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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop Appendixes

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop This page intially left blank

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop A Reports of the Working Groups WORKING GROUP A PREDICTION ON TIMESCALES OF YEARS TO DECADES AND SOLAR-CYCLE VARIABILITY The focus of Working Group A was to understand the issues involved in predicting long-term solar cycle variability over timescales of years to decades. Ultimately, levels of solar activity and magnetic variations of the Sun control the fluxes of solar energetic particles (SEPs) and galactic cosmic rays that are of key importance for human and robotic exploration of the Moon, Mars, and beyond. Current Assets—Observations and Models Sunspot number is well correlated with several other measures of solar activity, including sunspot area, 10.7 cm radio flux, x-ray flares, total irradiance, the geomagnetic aa index, and cosmic ray flux. The long record of sunspot number helps in the characterization of features of past solar cycles. Feature-recognition techniques may help to better characterize magnetic structures on the Sun. This ability could, in turn, lead to the prediction of activity levels with a lead time of several years. Cycle 23 was accurately predicted using a curve-fitting technique that used the cycle properties for the first year or two from the prior solar minimum. Thus, this type of technique can give several-year forecasting capability once a given cycle is under way. Dynamo models that incorporate deep meridional flow throughout the convection zone to transport magnetic flux (so-called flux-transport models) may have considerable predictive capability. These models transport flux toward the solar equator at the base of the convection zone. It is known that magnetic-field maps show the equatorward drift of active regions, Hale’s polarity law, differential solar rotation, and pole-ward meridional flow. “Magnetic persistence,” or the duration of the Sun’s memory of its own magnetic field, can be controlled by meridional circulation in the solar convection zone. There is a flux-transport, dynamo-based prediction scheme for solar activity. It describes the poloidal source on the basis of sunspot areas. In other words, the time variation of the poloidal source function within each sunspot cycle is derived from observations of the sunspot areas during that cycle. A predictive

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop model using this technique is able to correctly predict the sequence of sunspots (and sunspot peaks) for cycles 16 through 23. The method uses cycles 12 through 15 to model and simulate the Sun’s memory of its own magnetic-field history. Based on the successful simulations of cycles 16 through 23, recent work has predicted the strength of cycle 24. This physics-based (i.e., dynamo-based) prediction gives a peak amplitude of cycle 24 that is 1.2 to 1.5 times larger than that of cycle 23. If true, this predicted cycle amplitude would make cycle 24 the largest since cycle 19 (which occurred right at the beginning of the Space Age in about 1960). This physics-based prediction is in contrast to recent predictions made using so-called precursor methods. The precursor methods have uncertainty that decreases as the cycle approaches closer to its peak. By contrast, flux-transport dynamo-model-based predictions use the Sun’s magnetic memory, whose duration is controlled by the meridional circulation and magnetic diffusion. This class of solar dynamo model may also have some skill in predicting cycle amplitude two cycles ahead and therefore may be able to predict the amplitude of cycle 25. The speed of movement of the latitude of sunspot appearance toward the equator as the solar cycle advances is anticorrelated with sunspot cycle period (Hathaway et al., 2003). That is, the faster the drift rate, the shorter the period. Flux-transport dynamo models show similar behavior. The equatorward drift velocity of a cycle is correlated with the amplitude of the second following cycle, a property also seen in flux-transport dynamos with deep meridional circulation. This fact supports the possibility of predicting cycle amplitudes two cycles ahead. This correlation predicts that cycle 24 will be larger than average amplitude, and cycle 25 will be significantly smaller than average amplitude. This prediction is at odds with previous forecasts that have predicted that cycle 24 would be smaller in intensity. Other key evidence about long-term solar activity and radiation hazards comes from ice core records of nitrates and 10Be. For example, the highest nitrate (NOy) density in the ice core records from Greenland over the past several hundred years was associated with a large solar event in 1859 (the so-called Carrington solar flare event). The ice core record shows that solar particle events can occur at any time during the 11 year solar activity cycle, but there is evidence that solar particle event peak intensities have been somewhat lower in recent times than they were in the late 19th century. This raises questions about whether the present is a relatively benign period for deleterious radiation. If so, how long will this condition last? Distributions of solar particle events over the past five cycles have shown that SEPs roughly follow the sunspot number, but there tend to be proportionately more SEPs late in each cycle. There is a very significant variation from cycle to cycle. What Is Not Known Now But Needs to Be Known for the Vision for Space Exploration? The human spaceflight missions that will take place as part of the Vision for Space Exploration (VSE) require substantial advance planning (under current plans the Moon landing will not take place until approximately 2018, and the human exploration of Mars has been predicted as taking place no sooner than 25 to 30 years from now). Therefore, the ability to make longer-term predictions of solar-cycle activity is important. Will the radiation environment be significantly different in 2018 than it is today? Will it be different in 2035 compared to that of today? The answers to those questions will affect planning (for instance, when to launch crews) and spacecraft and mission design—for example, the amount of shielding that will have to be provided to protect the astronauts. Spacecraft are currently designed on the basis of an understanding and expectation of radiation levels that historical ice core data indicate could be abnormally low. If those levels increase to a more commonly occurring level, then the design of spacecraft will also have to change to accommodate them.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop How Do Scientists Address the Identified Needs? It is known that large solar particle events have occurred in the past. However, scientists need to characterize these events better. This can be accomplished through further investigation of ice core data. Also, 10Be measurements show that the recent experience with solar-cycle modulation of galactic cosmic radiation may not be a good predictor of future levels. Better calibration of ice core data vis-à-vis neutron monitor data needs to be obtained. Reference Hathaway, D.H., D. Nandy, R.M. Wilson, and E.J. Reichmann. 