remainder of September until its almost complete annihilation early in 1991.Geophys.Res.Let.19,821(1992).doi:10.1029/92GL00624 October 2012 6.X.Li et al,Simulation of the prompt energization and transport of radiation We note that powerful "injection"of high-energy electrons and pro- belt particles during the March 24,1991 SSC.Geophys.Res.Lett.20,2423 tons deep into the inner part of the Earth's magnetosphere on 24 March (1993).doi:10.1029/93GL02701 1991 (5,18,/9)was observed by instruments on board the CRRES 7.D.N.Baker et al.,Relativistic electron acceleration and decay time scales in the inner and outer radiation belts:SAMPEX.Geophys.Res.Left.21,409 spacecraft (/3).It was a highly impulsive event caused by an exception- (1994).doi:10.1029/93GL03532 ally strong interplanetary shock wave (5,6).This March 1991 event was 8.R.B.Home et al.,Timescale for radiation belt electron acceleration by whistler a stark example of the sudden appearance of a newly energized popula- mode chorus waves.J.Geophys.Res.110,A03225 (2005). tion of both protons and electrons in a localized portion of the slot region doi:10.10292004JA010811 of the radiation belts that normally is nearly devoid of very energetic 9.B.H.Mauk et al.,Science objectives and rationale for the Radiation Belt Storm particles(19,20).This prior event contrasts with the storage ring feature Probes mission.Space Sci.Rev.10.1007/s1 1214-012-9908-y (2012). observed by the Van Allen Probes sensors:The storage ring clearly re- 10.D.N.Baker et al.,The Relativistic Electron-Proton Telescope (REPT) sulted largely from loss of the more distant portion of the outer zone instrument on board the Radiation Belt Storm Probes (RBSP)spacecraft: electron population rather than fresh,localized injection of the March Characterization of Earth's radiation belt high-energy particle populations. Space Sci.Rev.10.1007/s11214-012-9950-9(2012). 1991 type.The original acceleration of the electron population(prior to the turn-on of REPT on I September)that eventually formed the storage 11.Since the initial Van Allen belt discovery,there have been many missions that have measured key aspects of the radiation properties around the Earth.Some ring may have resulted either from local wave heating (2/,22)or from of these have been from "operational"satellite systems such as the National enhanced radial diffusion (23,24)or both. Oceanic and Atmospheric Administration (NOAA)weather satellites in Based on prior radiation belt research [e.g.,(7,15)],the outer Van geostationary Earth orbit (GEO)(www.oso.noaa.gov/goesstatus)or polar low- m Allen zone electron populations would be expected to respond rather Earth (LEO)(www.oso.noaa.gov/poesstatus)orbits.Other measurements have directly to changes in the solar wind,interplanetary magnetic field been made using sor on board perational GE spacecraft or theGloba (IMF),and geomagnetic activity.Indeed,the development of the storage Positioning Satellite(GPS)timing and navigation constellation of spacecraft ring feature itself (Fig.3)was closely associated with loss of outer belt as well as the Polar and Cluster scientific satellites (/2).These prior satellites electrons following passage of an interplanetary shock wave on 3 Sep- have provided key long-term monitoring of radiation belt changes,but have tember,seen as a sharp increase in solar wind speed(Fig.3B)and abrupt generally not made measurements directly in the heart of the radiation belt 乏 regions.Only the Combined Release and Radiation Effects Satellite (CRRES) change in the IMF(Fig.3C).Subsequently,a new population of highly relativistic electrons emerged at a region around L4.0 and grew in mission (/3)operated briefly (1990-91)in the heart of the radiation belts but lacked the background rejection and the temporal,energy,and spatial intensity and spatial extent(Fig.3A)following a high-speed solar wind resolution now provided by the dual Van Allen Probes. episode(Fig.3B)on 5 September.Another such period of high-energy 12.R.H.W.Friedel,G.D.Reeves,T.Obara,Relativistic electron dynamics in electron flux diminution,reappearance,and intensification was seen the inner magnetosphere -a review.J.Atmos.Sol.Terr.Phys.64,265 from ~21 September through to 1 October(Fig.3A),again this sequence (2002).doi:10.1016/S1364-68260100088-8 