Sciencexpress Reports A Long-Lived Relativistic Electron Prior key measurements of Earth's radiation environment have been made (//-13),but some of the longest and Storage Ring Embedded in Earth's most comprehensive radiation belt ob- servations previously have come from Outer Van Allen Belt sensors on board the Solar,Anomalous. and Magnetospheric Particle Explorer (SAMPEX)mission (/4).This space- D.N.Baker,1*S.G.Kanekal,2 V.C.Hoxie,1 M.G.Henderson,3 X.Li,1 H.E. craft made low-Earth orbit (LEO)ob- Spence,4 S.R.Elkington,1 R.H.W.Friedel,3 J.Goldstein,5 M.K.Hudson,5 G.D. servations of inner and outer zone particles from its launch in July 1992 Reeves,3 R.M.Thorne,?C.A.Kletzing,8 S.G.Claudepierre until its recent atmospheric reentry and 'Laboratory for Atmospheric and Space Physics,University of Colorado,Boulder,CO,USA.2Goddard demise on 13 November 2012 (/5.16) Space Flight Center,Greenbelt,MD,USA.Los Alamos National Laboratory,Los Alamos,NM,USA. SAMPEX measured E>I MeV elec- Institute for the Study of Earth,Oceans,and Space,University of New Hampshire,Durham,NH,USA. trons at the near-Earth foot of magnetic Space Science and Engineering Division,Southwest Research Institute,San Antonio,TX,USA field lines but never was able to look Department of Physics and Astronomy,Dartmouth College,Hanover,NH,USA.'Department of into the "throat"of the radiation belt Atmospheric and Oceanic Sciences.University of California-Los Angeles,Los Angeles,CA,USA Department of Physics and Astronomy,University of lowa,lowa City,IA,USA.The Aerospace accelerator in the magnetospheric equa- Corporation,Los Angeles,CA,USA torial plane.This contrasts dramatically with the REPT-A and REPT-B instru- "To whom correspondence should be addressed.E-mail:daniel.baker@lasp.colorado.edu ent data collected by the Vanllen Probes from 1 September 2012 through Since their discovery over 50 years ago,the Earth's Van Allen radiation belts have early October 2012(Fig.1).These data5 been considered to consist of two distinct zones of trapped,highly energetic show that a powerful electron accelera- charged particles.The outer zone is comprised predominantly of mega-electron volt tion event was already in progress as 乏 (Mev)electrons that wax and wane in intensity on time scales ranging from hours to the instruments were first turned on. days depending primarily on external forcing by the solar wind.The spatially The entire outer radiation belt was en- g separated inner zone is comprised of commingled high-energy electrons and very hanced in electron flux from E ~2.0 energetic positive ions(mostly protons),the latter being stable in intensity levels MeV (Fig.1A)up to energies well over years to decades.In situ energy-specific and temporally resolved spacecraft above the 6.2<E<7.5 MeV channel E observations reveal an isolated third ring,or torus,of high-energy(E>2 Mev) (Fig.1C).At this time,the radiation electrons that formed on 2 September 2012 and persisted largely unchanged in the belt populations clearly had the ex- geocentric radial range of 3.0 to ~3.5 Earth radii for over four weeks before being pected double-belt structure with an disrupted(and virtually annihilated)by a powerful interplanetary shock wave inner zone,an outer zone,anda“slot” passage region of greatly diminished intensity separating the two. E The magnetically confined radiation zones surrounding the Earth were What is most striking (and unex- the first major discovery of the Space Age in 1958(1-4).Long-term pected)is the clear emergence of a separate,previously unseen belt,or observations of these energetic particle populations subsequently have "storage ring,"of high-energy electrons that stands out clearly after 2 shown dramatic,highly dynamic changes of the outer Van Allen belt. September.This belt is evident in the E=4.0-5.0 MeV range(Fig.IB) Previous,rather sparse measurements of the radiation environment sug- and is the dominant flux feature in the E=5.0-6.2 MeV energy range gested that powerful acceleration events for relativistic electrons occur (Fig.IC).This distinctive ring of highly relativistic electrons persists, on time scales ranging from minutes (5,6)to many hours (7,8).Thus. changing only gradually,until its abrupt and almost complete disappear- there has been direct as well as circumstantial evidence that an immense- ance late on I October.While the inner zone,the slot region,and the ly powerful and efficient accelerator operates within the terrestrial mag- relativistic