**3. Radiation doses within the near-earth space environment from SPEs**

Energetic particles driven by CME shocks and flares and GCRs that escape the magnetospheric shielding through the polar regions get trapped by the earth's magnetic field in the Van Allen Belts (VAB). The outer belts consist of energetic electrons with energies up to ≈10 MeV while the inner belt consists of electrons and protons with energies ranging from KeV to ≈10 MeV [24, 25]. Over the South Atlantic Anomaly (SAA) which extends from South America to West Africa, the VAB dips

*Solar Proton Activity over the Solar Cycle 24 and Associated Space Radiation Doses DOI: http://dx.doi.org/10.5772/intechopen.103832*


#### **Table 1.**

*A table showing the properties of proton events associated with GLE in the solar cycle 24.*

down to about 200 km into the upper atmospheric region; the increased energetic particle flux in this region contributes the largest radiation dose during low inclination flights and space missions. The secondary particle showers from the ionization of GCRs contribute mostly to the background radiation levels within our earth's atmosphere throughout the solar cycle. Several spacecraft and satellites launched into different earth orbits and altitudes have inbuilt particle detectors for detecting the energetic particle flux. The energy deposited by these particles is evaluated by models developed using either radiation transport equations or Monte-Carlo calculations to calculate the amount of absorbed energy as radiation dose to the spacecraft.

**Figure 3.** *Dose recorded by a micro-dosimeter on CRaTER from 2010 to 2017.*

**Figure 3** shows the dose recorded by a micro-dosimeter from 2009 to 2017 on CRaTER (https://prediccs.sr.unh.edu//data/craterProducts/doserates/data/2017365/ doserates\_micro\_2017365\_alldays\_allevents.txt). The sudden increase/spikes in dose were due to protons events which were more frequent during the solar maxima periods. The CRaTER is an experiment on the Lunar Reconnaissance Orbiter (LRO), a NASA spacecraft orbiting the moon. CRaTER characterizes the radiation environment with measurement of effects of ionizing energy loss on silicon solid detectors (three pairs of thin and thick silicon detectors and a micro dosimeter) due to penetrating energetic particles and GCRs. The micro dosimeter is on the analog electronics board with the aluminum shield of a thickness of 2*:*28 g/cm<sup>2</sup> facing space [26].

**Figure 4** shows the correlation relationship (*R*) between dose from 2010 to 2017 and GCR intensity and sunspot number. GCR intensity was got from Oulu Neutron monitor in the NEST database (https://www.nmdb.eu/nest/) and sunspot number was got Omniweb database (https://omniweb.gsfc.nasa.gov/form/dx1.html). **Figure 4a** clearly shows that there is a high positive correlation between the dose and the GCR intensity while **Figure 4b** shows a significant negative correlation between dose and sunspot number. Solar activity modulates GCRs with 90% of the GCRs filtered out by heliospheric solar wind [27]. This effect decreases with solar wind pressure decrease and this results in change in the controlling heliospheric transport processes. The heliospheric transport processes (diffusion, convection, and adiabatic acceleration) are dominated by cosmic rays during solar minima however this shifts to solar wind dominated transport processes during the solar maxima. This suppresses GCRs propagation, hence a decrease in count rate is observed on the ground neutron detectors drinng solar maxima. The correlation relationship between dose and GCRs and SSN conforms to an anti-corelation relation observed between GCRs and sunspot in **Figure 2**, this has been studied by a number of researchers [28, 29]. It should be noted that the modulation of GCR intensity does not only depend on solar activity but also on interplanetary magnetic field which further modifies the transport processes [29]. We can observe from **Figure 4b** that the sunspot number has a less effect on space radiation but only a factor that describes solar activity variation and therefore has a positive relationship with occurrence of proton events.

