Solar Energetic Particles and Space Weather: Science and Applications
This chapter provides an overview on solar energetic particles (SEPs) and their association to space weather, both from the scientific as well as from the applications perspective. A historical overview is presented on how SEPs were discovered in the 1940s and how our understanding has increased and evolved since then. Current state-of-the-art based on unique measurements obtained in the 3-dimensional heliosphere (e.g. by the Ulysses, ACE, STEREO spacecraft) and their analysis is also presented. Key open questions on SEP research expected to be answered in view of future missions that will explore the solar corona and inner heliosphere are highlighted. This is followed by an introduction to why SEPs are studied, describing the risks that SEP events pose on technology and human health. Mitigation strategies for solar radiation storms as well as examples of current SEP forecasting systems are reviewed, in context of the two novel real-time SEP forecasting tools developed within the EU H2020 HESPERIA project.
1.1.1 Historical Perspective of Solar Energetic Particle (SEP) Events
It is widely accepted that protons, electrons, and heavier nuclei such as He-Fe are accelerated from a few keV up to GeV energies in at least two distinct locations, namely the solar flare and the coronal mass ejection (CME)-driven interplanetary (IP) shock. The particles observed in IP space and near Earth are commonly referred to as solar energetic particles (SEPs). Those accelerated at flares are known as impulsive SEP events, particle populations accelerated by near-Sun CME-shocks are termed as gradual SEP events, and those associated with CME shocks observed near Earth are known as energetic storm particle (ESP) events (Desai and Giacalone 2016).
Since then, observations have indicated that there are ‘hybrid’ or mixed event cases, where both mechanisms appear to contribute, with one accelerating mechanism operating in the flare while the other operates at the CME-driven shock (Kallenrode 2003). Such hybrid events may result from the re-acceleration of remnant flare suprathermals by shock waves (Mason et al. 1999; Desai et al. 2006) or from the interaction of CMEs (Gopalswamy et al. 2002). It is noteworthy however, that based on large enhancements in the Fe/O during the initial phases of two large SEP events observed by Wind and Ulysses when the two spacecraft (s/c) were separated by 60° in longitude (Tylka et al. 2013) argued that the initial Fe/O enhancements cannot be cited as evidence for a direct flare component, but instead they are better understood as a transport effect, driven by the different mass-to-charge ratios of Fe and O.
High-energy protons in the largest SEP events can pose significant radiation hazards for astronauts and technological systems in space, particularly beyond the Earth’s protective magnetic field (National Research Council (NCR) 2008; Cucinotta et al. 2010; Xapsos et al. 2012) (see Sect. 1.2 for more details).
1.1.2 Large Gradual SEP Events
A flatter size distribution of SEP events relative to that of flare soft X-ray (SXR) events has been previously reported, with the power-law characterizing SEP size being significantly flatter than that of the SXR flux (e.g. Hudson 1978; Belov et al. 2007; Cliver et al. 2012). Cliver et al. (2012) have shown that this difference is primarily due to the fact that flares associated with large gradual SEP events are an energetic subset of all flares also characteristically accompanied by fast (>1000 km/s) CMEs that drive coronal/IP shock waves. They also concluded that the difference of ∼0.15 between the slopes of the SEP event distributions and SEP SXR flares is consistent with the observed variation of SEP event peak flux with SXR peak flux. Kahler (2013b) presented arguments against using scaling laws for the description of the relationship between the size distributions of SXR flares and SEP events. They suggested an alternative explanation for flatter SEP power-law distributions in terms of the recent model of fractal-diffusive self-organized criticality proposed by Aschwanden (2012), providing evidence against a close physical connection of flares with SEP production. Trottet et al. (2015), although based on a limited SEP event sample, have recently studied the statistical relationships between SEP peak intensities of deka-MeV and near-relativistic electrons and characteristic parameters of CME and solar flares: the CME speed as well as the peak flux and fluence of SXR emission and the fluence of microwave emission. Via a partial correlation analysis they showed that the CME speed and SXR fluence are the only parameters that significantly affect the SEP intensity and concluded that both flare acceleration and CME shock acceleration contribute to the deka-MeV proton and near-relativistic electron populations in large SEP events.