storage ring (3.0<L*<~3.5)change relatively little over netosphere just a few thousand kilometers above the Earth's surface. this four-week period,the more distant part of the outer Van Allen belt On 30 August 2012,twin NASA spacecraft,the Radiation Belt shows huge dynamical changes with new electron populations appearing Storm Probes (RBSP),were launched into highly elliptical,low- at L*>4.0 beginning on about 7 September and intensifying greatly inclination orbits around the Earth.The RBSP satellites are fully instru- over a period of two weeks.Subsequently,the outermost parts of the mented with identical energetic particle,plasma,magnetic field,and outer Van Allen zone grew and diminished further with little effect on plasma wave sensors to measure and thoroughly characterize the radia- the storage ring feature until the abrupt demise of virtually the entire tion belt regions(9).The scientific payloads on board the RBSP space- outer zone electron population at the end of 1 October.Other electron craft(renamed the Van Allen Probes mission by NASA at a formal sensor systems on board the Van Allen Probes spacecraft,overlapping ceremony on 9 November 2012)have unprecedented detection sensitivi- partially in energy coverage with the REPT sensors,also detected the ty,energy resolution,and temporal sampling capability.In particular,the storage ring feature (/7). Relativistic Electron-Proton Telescope (REPT)experiment (/0) The distinct storage ring feature is more clearly evident in the merid- measures the key ~1 MeV to ~20 MeV electron population throughout ional plane projection of 4.0-5.0 MeV electrons from the combined the RBSP orbit which extends from geocentric distances of r 1.2 Re to REPT-A and REPT-B instrument records (Fig.2).In the earliest obser- r=5.8 RE (IRE,Earth radius-6372 km).The REPT sensors were vational phase (1-3 September)the expected two-belt structure of the among the first instruments turned on(1 September 2012)and have been Van Allen zones is clear(Fig.2A).In the next phase from 3 to 6 Sep- returning nearly continuous data since that time from both Van Allen tember,the relativistic storage ring was formed(Fig.2B)probably large- Probes spacecraft. ly by erosion and loss of the more distant parts of the outer zone.It then persisted in a remarkably stable fashion(Fig.2,C and D)throughout the Sciencexpress/http://www.sciencemag.org/content/early/recent/28 February 2013/Page 1/10.1126/science.1233518Reports / http://www.sciencemag.org/content/early/recent / 28 February 2013 / Page 1 / 10.1126/science.1233518 The magnetically confined radiation zones surrounding the Earth were the first major discovery of the Space Age in 1958 (1–4). Long-term observations of these energetic particle populations subsequently have shown dramatic, highly dynamic changes of the outer Van Allen belt. Previous, rather sparse measurements of the radiation environment sug￾gested that powerful acceleration events for relativistic electrons occur on time scales ranging from minutes (5, 6) to many hours (7, 8). Thus, there has been direct as well as circumstantial evidence that an immense￾ly powerful and efficient accelerator operates within the terrestrial mag￾netosphere just a few thousand kilometers above the Earth’s surface. On 30 August 2012, twin NASA spacecraft, the Radiation Belt Storm Probes (RBSP), were launched into highly elliptical, low￾inclination orbits around the Earth. The RBSP satellites are fully instru￾mented with identical energetic particle, plasma, magnetic field, and plasma wave sensors to measure and thoroughly characterize the radia￾tion belt regions (9). The scientific payloads on board the RBSP space￾craft (renamed the Van Allen Probes mission by NASA at a formal ceremony on 9 November 2012) have unprecedented detection sensitivi￾ty, energy resolution, and temporal sampling capability. In particular, the Relativistic Electron-Proton Telescope (REPT) experiment (10) measures the key ~1 MeV to ~20 MeV electron population throughout the RBSP orbit which extends from geocentric distances of r = 1.2 RE to r = 5.8 RE (1RE, Earth radius – 6372 km). The REPT sensors were among the first instruments turned on (1 September 2012) and have been returning nearly continuous data since that time from both Van Allen Probes spacecraft. Prior key measurements of Earth’s radiation environment have been made (11–13), but some of the longest and most comprehensive radiation belt ob￾servations previously have come from