occurring in the wake of a powerful high-speed solar wind stream on 20- 13.M.H.Johnson,J.Kierein,Combined Release and Radiation Effects Satellite 21 September (Fig.3B).As noted above,one of the most abrupt and (CRRES):Spacecraft and mission.J.Spacecr.Rockets 29,556 (1992). doi:10.2514/3.55641 striking features of the entire data set was the nearly complete disappear- 14.D.N.Baker et al.,An overview of the Solar Anomalous,and Magnetospheric ance of the entire outer zone electron population late on 1 October asso- Particle Explorer(SAMPEX)mission.IEEE Trans.Geosci.Rem.Sens.31, ciated with another interplanetary shock wave (Fig.3,B and C)and 531(1993).doi:101109/36.225519 relatively strong geomagnetic storm (seen in Dst,which measures global 15.X.Li,M.Temerin,D.N.Baker,G.D.Reeves,Behavior of MeV electrons at magnetic field disturbance,Fig.3D) geosynchronous orbit during last two solar cycles.J.Geophys.Res.116, Figure 3A shows that for the period of 1-4 September,the average A11207(2011).doi:10.10292011JA016934 品 plasmapause boundary was relatively close to the Earth(L*~3)and a 16.D.N.Baker,J.E.Mazur,G.M.Mason,SAMPEX to reenter atmosphere: powerful outer zone electron acceleration event was occurring in the low Twenty-year mission will end.Space Weather 10,S05006 (2012). doi:10.10292012SW000804 plasma-density region outside the plasmasphere.However,from~4 Sep- tember until ~6 October,the plasmapause was much farther outward, 17.See data and methods in the accompanying supplementary materials on Science Online. ranging at L*>4.Thus,the storage ring feature as well as most of the 18.A.L.Vampola,A.Korth,electron drift echoes in the inner magnetosphere. outer Van Allen zone E>4.5 MeV electron population was inside the Geophys.Res.Let.19,625(1992).doi10.1029/92GL00121 high-density plasmasphere.However,in the traditional picture the outer 19.E.G.Mullen,M.S.Gussenhoven,K.Ray,M.Violet,A double-peaked inner zone electron belt would largely be outside the plasmasphere and the slot radiation belt:cause and effect as seen on CRRES.IEEE Trans.Nucl.Sci.38. region inside the plasmasphere outer boundary (2/-23,25). 1713(1991).doi10110923.124167 The radiation belt particle populations are determined by a complex 20.D.H.Brautigam,JASTP 64,1709 (2002). superposition of acceleration,transport,and loss processes modulated by 21.R.B.Horne et al.,Wave acceleration of electrons in the Van Allen radiation their interactions with plasma waves (24).We are now seeing unex- belts.Nature 437,227(2005).doi:10.1038/nature03939 Medline pected radiation belt structures (Fig.4),but have yet to fully understand 22.Y.Y.Shprits et al.,Acceleration mechanism responsible for the formation of them in the context of present radiation belt theory. the new radiation belt during the 2003 Halloween solar storm.Geoplrys.Res Lett.33,L05104(2006).doi:10.10292005GL024256 23.X.Li,D.N.Baker,T.P.O'Brien,L.Xie,Q.G.Zong,Correlation between References and Notes the inner edge of outer radiation belt electrons and the innermost plasmapause 1.J.A.Van Allen et al.,Jet Propuls.28,588(1958). location.Geophys.Res.Left.33,L14107 (2006).doi:10.1029/2006GL026294 2.J.A.Van Allen,The geomagnetically trapped corpuscular radiation.. 24.R.M.Thorne,Radiation belt dynamics:The importance of wave-particle Geophys..Res.64,1683(1959).doi:10.1029/JZ064i011p01683 interactions. Geophys. Res. Lett. 37, L22107 (2010). 3.J.A.Van Allen,L.A.Frank,Radiation around the Earth to a radial fistance of doi:10.1029/2010GL044990 107,400km.Nature183,430(1959).doiI0.1038/183430a0 25.L.R.Lyons,R.M.Thorne,Equilibrium structure of radiation belt electrons 4.S.C.Freden,R.S.White,Protons in the Earth's magnetic field.Phys.Rev J.Geophys.Res.78,2142(1973).doi10.10290A078i013p02142 Lett.3,9 (1959).doi:10.1103/PhvsRevLett.3.9 5.J.B.Blake,W.A.Kolasinski,R.W.Fillius,E.G.Mullen,Injection of 26.J.Goldstein,Plasmasphere response:Tutorial and review of recent imaging results.Space Sci.Rev.124,203 (2006). electrons and protons with energies of tens of MeV into L<3 on 24 March Sciencexpress/http://www.sciencemag.org/content/early/recent /28 February 2013/Page 2/10.1126/science.1233518/ http://www.sciencemag.org/content/early/recent / 28 February 2013 / Page 2 / 10.1126/science.1233518 remainder of September until its almost complete annihilation