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Much of the concern for operational space systems arises from the Earth’s radiation belts. A modern view of the radiation belts derived from the Van Allen Probes observations of Baker et al. (Baker, Kanekal, Hoxie, Henderson, et al., 2013) is shown here as Figure 1.1. Closest to the Earth’s surface is the inner Van Allen belt. This belt extends from just above the dense atmosphere out to an equatorial altitude of about 10,000 km above the Earth’s surface. The inner Van Allen belt is comprised dominantly of very energetic protons (ranging up to multiple GeV energies). Recent results demonstrate that protons with energies from ~10 MeV to ~100 MeV are quite stable in time near the geocentric radial distance of r ~ 1.5R_E (Earth radii = 6372 km), at which the inner zone proton fluxes peak. However, an outer “shoulder” of the radial distribution from 1.7 ≤ r ≤ 2.5 R_E shows tremendous temporal variability for protons with E ≤ 60 MeV. These variable proton fluxes are probably due primarily to evolution of trapped solar energetic protons (Selesnick et al., 2014).

Figure 1.1 A modern‐day view of the Earth’s radiation belts as observed by the Van Allen Probes mission.

(Adapted from Baker, Kanekal, Hoxie, Henderson, et al., 2013.)

The inner Van Allen belt also has copious fluxes of low‐and medium‐energy electrons (Fennell et al., 2015; Li et al., 2015) as revealed by Van Allen Probes and other spacecraft data sets (Baker, Kanekal, Hoxie, Batiste, et al., 2013). However, the Van Allen Probes era (September 2012–present) has provided many new discoveries about inner magnetospheric ultrarelativistic electrons with energies E ≳ 5 MeV. In particular, initial results after major storm intervals have shown essentially no detectable prompt ultrarelativistic electron fluxes in the region r ≤ 2.8 R_E (Baker et al., 2014). The paucity of very energetic electrons in the inner magnetosphere immediately following major magnetic storms is quite striking (Foster, 2016; Baker et al., 2016) with a fascinating dependence on plasmasphere conditions before the storm and perhaps even on in‐situ radio signals of terrestrial origin (Foster et al., 2016). The consequences of this energetic electron paucity for space radiation effects will be discussed further in this chapter.

The space weather concerns for the inner radiation zone are several. The intense, high‐energy trapped protons are extremely damaging to space systems (Vette et al., 1966) and are quite hard to shield against. There are also more variable, trapped solar energetic ions in the inner zone that can cause dose and single‐event effects (see Lorentzen et al., 2002; Baker, 2002). The hazardous proton populations of the inner zone and slot regions have two fundamental sources. In the inner zone, the primary source is the neutron albedo decay process, which has been well defined (Selesnick et al., 2007, and references therein). The outer edges of the inner zone and the lower part of the slot region also host energetic protons and ions that consist of entrained solar particles (Mazur et al., 2005; Selesnick et al., 2007). The inner zone also has trapped galactic cosmic rays (see Klecker et al., 1995; Cummings et al., 1993). Finally, the trapped energetic electrons with E ≤ 1MeV (Claudepierre et al., 2017; Fennell et al., 2015) also represent a further significant source causing total dose effects.

The so‐called “slot region” of the radiation belts extends from roughly L ~ 2.0 to L ~ 3.0 depending on particle energy and species. (L is the geocentric distance in Earth radii at which a dipole magnetic field line crosses the magnetic equatorial plane.) The slot is a region often relatively devoid of energetic electrons. However, during strong geomagnetic storm periods, the gap between the inner and outer zone can be filled to a large degree by moderate (and even high) energy electrons (Fennell et al., 2005). For example, in the intense “Halloween” storm period of late October and November 2003, the slot region was filled with multi‐MeV electrons for several weeks (Baker et al., 2004). Thus, the slot region can present several space weather concerns including low‐ and medium‐energy electron enhancements, multi‐MeV electrons (on rare occasions), and strong solar energetic particle events (again on relatively rare occasions).

Finally, the outer Van Allen radiation belt represents in many ways the most pervasive space weather risks to operating spacecraft. The outer radiation belt is broad in spatial extent (from r ~ 3R_E to r ≳ 6.5 R_E). It is comprised of mildly to highly energetic electrons (~100keV to ≳10 MeV) and varies widely in particle intensity. Commercial, military, and scientific satellites operating in medium‐Earth orbit and geostationary Earth orbit number in the multiple hundreds worldwide. All of these operating spacecrafts are subject to outer Van Allen belt space radiation impacts.

Obviously, from the above brief description it is clear that the space radiation environment can cause a wide variety of impacts on space systems. Having a deeper understanding of radiation properties including dynamics and temporal trends is crucial for our technological society. This chapter describes the current knowledge of these space radiation aspects.