1. INVITED: Radiation Environments for Future Human Exploration Throughout the Solar System
N. A. Schwadron1, M. Gorby2, J. Linker2, P. Riley2, T. Torok2, C. Downs2, H. E. Spence1, M. Desai3, Z. Mikic2,
C. Joyce1, K. Kozarev4, L. Townsend5, and and R. Wimmer-Schweingruber6
1University of New Hampshire,2Predictive Science Inc.,San Diego, CA,3Southwest Research Institute, San Antonio, TX , 4Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 5University of Tennessee, Knoxville, TN, 6 University of Kiel, Germany
SA41B-2371
SA41B-2371
II. Origin of Solar Energetic Particle Events from the Low Corona
III. SEP & GCR Propagation Through Our Solar System
Abstract
Acute space radiation hazards pose one of the most serious risks to future human and robotic exploration. The ability to predict when and where large events will occur is necessary in order to mitigate their hazards. The largest events are usually associated with complex sunspot groups (also known as active regions) that harbor strong, stressed magnetic fields. Highly energetic protons accelerated very low in the corona by the passage of coronal mass ejection (CME)-driven compressions or shocks and from flares travel near the speed of light, arriving at Earth minutes after the eruptive event. Whether these particles actually reach Earth, the Moon, Mars (or any other point) depends on their transport in the interplanetary magnetic field and their magnetic connection to the shock. Recent contemporaneous observations during the largest events in almost a decade show the unique longitudinal distributions of this ionizing radiation broadly distributed from sources near the Sun and yet highly isolated during the passage of CME shocks. Over the last decade, we have observed space weather events as the solar wind exhibits extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity during cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small particle radiation events. We have used observations from LRO/CRaTER to examine the implications of these highly unusual solar conditions for human space exploration throughout the inner solar system. While these conditions are not a show-stopper for long-duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we examine the rapidly changing radiation environment and its implications for human exploration destinations throughout the inner solar system.
Key Points
• Emergence of strong SEP events from low corona (2-5 Rs)
• Low corona supports rapid acceleration time (minutes) and from flanks of accelerator (motion of footpoints along shock or compression)
Key Points
• Broad longitudinal distribution from emergence of low coronal shocks or compressions
• Large flux variability of SEPs and GCRs through the solar system
Figure 6: Longitudinal distribution of SEPs in the July event. Shown is the ENLIL MHD model output at the time the shock arrives at 1 AU along with the dose rates observed at CRaTER (left, blue), derived at STEREO A (middle, red) and STEREO B (right, green), representing the observers located at the eastern flank (retrograde), center flank (prograde) of the shock front respectively.
I. Background
Figure 1:Radiation hazards posed by space radiation: (top) galactic cosmic rays from outside our solar system and from (middle) shocks and (bottom left) are often driven by (bottom right) coronal mass ejections. Figure taken from Schwadron et al. (2010).
Figure 2: Eruption of a rapidly accelerating CME: the eruption begins with a strong flux rope low in the corona; as the flux rope loses equilibrium, the CME eruption occurs (panel a), leading to a large outward speed of the plasma within the CME (panel b). The rapid acceleration of the CME causes a large divergence of the plasma velocity (panel c), particularly on the flanks of the CME. The CME-driven shock forms at a height of 1.4 solar radii. The compression (negative divergence in velocity) is stronger at the flanks of the CME than at its front (Ontiveros and Vourlidas 2009; Hudson 2011). Strong compression is also found in the wake of the CME (panel c) due to reconnection outfllows (e.g., Shibata and Magara 2011).
Figure 7: Variability in GCR fluxes and SEP
fluxes throughout the solar system. Shown here are fluxes of GCR protons from MeV (red) based on the empirical model of Webber and Lockwood (2004). Also shown is the flux of the March 2012 SEP event, projected throughout the solar system. The results demonstrate the large variability in the radiation environment throughout the solar system, with inner planets experiencing higher levels of SEP radiation and outer planets experiencing higher levels of GCRs.