2003. Evidence that a deep meridional flow sets the sunspot cycle period. Astrophys. J. 589:665. WORKING GROUP B SOLAR ACTIVE REGIONS, FLARES, AND CORONAL MASS EJECTIONS The focus of Working Group B was to understand the relationship between solar phenomena that lead to particle acceleration, solar flares, and coronal mass ejections (CMEs), and the solar activity observed on the disk. Active regions, the concentrated magnetic-field configurations that result in plage, sunspots, and bright x-ray emission, also provide the energy source for the most powerful solar flares and CMEs. Active regions form over a period of days and typically live for weeks to months. Some regions appear to be especially dangerous from a space weather perspective, while others do not. Any capability for predicting solar drivers for space weather on timescales ranging from hours to weeks must characterize dangerous active regions. Long-time prediction capability, such as that needed for operational planning of long-duration missions outside of Earth’s magnetosphere, critically depends on scientists’ ability to enhance the predictive power based on currently available data, model development for flare and CME generation, and novel predictive schemes that involve subsurface analysis through helioseismology, and accurate determinations of the full three-dimensional structure of the solar magnetic field. Current Assets—Observations and Models The understanding of solar activity and its relation to CMEs and flares has made tremendous progress based on the contributions of a series of spacecraft, such as Yohkoh, the Solar and Heliospheric Observatory (SOHO), and most recently the Ramati High Energy Solar Spectrographic Imager (RHESSI). Furthermore, in situ data of CMEs and their global structure, such as from the Advanced Composition Explorer (ACE), WIND, and Ulysses, have provided important constraints for the understanding of these ejecta and their evolution in the structured heliosphere. Magnetic flux is entering the solar atmosphere everywhere on the disk, through the emergence of so-called ephemeral regions. These bipolar structures emerge at a rate that allows replenishment of the entire solar magnetic field within a timescale of 40 hours or less. In addition, large-scale strong fields are emerging, with large temporal and spatial dependence. These active regions are more localized and dominate the local structure of the local solar atmosphere. The relation of these two sources of magnetic flux, and their relation to the thermodynamics of the corona, and the occurrence of flares and CMEs are still subject to the active research of a sizable community that was represented in this working group.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE A.1 Vector-magnetogram-based predictions of coronal mass ejections. Red symbols indicate active regions that lead to eruptions within a certain time period, and green symbols indicate active regions without eruptions. The vertical axis shows values for the total shear-weighted length, L_SS, of strong shear regions along the main neutral line (in units of radians*km). The horizontal axis shows values for the total gradient-weighted length, L_SG, of strong magnetic gradient regions along the neutral line (in units of G). SOURCE: Courtesy of D.A. Falconer, University of Alabama, Huntsville. (See also Falconer et al., 2002.) Observational Most investigations attempting to relate the evolution of the solar photospheric magnetic field and its coronal responses still rely on the measurement of a single component, along the Sun-Earth line. Owing to the highly nonpotential character of source regions giving rise to flares and CMEs, extrapolations to full three-dimensional field topologies, their reconnection geometries, and currents are almost impossible. Owing to the available vector magnetic-field data of strong field regions, new empirical predictors have become available, as shown in Figure A.1. Owing to the evolutionary behavior of the photospheric magnetic field on many timescales, the accurate prediction of the time of a flare/CME event is still not possible. Emerging technologies developed for heliosesimology have shown their ability to forecast active regions before these regions come around the solar limb. This allows predictive power for large solar

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop active regions, which are the source of most of the strongest flares and the fastest, most hazardous CMEs. These helioseismological techniques are currently implemented and perfected to allow following active regions throughout the entire solar rotation. There are other observational techniques that are being implemented, many of them in early stages of development. They involve global measures of the free magnetic field before eruptions, the total transport of magnetic-free energy through the photosphere on all relevant temporal scales, and the identification of coronal morphology changes up to 1 day before eruption, for example, through the identification of coronal density enhancement. Modeling Modeling of the photosphere coronal interfaces falls within two major sets of models. The first set is fundamentally time-stationary and focuses on the overall topology of magnetic fields with simplifying assumptions, such as force-free assumptions, or potential field assumptions. These models have been successfully used to analyze the overall magnetic structure of active regions, and the corona as a global entity. However, these models are not useful for developing predictions for the timing of eruptions, or even the overall topology of CMEs. Secondly, there is a set of CME initiation models that seek to establish the linkage between photospheric forcing and eruptions (Klimchuk, 2001). These models have been limited by computational technologies owing to the necessity of describing, simultaneously, the small-scale reconnection process and the global-scale CME eruption. There has been important progress in these fields, with a number of candidate models becoming available that can lead to testable predictions. These models are far from applications in a predictive environment, such as would be useful for the Space Environment Center of the National Oceanic and Atmospheric Administration (NOAA). What Is Not Known Now But Needs to Be Known for the Vision for Space Exploration? As part of the Vision for Space Exploration, NASA seeks to establish a human presence on the Moon for short (days to weeks) and long-term (weeks to months) stays. For such a trip, there is an operational need to predict solar eruptions and their effects for particle acceleration (addressed by Working Group C). For any prediction capability exceeding 2 days, the physical processes addressed by this working group become crucial. The workshop participants identified the following important issues that need to be addressed by future observational and modeling efforts. It is clear from these questions that this research is very much exploratory. Predictive schemes should not be expected very soon. How accurately can the coronal magnetic field now be characterized in three dimensions, and how much better will this be with future data-gathering capabilities? The physics of CME initiation is only poorly understood. How can the most effective progress in the scientific understanding of CME initiation be made? To what extent can CME initiation be predicted without detailed understanding of the CME initiation physics? (I.e., what is the usefulness of empirical and statistical tools?) The most dangerous space weather events can be traced back to active regions with “dangerous” (e.g., delta-spot) configurations. What are the physical mechanisms that lead to such active region configurations? Can these mechanisms be predicted? To what extent is it possible to predict the emergence of active regions before they reach the photosphere, or to predict the rotation of an active region from behind the limb?