#### **Figure 4.**

*Correlation plot between micro-dosimeter reading from 2010 to 2017 and cosmic ray intensity (a) and sunspot number (b).*

*Solar Proton Activity over the Solar Cycle 24 and Associated Space Radiation Doses DOI: http://dx.doi.org/10.5772/intechopen.103832*

#### **3.1 Effects of radiation doses**

Radiation damage effects are discussed extensively by several scientists including [7, 9, 30] and references therein. In transistors, the accumulation of total ionizing dose (TID) builds up positive charges on the gate oxide of field-effect transistors causing threshold shifts and off-state leakage currents. At very high dose rates, shifts become large enough to exceed the threshold, this results in defective and poor responsiveness of the critical spacecraft component [30]. In semiconductors, the deposited energy by non-ionization processes is quantified as the displacement damage dose (DDD). The DDD displaces electrons from their initial positions forming vacancies and interstitials— Frenkel pairs. The localized groupings of Frenkel pairs result in the formation of material defects which alters the material properties [6, 31]. This affects satellite components mostly utilizing semiconductors; the most observable effect is the loss of maximum power output due to the degradation of solar cells. Hands et al. [30] evaluated degradation performance on solar cells by evaluating cumulative damage effects of TID and DDD on a Galileo satellite and concluded that the most damage effects of extreme space weather come from trapped electrons rather than solar protons.

Astronauts on long space missions may be affected by cumulative radiation effects and exposed to short periods of high radiation levels during proton events. The radiation effects on humans are either deterministic resulting from immediate effects due to high sudden dose levels or stochastic associated with long-term effects of abnormal tissue growth and cancer arising from cumulative doses. Previous studies using space environment radiation models have calculated an average total body dose of ≈ 0.5–1.5 μGy/min for space missions which may increase in case of SPE occurrence [32, 33]. In comparison, the dose rates are <sup>≈</sup>10<sup>8</sup> <sup>10</sup><sup>10</sup> times lower than the medical exposure dose rates (8, 4 and 2 mGy for abdominal computed tomography, mammography, and radiotherapy sessions respectively). However, radiation-induced cataracts, DNA damage, cancer risks, cell damage, chromatin decondensation are possible biological effects from repeated doses and nonlinear low dose effects [33]. The estimation of biological space radiation damage is more complex attributed to the probabilistic nature of SPE occurrence, very low doses, individual susceptibility and tissue response to radiation, quantification of secondary radiation, and limited dose data for spacecraft crews on long space missions to develop more representative models for radiation effects.

Particle fluxes of secondary cosmic radiation at 18 and 12 km flight altitude are about 500 and 300 times greater than at sea level respectively [34]. These are the main sources of background radiation at aviation flight altitudes. However, very large and intense SPEs and GLEs may increase the radiation exposure levels to exceed the recommended threshold safety levels (effective dose limits of 20 mSv/year averaged over five years for radiation workers and 1 mSv/year for the public) by the international radiation protection community and the International Civil Aviation Organization (ICAO) [35]. Dosimeter measurements and model simulations have shown an increase in radiation levels during specific proton and GLE events, for example, radiation dose levels on Lufthansa flights between Munich and Chicago rose from 3.4 to 5.7 μSv/hr. during GLE 65 [36] while on Qantas 747 flight from Los Angeles to New York City, the radiation levels increased from 3.4 to 4.7 μSv/hr. during the GLE 66 [37], simulated effective dose rates at 12 km altitude were 5.8 μSv/hr. for GLE 70 and 4.5 μSv/hr. for non-GLE SPE (9th November 2000) [8]. Mishev and Usoskin [38] estimated about 100 μSv for a polar flight at 12.5 km altitude for 3.5 hours from during GLE 72 using rigidity spectrum. While the modeled and simulated doses obtained may not be the actual effective radiation doses to the aircrew but it's indicative of a possibility of radiation hazard on the aviation industry associated with SPEs. A survey on radiation exposure to Canadian pilots showed an annual dose of about 3 mSv [39]. However, the estimation of total effective radiation doses to the aircrew requires consideration of flight path, time of flight, flight altitude, the position of the aircrew within the aircraft, and material component of the aircraft (shielding design). The International Commission on Radiation Protection (ICRP) recommendations in 2007 recommended that the aircrew be considered as occupationally exposed workers with the implementation of practical regulatory measures including monitoring programs and individual dose assessment for radiation protection by aviation operators [40]. Several warning systems of aviation dosimetry [10, 41, 42] have been developed in addition to S-scale provided by NOAA SWPC alerts the aviation operators to initiate the safety protocols which include rerouting, flying at low altitudes, avoiding polar regions, and flight delay. However, they are challenged by the probabilistic occurrence of SPEs and the description of the broad energy spectrum from low to high energies [10].

