Specifying and Predicting the Space Radiation Environment
The Sun clearly has many effects on Earth. The energetic radiations, mainly charged particles, that present a significant hazard for space exploration arise from magnetic processes in the solar atmosphere. These in turn have their origin farther down in the Sun’s outer layers, where the magnetic field originates.
HELIOSPHERIC MAGNETIC FIELD
The Sun’s surface rotates once every 27 days near its equator (and about 30 percent slower at the poles), which results in a clear 27 day cycle in many manifestations of solar activity at Earth. Solar activity is most often associated with solar active regions, which generally last for several solar rotations and that are localized regions on the solar surface where sunspots, flares, and other magnetically related phenomena occur.
The Sun’s magnetic field and its associated activity exhibit a 22 year cycle—twice the familiar 11 year sunspot cycle, because the dominant polar magnetic fields have opposite polarities for consecutive 11 year sunspot cycles. The index of sunspots has an average period between successive maxima or minima of approximately 11 years, and the Sun’s large-scale magnetic direction changes near each sunspot maximum to produce the total 22 year magnetic cycle. Again, many manifestations at Earth reflect this cyclic behavior.
The large-scale solar magnetic field has a clear dipolar structure (with significant smaller-scale variations) in the years around sunspot minimum, which reverses sign at sunspot maximum. Recent observations from the Ulysses spacecraft suggest that the field remains somewhat dipolar during the change in sign, with the dipole rotating to change its direction.
Helioseismological observations have led to a picture in which the origin of the Sun’s magnetic field is a dynamo acting at the base of a region of convective overturning in the outer layers of the solar interior. Models of the dynamo suggest that the turbulent convection couples with solar rotation to produce the large-scale magnetic field. A full understanding of solar activity and its resulting radiations depends in part on an understanding of the dynamo mechanism. Scientific knowledge is still not yet complete enough to make short- or long-term predictions adequate to possibly predict the particle environment. At present,
more-phenomenological models that use observations at the solar surface to make short-term predictions must be relied on.
GALACTIC COSMIC RAYS
The phenomenon of the solar modulation of cosmic rays is the result of the solar wind and its magnetic field inhibiting the interstellar cosmic rays from entering the inner solar system. Since the effect is at a maximum during high sunspot activity, the cosmic ray intensity is a minimum at Earth during the period around sunspot maximum. Conversely, it reaches a maximum at Earth during sunspot minimum. This reflects a general, heliospheric depression of the intensity of galactic cosmic rays, which are entering the solar system from the outside.
The heliosphere, the bubble with the Sun at its center, is carved out of the interstellar gas by the solar wind, which blows radially outward, carrying with it the solar magnetic field, producing a classical Archimedean spiral magnetic field. (See Figure 2.1.) At a radius of about 100 AU, because of the resistance of the interstellar gas, the wind undergoes a shock transition to subsonic flow. This shock is called the termination shock. Some distance (probably about 30 to 50 AU) beyond the shock is the contact surface that separates the ionized part of the interstellar gas from the solar wind gas. A possible second, outer shock in the interstellar plasma is also present. The outer portions of this picture, beyond the inner shock, are not well understood. Fortunately, the general properties of the modulated intensity of approximately
1 GeV particles are not sensitive to these uncertainties, and a reasonable quantitative understanding of the physics has been attained.
The heliosphere is bathed by an essentially isotropic, uniform distribution of galactic cosmic rays that is expected to remain steady over periods of thousands of years. These cosmic rays have difficulty in traveling into the inner solar system, resulting in a depressed intensity there. The problem is to understand this quantitatively in terms of what is known of the solar wind and cosmic ray transport. Since the solar wind is supersonic, the inner part of the heliospheric plasma and magnetic field are not affected much by the uncertainties at the outer boundary. The transport of the cosmic rays is determined by solar wind and its embedded magnetic field, both of which are convected out by the supersonic wind flow.
The large-scale structure of the magnetic field has been clarified considerably by observations carried out on the Pioneer, Voyager, and Ulysses spacecraft, and by the inferred relationship to observed coronal structure. During the years around each solar sunspot minimum, the field is generally organized into two hemispheres, separated by a thin current sheet at low heliographic latitude across which the field reverses direction. In each hemisphere the field is generally assumed to be the Archimedean spiral, with the sense of the field being outward in one hemisphere and inward in the other. At sunspot minimum, the current sheet is nearly equatorial. The structure for the years near sunspot maximum is not simple, with transient solar activity causing significant, large-scale propagating disturbances.
