance of pitch-angle dependent 3D particle modelling. We observe that interplanetary cross-field diffusion can mitigate the effects of particle drifts. Conclusions. Particle drifts can substantially modify the decay phase of SEP events, especially if the solar wind contains compression regions or shock waves where the drifts are enhanced. This is, for example, the case for our CIR solar wind configuration generated with a 3D MHD model, where the effect of drifts is strong. A similar decay rate in different energy channels and for different observers requires the mitigation of the effect of drifts. One way to accomplish this is through interplanetary cross-field diffusion, suggesting thus a way to determine a minimum value for the cross-field diffusion strength.

Prediction Model for Solar Energetic Proton Events: Analysis and Verification

AbstractSolar energetic protons (SEPs) can cause radiation damage to satellites. The Space Environment Prediction Center of China defines the start of a SEP event as the time when three consecutive instances of the 5‐min‐averaged integral flux of >10 MeV protons equal or exceed the 10 proton flux unit threshold. In this study, we analyzed the 5‐min‐averaged soft X‐ray flux and the differential and integral proton flux obtained by the Geostationary Operational Environmental Satellites for the period from January 1990 to September 2017 and developed SEP prediction models and their products. The statistical models take one or two data products and the given thresholds as the predictor and predict whether a SEP event will occur or not in the next 24 hr. The quality of the forecast models was measured by comparing the model results against certain verification metrics. Our model taking the products of 5‐min‐averaged integral flux of >10 MeV protons and long wavelength of soft X‐ray flux as predictors can provide a probability of detection of 0.80 (152/190), a false alarm ratio of 0.26 (53/205), and an average warning time of 2.6 hr for the correctly predicted events. In addition, the model can provide a critical success index of 0.63, a Gilbert skill score of 0.62, and a Heidke skill score of 0.76.

Predicting well-connected SEP events from observations of solar EUVs and energetic protons

This study shows a quantitative assessment of the use of Extreme Ultraviolet (EUV) observations in the prediction of Solar Energetic Proton (SEP) events. The UMASEP scheme (Space Weather, 9, S07003, 2011; 13, 2015, 807–819) forecasts the occurrence and the intensity of the first hours of SEP events. In order to predict well-connected events, this scheme correlates Solar Soft X-rays (SXR) with differential proton fluxes of the GOES satellites. In this study, we explore the use of the EUV time history from GOES-EUVS and SDO-AIA instruments in the UMASEP scheme. This study presents the results of the prediction of the occurrence of well-connected >10 MeV SEP events, for the period from May 2010 to December 2017, in terms of Probability of Detection (POD), False Alarm Ratio (FAR), Critical Success Index (CSI), and the average and median of the warning times. The UMASEP/EUV-based models were calibrated using GOES and SDO data from May 2010 to October 2014, and validated using out-of-sample SDO data from November 2014 to December 2017. The best results were obtained by those models that used EUV data in the range 50–340 Å. We conclude that the UMASEP/EUV-based models yield similar or better POD results, and similar or worse FAR results, than those of the current real-time UMASEP/SXR-based model. The reason for the higher POD of the UMASEP/EUV-based models in the range 50–340 Å, was due to the high percentage of successful predictions of well-connected SEP events associated with <C4 flares and behind-the-limb flares, which amounted to 25% of all the well-connected events during the period May 2010 to December 2017. By using all the available data (2010–2017), this study also concluded that the simultaneous use of SXRs and EUVs in 94 Å in the UMASEP-10 tool for predicting all >10 MeV SEP events, improves the overall performance, obtaining a POD of 92.9% (39/42) compared with 81% (34/42) of the current tool, and a slightly worse FAR of 31.6% (18/57) compared with 29.2% (14/58) of the current tool.

From solar to stellar flare characteristics

Context. The connection between solar energetic proton events and X-ray flares has been the focus of many studies over the past 13 yr. In the course of these investigations several peak size distribution functions based on Geostationary Operational Environmental Satellite (GOES) measurements of both quantities have been developed. In more recent studies one of these functions has been used to estimate the stellar proton fluence around the M-dwarf star AD Leonis. However, a comparison of the existing peak size distribution functions reveals strong discrepancies with respect to each other. Aims. The aim of this paper is to derive a new peak size distribution function that can be utilized to give a more realistic estimate of the stellar proton flux of G-, K-, and M-dwarf stars. Methods. By updating and extending the GOES-based peak size distribution down to B-class X-ray flare intensities with the help of SphinX data from the solar minimum conditions of 2009 and newly derived GOES data between 1975 and 2005, we developed a new power-law peak size distribution function for solar proton fluxes (E > 10 MeV). However, its resulting slope differs from values reported in the literature. Therefore, we also developed a double-power-law peak size distribution function. An extension to much higher X-ray flare intensities (10−1) W m−2 and above, for the first time, results in an approximation of best- and worst-case scenarios of the stellar proton flux around G-, K-, and M-dwarf stars. Results. Investigating the impact of the newly developed peak size distribution function for G-, K-, and M-dwarf star flare intensities we show that in the worst-case scenario previous studies may underestimate the stellar proton flux by roughly one to five orders of magnitude.