1.1.3 Ground Level Enhancement (GLE) Events
Ground Level Enhancement (GLE) events form a particular case of high-energy SEP events associated with ∼GeV protons. These events pose severe radiation hazards to astronauts and technological assets in space and disrupt airline communications (Shea and Smart 2012). GLEs are nowadays measured with better coverage from space than at ground level, including ∼80 MeV/amu to ∼3 GeV/amu H and He spectra (Adriani et al. 2011), onsets (Reames 2009a, b), energy spectral shapes and abundances (Mewaldt et al. 2012b), electrons (Kahler 2007, 2012; Tan et al. 2013) and general properties (Gopalswamy et al. 2012). Rouillard et al. (2016) recently studied the link between an expanding coronal shock and the energetic particles measured near Earth during the GLE of 17 May 2012. The analysis showed that the GLE event occurred inside a clear magnetic cloud (see e.g. Malandraki et al. 2002). Using a new technique developed to triangulate the three-dimensional (3D) expansion of the shock forming in the corona it was found that the highest Mach number (MFM) values appear near the coronal neutral line within a few minutes of the CME onset. This neutral line is usually associated with the source of the heliospheric current sheet (HCS) and plasma sheet. It was shown that the release time of GeV particles occurs when the coronal shock becomes super-critical (MFM > 3).
1.1.4 Multi-Spacecraft Observations of SEP Events
The Ulysses European Space Agency (ESA)/National Aeronautics and Space Administration (NASA) mission provided unprecedented observations of the 3D heliosphere inside ∼5 AU. Comparison of simultaneous SEP observations near the ecliptic plane with the Ulysses observations at high latitudes showed that most events that produce large high-energy (>20 MeV) proton and near-relativistic electron flux increases near Earth also produce flux increases at high latitudes, regardless of the longitudinal, latitudinal and radial separation between the s/c, although with somewhat lower maximum intensities and slower rise at Ulysses (McKibben et al. 2003; Lario and Pick 2008; Malandraki et al. 2009). Particle anisotropies during SEP events at high latitudes are typically directed outward from the Sun and aligned with the local magnetic field (McKibben et al. 2003; Malandraki et al. 2009).
The observed field-aligned anisotropies, with components perpendicular to the local magnetic field that are essentially zero, indicate that there is no net flow of particles across the local magnetic field. The Ulysses observations revealed the 3D nature of the reservoir effects in the heliosphere. Dalla et al. (2003) concluded that the presence of a shock is not necessary for creating the near-equality observed at Ulysses and near Earth decay phases, but that these observations are better explained by diffusion across the interplanetary magnetic field (IMF).
More recently, combined observations by the twin STEREO s/c as well as near-Earth observatories revealed the wide longitudinal spreads of large gradual SEP events in the heliosphere and even strongly questioned the constraint of a narrow spread for 3He-rich events (Wiedenbeck et al. 2013). A combination of physical processes appears to cause the large longitudinal spread of high-energy particles. Dresing et al. (2014) concluded that both an extended source region at the Sun and perpendicular transport in the IP medium are involved for most of the wide-spread events under study. The studies (Rouillard et al. 2011, 2012) found that the delayed SEP release times at STEREO and L1 are consistent with either the time required for the CME shock to reach field lines connected to the s/c or with the time required (∼30–40 min) for the CME to perturb the corona over a wide range of longitudes. Observations by Gómez-Herrero et al. (2015) indicated that higher SEP fluxes, harder SEP spectra and direct injections of SEPs onto well-connected IMF lines are associated with lateral expansions of CME-driven shocks in the low corona, and may therefore be responsible for the rapid longitudinal spread as observed at vastly distributed s/c in many SEP events. Other factors that may also play a role in distributing SEP events longitudinally include the large-scale IMF configuration inside interplanetary CMEs (ICMEs) (e.g. Kahler and Vourlidas 2013) and the relative strength of the CME shock, which depends on the local Alfvén speed, rather than the actual speed of the CME (e.g. Gopalswamy et al. 2014).