One other aspect of the theory of modulation and transport in the heliosphere is the study of the anomalous component of the cosmic rays, which are an important component at energies around a few hundred MeV. It appears that these particles are freshly ionized interstellar particles accelerated at the termination shock by the mechanism of diffusive shock acceleration.
The spacecraft Voyagers 1 and 2 are at present studying the outer parts of the heliosphere, and Voyager 1 has actually crossed the termination shock. This extended Voyager mission, to study the outer heliosphere, has provided important insights into the shape of the heliosphere and the mechanism of energetic particle acceleration. Other spacecraft, including Ulysses and the Advanced Composition Explorer (ACE), are observing the inner heliosphere.
To summarize the present knowledge, the modulation of galactic cosmic rays and the anomalous component are understood well enough to enable a confident prediction that the intensity will continue to vary in antiphase with the sunspot cycle, with variations of the order of 30 percent or so at GeV energies from sunspot minimum to sunspot maximum. It is possible that unexpected solar phenomena could produce lesser or larger effects.
SOLAR ENERGETIC PARTICLES
Energetic particles with energies occasionally exceeding several GeV are often produced in sporadic events at the Sun associated with solar activity. Solar flares and coronal mass ejections (CMEs) produce the energetic particles by processes whose specific nature is still being studied. The energy spectrum of the solar energetic particles is softer than that of galactic cosmic rays, and the events typically last for periods of hours to days. The intensities during the events can be quite large, although at energies above several hundred MeV, the time-integrated galactic cosmic ray flux is larger than that of solar cosmic rays. The events occur sporadically, although less frequently near sunspot minimum, and cannot be easily predicted. The basic energy source for the particles is the solar magnetic field, which becomes unstable and dissipates magnetic energy very rapidly, producing an explosive event. The explosion produces transient effects in the surrounding plasma, which then accelerates the solar energetic particles (SEPs). In addition to observations of the particles themselves, solar energetic particles produce variable electromagnetic
radiations such as radio waves, x-rays, and gamma rays, which serve as remote diagnostics of the particle acceleration and transport.
Solar energetic particle events can be classified as gradual (lasting days) and prompt (lasting hours). The particles in gradual events are mainly accelerated by propagating shocks generated by CMEs that propagate from very near the Sun to beyond the orbit of Earth. The acceleration process at the CME shock is fundamentally the same as the process producing the anomalous cosmic rays at the heliospheric termination shock. The time profile and energies produced by this acceleration are defined by both the nature of the CME and the properties of the interplanetary medium through which the shock propagates. The details of this process are still being debated, and only the most basic properties are understood. Detailed predictions are still a long way off. The composition of these events indicates that the process of acceleration may involve the existence of “seed” particles (low-energy superthermal particles) that were accelerated earlier in prompt solar energetic particle events.
The prompt solar energetic events are much shorter in duration and are apparently directly related to an explosive event in the solar atmosphere. It is possible that the event produces turbulent fluctuations or waves in the ambient plasma and magnetic field, which then accelerate a fraction of the ambient ions and electrons to very high energies. Alternatively, the event could produce localized shock waves, which would then be expected to accelerate the particles by the process of diffusive shock acceleration, as do the propagating CME shocks and the heliospheric termination shock (Jones and Ellison, 1991). The transport processes, although understood in broad outline, still are not understood well enough to permit detailed predictions.
Methods involving artificial intelligence, Bayesian inference, and locally weighted regression have demonstrated promise in providing “nowcasting” capabilities after SEP event particles begin to arrive. These methods are capable of predicting, with reasonable accuracy, total doses and the future temporal evolution of the dose as particles arrive very early in the evolution of the event. However, these methods are at present unable to forecast SEP event fluence levels and their associated doses until after particles begin to arrive. Hence, they could provide a much-needed short-term capability for mission operations, but in a merely stopgap role. Computer codes implementing these models are currently research codes and not in the form of operational tools that are usable by mission operations personnel (see the report of Working Group E in Appendix A of this report).