Magnetic Flux Reconnection in Flaring Active Regions with Sustained Gamma-Ray Emission

Abstract Characteristics of sustained >100 MeV γ-ray emission (SGRE) events observed by the Large Area Telescope on Fermi were recently reported by Share et al. Their spectra are consistent with the decay of pions produced by >300 MeV protons and appear spectrally and spatially distinct from preceding associated flares. The source(s) of the sustained production of the >300 MeV protons is uncertain, but acceleration in coronal/interplanetary shock waves driven by coronal mass ejections, followed by a return of the protons back to the Sun, is favored. This scenario requires proton transport through converging magnetic fields behind the shock, which might result in considerable reflection of the protons back into space, and 1 au observations of the associated solar energetic proton (SEP) events do not always include a population of E > 300 MeV protons. Alternative source candidates that involve trapping or continued acceleration of SEPs in coronal loops have been considered. The energy release rates from magnetic reconnection in flaring active regions (ARs) have been measured with a new technique, and in this work we compare those measured flux reconnection rates with emission profiles in 11 SGRE events. In general, the magnetic reconnection event is nearly or completely finished before the bulk of the γ-ray emission, which argues against scenarios of continued proton acceleration in the flaring ARs.

Size Distributions of Solar Proton Events and Their Associated Soft X-Ray Flares: Application of the Maximum Likelihood Estimator

Abstract We use the maximum likelihood estimator to determine the slope (α) of the power-law size distribution of the peak proton fluxes of a subsampled set of 106 ∼ 25 MeV solar energetic proton (SEP) events from 1997 to 2016 associated with western hemisphere soft X-ray (SXR) flares: α = 1.28 ± 0.03. For the peak SXR fluxes of a subsample of 110 SEP-associated flares, we find α = 1.51 ± 0.05. In addition, we obtained a slope of 1.61 ± 0.05 for the peak SXR fluxes of a sample of 128 ≥M1 SXR flares from 1996 to 2005 that were associated with coronal mass ejections (CMEs) with speeds ≥1000 km s−1. The slopes of both of these SXR peak-flux distributions (∼1.5 and ∼1.6) are closer to that for proton events (∼1.3) than to the α value of ∼2.1 (2.09 ± 0.08) determined for a subsample of 177 western hemisphere ≥M1 SXR flares considered from 1996 to 2005. These results are consistent with those of a previous study, based on a less reliable method (for small samples), in which it was argued that the flatter size distribution generally found for SEP events versus those for flare electromagnetic emissions was due to the fact that SEP flares are an energetic subset of all flares, characterized by their ∼100% association with fast/wide CMEs that drive coronal/interplanetary shock waves. Shock formation in the corona requires CMEs with speeds ≳400 km s−1, a threshold effect that further distinguishes SEP flares from the general population of all flares.

A Statistical Study to Determine the Origin of Long-duration Gamma-Ray Flares

Abstract Two scenarios have been proposed to account for sustained ≥30 MeV gamma-ray emission in solar flares: (1) prolonged particle acceleration/trapping involving large-scale magnetic loops at the flare site, and (2) precipitation of high-energy (>300 MeV) protons accelerated at coronal/interplanetary shock waves. To determine which of these scenarios is more likely, we examine the associated soft X-ray flares, coronal mass ejections (CMEs), and solar energetic proton events for (a) the long-duration gamma-ray flares (LDGRFs) observed by the Large Area Telescope on Fermi, and (b) delayed and/or spatially extended high-energy gamma-ray flares observed by the Gamma-ray Spectrometer on the Solar Maximum Mission, the Gamma-1 telescope on the Gamma satellite, and the Energetic Gamma-Ray Experiment Telescope on the Compton Gamma-Ray Observatory. For the Fermi data set of 11 LDGRFs with >100 MeV emission lasting for ≥∼2 hr, we search for associations and reverse associations between LDGRFs, X-ray flares, CMEs, and SEPs, i.e., beginning with the gamma-ray flares and also, in turn, with X-class soft X-ray flares, fast (≥1500 km s−1) and wide CMEs, and intense (peak flux ≥2.67 × 10−3 protons cm−2 s−1 sr−1, with peak to background ratio >1.38) >300 MeV SEPs at 1 au. While LDGRFs tend to be associated with bright X-class flares, we find that only one-third of the X-class flares during the time of Fermi monitoring coincide with an LDGRF. However, nearly all fast, wide CMEs are associated with an LDGRF. These preliminary association analyses favor the proton precipitation scenario, although there is a prominent counter-example of a potentially magnetically well-connected solar eruption with >100 MeV emission for ∼10 hr for which the near-Earth >300 MeV proton intensity did not rise above background.