early in October 2012. We note that powerful “injection” of high-energy electrons and pro￾tons deep into the inner part of the Earth’s magnetosphere on 24 March 1991 (5, 18, 19) was observed by instruments on board the CRRES spacecraft (13). It was a highly impulsive event caused by an exception￾ally strong interplanetary shock wave (5, 6). This March 1991 event was a stark example of the sudden appearance of a newly energized popula￾tion of both protons and electrons in a localized portion of the slot region of the radiation belts that normally is nearly devoid of very energetic particles (19, 20). This prior event contrasts with the storage ring feature observed by the Van Allen Probes sensors: The storage ring clearly re￾sulted largely from loss of the more distant portion of the outer zone electron population rather than fresh, localized injection of the March 1991 type. The original acceleration of the electron population (prior to the turn-on of REPT on 1 September) that eventually formed the storage ring may have resulted either from local wave heating (21, 22) or from enhanced radial diffusion (23, 24) or both. Based on prior radiation belt research [e.g., (7, 15)], the outer Van Allen zone electron populations would be expected to respond rather directly to changes in the solar wind, interplanetary magnetic field (IMF), and geomagnetic activity. Indeed, the development of the storage ring feature itself (Fig. 3) was closely associated with loss of outer belt electrons following passage of an interplanetary shock wave on 3 Sep￾tember, seen as a sharp increase in solar wind speed (Fig. 3B) and abrupt change in the IMF (Fig. 3C). Subsequently, a new population of highly relativistic electrons emerged at a region around L* ~4.0 and grew in intensity and spatial extent (Fig. 3A) following a high-speed solar wind episode (Fig. 3B) on 5 September. Another such period of high-energy electron flux diminution, reappearance, and intensification was seen from ~21 September through to 1 October (Fig. 3A), again this sequence occurring in the wake of a powerful high-speed solar wind stream on 20- 21 September (Fig. 3B). As noted above, one of the most abrupt and striking features of the entire data set was the nearly complete disappear￾ance of the entire outer zone electron population late on 1 October asso￾ciated with another interplanetary shock wave (Fig. 3, B and C) and relatively strong geomagnetic storm (seen in Dst, which measures global magnetic field disturbance, Fig. 3D). Figure 3A shows that for the period of 1-4 September, the average plasmapause boundary was relatively close to the Earth (L* ~ 3) and a powerful outer zone electron acceleration event was occurring in the low plasma-density region outside the plasmasphere. However, from ~4 Sep￾tember until ~6 October, the plasmapause was much farther outward, ranging at L* > 4. Thus, the storage ring feature as well as most of the outer Van Allen zone E > 4.5 MeV electron population was inside the high-density plasmasphere. However, in the traditional picture the outer zone electron belt would largely be outside the plasmasphere and the slot region inside the plasmasphere outer boundary (21–23, 25). The radiation belt particle populations are determined by a complex superposition of acceleration, transport, and loss processes modulated by their interactions with plasma waves (24). We are now seeing unex￾pected radiation belt structures (Fig. 4), but have yet to fully understand them in the context of present radiation belt theory. References and Notes 1. J. A. Van Allen et al., Jet Propuls. 28, 588 (1958). 2. J. A. Van Allen, The geomagnetically trapped corpuscular radiation. J. Geophys. Res. 64, 1683 (1959). doi:10.1029/JZ064i011p01683 3. J. A. Van Allen, L. A. Frank, Radiation around the Earth to a radial fistance of 107,400 km. Nature 183, 430 (1959). doi:10.1038/183430a0 4. S. C. Freden, R. S. White, Protons in the Earth’s magnetic field. Phys. Rev. Lett. 3, 9 (1959). doi:10.1103/PhysRevLett.3.9 5. J. B. Blake, W. A. Kolasinski, R. W. Fillius, E. G. Mullen, Injection of electrons and protons with energies of tens of MeV into L < 3 on 24 March 1991. Geophys. Res. Lett. 19, 821 (1992). doi:10.1029/92GL00624 6. X. Li et al., Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC. Geophys. Res. Lett. 20, 2423 (1993). doi:10.1029/93GL02701 7. D. N. Baker et al., Relativistic electron acceleration and decay time scales in the inner and outer radiation belts: SAMPEX. Geophys. Res. Lett. 21, 409 (1994). doi:10.1029/93GL03532 8. R. B. Horne et al., Timescale for radiation belt electron acceleration by whistler mode chorus waves. J. Geophys. Res. 110, A03225 (2005). doi:10.1029/2004JA010811 9. B. H. Mauk et al., Science objectives and rationale for the Radiation Belt Storm Probes mission. Space Sci. Rev. 10.1007/s11214-012-9908-y (2012). 