Current SEP event forecasting models are climatological and empirically based and trigger on flare electromagnetic emission. Predictions from the National Oceanic and Atmospheric Administration (NOAA) use x-rays; the U.S. Air Force (USAF) uses microwaves or x-rays; both use optical position of active region. NOAA predicts onset, duration, and peak flux, while the USAF predicts time, intensity, and spectra (assumes 404 km/s solar wind and Archimedes spiral). NOAA supports the NASA Space Radiation Analysis Group (SRAG) via Geostationary Operational Environmental Satellite (GOES) measurements. NASA then takes GOES data and predicts doses inside the International Space Station (ISS). For the average well-connected event, these methods predict the maximum intensity within an order of magnitude and the timing of the maximum within a couple of hours. These methods, however, fall apart for shock-dominated events. Hence, current forecasting models do badly in predicting extreme events, and extreme events represent the greatest danger to human spaceflight crews. The consensus of the workshop’s relevant task group (see the report of Working Group C in Appendix A) on current models is that known physics is not included in current models and needs to be added to them. Exactly how to do so is not clear at the present time, but major improvements to a predictive capability can be made if the physics-based models can incorporate measurements at the solar energetic particle source regions in order to pin down crucial model parameters that are currently missing. A large SEP event is illustrated in Figure 2.2.
Coronal Mass Ejections
Coronal mass ejections consist of large, balloon-shaped clouds of solar plasma and magnetic field that contain up to 1016 grams of matter and reach speeds in excess of 2,500 km/s. The kinetic energy alone is sufficient to boil the North Atlantic Ocean. CMEs are associated with solar flares and solar energetic particle events and occur most often during sunspot maximum. Most of the energy in such events is associated with the CME and not the flare. Such a cloud of gas drives a large-scale “bow” shock wave, which precedes the spheroidal cloud through interplanetary space and that accelerates the energetic particles in gradual solar energetic particle events. When this shock wave and cloud strike Earth’s magnetosphere, there are usually significant geomagnetic and ionospheric effects.
In white light images, CMEs often appear to have a bright, leading, looplike structure within which exists a dark cavity and a core of denser material—suggesting the eruption of a pre-event prominence, its overlying coronal cavity, and the ambient corona. The energy is thought to come from magnetic
rearrangement (reconnection) near the base of a coronal loop, triggered by motions of the footpoints (the point where the field lines enter the Sun) of the magnetic field lower in the solar atmosphere. Thus, the energy ultimately comes from the conversion of magnetic-field energy to thermal and kinetic energy. It is further observed that CMEs tend to arise in coronal streamers in an equatorial belt that encircles the Sun. Knowledge of CMEs has advanced to the point of attempting to understand the trigger mechanism, which causes the sudden instability leading to the magnetic rearrangement after a relatively slow buildup of stresses. The processes leading to the buildup of stress are at present very poorly understood; hence, the basic causes of CMEs are still not known in any detail, so prediction is difficult.
Figure 2.3 shows the structure of a CME and the sources of particle radiation. Figure 2.4 is a Solar and Heliospheric Observatory (SOHO) image that illustrates the production of particles and also their effects on spacecraft sensors.
Flares and Active Regions
Flares occur during the rapid conversion of energy from the solar magnetic field to the kinetic energy of particles in localized regions at the base of the solar corona. The accelerated particles and their interactions with the surrounding plasmas and fields can also produce electromagnetic emission in a broad range of frequencies, from microwaves to gamma rays. Flares are one of the primary sites for the acceleration of electrons (up to 10 MeV) and protons and heavy ions (up to >100 MeV per nucleon), but the exact nature of the acceleration and their propagation out to the heliosphere is still being investigated. Flare-produced SEPs are of great concern because of their fast arrival times at Earth. In some cases, it takes just minutes for relativistic particles to travel along magnetic-field lines that connect directly to the source regions.
Flares have traditionally been associated with sources of SEPs, although whether they are a source distinct from the CMEs is still controversial (see, e.g., Gosling, 1993). It is very likely that flares and CMEs are different aspects of the same general phenomenon of magnetic reconnection on the Sun (Harrison, 1995). Long-duration flares (with x-ray emission lasting hours to days) have been associated with CME shocks that produce the largest, most energetic SEP events, whereas the impulsive x-ray flares with relatively short rise times and durations (seconds to hours) produce the less energetic SEPs. A glaring exception to this tendency is the January 20, 2005, SEP event that produced one of the most energetic particle events
ever recorded, although it has many of the characteristics of an impulsive, fast rise time flare event (see Simnett, 2006). One possibility is that large SEP events may involve both flares and CME shocks. The flare may provide the suprathermal seed particles that are then processed in the CME shock.