The Wind/EPACT Proton Event Catalog (1996 – 2016)

We present the finalized catalog of solar energetic proton events detected by Wind/EPACT instrument over the period 1996-2016. Onset times, peak times, peak proton intensity and onset-to-peak proton fluence are evaluated for the two available energy channels, at about 25 and 50 MeV. We describe the procedure utilized to identify the proton events and to relate them to their solar origin (in terms of flares and coronal mass ejections). The statistical relationships between the energetic protons and their origin (linear and partial correlation analysis) are reported and discussed in view of earlier findings. Finally, the different trends found in the first eight years of solar cycles 23 and 24 are discussed.

Predicting well-connected SEP events from observations of solar soft X-rays and near-relativistic electrons

This paper studies the use of electron data from the Electron Proton Alpha Monitor (EPAM) on board the Advanced Composition Explorer (ACE) in the UMASEP (University of Málaga Solar particle Event Predictor) scheme [Núñez, Space Weather 9 (2011) S07003; Núñez, Space Weather 13 (2015)] for predicting well-connected >10 MeV Solar Energetic Proton (SEP) events. In this study, the identification of magnetic connection to a solar particle source is done by correlating Geostationary Operational Environmental Satellites (GOES) Soft X-Ray (SXR) fluxes with ACE EPAM electrons fluxes with energies of 0.175–0.375 MeV. The forecasting performance of this model, called Well-Connected Prediction with electrons (WCP-electrons), was evaluated for a 16-year period from November 2001 to October 2017. This performance is compared with that of the component of current real-time tool UMASEP-10, called here WCP-protons model, which predicts the same type of events by correlating GOES SXR with differential proton fluxes with energies of 9–500 MeV. For the aforementioned period, the WCP-electrons model obtained a Probability of Detection (POD) of 50.0%, a False Alarm Ratio (FAR) of 39% and an Average Warning Time (AWT) of 1 h 44 min. The WCP-protons model obtained a POD of 78.0%, a FAR of 22% and an AWT of 1 h 3 min. These results show that the use of ACE EPAM electron data in the UMASEP scheme obtained a better anticipation time (additional 41 min on average) but a lower performance in terms of POD and FAR. We also analyzed the use of a combined model, composed of WCP-electrons and WCP-protons, working in parallel (i.e. the combined model issues a forecast when any of the individual models emits a forecast). The combined model obtained the best POD (84%), and a FAR and AWT (34.4% and 1 h 34 min, respectively) which is in between those of the individual models.

Solar Activity from 2006 to 2014 and Short-term Forecasts of Solar Proton Events Using the ESPERTA Model

Abstract To evaluate the solar energetic proton (SEP) forecast model of Laurenza et al., here termed ESPERTA, we computed the input parameters (soft X-ray (SXR) fluence and ∼1 MHz radio fluence) for all ≥M2 SXR flares from 2006 to 2014. This database is outside the 1995–2005 interval on which ESPERTA was developed. To assess the difference in the general level of activity between these two intervals, we compared the occurrence frequencies of SXR flares and SEP events for the first six years of cycles 23 (1996 September–2002 September) and 24 (2008 December–2014 December). We found a reduction of SXR flares and SEP events of 40% and 46%, respectively, in the latter period. Moreover, the numbers of ≥M2 flares with high values of SXR and ∼1 MHz fluences (>0.1 J m−2 and >6 × 105 sfu × minute, respectively) are both reduced by ∼30%. A somewhat larger percentage decrease of these two parameters (∼40% versus ∼30%) is obtained for the 2006–2014 interval in comparison with 1995–2005. Despite these differences, ESPERTA performance was comparable for the two intervals. For the 2006–2014 interval, ESPERTA had a probability of detection (POD) of 59% (19/32) and a false alarm rate (FAR) of 30% (8/27), versus a POD = 63% (47/75) and an FAR = 42% (34/81) for the original 1995–2005 data set. In addition, for the 2006–2014 interval the median (average) warning time was estimated to be ∼2 hr (∼7 hr), versus ∼6 hr (∼9 hr), for the 1995–2005 data set.