1.1.5 Particle Acceleration
Recently, important progress has occurred, both from the theoretical and the observational perspective in the research of small-scale magnetic islands in the solar wind and their role in particle acceleration. Khabarova et al. (2015) presented observations that show the occurrence of small-scale magnetic islands and related plasma energization in the vicinity of the HCS. They found evidence that magnetic islands experience dynamical merging in the solar wind and that increases of energetic particle fluxes in the keV–MeV range are found to coincide with the presence of magnetic islands confined by strong current sheets (CSs). Moreover, the interaction of ICMEs with the HCS can lead to significant particle acceleration due to plasma confinement. Their observations confirmed the rippled structure of the HCS and since such a structure confines plasma, it makes possible the strong energization of particles trapped inside small-scale magnetic islands. They concluded that although initial particle acceleration due to magnetic reconnection at the HCS may be insufficient to obtain high energies, the presence of magnetic islands inside the ripples of the HCS or between two CSs with a strong guide field offers the possibility of re-accelerating particles in the ways discussed theoretically by Zank et al. (2014) and le Roux et al. (2015a).
Khabarova et al. (2016) further explored the role of the heliospheric magnetic field configurations and conditions that favor the generation and confinement of small-scale magnetic islands associated with the so-called atypical energetic particle events (AEPEs) in the solar wind. Some AEPEs have been found not to align with standard particle acceleration mechanisms, such as flare-related or simple diffusive shock acceleration processes related to ICMEs and corotating interaction regions (CIRs). They provided more observations fully supporting the idea and the theory of particle energization by small-scale-flux-rope dynamics previously developed by Zank et al. (2014, 2015a, b) and le Roux et al. (2015a, b). If the particles are pre-accelerated to keV energies via classical mechanisms, they may be additionally accelerated up to 1–1.5 MeV inside magnetically confined cavities of various origins. Khabarova et al. (2016) showed that particle acceleration inside magnetic cavities may explain puzzling AEPEs occurring far beyond IP shocks, within ICMEs, before approaching CIRs, as well as between CIRs. SEP transport processes are described in detail in Chap. 4 of this volume (see also Desai and Giacalone (2016) which includes a review on this topic).
1.1.6 Key Open Questions and Future Missions
Solar Orbiter (SolO) is a unique ESA/NASA joint mission conceived to unveil the Sun-heliosphere connection (Mueller et al. 2013), expected to be launched in 2019. The orbital configuration includes a close perihelion, high inclination intervals allowing the observation of the solar polar regions and quasi-co-rotation periods. One of the top-level science questions is “How do solar eruptions produce energetic particle radiation that fills the heliosphere?” which can be broken down into three inter-related key topics: What are the seed populations for energetic particles? How and where are energetic particles accelerated at the Sun? How are energetic particles released from their sources and distributed in space and time?
The Energetic Particle Detector (EPD) instrument suite onboard SolO (Principal Investigator: Prof. J. Rodríguez-Pacheco, Spain) will measure energetic electrons, protons and ions, operating at partly overlapping energy ranges covering from a few keV to 450 MeV/nucleon. The EPD sensors will measure the composition, spectra and anisotropies of energetic particles with sufficient temporal, spectral, angular and mass resolution to achieve the mission goals (Gómez-Herrero et al. 2016). Energetic particles escaping from the acceleration sites propagate through the turbulent IMF. Previous observations by the Helios s/c have shown that SEP events near the Sun are much less affected by IP transport effects compared to 1 AU observations. As the s/c moves further away from the Sun, scattering and diffusion processes become more important and multiple injections closely spaced in time cannot be resolved (Wibberenz and Cane 2006). Thus, SolO observations close to the perihelion will be crucial to unveil SEP injection, acceleration, release and transport processes, in view of the ongoing debate about the SEP acceleration sites, disentangling the acceleration at CME-driven shocks and at reconnection sites in solar flares (e.g. Malandraki et al. 2006).