Active regions are places where the Sun’s magnetic fields are changing rapidly owing to flux emergence or cancellation. The study of these regions is important in the context of predicting where flares are most likely to occur on the Sun. Decades of observations of active regions show that magnetic-field configurations that are highly sheared tend to produce the most flares. Determining which of these flares will produce large SEP fluxes would be of extreme importance for the prediction of the radiation environment in the heliosphere.
PROSPECTS FOR LONG- AND SHORT-TERM FORECAST MODELS
On the basis of current knowledge and present progress, participants at the workshop expressed optimism that solar and space physics researchers will be able to specify the space radiation environment accurately and will be able to forecast both long-term trends and short-term events. However, the success of such a specification and prediction program will require a careful and continuing effort to be conducted by the research programs of NASA and NOAA (and other agencies).
Long-Term Forecasts (Years to Decades)
It is well known that sunspot number is correlated with various 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. There have been successful empirical predictions of the peak of a solar cycle using sunspot trends in the earliest years of that cycle. In spite of these partial successes, accurate, long-term predictions of the solar-cycle properties and the physics underlying solar-cycle variability remain a major challenge in solar physics.
A promising approach that was presented at the workshop and subsequently published (for instance, in Dikpati et al., 2006) appears to have substantial predictive capabilities. This approach relates to dynamo models that incorporate meridional flow throughout the convection zone to transport magnetic flux (so-called flux-transport models) toward the poles at the surface and toward the equator near the convection zone base (see Figure 2.5). This approach may be very useful, but remains controversial.
A current flux-transport model correctly simulates the relative peaks of the past 8 solar cycles, and predicts that cycle 24, beginning in late 2006 or early 2007, will be 1.3 to 1.5 times the amplitude of cycle 23. This prediction is in contrast to recent predictions made using so-called precursor methods (Svalgaard et al., 2005; Schatten, 2005). It has been acknowledged that precursor methods have uncertainty that reduces 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 considerable success in predicting cycle amplitude 2 cycles ahead and therefore may be able to predict the amplitude of cycles 20 to 25 years in the future (Figure 2.6).
The latitudinal speed of movement 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. This property is also seen in flux-transport dynamos with deep meridional circulation. This fact supports the possibility of predicting cycle amplitudes two cycles ahead.
Short-Term Forecasts (Nowcasts to Days and Weeks)
Current models are inadequate to predict with confidence the onset or severity of a solar radiation event. These models typically describe separate parts of the problem (the flare or CME initiation, heliospheric propagation, interactions with planetary magnetospheres and atmospheres), but the latest models are now capable of describing the complete end-to-end system using various simplifications in their details.
Active regions contain complex magnetic structures that often erupt to produce flares and CMEs. While models exist that describe the magnetic-field evolution leading up to the eruption, the timing of the eruption is not yet predictable with these models. One method for predicting the probability of eruption is to identify S-shaped magnetic-field structures, called sigmoids, which are observed in x-ray and extreme ultraviolet (EUV) images of the corona (Canfield et al., 1999). The twisted configurations are thought to be related to the amount of magnetic helicity in the magnetic field. While the method is somewhat successful in correlating sigmoidal shape to active region eruptions, there are also nonsigmoidal active regions that can lead to eruptions but which are not predicted. Thus, this method needs improvement before it can be used reliably.
Other tools for predicting active region eruptions use photospheric magnetograms to determine the degree of nonpotentiality (overall twist and shear) and the amount of free energy in coronal magnetic fields (e.g., Falconer et al., 2003; Beveridge and Longcope, 2006). These research models are promising as future
forecasting tools because they include much of the relevant physics. All of the above methods require the active regions to be on the disk before making assessments.
The possibility for longer-term predictions may come from helioseismological models of active region formation beneath the photosphere, before their appearance on the surface (Braun and Lindsey, 2000). Helioseismology is also being used to track active regions while they are on the back side of the Sun (Braun and Lindsey, 2001). Currently, back-side tracking can only follow the largest sunspot groups, and the tech-
nique is not yet powerful enough to detect individual sunspots. However, the largest sunspots also tend to produce the biggest events. The most useful input data for forecasting active region eruptions are coronal vector magnetograms (not yet available), x-ray and EUV images, and helioseismological measurements.