BARREL observations of a solar energetic electron and solar energetic proton event

AbstractDuring the second Balloon Array for Radiation Belt Relativistic Electron Losses (BARREL) campaign two solar energetic proton (SEP) events were observed. Although BARREL was designed to observe X‐rays created during electron precipitation events, it is sensitive to X‐rays from other sources. The gamma lines produced when energetic protons hit the upper atmosphere are used in this paper to study SEP events. During the second SEP event starting on 7 January 2014 and lasting ∼3 days, which also had a solar energetic electron (SEE) event occurring simultaneously, BARREL had six payloads afloat spanning all magnetic local time (MLT) sectors and L values. Three payloads were in a tight array (∼2 h in MLT and ∼2 ΔL) inside the inner magnetosphere and at times conjugate in both L and MLT with the Van Allen Probes (approximately once per day). The other three payloads mapped to higher L values with one payload on open field lines for the entire event, while the other two appear to be crossing from open to closed field lines. Using the observations of the SEE and SEP events, we are able to map the open‐closed boundary. Halford et al. (2015) demonstrated how BARREL can monitor electron precipitation following an interplanetary shock created by a coronal mass ejection (ICME‐shock) arrival at Earth, while in this study we look at the SEP event precursor to the arrival of the ICME‐Shock in our cradle‐to‐grave view: from flare, to SEE and SEP events, to radiation belt electron precipitation.

Extreme Geomagnetic Storms – 1868 – 2010

We present the first large statistical study of extreme geomagnetic storms based on historical data from the time period 1868 – 2010. This article is the first of two companion papers. Here we describe how the storms were selected and focus on their near-Earth characteristics. The second article presents our investigation of the corresponding solar events and their characteristics. The storms were selected based on their intensity in the aa index, which constitutes the longest existing continuous series of geomagnetic activity. They are analyzed statistically in the context of more well-known geomagnetic indices, such as the Kp and Dcx/Dst index. This reveals that neither Kp nor Dcx/Dst provide a comprehensive geomagnetic measure of the extreme storms. We rank the storms by including long series of single magnetic observatory data. The top storms on the rank list are the New York Railroad storm occurring in May 1921 and the Quebec storm from March 1989. We identify key characteristics of the storms by combining several different available data sources, lists of storm sudden commencements (SSCs) signifying occurrence of interplanetary shocks, solar wind in-situ measurements, neutron monitor data, and associated identifications of Forbush decreases as well as satellite measurements of energetic proton fluxes in the near-Earth space environment. From this we find, among other results, that the extreme storms are very strongly correlated with the occurrence of interplanetary shocks (91 – 100 %), Forbush decreases (100 %), and energetic solar proton events (70 %). A quantitative comparison of these associations relative to less intense storms is also presented. Most notably, we find that most often the extreme storms are characterized by a complexity that is associated with multiple, often interacting, solar wind disturbances and that they frequently occur when the geomagnetic activity is already elevated. We also investigate the semiannual variation in storm occurrence and confirm previous findings that geomagnetic storms tend to occur less frequently near solstices and that this tendency increases with storm intensity. However, we find that the semiannual variation depends on both the solar wind source and the storm level. Storms associated with weak SSC do not show any semiannual variation, in contrast to weak storms without SSC.

Real‐time prediction of the occurrence and intensity of the first hours of >100 MeV solar energetic proton events

AbstractA new model for predicting the occurrence of >100 MeV solar energetic proton (SEP) events and the first hours of the >100 MeV integral proton flux is presented. This model uses a novel approach based on the lag correlation between strong positive derivatives of X‐ray flux and proton flux. The new model has been validated with data from January 1994 to September 2013, obtaining a probability of detection of all >100 MeV SEP events of 80.85%, a false alarm ratio of 29.62%, and an average warning time of 1 h and 6 min. The model identifies the associated flare and active region. Currently, there is no other automatic empirical or physics‐based system able to predict SEP events of energies in the interval of 100 MeV to ~430 MeV (lower GLE cutoff according to Clem and Dorman (2000)). This paper also proposes the combined use of the new prediction model and the existing one for predicting >10 MeV SEP events. The combined SEP prediction models have been developed to improve mitigation of adverse effects on near‐Earth and interplanetary missions.