10. D. N. Baker et al., The Relativistic Electron-Proton Telescope (REPT) instrument on board the Radiation Belt Storm Probes (RBSP) spacecraft: Characterization of Earth’s radiation belt high-energy particle populations. Space Sci. Rev. 10.1007/s11214-012-9950-9 (2012). 11. Since the initial Van Allen belt discovery, there have been many missions that have measured key aspects of the radiation properties around the Earth. Some of these have been from “operational” satellite systems such as the National Oceanic and Atmospheric Administration (NOAA) weather satellites in geostationary Earth orbit (GEO) (www.oso.noaa.gov/goesstatus) or polar low￾Earth (LEO) (www.oso.noaa.gov/poesstatus) orbits. Other measurements have been made using sensors on board operational GEO spacecraft or the Global Positioning Satellite (GPS) timing and navigation constellation of spacecraft as well as the Polar and Cluster scientific satellites (12). These prior satellites have provided key long-term monitoring of radiation belt changes, but have generally not made measurements directly in the heart of the radiation belt regions. Only the Combined Release and Radiation Effects Satellite (CRRES) mission (13) operated briefly (1990-91) in the heart of the radiation belts but lacked the background rejection and the temporal, energy, and spatial resolution now provided by the dual Van Allen Probes. 12. R. H. W. Friedel, G. D. Reeves, T. Obara, Relativistic electron dynamics in the inner magnetosphere — a review. J. Atmos. Sol. Terr. Phys. 64, 265 (2002). doi:10.1016/S1364-6826(01)00088-8 13. M. H. Johnson, J. Kierein, Combined Release and Radiation Effects Satellite (CRRES): Spacecraft and mission. J. Spacecr. Rockets 29, 556 (1992). doi:10.2514/3.55641 14. D. N. Baker et al., An overview of the Solar Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission. IEEE Trans. Geosci. Rem. Sens. 31, 531 (1993). doi:10.1109/36.225519 15. X. Li, M. Temerin, D. N. Baker, G. D. Reeves, Behavior of MeV electrons at geosynchronous orbit during last two solar cycles. J. Geophys. Res. 116, A11207 (2011). doi:10.1029/2011JA016934 16. D. N. Baker, J. E. Mazur, G. M. Mason, SAMPEX to reenter atmosphere: Twenty-year mission will end. Space Weather 10, S05006 (2012). doi:10.1029/2012SW000804 17. See data and methods in the accompanying supplementary materials on Science Online. 18. A. L. Vampola, A. Korth, electron drift echoes in the inner magnetosphere. Geophys. Res. Lett. 19, 625 (1992). doi:10.1029/92GL00121 19. E. G. Mullen, M. S. Gussenhoven, K. Ray, M. Violet, A double-peaked inner radiation belt: cause and effect as seen on CRRES. IEEE Trans. Nucl. Sci. 38, 1713 (1991). doi:10.1109/23.124167 20. D. H. Brautigam, JASTP 64, 1709 (2002). 21. R. B. Horne et al., Wave acceleration of electrons in the Van Allen radiation belts. Nature 437, 227 (2005). doi:10.1038/nature03939 Medline 22. Y. Y. Shprits et al., Acceleration mechanism responsible for the formation of the new radiation belt during the 2003 Halloween solar storm. Geophys. Res. Lett. 33, L05104 (2006). doi:10.1029/2005GL024256 23. X. Li, D. N. Baker, T. P. O’Brien, L. Xie, Q. G. Zong, Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location. Geophys. Res. Lett. 33, L14107 (2006). doi:10.1029/2006GL026294 24. R. M. Thorne, Radiation belt dynamics: The importance of wave-particle interactions. Geophys. Res. Lett. 37, L22107 (2010). doi:10.1029/2010GL044990 25. L. R. Lyons, R. M. Thorne, Equilibrium structure of radiation belt electrons. J. Geophys. Res. 78, 2142 (1973). doi:10.1029/JA078i013p02142 26. J. Goldstein, Plasmasphere response: Tutorial and review of recent imaging results. Space Sci. Rev. 124, 203 (2006). on March 6, 2013 www.sciencemag.org Downloaded from