#### **3.2 Quantification of space radiation doses from SPEs**

Energetic particles incident on spacecraft and satellite materials deposits energy by ionization process, these particles include proton, trapped protons, and electrons in the VAB, secondary photons, and GCRs. The energy absorbed is measured as TID of which cumulative effects can result in device failure of critical spacecraft electronics and biological damage to astronauts. TID is quantified by absorbed dose (SI unit called rad, that is 1 gray (Gy) = 100 rads = 1 J/kg). The absorbed dose (*D*) is a function of fluence spectrum at different energy levels (∅) and the mass stopping power of the material (d*E=ρ*d*x*) through which the incident particle penetrates (Eq. (1)); this is dependent on the orbit altitude, time taken in the orbit, and spacecraft/satellite orientation [43].

$$D = \mathcal{Q} \frac{\text{d}E}{\rho \text{d}\mathbf{x}} \left( \mathbf{1.6} \times \mathbf{10}^{-10} \right) \text{Gy} \tag{1}$$

The Bethe-Bloch formula [44] defines stopping power using relativistic quantum mechanics (Eq. (2)):

$$\frac{\text{dE}}{\text{d}\infty} = \frac{4\pi z^2 k\_o^2 e^4}{mv^2} \left[ \ln \frac{2mv^2}{I} - \ln \left( 1 - \frac{v^2}{c^2} \right) - \frac{v^2}{c^2} \right] \tag{2}$$

where *z* is the atomic number of the heavy particle, *e* is the magnitude of the electron charge, *m* is the electron rest mass, *c* is the speed of light, *I* is the mean excitation energy of the medium, *v* is the velocity of the particle and *k*<sup>o</sup> is the Boltzmann constant.

#### **3.3 Fluence and energy spectra dose relationship**

Space Environments and Effects section at the European Space Agency maintains an archive of SPEs with plots and fit data for event fluence and radiation effects (https://space-env.esa.int/noaa-solar-proton-event-archive/) from 1997 up to date. The event selection is based on the NOAA/GOES-p\_ 5 min averaged >10 MeV p + flux *Solar Proton Activity over the Solar Cycle 24 and Associated Space Radiation Doses DOI: http://dx.doi.org/10.5772/intechopen.103832*

exceeds 2.0 p+/cm<sup>2</sup> /s/sr and ends when the flux returns to below 1 p+/cm2 /s/sr. The estimation of radiation effects is done using the best fit from a comparison of three forms of fit that is exponential in power rigidity *A* ∗ eRm*=B* ), exponential in energy *A* ∗ e*<sup>E</sup>=B* and power law *<sup>A</sup>* <sup>∗</sup> *EB* , where *<sup>A</sup>* and *<sup>B</sup>* are constants, Rm is magnetic rigidity and *E* is energy. **Figure 5a–f** shows the correlation relationship (*R*) of the SPEs integrated fluence spectra from 2010 to 2017 at different proton energies of 1, 5, 10, 30, 50 and 100 MeV with the dose behind 0.05 mm Al shield in rads. The *R* values show that the integrated fluence spectrum at all proton energies has a positive correlation with the magnitude of the dose. However, the integrated fluence at low energies from 1 to 30 MeV is more correlated to dose with high *R* values compared to higher

**Figure 5.**

*Scatter graphs with line of best fit showing correlation relationship between integrated fluence spectrum and dose at different energy levels; (a) 30 MeV, (b) 50 MeV, (c) 100 MeV, (d) are 1 MeV, (e) 5 MeV and (f) 10 MeV.*

energies from 50 to 100 MeV. This observation is a manifestation of the contribution of slowed-down proton spectra to dose build-up on a material.
