Van Allen Probes Mission Overview and Discoveries to Date

Elena Y. Adams, Kristin A. Fretz, Aleksandr Y. Ukhorskiy, and Nicola J. Fox

ABSTRACT The morning of 30 August 2012 saw an Atlas V rocket launch NASA’s second Living With a Star spacecraft mission, the twin Radiation Belt Storm Probes (RBSP), into orbit for an epic mission to understand Earth’s space radiation environment. Renamed the Van Allen Probes soon after launch, the Probes are designed to determine how populations of high-energy charged particles within Earth’s radiation belts, dangerous to astronauts and satellites, are created, respond to solar variations, and evolve in space environments. Operating in nearly identical geocentric

1.1 × 5.8 RE (10° inclination) orbits, the two Probes have advanced the goals of NASA’s Science Plan (2014) by radically revising our understanding of Earth’s radiation belts and inner magnetosphere. As a key new member of the Heliophysics System Observatory, the mission has led to the discovery of several new and unanticipated structures, the verification and quantification of previously suggested energization processes, and the discovery of new energization processes. It has revealed the critical importance of dynamic injections into the innermost magnetosphere and has coordinated with the high-altitude Antarctic balloon mission Balloon Array for RBSP Relativistic Electron Losses (BARREL) to determine the nature of particle precipitation. The uniquely capable instruments have revealed inner radiation belt features that were all but invisible to previous sensors, and the mission has profoundly advanced our understanding of the measurements critical for predictive modeling of space weather. This article describes the science objectives of the Van Allen Probes mission, highlights the discoveries made so far, and explains the objectives and observation plan for the extended mission.

INTRODUCTION The NASA Van Allen Probes mission (previously ation belts. The Probes were launched 30 August 2012 known as Radiation Belt Storm Probes, or RBSP) and entered into their 2-year prime mission science addresses how populations of high-energy charged parti- phase on 1 November 2012. The mission achieved all cles are created, vary, and evolve in space environments, of its level-1 requirements by the end of the prime mis- and specifically within Earth’s magnetically trapped radi- sion on 31 October 2014. Because the mission was out

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of synchronization with NASA’s Senior Review process, has been accepted for several more years of operation, NASA granted a “bridge phase” that allowed the mis- specifically through the expected lifetime of the Probes, sion to continue operating for an additional 11 months ~1 January 2019, limited by the fuel needed to maintain through September 2015. The mission was proposed and Sun-aligned positive power orientation.

Table 1. Van Allen Probes level-1 observations Observations Purposes Determine spatial/temporal variations of medium- and high- Determine time history of energization, loss, and transport for energy electron and proton angle and energy distributions, faster hazardous particles. Understand/quantify sources of these par- than drift times, interior and exterior to acceleration events. ticles and source paths. Enable improved particle models. Derive electron and proton radial phase space density (PSD) Distinguish between candidate processes of acceleration, profiles for medium- and high-energy electrons and protons on transport, and loss and statistically characterize these processes short timescales compared to storm times. versus solar input conditions. Determine spatial/temporal variations of charged particle Understand how large-scale magnetic and electric fields in the partial pressures and their gradients within the inner magneto- inner magnetosphere are generated and evolve, their role in the sphere with fidelity to calculate pressure-driven currents. dynamics of radiation belt particles, and their role in the creation and evolution of the plasma environments for other processes. Determine spatial/temporal variations of low- to medium- Understand/quantify the conditions that control the production energy electron and ion energy, composition, and angle distri- and propagation of waves (e.g., electromagnetic ion cyclotron, butions on short timescales compared to drift periods. whistler-mode chorus and hiss) and determine the wave propagation medium. Determine the local steady and impulsive electric and magnetic Determine convective and impulsive flows causing particle fields with fidelity to determine the amplitude, vector direction, transport and energization, determine propagation properties and time history of variations on a short timescale compared to of shock-generated propagation fronts, and infer total plasma times required for particle measurements. densities. Determine spatial/temporal variations of electrostatic and Determine the types/characteristics of plasma waves causing electromagnetic field amplitudes, frequencies, intensities, direc- particle energization and loss, including wave growth rates; tions, and temporal evolutions with fidelity to calculate wave energization and loss mechanisms; diffusion coefficients and energy, polarization, saturation, coherence, wave angle, and loss rates; plasma densities; ULF waves’ versus irregular fluctua- phase velocity for (A) very low-frequency and extremely low- tions’ effects on radial transport; and statistical maps of wave frequency waves and (B) random, ultralow-frequency (ULF), fields versus position and conditions. and quasi-periodic fluctuations. Provide concurrent, multipoint measurements sufficient to con- Covert particle measurements to invariant coordinate systems; strain global convective electric field and storm-time electric infer loss cone sizes; and model effects of global electric and and magnetic field models. magnetic field variations on particle distributions. Track/characterize transient structures propagating through Determine which shock-related pressure pulses produce sig- the inner magnetosphere with fidelity to determine amplitude, nificant acceleration and estimate their significance relative to arrival times, and propagation directions. other energization mechanisms.