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop To what extent does the pre-event corona allow scientists to predict eruption owing to new emergence? Is it possible to predict significant long-term changes in the flux of galactic cosmic rays caused by changes at the Sun propagating out into the large-scale heliosphere? How Do Scientists Address the Identified Needs? Major progress in the predictive capabilities is expected to come from a number of parallel thrusts, which were addressed during this workshop. For example: An improvement of observations of the boundary conditions in the corona; this improvement can include “force-free” vector magnetograms in the chromosphere or the corona; The assimilation of data to the global coronal magnetic-field specification from radio, x-ray/extreme ultraviolet radiation and imaging spectroscopy, as well as coronal seismology; Detailed observational determination of the magnetic topology in filament channels to determine the CME eruption mechanism; and The development of self-consistent magnetohydrodynamic models that couple the photosphere and the corona, with a vigorous investigation of CME initiation processes. These efforts are expected to benefit from upcoming missions and programs (see the list in the following report, by Working Group C). These investments will be best exploited if they are coupled with comprehensive modeling efforts. These models need to establish connections between different kinds of observations of solar eruptions, combining remote imaging and spectroscopy and in situ ground-truth measurements of CMEs. References Falconer, D.A., R.L. Moore, and G.A. Gary. 2002. Correlation of the coronal mass ejection productivity of solar active regions with measures of their global nonpotentiality from vector magnetograms: Baseline results. Astrophys. J. 569:1016. Klimchuk, J.A. 2001. Theory of coronal mass ejections. P. 143 in Space Weather (P. Song, H.J. Singer, and G.L. Siscoe, eds.), AGU Monograph 125, AGU Code GM1259841. American Geophysical Union, Washington, D.C. WORKING GROUP C PROPAGATION OF EVENTS IN PROGRESS The focus of Working Group C was on understanding the propagation and evolution of solar energetic particles. SEPs are a potentially serious risk to humans and their sensitive instruments when they journey beyond Earth’s protective magnetosphere to reach the Moon and other locations called for in the Vision for Space Exploration. From a science standpoint, it is important to understand the variability and extreme values for properties of SEP events in order to develop a predictive capability that could eventually be incorporated into an operational space weather warning system. The science community recognizes that an accurate and reliable warning system requires a better understanding of the physical processes that produce SEP events. A firm foundation for such a system requires the development of physical models for particle production, acceleration, and transport. These models should be validated by appropriate observations before they are transitioned to operational use.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop Current Assets—Observations and Models There are currently more than a dozen NASA spacecraft obtaining scientific measurements of solar wind, energetic particles, magnetic fields, and electromagnetic radiation from many vantage points in the heliosphere. They provide data to test and guide the development of theoretical models as well as supporting the operational space weather community. The supporting NASA operating missions include ACE, Cluster, Fast Auroral Snapshot Explorer, Geotail, IMAGE, Polar, Ramati High Energy Solar Spectrographic Imager (RHESSI), Solar and Heliospheric Observatory (SOHO), Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission (TIMED), Transition Region and Coronal Explorer (TRACE), Ulysses, Voyagers 1 and 2, and WIND. These spacecraft are located at strategic vantage points in the heliosphere from the L1 Lagrange point (1.5 million km upstream from Earth), to inside Earth’s magnetosphere, and out to the termination shock (near the boundary with interstellar medium where the solar wind slows down from supersonic to subsonic speeds). Spacecraft missions from other agencies (NOAA and DOD) also provide valuable data for inputs to space weather models. Current space weather models are climatological and empirically based and therefore do badly in predicting extreme events. Two well-known types of models are the ones used by (1) the Air Force Weather Agency (AFWA), which triggers on x-ray or microwave emission from flares; and (2) the NOAA SEC, which triggers on Geostationary Operational Environmental Satellite (GOES) x-ray data. NOAA supports NASA’s Space Radiation Analysis Group (SRAG) in its forecasts for the International Space Station (ISS). The AFWA models are only accurate to an order of magnitude in the estimates for peak flux and can estimate the timing of the maximum flux (to within about 2 hours after onset). There are many physics-based models in the research community. Those presented at the workshop are being developed at the Boston University Center of Integrated Space Weather Modeling, the University of California (Berkeley and Riverside), the Johns Hopkins University Applied Physics Laboratory, and the University of Michigan. The challenge to the research community is to know how improved physics can be included in these models without making them too difficult for transitioning to operational use. What Is Not Known Now But Needs to Be Known for the Vision for Space Exploration? The first major goal of the Vision for Space Exploration is to establish a human presence on the Moon for short-term (days to weeks) and long-term (weeks to months) stays. For such a trip the primary radiation danger is from acute exposure from high-energy particles emitted by solar flares and coronal mass ejection shocks. Important quantities that space weather modelers need for better forecasting of SEP events are the following: (1) the onset time for an SEP event, (2) its time-intensity profile, (3) the “spectral indices” of the energy spectrum, (4) the shock arrival time, and (5) the anisotropy in the particle velocity distribution (lower priority). For the near-term need, it would be very helpful to be able to predict, with high assurance, “all-clear” periods when there is a very low probability of an SEP event expected. Table A.1 shows the energetic particles and their energies of most interest to astronaut safety and preventing hardware disruption or damage. TABLE A.1 Energetic Particles and Their Energies of Most Relevance to the Vision for Space Exploration Application Ion Species Particle Energies Astronaut safety H, He, heavy ions >10 MeV per nucleon Dosimetry e >1 MeV Hardware systems H, He, e, plus heavy ions Greater than a few MeV per nucleon