Solar Wind and Heliospheric Models
Solar energetic particles propagate through the background solar wind and interplanetary magnetic field. Reliable predictions of SEP onset and severity cannot be made without understanding the heliospheric environment through which these particles propagate. In this regard, several groups are developing large-scale models to predict the plasma and magnetic-field environment of the global heliosphere. The NOAA Wang-Sheeley-Arge model (Arge and Pizzo, 2000) is an empirical model for forecasting solar wind speeds based on measurements of photospheric fields to obtain flux tube expansion factors. The derived values for flux tube expansion at the Sun are inversely related to solar wind speeds at 1 AU. More sophisticated three-dimensional magnetohydrodynamic models (e.g., Riley et al., 2001; Roussev et al., 2003) can now map velocity and magnetic-field structures in the heliosphere to specific structures in the solar corona. Such models can reproduce the large-scale features of the solar wind from solar magnetogram data and initial estimates of the density, temperatures, and velocities at the coronal base. These models are typically run until a steady-state solution is achieved. These magnetohydrodynamic models can be improved with more frequent magnetographic observations (typically, magnetograms are averaged over a solar rotation). Also, model validation can be improved with a more complete data coverage from remote sensing and in situ observations. These numerical models have not yet reached the stage of being able to routinely and reliably forecast solar wind conditions at Earth and other locations, and a major challenge for the models is to specify the north-south component of the interplanetary magnetic field that controls energy transfer at, for example, Earth’s magnetopause.
Models for Coronal Mass Ejections and Flares and for Solar Energetic Particles
CMEs and flares are the primary sources of solar energetic particles, so understanding their onset and evolution are active areas of research. Predicting the onset of CME and flare eruptions is not possible with any high degree of accuracy with present models. However, statistical methods can be used for certain cases, such as well-connected energetic electron events, which can be used to specify CME launch times. Once an eruption has occurred, it is not yet possible to predict the properties of the SEPs produced, for example, the event duration, maximum flux, energy spectrum, and so on. Some models do better than others in predicting certain aspects of SEP events: for example, models by Zank et al. (2000) can produce the observed “spectral breaks” in the energy spectrum of shock-produced SEPs (integrated over the transit time from the Sun to 1 AU). The spectral break may be an indication of the energy for which particles escape from the shock. The Solar Particle Engineering Code (SOLPENCO) model (Aran et al., 2006) is able to predict qualitatively the proton flux and fluence time profiles for a broad range of heliolongitudes—from the west limb (W90) to a far eastern location (E75), but the model relies on average parameter values from a large number of SEP test cases. In general, the SEP properties at the solar source regions vary from one event to another and also vary depending on where they are measured in the heliosphere. Current research involves understanding how the CME/flare system erupts at the Sun (e.g., the breakout model of Antiochos et al., 1999); how the CMEs are accelerated near the Sun (Chen and Krall, 2003); and how SEPs are transported in the heliosphere (e.g., Li et al., 2003; Manchester et al., 2005). Model validation using data from real SEP events is needed to better understand (1) the sources for seed particle populations, (2) particle injection into and escape from the shock, and (3) the role of turbulence in the particle acceleration process.
Required observations for specifying the input parameters include vector magnetograms to specify the coronal magnetic field, and coronal imaging and spectroscopy (radio through gamma ray) for specifying the SEP/CME/solar wind source regions. Real-time in situ plasma and magnetic-field measurements are essential for model validation and for nowcasting simulations.
ONGOING INTEGRATED MODELING ACTIVITIES
It is an enormous challenge to predict the radiation environment from first principles starting with a disturbance at the Sun. This is so for three main reasons: (1) the multiple size scales involved for the relevant physical processes, (2) the different environments where these processes occur, and (3) an incomplete knowledge of the underlying physics. A complete end-to-end model should link the separate space weather models for the solar origin, heliospheric propagation, and interactions with planetary magnetospheres and atmospheres. Several research collaborations are involved in the development of such integrated models. They are listed below:
Community Coordinated Modeling Center (CCMC)—A multiagency partnership that provides access to modern space science simulations for research and supports the transition to space weather operations of modern space research models (http://ccmc.gsfc.nasa.gov/).