NASA’s Solar Probe Plus mission, recently re-named to Parker Solar Probe to honour pioneering physicist Prof. Eugene Parker, will fly within nine solar radii of the Sun’s surface and is scheduled to be launched in July 2018. The two Energetic Particle Instruments (EPI) of the Integrated Science Investigation of the Sun (ISIS) (Principal Investigator: Prof. D. J. McComas, USA) will measure lower (EPI-Lo) and higher (EPI-Hi) energy particles. EPI-Lo will measure ions and ion composition from ∼20 keV/nucleon–15 MeV total energy and electrons from ∼25–1000 keV. EPI-Hi measures ions from ∼1–200 MeV/nucleon and electrons from ∼0.5–6 MeV. The unique ISIS observations will allow the exploration of the mechanisms of energetic particles dynamics, including their (1) Origin: defining the seed populations and physical conditions necessary for energetic particle acceleration; (2) Acceleration: determining the role of shocks, reconnection, waves and turbulence in accelerating energetic particles; (3) Transport: revealing how energetic particles propagate from the corona out into the heliosphere (McComas et al. 2016).
It is evident that the next decade is expected to revolutionize our understanding of SEP acceleration and transport, by means of state-of-the-art sensors on board these two upcoming missions providing unique and unprecedented measurements for the exploration of the solar corona and inner heliosphere. Synergies between the two missions are of particular relevance, since both missions have overlapping timelines and the Parker Solar Probe perihelion, reaching up to ∼9 solar radii, will permit simultaneous in-situ observations at the SEP acceleration region close to the Sun and at larger radial distances, with continuous remote sensing coverage provided by SolO and near-Earth s/c. Radial alignments between the two s/c will enable the observations of plasma ‘entities’ from the same solar source region at progressive radial distances as well as the study of energetic particle radial gradients. Furthermore, other useful configurations for the optimization of the science return are alignments along the same IMF line allowing the observation of SEPs originating at the same acceleration site by two or more s/c located at different radial distances. SEP event observations by multiple s/c located at widely separated points in the heliosphere, both in longitude and in latitude, will be valuable for the investigation of the spatial distribution of SEPs and the unraveling of the physical mechanisms responsible for producing wide-spread SEP events (see e.g. Sect. 1.1.4) (Gómez-Herrero et al. 2016).
1.2.1 Why Study SEP Events?
In the case of nuclear power a Carrington event may not be a sufficient case since relevant timescales for risk assessment may be as long as 10,000 years.
(Paul Cannon (Cannon et al. 2013))
In the following some of the most important and common SEP induced effects are presented, as well as mitigation strategies currently being relied on.
1.2.2 SEP Effects on Technology
Single Event Upsets: Occurs in logical circuits and is defined as a bit switching from an initial logical state to an opposite logical state.
Single Event Latchup: Results in a high operating current, above device specifications, and must be cleared by a power reset.
Single Event Gate Rupture: Occurs in powerful transistors and is manifested by an increase in gate leakage current
Single Event Burnout: A condition that can cause device destruction due to a high current state in a power transistor.