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop The workshop participants identified the following important issues that need to be addressed by future observational and modeling efforts: Why does the time evolution of large SEP events show a great deal of variability in such features as the initial rise of intensity, time of maximum intensity, and decay time? Why do the maximum intensities and distributions of particles (with different energies, masses, and ionization states) vary widely? What are the effects of overlapping, multiple events, which often happen during the most active periods? How does the passage of an interplanetary shock at Earth orbit increase the SEP intensity? How Do Scientists Address the Identified Needs? The consensus of Working Group C is that present operational models are too simplistic, while research models have too many unknown input parameters for making the required space weather predictions. Progress can be made through the vigorous development of models that can describe the following processes: (1) flare/CME/shock initiation, (2) particle acceleration at or close to the Sun, (3) three-dimensional transport in heliosphere, and (4) particle acceleration near 1 AU (for lunar exploration sites). New observations from current and future space missions will also be needed to provide inputs for testing and validating of these models. Finally, the models and missions that provide the input data need to be transitioned to operational use. Present one- and two-dimensional models of CME propagation and shock acceleration need to be extended to full three-dimensional models that can model the global structure of the heliosphere from the Sun to 1 AU and beyond. Some success in modeling shock-accelerated SEPs has been made with quasi-parallel and quasi-perpendicular shock models, but these models cannot yet model specific events (e.g., Zank et al. [2000]). Better understanding of how the magnetic field geometry, shock speed, seed particle population, and interplanetary transport affect the SEP output (energy spectra, fluence, intensity profile, maximum energies, and velocity anisotropies) are still needed. Also, some SEPs may be produced in compression regions without a shock and by current sheets in flares and CMEs. Realistic models for these types of events are also needed. Progress in model improvement requires the systematic validation of models against large numbers of past and future events. The Living With a Star Targeted Research and Technology program could encourage this sort of testing by making such validation efforts one of the program’s focus areas. Currently, it is difficult to validate models because the available data are spotty. In other cases, the required data at the inner boundary conditions near the Sun do not yet exist. What is required are accurate solar wind and magnetic-field parameters at the inner boundary near the Sun and at locations of interest to the VSE. The models are then required to forecast conditions based on these inputs at the coronal boundary. Some current missions have demonstrated the types of new measurements that are required. RHESSI observations indicate some important differences in the source regions for electrons and ions in solar flares. Recent SOHO Ultraviolet Spectroscopic Coronagraph observations demonstrate how CME shock parameters (including information on suprathermal seed particles) can be obtained in coronal regions within a few solar radii of the Sun. Ground-based radio observations of Type-II radio bursts can reveal information about the magnetic fields and temperatures within CME shocks that produce energetic particles. Knowledge of the turbulence spectrum in the corona and solar wind is another key input. Many models for predicting SEPs that are currently under development can benefit by using these types of data to validate their results.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop In situ measurements from the ACE and WIND provide much of the ground-truth data needed to test SEP acceleration models. However, currently there are no funds identified in NASA’s long-range plan for follow-on missions. Planned near-term facilities and missions will provide the new diagnostics for obtaining better information about conditions at the Sun before and during eruptive events: Advanced Technology Solar Telescope (ATST) will provide routine high-cadence, ultrahigh-resolution measurements of active region magnetic fields. Frequency Agile Solar Radio (FASR) telescope array will provide high-resolution, full-Sun images of coronal shocks and other SEP sources. Solar Dynamics Observatory (SDO) will have a 10-sec cadence for measurements of magnetic fields, coronal x-rays, and ultraviolet emission to provide a better understanding of CME and flare source regions. Solar Terrestrial Relations Observatory (STEREO) will provide three-dimensional views of CMEs and measure associated particles and fields from two spacecraft locations. The Japanese-U.S. Solar-B mission will obtain routine vector magnetic-field measurements of CME and flare initiation sites on the Sun. New missions that have yet to start their hardware development phases are also vital: Inner Heliosphere Sentinels will fly multiple spacecraft to better understand the evolution of solar disturbances from 0.25 AU out to beyond Earth’s orbit. Solar Probe will investigate the only unexplored region of the heliosphere from 4 to 30 solar radii to measure particles and fields at SEP source regions. The European Space Agency’s Solar Orbiter will make the first high-inclination measurements of the Sun and solar wind from a near-Sun location (0.21 AU). Other future mission concepts that could provide space weather warning to locations that are far from the Earth-Sun line are the multispacecraft Solar Weather Buoys stationed at 0.9 AU, the Mars Aeronomy Orbiter stationed at Mars, and the Interstellar Probe at our solar system’s outer boundary. To summarize, the heliophysics community recognizes its responsibility to contribute in a timely manner to the Vision for Space Exploration by engaging in the following activities: Improving basic understanding of governing physical processes of the space radiation environment; Developing models that can form the basis of greatly improved predictive models; Validating these models with improved observations; Developing the conceptual and hardware infrastructure for an operational space weather system; and Carrying out exciting exploratory missions such as those recommended by the 2005 NASA Roadmaps and the NRC (2002) decadal survey, The Sun to the Earth—and Beyond. References NRC (National Research Council). 2002. The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. The National Academies Press, Washington, D.C. Zank, G.P., W.K.M. Rice, and C.C. Wu. 2000. Particle acceleration and coronal mass ejection driven shocks: A theoretical model. J. Geophys. Res. 105:25079.