The hazards posed by particle radiation in space depicted in Figure 1 pose a serious challenge to human and robotic exploration missions to the Moon, Mars and beyond. The hazards include the following:
- Galactic cosmic rays (GCRs), which are always present in the near Earth space environment and throughout the solar system, originate from beyond our heliosphere and produce chronic but not acute exposures. GCRs are very difficult to shield against beyond the Earth's protective atmosphere and magnetosphere. At ground level on Earth we live under the shielding equivalent of a swimming pool ten meters in depth, while the geomagnetic eld reduces ground level radiation (e.g., secondary neutrons) by another
factor of four at the magnetic equator. A ten-meter thick layer of water covering an area of ten square meters, the size of a small habitat for one person, would weigh 100 metric tons, near maximum capacity for currently envisaged launch vehicles. It is more likely, therefore, that astronauts will make use of natural shielding materials at their destinations than that artificial materials will be launched from Earth. For the same habitable area an artificial magnetic field of 60 T, far too strong for habitation by humans, would be required to provide magnetic shielding equivalent to that at the Earths magnetic equator. Otherwise, astronauts operating in space under typical shielding of a few-10 g/cm2 of aluminum by spacecraft structures could reach their career limit of radiation exposure from GCRs in one or several years (Cucinotta et al. 2001; Schwadron et al. 2010). Current research in this area is focused on understanding the constraints imposed by GCRs and how they vary with mission transit time, shielding type and thickness, and on developing better techniques to shield against GCRs.
The intensities of GCRs vary with the solar cycle with the largest intensities occurring near solar minimum when interplanetary eld strengths (e.g., Le Roux and Potgieter 1995) are weakest and there are the fewest number of interplanetary disturbances from transient disturbances such as coronal mass ejections (e.g., Owens and Crooker 2006; Schwadron et al. 2008). GCRs are modulated by the outflowing solar wind and its embedded magnetic field; the modulation is therefore weakest when the interplanetary field strength is low (e.g., Potgieter et al. 2001) and the associated intensities of GCRs are commensurately high.
IV. SEP & GCR Evolution over Weakest Solar Cycles in 100 Years
- Solar energetic particle (SEP) events (which we dene to include ions; also solar particle events, SPEs) are also dangerous to astronauts outside of Earths protective layers (the atmosphere and magnetosphere). Current research in this area focuses on developing the ability to predict when and where SEP events will occur and nding ways to adequately shield against SEP-associated particle radiation.
Key Points
• Decreasing activity leading to rapidly increasing GCR fluxes in subsequent solar minima
• Intensity of SEP events dropping strongly in recent weak cycles
- Unique radiation environments at each planet and their satellites. We have thoroughly characterized the locations of the radiation belts at Earth, which allows us to reduce the hazard they pose by rapidly transiting them. Human and robotic exploration of other planets and satellites requires that we adequately characterize planetary radiation environments and develop appropriate mitigation strategies and adequate shielding. Shielding is often considered the solution to space radiation hazards. Very high energy radiation (e.g., > 100 MeV), however, produces secondary penetrating particles such as neutrons and nuclear fragments in shielding material. Some types of shielding material may actually increase the radiation hazard (Wilson et al. 1999). Recent work has shown that relatively lightweight material
Figure 3: Energetic particles are accelerated over a broad latitudinal and longitudinal spread from the CME released following destabilization (in Figure 2). The colored magnetic field lines (left top) show strong distortions by the plasma expansion, driven by reconnection jets. Using coupled MAS-EPREM simulations, we link coronal conditions, CMEs and associated shocks and transients to solar energetic particles (top right), solar wind conditions, and time dependent radiation exposure (bottom panels). Shown in the right top panel and bottom panels are the results for particle dierential energy fluxes at 1 AU from the event shown in Figure 2. The bottom panels shows the resulting integrated dose equivalents for Lens and Blood Forming Organs (BFO) behind dierent levels of shielding. We nd 10's of cSv even for well-shielded (10 g/cm2 Al) BFO dose equivalents, indicating a radiation hazard that approaches the 30-Day Limit (25 cSv) in roughly 2 hours after CME initiation.The CME-onset begins at t = 1.43 hours, and then strong compression and energetic particle acceleration begins at 1.45 hours. Integrated doses at prograde (bottom-right) and retrograde observers (bottom-left) are shown in addition to the near-Earth observer. The retrograde observer is well connected magnetically to the CME driver, which results in higher and more prolonged SEP fluxes at this observer. (Schwadron et al. 2014b).
Figure 8. SEP proton fluences in three successive solar cycles highlighting the effects of dropping solar activity for lowering the fluxes of SEP events (Mewaldt et al. 2015).
Figure 4: The acceleration time to the indicated energies as a function of the magnetic eld shock-normal angle. Small angles represent quasi-parallel shocks or compressions, whereas large angles represent quasi-perpendicular shocks or compressions. We have adopted a compression ratio of 3. Prompt acceleration (on the scale of minutes) to high energies (> 30 MeV) requires a quasi-perpendicular shock or compression.