Particle effects on technology observed as a function of the particle energy range
Protons <10 MeV
Material and solar cell effects over time as a result of cumulated dose (e.g. solar cell degradation)
Protons >10 MeV
Nuclear interactions (e.g., sensor background noise, ionization, displacement damage)
Protons >50 MeV and
Ions >10 MeV nucleon−1
Nuclear interactions (e.g., single event effects in electronics onboard satellites, as well as aircraft)
For s/c mission planning and operations SEP events are considered. In regard to launch operations the SEP environment is also a decisive factor whether to give the go ahead to launch or not for several reasons. Launch vehicles and s/c reaching sufficiently high geomagnetic latitudes could for example see an increase in SEE rates at times of significant SEP events. On the other hand, optical instruments are also vulnerable to SEPs and induced sensor interference can disrupt the operation of star trackers and put critical s/c manoeuvres at risk.
Under normal space weather conditions Earth’s magnetosphere acts as a shield and protects us from charged particles and magnetic clouds. Nevertheless at times SEPs may have sufficient energies to “break” through this shield and enter the ionosphere; SEPs have easier access to the polar regions near Earth’s magnetic poles than at the equator due to the “open” magnetic field lines. The cutoff latitude is a function of a particle’s momentum per unit charge and is referred to as its rigidity (see Chap. 5). Variations in SEP access to latitudes can occur on time scales of an hour or less in response to changes in the solar wind dynamic pressure and IMF (Kress et al. 2010). For this reason high inclination LEO satellites can at times be vulnerable to SEPs, as well as the International Space Station that has an orbital inclination of 51.64°.
SEP events can also effect signal propagation between Earth and satellites. Polar cap absorption (PCA) events result from intense ionisation of the D-layer of the polar ionosphere by strong (>10 MeV) SEP events. Due mainly to protons with energy in the range of 1–100 MeV (corresponds to an altitude between 30 and 80 km) the increased ionisation absorbs radio waves in the HF and VHF bands, resulting in problems for communications (degraded radio propagation through the polar regions) and navigation position errors with the importance being a function of the individual SEP event.
Despite the relative steepness of SEP energy spectra, the small percentage of protons accelerating up to high energies (>500 MeV) still pose considerable problems. These high-energy SEP events such as the September 1989 SEP event (Fig. 1.3) are often associated with GLE events and can result in secondary radiation caused by particles interacting with s/c shielding and other material. This results in the production of particles such as lower energetic protons, neutrons, and pions that in some cases may be more of an obstacle for the s/c designer than the primary SEPs themselves. While the former can induce SEEs, secondary particle background can have more profound effects on sensitive space-borne instrumentation.
Technology onboard commercial airline operations can also be affected by SEP events including avionics (electronic systems), communications and GPS navigation systems (Jones et al. 2005). Specifically ultra-long-haul “over-the-pole” routes and high-latitude flights are susceptible to these SEP induced effects.
1.2.3 SEPs and Human Health Effects
In addition to being a threat to technology, SEP events are also an important risk to human health. Since the Apollo missions to Earth’s Moon in the 1970s human space exploration has mainly been focused on low-Earth orbit altitudes (e.g. Space Shuttle, International Space Station) and suborbital flights. Outside Earth’s magnetosphere SEP events have for the most part been a concern for robotic flight missions up until now. During the last decades the vision for space exploration has changed as space agencies and private companies are contemplating sending humans to Mars and asteroids, and as the population on Earth increases colonizing such targets and pursuing deep space exploration will only become more and more attractive. The downside is that human interplanetary exploration will expose astronauts not only to the galactic cosmic ray background but at times also to increased levels of radiation during SEP events and this may indeed be the most important obstacle to overcome.
Deterministic (early) Effects: Due to exposure to a large dose of radiation for a limited time (ranges from hair loss, nausea, acute sickness, death)
Stochastic (late) Effects: Due to random radiation-induced changes at the deoxyribonucleic acid (DNA) molecule level (cancer).
As already mentioned in Sect. 1.1.1 protons with high energies (>30 MeV) are a health risk for astronauts. For this reason protons with energies >10 MeV are continuously monitored and taken into account when planning extra-vehicular activities.