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop WORKING GROUP D EARTH, LUNAR, AND PLANETARY (MARS) ENVIRONMENTS Working Group D focused on how the space environments around the Moon and Mars are impacted by space weather. It discussed extreme events, including the effects of SEPs. Discussions started with basic scenarios and concepts and progressed to the identification and uses of existing data, currently planned measurements, and the identification of additional new measurements necessary to resolve key unknowns. The group also addressed lessons learned from the study of Earth’s upper atmosphere and ionosphere concerning solar-activity-related disturbances that could affect Mars exploration. The group then asked what is currently known, what is not known and needs to be known for the Vision for Space Exploration, and how those issues could be addressed. Each of these questions was considered for the Earth environment, lunar orbit and surface, and Mars orbit and surface. Earth Environment Because of its importance to understanding and mitigating the impacts of space weather on and near Earth, Earth’s environment is an area in which many scientists are conducting ongoing research. There are models of the interaction of Earth’s magnetosphere with the surrounding plasmas and fields of the heliosphere. There are standard models of the low-Earth-orbit, geosynchronous radiation belt with solar-cycle variations, a history of SEP observations and impact studies, models of space radiation effects on satellite systems in this environment, and an extensive archive of science data and space systems anomalies. Closer to Earth, there are standard operational models of the ionosphere and upper atmosphere. Most of these empirically include the effects of solar and geomagnetic activity, but generally through surrogate parameters (extreme ultraviolet [EUV] radiation or radio flux or magnetic fluctuations measures at locations on the surface of Earth). There are archives of ground- and space-based observations. There are archives of documented impacts on communications and satellite operations. Much remains to be done to improve the understanding of Earth’s environment as a system. In each domain there is a need to better understand and dynamically model the magnetosphere, the radiation belts, and the ionosphere, especially during periods of extreme events. The models need to transition from statistical and climatological models to truly predictive models. The magnetosphere needs to be coupled through physics-based global models to the surrounding heliosphere. The radiation belts and ionosphere need to be more intimately coupled to influences of changes in the heliosphere, including during periods of solar activity and increased flux of energetic particles. Suggestions on how to proceed, raised during the working group’s breakout session, included the following: Provide the necessary data-mining infrastructure, together with new observations targeted toward this goal, for both model parameterization and validation (note: using these effectively requires solar and upstream interplanetary supporting information); Use Earth high-latitude upper atmosphere as a local “laboratory” for Mars (to investigate matters from atmospheric transport code validations with balloon experiments, to studying responses of a thin atmosphere, to solar photon and particle inputs expected at Mars); and Collaborate on and cooperate in and sponsor joint studies and investigations with like-minded programs with similar goals—especially the Living With a Star program.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop Lunar Environment (Orbit and Surface) A significant amount of information on the lunar environment (surface and orbital) exists from past relevant observations by Explorer 35, Apollo, Clementine, Lunar Prospector, Geotail, and WIND spacecraft. The existing data provide insight into the lunar plasma and field environment, crustal magnetic fields, lunar surface charging, dust, lunar atmosphere (also remote detections), SEP events, and dosimetry at the Moon. There is surface neutron information from Lunar Prospector, plus surface neutron models (Space Ionizing Radiation Environments and Shielding Tools [SIREST] transport codes and others). There is also relevant surface composition information from Clementine, Lunar Prospector, and the archive of lunar samples. Some information about the lunar radiation exposure history has been derived from lunar samples. There are still gaps in understanding of the lunar environment. There is also concern that the available information is not sufficiently comprehensive or adequately validated, either on the lunar surface or in lunar orbit, to meet the needs of the VSE. One such gap addressed by the working group was the issue of electrostatic charging on the lunar surface. The temporal dynamics of surface electrostatics, particularly at dawn and dusk, and the spatial variability of surface charging, and how it varies with lunar geology are not well understood. The working group also observed that the steady-state description of the lunar environment may be inadequate during periods when the Moon transits Earth’s magnetotail. This occurs in the middle of the lunar day on the Earth facing side, so it is likely that astronaut activities will be under way under transit conditions. Suggestions on how to proceed, raised during the breakout session, included the following: Restore (as necessary) existing lunar environment data (including those from Apollo and Clementine). Convene a splinter group to revisit these and all existing environment data relevant to the Vision for Space Exploration from all sources—to reestablish the environment knowledge base; Install more-comprehensive lunar surface models in surface radiation environment codes; Determine (now) whether Lunar Reconnaissance Orbiter measurements will sufficiently validate models of the radiation environment in orbit and on the surface. Determine what further orbital and surface measurements are needed (e.g., neutron, dust, electric-field measurements on the Lunar Robotic Lander) to establish environment knowledge for human expeditions; and The working group emphasized the importance of the Lunar Reconnaissance Orbiter and a subsequent lunar lander to aid in understanding the space environment. Mars Environment (Orbit and Surface) Properties of the primary galactic cosmic radiation (GCR) and SEP environment are generally extrapolated from observations near Earth and from several deep space missions. The GCR flux increases by radial distance from the Sun to a degree that is generally understood. The secondary particles generated by collisions between the primary particles and the atmosphere and surface of Mars have been estimated by standard transport codes (e.g., SIREST, which requires other surface and atmosphere models). In spite of the number of spacecraft that have been sent to Mars, there have been no comprehensive measurements of the radiation environment in Mars orbit and on the martian surface. Direct on-orbit measures of a portion of the neutron flux and energetic protons in a limited energy range have been made by instruments on Odyssey and inferred indirectly from instruments on Mars Global Surveyor.