References
Figure 9: Evolving and increasing radiation levels in space (Schwadron et al. 2014a). ACE dose rates (red points) are derived from ts to CRIS spectra (O'Neill 2006), CRaTER measurements (green points) are from the zenith facing D1/D2 detectors, commonly used as proxies for lens dose rates behind 0.3 g/cm2 Al shielding (Schwadron et al. 2012). Sunspot number predictions (upper panel, black dashed lines) shown are based on a Gleissberg-like and a Dalton-like minimum. Dose predictions (solid blue line and the upper black and blue dashed curves) are from a sunspot-based model of the heliospheric magnetic field and the correlated variation in modulation of GCRs (Schwadron et al. 2014a) . The dose rates are projected to the lunar surface. The bottom panel is the same as top panel but for a longer time span.
F. Cucinotta, Issues in risk assessment from solar particle events. Radiation Measurements 30, 261 (1999)
F.A. Cucinotta, et al, Space radiation cancer risks and uncertainties for mars missions. Radiation Research 156, 682 (2001)
F.A. Cucinotta,et al, Space radiation risk limits and Earth-Moon-Mars environmental models. Space Weather 8, 0 (2010). doi: /2010SW000572
C.J. Joyce, et al., Validation of PREDICCS using LRO/CRaTER observations during three major solar events in Space Weather 11, 350{360 (2013). doi: /swe.20059
C.J. Joyce, et al., Analysis of the potential radiation hazard of the 23 July 2012 SEP event observed by STEREO A using the EMMREM model and LRO/CRaTER. Space Weather 13, 560{567 (2015). doi: /2015SW001208
C.J. Joyce et al., Atmospheric radiation modeling of galactic cosmic rays using LRO/CRaTER and the EMMREM model with comparisons to balloon and airline based measurements . SpaceWeather (2016)
K.A. Kozarev et al, O-limb SolarCoronal Wavefronts from SDO/AIA Extreme-ultraviolet Observations - Implicationsfor Particle Production. Astrophys. J. Lett. 733, 25 (2011). doi: / /733/2/L25
K.A. Kozarev et al., Global Numerical Modeling of Energetic Proton Acceleration in a CoronalMass Ejection Traveling through the Solar Corona. Astrophys. J. 778, 43 (2013).doi: / X/778/1/43
N.A. Schwadron, M. Owens, N.U. Crooker, The heliospheric magnetic eld over the hale cycle. Astrophysics and Space Sciences Transactions 4, 19{26 (2008). doi: /astra
N.A. Schwadron, H.E. Spence, R. Came, Does the space environment aect the ecosphere? EOS Transactions 92, 297{298 (2011). doi: /2011EO360001
N.A. Schwadron et al., Earth-Moon-Mars Radiation Environment Module framework. Space Weather 8, 0 (2010). doi: /2009SW000523
N.A. Schwadron, Coronal electron temperature from the solar wind scaling law throughout the space age. The Astrophysical Journal 739, 9 (2011). doi: / X/739/1/9.
N.A. Schwadron, et al., Lunar radiation environment and space weathering from the cosmic ray telescope for the eects of radiation (CRaTER). Journal of Geophysical Research (Planets) 117 (2012). doi: /2011JE
N.A. Schwadron, et al., Coronal electron temperature in the protracted solar minimum, the cycle 24 mini maximum, and over centuries. Journal of Geophysical Research: Space Physics, (2014). doi: /2013JA
N.A. Schwadron, et al., Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration? Space Weather 12, 622{ 632 (2014a). doi: /2014SW001084
N.A. Schwadron et al., Synthesis of 3-D Coronal-Solar Wind Energetic Particle Acceleration Modules. Space Weather 12, 323{328 (2014b). doi: /2014SW001086
N.A. Schwadron, et al., Broken Power-law Distributions from Low Coronal Compression Regions or Shocks. Journal of Physics Conference Series 642(1), (2015a). doi: / /642/1/012025
N.A. Schwadron, et al., Particle Acceleration at Low Coronal Compression Regions and Shocks. Astrophys. J. 810, 97 (2015b). doi: / X/810/2/97
V. Conclusions
• Emergence of strong SEP events from low corona (2-5 Rs)
• 100 year low in solar activity, increasing GCR fluxes , decreasing SEP event intensities
• Increasingly broad longitudinal distribution of SEPs from low coronal accelerators
• Large variability of fluxes through the solar system
Figure 5: Break energy of accelerated particles as a function of the magnetic eld shock-normal angle. Size-limited shocks and compressions limit break energies for large shock angles (> 30 deg), whereas time-limited acceleration limits break energies for lower shock angles (< 30 deg). Note generally that larger shock angles lead to higher break energies.