In those instances SEP events reach aviation altitudes they become also a concern for human health as the radiation dose received can increase. This specifically applies to high-latitude flights (>50°N) and polar routes (>78°N). For commercial aviation this can be a risk for frequent flyers and particularly for aircrew. Effective pilot training programs as well as monitoring, measuring, and regulatory measures in regard to radiation exposure risks for both human tissue and avionics are recognized by a broad community (Tobiska et al. 2015).
1.2.4 Mitigating the Effects of SEPs
The SEP radiation environment is assessed when designing s/c, for s/c mission planning and operations, and when human spaceflight is involved. How does one best go about protecting assets in space and on the ground from the effects of SEP events? For this purpose post-event analysis “hazard assessment” is performed after an anomaly occurs (is recorded). Furthermore, mitigation procedures are put in place before launch (e.g. s/c shielding, redundancy onboard) and during operations SEP forecasting takes place.
18.104.22.168 Hazard Assessment
Information obtained from hazard assessment can provide useful input for both engineering (mitigation) and scientific approaches (forecasting), and establish a one-to-one correction between space environment conditions and technical failures. However, s/c operators are sometimes reluctant in providing anomaly data due to confidentiality issues specifically when the anomaly is an important one (e.g. loss of s/c); this makes it sometimes difficult to assess whether a failure was due to the space environment or not. To complement hazard assessment one therefore relies on efficient mitigation strategies such as s/c shielding and forecasting techniques.
22.214.171.124 Mitigation Procedures
The classical engineering approach is based on passive shielding that can protect the crew and hardware (exterior and interior) of the s/c and understanding how the space environment interacts with the shielding. For this reason SEP energy spectra are used as input in engineering tools when computing induced effects such as the dose encountered on technology and humans during SEP events. The shape of the spectrum is important as worst-case scenarios are application dependent, meaning that the flux intensity at lower energies are important for material and solar cell effects. At the other end of the energy spectrum, flux intensities at higher energies are more important for nuclear interactions (e.g., background noise, single event upsets).
theoretical understanding (e.g. physical models),
remote sensing of phenomena such as solar flares, CMEs and active regions
space-based in-situ observations at L1 (shock arrival, energetic storm particles) and GEO
ground-based observations (e.g. radio, neutron monitors).
Physics-based numerical models (e.g. Earth-Moon-Mars Radiation Environment Module (EMMREM) (Schwadron 2010), Predictions of radiation from REleASE, EMMREM and Data Incorporating CRaTER, COSTEP and other SEP measurements (PREDICCS) (Schwadron 2012), Solar Energetic Particle MODel (SEPMOD) (Luhmann et al. 2010), SOLar Particle ENgineering Code (SOLPENCO) (Aran et al. 2006), and SOLPENCO2 (provides SEP modelling away from 1 AU to the SEP statistical model of the SEPEM project (Crosby et al. 2015)))
Empirical models (e.g. University of Malaga Solar Energetic Particle (UMASEP) system (Núñez 2011), Relativistic Electron Alert System for Exploration (REleASE) (Posner 2007), Proton Prediction System (PPS) (Kahler et al. 2007), PROTONS system (Balch 2008), GLE Alert Plus (Kuwabara et al. 2006; Souvatzoglou et al. 2014) and Laurenza’s approach (Laurenza et al. 2009))
In some cases forecasting systems rely on methods from both categories such as the SEPForecast tool built under the EU FP7 COMESEP project (263252) (Crosby et al. 2012), (http://www.comesep.eu/alert/).
The HESPERIA UMASEP-500 tool makes real-time predictions of the occurrence of GLE events, from the analysis of SXR and differential proton flux measured by the GOES satellite network.
The HESPERIA REleASE tool generates expected proton flux alerts at two energy ranges (15.8–39.8 MeV and 28.2–50.1 MeV) making use of relativistic electrons (v > 0.9 c) provided by the Electron Proton Helium Instrument (EPHIN) on SOHO and near-relativistic (v < 0.8 c) electron measurements from the Electron Proton Alpha Monitor (EPAM) aboard the Advanced Composition Explorer (ACE).