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop Surface radiation calculations have been performed for Earth’s Moon, Mars, and Callisto. These calculations show that radiation shielding will be an important consideration in the planning of long-term missions to these surfaces. These calculations also demonstrate the large variation in exposure rates due to solar cycle. Previous and new measurements show dynamic, structured, variable ionosphere and upper-atmosphere systems (depending on season, dust, crustal magnetic fields, solar EUV flux, solar wind conditions, and so on). Some models exist: global circulation models and thermospheric global circulation models for lower and upper atmosphere, and more limited ionosphere models. Upper-atmosphere models are used in Mars mission aerobraking plans. The main missing element for the radiation environment is a validation of orbital and surface radiation models and transport codes, for both spectrally hard (GCR) and softer (SEP) components. Validation is also needed of atmosphere and ionosphere models to be used in applications. Upper-atmosphere models will be used in entry and orbital planning, global dust transport dynamics, and atmospheric shielding against radiation. Ionosphere models will be considered in communications technologies planning. Validation requires supporting solar and interplanetary data together with upper-atmospheric and ionospheric measurements. Electrostatic properties of the martian surface have been estimated, but good temporal and spatial dynamics models do not exist. Model validation will be critical to support a “Be there before you get there” paradigm to design system architectures and to simulate VSE situations. Suggestions on how to proceed, raised during the working group’s breakout session, included the following: Validate radiation transport codes, for example, with measurements on stratospheric balloons at Earth and on the martian surface. There is a need for a science-based review to determine if the planned Mars Science Laboratory measurements are sufficient for the validation of transport codes. If not, what measurements are needed and what instruments are required to make those measurements? It was pointed out that a lack of local, upstream measurement of SEP fluxes and spectra limits transport code validation. Codes need to be validated in both hard and soft spectral ranges. A suggestion was made to consider convening a splinter group to answer this question and to make recommendations. Communication with the exploration and the Mars science communities will be critical to the success of model-validation efforts. Undertake a special initiative to collect and synthesize the full range of available atmospheric and ionospheric measurements relevant to VSE orbiters and landers, and to make model improvements to include all significant parameters (e.g., dust, flare effects, and so on). This effort would lead to recommendations to make specific on-orbit and surface measurements to validate models in areas of VSE interest, including orbital evolution and craft entry, and on-surface operations (e.g., dust, E and B field effects, ionospheric conditions relevant to communications and operations). Cooperate with and coordinate plans and activities with the Mars Exploration Program where appropriate and advantageous, which is critical. Timelines The working group recognized that some measurements are needed immediately, either because decisions are being made right now in the exploration community (shielding and operations strategies for lunar exploration, for example) or because a long lead time is needed to develop and deploy appropriate

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop sensors on appropriate platforms. The working group prepared a strawman list of near-, mid-, and far-term activities. This list should be used as a starting point and could be the focus of a workshop. Near-term activities: Updates and upgrades of radiation transport codes; Radiation-transport-codes validation on stratospheric balloons; Lunar data restorations and synthesis activity; Creation of data-mining infrastructures: Earth, Moon, and Mars; Collection and synthesis of Mars atmosphere data specific to VSE needs; Radiation environment and upper-atmosphere/ionosphere model developments based on data mining and ongoing observations. In-process and midterm activities: Lunar Reconnaissance Orbiter; Mars Science Laboratory; Support of solar and helio missions (Sentinel, SDO, STEREO); and Validation of Earth radiation environment and lunar and Mars surface radiation models. Future or needed study: Mars upper-atmosphere characterization missions, and Mars upstream solar monitoring or validated models. Other Comments The working group discussed additional issues that were felt to be overarching issues, relevant generally to other working groups. In summary, there is a need to acknowledge, learn from, and communicate with the wide range of groups conducting and generating overlapping activities, studies, and reports (e.g., Mars Exploration Program Analysis Group; Living With a Star; other National Research Council committees; the Committee on Space Research of the International Council for Science; the European Space Agency geospace, Mars, and lunar study groups and missions; NASA Roadmap groups; and the astrobiology community). There is concern that the myriad of groups and reports may be confusing or conflicting, or that efforts may be needlessly redundant, or that relevant good ideas, results, and/or opportunities from others may not be folded in to the Vision for Space Exploration and NASA’s advantage. WORKING GROUP E DOSIMETRY The focus of Working Group E was to understand the systems and techniques used to measure space radiation and efforts to improve these measurements. Radiation exposures of humans on crewed missions in low Earth orbit (LEO) have been measured using radiation-detection and dosimetry systems, including thermoluminescent dosimeters, nuclear emulsions, plastic track detectors, charged-particle telescopes, and tissue-equivalent materials, since the early days of space travel. Databases of doses to crews traveling into space during the Mercury, Apollo, Skylab, shuttle, and International Space Station era have been archived. In some cases, the original detector materials are stored and available for reanalysis and further data mining. Unmanned missions beyond LEO, such as the Interplanetary Monitoring Platform series of missions, the ACE mission at the L1 point, and the recent Odyssey mission to Mars, have at times also carried instruments, such as the Mars Radiation Environment Experiment (MARIE) detector, that have recorded particle