Center for Integrated Space Weather Modeling (CISM)—A multi-institutional center funded by the National Science Foundation (NSF) to create a physics-based numerical simulation model that describes the space environment from the Sun to Earth. Boston University is the lead institution (http://www.bu.edu/cism/).
Solar Multidisciplinary University Research Initiative (Solar MURI)—A collaborative Department of Defense (DOD)-funded project studying magnetic eruptions on the Sun and their effects on Earth’s space environment. The University of California, Berkeley, is the lead institution (http://solarmuri.ssl.berkeley.edu/).
Space Weather Multidisciplinary University Research Initiative (Space Weather MURI)—A collaborative DOD-funded project with a long-term goal to achieve significant progress in the quest for a predictive space weather modeling capability that can eventually be transitioned to use by civilian and DOD space weather forecasting centers. The University of Michigan is the lead institution (http://csem.engin.umich.edu/muri/).
Solar, Heliospheric, and Interplanetary Environment (SHINE)—An NSF-funded project to support an affiliation of researchers within the solar, interplanetary, and heliospheric communities whose goal is to enrich and strengthen both physical understanding and predictive capabilities for connecting events on the Sun with solar wind and disturbances in the inner heliosphere (http://www.shinegroup.org/).
Two other programs that provide funding opportunities for the development and testing of integrated models are the NASA-funded Interdisciplinary Exploration Science program and the Living With a Star Targeted Research and Technology program (with joint funding by NASA and NSF).
WHAT IS POSSIBLE FOR THE FUTURE?
Current efforts are leading to better, more realistic models of CMEs, flares, and SEPs. Future models will use more realistic physics and will be enabled by computers with increasing computational speed and capabilities that allow for the rendering of space weather events in three spatial dimensions, with high spatial and temporal resolution. These models will be global in the sense that they will be capable
of forecasting events anywhere in the inner heliosphere that are regions of interest to the Vision for Space Exploration (VSE).
Some global models are beginning to come online now, but they are difficult to tailor to specific events. One clear statement from the workshop is that there is a need for a better understanding of how to relate the observations to the models. The observations have a dual role: (1) they provide the inputs to drive models, and (2) they are required to validate the models (post facto). Some of the anticipated near-term and long-term results from the space physics community are described below.
Near-Term Results (Up to 2015)
For the near-term need (up to 2015), it should be possible to improve predictions of “all clear” periods when there is a very low probability that an SEP event will occur. This is possible with a better understanding of the signatures indicating that a flare or CME is about to erupt. New observations of solar magnetic structures with Solar-B, the Solar Dynamics Observatory, and the ground-based Advanced Technology Solar Telescope and the Frequency Agile Solar Radio Telescope will help in this regard.
Missions such as the Solar Terrestrial Relations Observatory (STEREO) will provide simultaneous data from two different positions off of the Sun-Earth line as interplanetary CMEs propagate outward from the Sun. These data will be useful for developing models that can specify, and in some cases forecast, the SEP environment at a wide range of longitudes in the inner heliosphere. This is an important point since Mars, a future exploration target, is rarely on the Sun-Earth line.
The physics-based models discussed above must be pared down to include only the most essential parameters that are capable of predicting the variability of the space radiation environment. These models must be validated before being transitioned to operational use.
Far-Term Results (After 2015)
Farther out in the future (after 2015), it is desirable to make predictions of solar events weeks before they occur. More than likely this will be possible only with models that use a statistical approach along with a suitable set of in situ and remote sensing measurements from multiple vantage points in the heliosphere. It will be most useful for the VSE if models can predict 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).
An effective SEP warning system will require an operational distributed network of observations from the Sun throughout the heliosphere (similar to the distributed network of weather stations on Earth). Near-Sun missions such as Inner Heliosphere Sentinels, Solar Orbiter, and Solar Probe will provide unique measurements to test more sophisticated models. One novel approach for a future mission, presented at the workshop, is to put multiple (~15) spacecraft around the Sun at 1 AU. Spacecraft situated in polar solar orbits, at the Earth-Sun L2 and L3 points, or spacecraft at the Mars-Sun L1 points can provide unique data for a global heliospheric warning system. Remote sensing observations of CME/flare/SEP source regions in the extended corona (2 to 10 solar radii) can provide unique precursor signatures for the severity of radiation events.
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