When observing images of the Sun east and west are reversed.
Olga E. Malandraki has been partly supported by the International Space Science Institute (ISSI) in the framework of International Team 504 entitled "Current Sheets, Turbulence, Structures and Particle Acceleration in the Heliosphere".
- Cane, H.V., et al.: J. Geophys. Res. 111(A10), A06S90 (2006)Google Scholar
- Cannon, P., et al. (eds.): Extreme space weather: impacts on engineered systems and infra-structure. Royal Academy of Engineering, London (2013)Google Scholar
- Cliver, E.W.: CP528: acceleration and transport of energetic particles observed in the heliosphere. In: Mewaldt, R.A., et al. (eds.) ACE 2000 Symposium, California, January 2000. AIP Conf. Proc., vol. 528, p. 21. AIP, New York (2000)Google Scholar
- Crosby, N.B., et al.: AIAC ’11: space weather: the space radiation environment. Hu, Q., et al. (eds.) 11th Annual International Astrophysics Conference, PalmSprings, USA, March 2012. AIP Conf. Proc., vol. 1500, p. 159. AIP, New York (2012)Google Scholar
- Desai, M.I., Giacalone, J.: J. Living Rev. Sol. Phys. 13, 3 (2016)Google Scholar
- Gómez-Herrero, R., et al.: XXV European Cosmic Ray Symposium, Turin, 4–9 September (2016)Google Scholar
- le Roux, J.A., et al.: AIAC ’14: Linear and nonlinear particle energization throughout the heliosphere and beyond. In: Zank, G.P. (eds). 14th Annual International Astrophysics Conference, Florida, April 2015. J. Phys.: Conc. Ser., vol. 642, p. 012015. IOP Publishing, USA (2015b)Google Scholar
- Malandraki, O.E., et al.: Proceedings of the second solar orbiter workshop, Athens, 16–20 October 2006. ESA Publication (2006)Google Scholar
- Mewaldt, R.A., et al.: AIAC ’11: Space weather: the space radiation environment. Hu, Q. et al. (eds.) 11th Annual International Astrophysics Conference, PalmSprings, USA, March 2012. AIP Conf. Proc., vol. 1500, p. 128. AIP, New York (2012a)Google Scholar
- National Research Council (NCR): Managing space radiation risk in the new era of space exploration. The National Academies Press, Washington, DC (2008)Google Scholar
- Roelof, E.C.: AIAC ’11: Space weather: the space radiation environment. In: Hu, Q., et al. (eds.) 11th Annual International Astrophysics Conference, PalmSprings, USA, March 2012. AIP Conf. Proc., vol. 1500, p. 174. AIP, New York (2012a)Google Scholar
- Roelof, E.C.: AIAC ’11: Space weather: the space radiation environment. In: Hu, Q., et al. 11th Annual International Astrophysics Conference, PalmSprings, USA, March 2012. AIP Conf. Proc., vol. 1500, p. 180. AIP, New York (2012b)Google Scholar
- Schwadron, N.A.: Space Weather. 8, 1 (2010)Google Scholar
- Tylka, A.J., et al.: CP528: Acceleration and transport of energetic particles observed in the heliosphere. In: Mewaldt, R.A., et al. (eds.) ACE 2000 Symposium, California, January 2000. AIP Conf. Proc., vol. 528, p. 147. AIP, New York (2000)Google Scholar
- Wilson, J.W., et al. (ed.): Shielding strategies for human space exploration, NASA Conference Publication, 3360 (1997)Google Scholar
- Zank, G.P., et al.: AIAC ’14: linear and nonlinear particle energization throughout the heliosphere and beyond. In: Zank, G.P. (ed.) 14th Annual International Astrophysics Conference, Florida, April 2015. J. Phys.: Conc. Ser., vol. 642, p. 012031. IOP Publishing, USA (2015b)Google Scholar
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.