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop fluxes and their associated doses. The latter has recently made measurements in martian orbit, but no measurements of doses or particle fluxes on the surface of Mars exist at present. Energetic-particle detectors on the GOES series of weather satellites have provided continuous measurements of charged-particle spectra in geostationary orbits for about three decades. What Is Not Known Now But Needs to Be Known for the Vision for Space Exploration? For solar energetic particle (SEP) events, predictive tools to enable mission controllers and crews in deep space to make informed operational decisions in real time are needed but are not yet available. Methods involving artificial intelligence and other predictive techniques, such as Bayesian inference and locally weighted regression, have been investigated and have demonstrated promise in providing nowcasting capabilities after SEP event particles begin to arrive (Hoff et al., 2003; Hines et al., 2005; Neal and Townsend, 2005). These methods are capable of predicting, with reasonable accuracy, total doses and the temporal evolution of the dose as particles arrive very early in the evolution of the event. They are also robust enough to adjust for the arrival of particles associated with interplanetary shocks. However, these methods are presently unable to forecast SEP fluence levels and their associated doses before particles begin to arrive. Hence, they could provide a much-needed capability for mission operations, but in a merely stopgap role. Unfortunately, computer codes implementing these nowcasting models are research codes and not in the form of operational tools that are usable by mission operations personnel. The greatest needs in the area of SEP events, as reported by personnel from the Space Radiation Analysis Group at the NASA Johnson Space Center, are the following: Predictions of the temporal evolution profile of the next most likely SEP event at selected energies with associated probabilities, before particles begin to arrive; Flux data from the actual event at the selected energies in real time; The capability to refine the temporal profiles and associated probabilities as the data arrive in real time; and Reliable forecasts of no solar activity of interest—that is, all-clear forecasts. Accomplishing the first item of the previous list involves developing methods for forecasting differential fluence rates (particles/cm2/time/energy) at various times τ before the event begins, that is, F(E, t, τ) at 1 AU, along the spacecraft trajectory to Mars, and at Mars for the following: Protons (H): Parameter ranges: E (energy): 30, 60, 100, 500, 1,000 MeV t (time): 5 minute increments from t = 0 (SEP event onset) until fluxes return to background levels τ : 0 (now) and for 3, 6, 12 , and 24 hour lead times. Helium (He): Same parameter ranges as above for protons (except now energies are in MeV/n, and the upper bound of interest is cut off at 500 MeV/n). Electrons (e): Same parameters as above, except: E: 0.5 to 5.0 MeV.

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop The limits of the long-term variability in the space radiation environment need to be determined, including those for GCR, SEP events, and Earth’s trapped radiation belts, with a view to prediction. This may require understanding the long-term secular changes in the GCR spectrum using 10Be from ice cores, Voyager measurements beyond the termination shock, nitrate measurements from ice cores, and other historical data. Dosimetric measurements on lunar and martian surfaces using tissue-equivalent detectors that include both charged-particle and neutron contributions to dose and dose equivalent are needed. Mechanisms and procedures for transferring knowledge and computer codes from the research realm to the operational realm need to be developed. How Can Scientists Obtain the Needed Knowledge? In order to obtain better information, the community must carry out the following: Obtain U.S. access to particle and dose data from missions conducted by other countries that enable addressing the needs mentioned previously. Monitoring at L1 is preferred because it is outside Earth’s magnetosphere, is approximately at 1 AU, and allows for continuous coverage of the deep space radiation environment. Monitoring of charged-particle and neutron radiation fields and associated doses on the lunar and martian surfaces is also needed. Develop a “technology transfer” mechanism for moving research into operational tools for use by mission planners and controllers. Conduct research that develops methods for reliably forecasting SEP event fluence rates and temporal profiles, and their associated probabilities, with lead times ranging from hours to 1 day, before particles actually begin to arrive from such an event. Methods of forecasting “all-clear” periods also need to be researched and developed. References Hines, J.W., L.W. Townsend, and T.F. Nichols. 2005. SPE dose prediction using locally weighted regression. Radiat. Prot. Dosim. 115(1-4):232-235. Hoff, J.L., L.W. Townsend, and J.W. Hines. 2003. Prediction of energetic solar particle event dose-time profiles using artificial neural networks. IEEE Trans. Nucl. Sci. 50(6):2296-2300. Neal, J.S., and L.W. Townsend. 2005. Multiple solar particle event dose time profile predictions using Bayesian inference. Radiat. Prot. Dosim. 115(1-4):38-42. WORKING GROUP F EFFECTS ON INSTRUMENTS, SPACECRAFT, AND COMMUNICATIONS During the intense and long-lasting solar storms in October and November 2003, the so-called Halloween Storms, the MARIE experiment was permanently lost (Barbieri and Mahmot, 2004). Ironically, the purpose of the MARIE experiment was to understand and characterize the radiation environment, not only near Mars and on its surface but also in the interplanetary space between Earth and Mars, in order to plan for future manned spacecraft and missions to Mars. It is almost always difficult to determine with certainty the cause of a spacecraft or instrument failure; however, in addition to the MARIE instrument, a number of other spacecraft and instruments in near-Earth space suffered during the Halloween storms. Fortunately, humans were not dependent on the operation of this instrument, but when humans venture again to the Moon and for the first time on the long voyage to Mars, it will be essential to ensure that

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop difficulties with instruments, spacecraft, and communications do not threaten mission success and human lives. Working Group F addressed these issues. The meeting description that introduced Working Group F stated: “Solar activity can affect instrumentation, spacecraft and communications in several ways. The solar energetic particle radiation has been found to degrade the performance of solar cells. This radiation may affect the electronics in all kinds of instrumentation primarily by causing single event effects. It can also interfere with various kinds of sensors both by direct ionization and by activation of the sensor or surrounding materials. For example, direct ionization can interfere with the imagery obtained using solid state cameras and may degrade optical and thermal control surfaces. Activation can interfere with gamma ray spectrometers used for scientific investigations. This group will discuss the effects of solar activity on these systems and identify approaches to avoid or mitigate these effects.” Space weather effects on instruments and spacecraft have received much attention in the near-Earth space environment. Among the space environment effects that are of