sensors on board the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission (14). This space￾craft made low-Earth orbit (LEO) ob￾servations of inner and outer zone particles from its launch in July 1992 until its recent atmospheric reentry and demise on 13 November 2012 (15, 16). SAMPEX measured E > 1 MeV elec￾trons at the near-Earth foot of magnetic field lines but never was able to look into the “throat” of the radiation belt accelerator in the magnetospheric equa￾torial plane. This contrasts dramatically with the REPT-A and REPT-B instru￾ment data collected by the Van Allen Probes from 1 September 2012 through early October 2012 (Fig. 1). These data show that a powerful electron accelera￾tion event was already in progress as the instruments were first turned on. The entire outer radiation belt was en￾hanced in electron flux from E ~2.0 MeV (Fig. 1A) up to energies well above the 6.2 < E < 7.5 MeV channel (Fig. 1C). At this time, the radiation belt populations clearly had the ex￾pected double-belt structure with an inner zone, an outer zone, and a “slot” region of greatly diminished intensity separating the two. What is most striking (and unex￾pected) is the clear emergence of a separate, previously unseen belt, or “storage ring,” of high-energy electrons that stands out clearly after 2 September. This belt is evident in the E = 4.0-5.0 MeV range (Fig. 1B) and is the dominant flux feature in the E = 5.0-6.2 MeV energy range (Fig. 1C). This distinctive ring of highly relativistic electrons persists, changing only gradually, until its abrupt and almost complete disappear￾ance late on 1 October. While the inner zone, the slot region, and the relativistic storage ring (3.0 < L* < ~3.5) change relatively little over this four-week period, the more distant part of the outer Van Allen belt shows huge dynamical changes with new electron populations appearing at L* > 4.0 beginning on about 7 September and intensifying greatly over a period of two weeks. Subsequently, the outermost parts of the outer Van Allen zone grew and diminished further with little effect on the storage ring feature until the abrupt demise of virtually the entire outer zone electron population at the end of 1 October. Other electron sensor systems on board the Van Allen Probes spacecraft, overlapping partially in energy coverage with the REPT sensors, also detected the storage ring feature (17). The distinct storage ring feature is more clearly evident in the merid￾ional plane projection of 4.0-5.0 MeV electrons from the combined REPT-A and REPT-B instrument records (Fig. 2). In the earliest obser￾vational phase (1-3 September) the expected two-belt structure of the Van Allen zones is clear (Fig. 2A). In the next phase from 3 to 6 Sep￾tember, the relativistic storage ring was formed (Fig. 2B) probably large￾ly by erosion and loss of the more distant parts of the outer zone. It then persisted in a remarkably stable fashion (Fig. 2, C and D) throughout the A Long-Lived Relativistic Electron Storage Ring Embedded in Earth’s Outer Van Allen Belt D. N. Baker,1 * S. G. Kanekal,2 V. C. Hoxie,1 M. G. Henderson,3 X. Li,1 H. E. Spence,4 S. R. Elkington,1 R. H. W. Friedel,3 J. Goldstein,5 M. K. Hudson,6 G. D. Reeves,3 R. M. Thorne,7 C. A. Kletzing,8 S. G. Claudepierre9 1 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA. 2 Goddard Space Flight Center, Greenbelt, MD, USA. 3 Los Alamos National Laboratory, Los Alamos, NM, USA. 4 Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA. 5 Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA. 6 Department of Physics and Astronomy, Dartmouth College, Hanover, NH, USA. 7 Department of Atmospheric and Oceanic Sciences, University of California – Los Angeles, Los Angeles, CA, USA. 8 Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA. 9 The Aerospace Corporation, Los Angeles, CA, USA. *To whom correspondence should be addressed. E-mail: daniel.baker@lasp.colorado.edu Since their discovery over 50 years ago, the Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is comprised predominantly of mega-electron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days depending primarily on external forcing by the solar wind. The spatially separated inner zone is comprised of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (E > 2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for over four weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage. on March 6, 2013 www.sciencemag.org Downloaded from