concern for instruments, spacecraft, and communications are these: single-event effects in electronics and sensors, total radiation dose to components, radiation damage to sensors and solar cells, and electrostatic charging. All of these effects are of concern for missions to the Moon and Mars. The working group discussed topics that ranged from test facilities needed for electronic components and systems, to the role of environmental models, to the needs for forecasting and specifying space weather conditions. These topics are discussed below. In addition, the group recognized the value of collecting, preserving, and accessing long-term space weather data sets for design, modeling, and operations activities. Instruments and equipment that will be used on missions to the Moon and Mars need to be tested for their suitability and robustness in a variety of space environments. These environments include diverse regimes that range from conditions near Earth to interplanetary space, to the Moon and Mars. Instruments and equipment will be exposed to a broad range of particle energies and composition from sources such as galactic cosmic rays, solar energetic particle events, and trapped radiation environments. There is also an expectation that commercial off-the-shelf parts and systems such as personal computers and video-cameras will be heavily utilized and will need testing to ensure performance in the disparate environments. Therefore, there is a need for the availability of and access to adequate high-energy particle beams at accelerators for testing and related performance measurements to simulate the space radiation environment under controlled conditions. NASA engineers have been using various accelerators around the country, but one facility exists where beams of all relevant cosmic ray energies and particle species are available. This is the NASA Space Radiation Laboratory at the Brookhaven National Laboratory. For Vision for Space Exploration projects to gain access to this facility, there is a need for a Memorandum of Understanding between the facility and the VSE program so that proposal-evaluation procedures for using the facility are responsive to VSE engineering activities as well as science investigations. In addition to the need for testing before launch, the group also described the value and importance of on-orbit testing for critical system tests and model validation. Space environment modeling plays a vital enabling role for missions to the Moon and Mars. At Earth, 1970s vintage, static trapped radiation belt models such as AE8 and AP8 that predict electron and proton flux spectra in Earth’s radiation belts are inadequate and outdated. Even the more recent, 1990s Combined Release and Radiation Effects Satellite models are limited because they were based on a very brief interval (about 1 year of data). Updated models that are dynamic, taking into account current solar wind and magnetospheric conditions, are needed to provide the history of variations on timescales that range from solar cycles to minutes. If Mars mission architecture includes parking a transit vehicle at geosynchronous orbit, it will be necessary to better understand the spacecraft charging environment, including short-term variations at

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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop that location. Regarding SEP events, more work is needed to understand the most appropriate models and methods that characterize these conditions, including extreme-event studies, risk-based models, and databased analysis of long-term records. Improved models of proton and heavy-ion environments (flux, fluence, and energy spectra) in SEP events are needed because of their effects on systems. Between Earth and Mars, the data on helio-radial dependence of the flux and fluence of SEP events are needed. To better design, protect, and test electronics, new models are needed to better incorporate SEP event conditions and galactic cosmic ray models, including correct solar-cycle modulation of composition. These models need improvements that better address geometry complexity, decreasing feature size, track effects, and single-event transient effects. New physics-based modeling, incorporating particle interactions and device physics, offers improved guidance for design, selection, and protection of devices and instruments. Real-time knowledge of space weather conditions during flights to the Moon and Mars is important for mission success, but it requires improved observations, modeling, and understanding. Without a doubt, new tools will be needed for forecasting space environmental conditions on Mars missions. Solar particle event occurrence and the expected time profile at the vehicle location are among the most serious environmental conditions to contend with, yet they are also among the most difficult to forecast. For the moment, since nowcasts are expected to be more reliable than forecasts, the ability to provide nowcasts was given greater importance by the group than forecasting. It was recognized that missions not only depend on warnings, forecasts, and nowcasts of space weather events, but they need “all-clears” so that they know when they can resume normal operations. In some situations, the time it takes for signals of solar events to propagate from the Sun to Earth and then to the vehicle, including the time for processing signals at Earth, will take far more time than is desirable for the protection of instruments and astronauts on the vehicle. Therefore, crews will want to have autonomous crew situational awareness for vehicle operations. It has been pointed out by Kunches et al. (1991) that astronauts are a “proactive group” with a “spirit of adventure and a desire to chart their own destiny.” It would therefore be advantageous to provide the crew with tools to monitor space weather from their vantage point, not only to give them the ability to rapidly respond, but for their own psychological well-being and to maintain a long-term record of actual mission radiation conditions. Generally, the engineering approach is to harden systems against worst cases; however, the unexpected can always occur. In such circumstances, a number of actions can be taken in response to predictions of poor space weather. Sensors can be safed, noncritical systems can be shut down to prevent damage and latch-up, sensors can be oriented in a direction that is least susceptible to damage, increased attention can be given to monitoring operations and to the interpretation of sensor data, and mission activities can be limited during high-background events. References Barbieri, L.P., and R.E. Mahmot. 2004. October–November 2003’s space weather and operations lessons learned. Space Weather 2: S09002, doi:10.1029/2004SW000064. Kunches, J.M., G.R. Heckman, E. Hildner, and S.T. Seuss. 1991. Solar Radiation Forecasting and Research to Support the Space Exploration Initiative. Space Environment Laboratory (SEL) Special Report. National Oceanic and Atmospheric Administration SEL, Boulder, Colo. February.