Table 2. Van Allen particle measurement parameter requirements as specified in the Van Allen program-level requirements document Require- Cadence Energy Range Angular Energy Measurement ment (distribution/min) (MeV) Resolution (deg) Resolution 4.1.1.1 High-energy electrons 1 1–10 30 30% at 3 MeV 4.1.1.2 Medium-energy electrons 1 0.05–1 20 30% at 0.3 MeV 4.1.1.3 High-energy protons 1 20–75 30 40% at 30 MeV 4.1.1.4 Medium-energy protons 1 0.1–1 20 40% at 0.3 MeV 4.1.1.5 Medium-energy ion composition 1 0.02–0.3 30 40% at 0.05 MeV 4.1.1.6 Low-energy ion/electron composition 1 50 eV–0.05 MeV 30 40% at 0.01 MeV

Measurement requirements are derived from the observation needs summarized in Table 1. All required on two platforms.

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The Probes comprise two spacecraft taking in situ all aspects of the mission are available in a 2013 volume measurements in nearly the same highly elliptical, low- of Space Science Reviews (volume 179). inclination orbits (1.1 × 5.8 RE, 10°). For the prime mission, the orbits are slightly different so that one spacecraft laps the other spacecraft about every 67 days, SCIENCE OBJECTIVES allowing separation of spatial from temporal effects over The fundamental science objective of the Probes mis- spatial scales ranging from ~0.1 to 5 RE. By adjusting sion is to promote understanding, ideally to the point the orbital phase of one spacecraft slightly with respect of predictability, of how populations of relativistic elec- to the other during lapping events, the spacecraft can be trons and penetrating ions in the radiation belts form roughly aligned along the same magnetic field line and or change in response to variable inputs of energy from thus sample field-aligned evolution of particles and wave the Sun. fields. A comprehensive suite of instruments, identical This broad objective was parsed into three overarch- on the two spacecraft, measures all of the particles (electrons, ions, ion composition), fields (E and B), and wave ing science questions: distributions (dE and dB) that are needed to resolve the 1. Which physical processes produce radiation belt most critical science questions. Detailed descriptions of enhancements, oftentimes with excess acceleration?

Table 3. Van Allen field and wave measurement parameter requirements as specified in the Van Allen program-level requirements document Requirement Measurement Cadence Frequency Range Frequency Resolution 4.1.1.7 3-D magnetic field 20 vectors/s DC–10 Hz n/a 4.1.1.8 3-D wave magnetic field 6 s 10 Hz–10 kHz 20 channels per decade 4.1.1.9 3-D wave electric field 6 s 10 Hz–10 kHz 20 channels per decade 4.1.1.10 3-D electric field 20 vectors/s DC–10 Hz n/a 4.1.1.11 Plasma density 10 s n/a n/a Measurement requirements are derived from the observation needs summarized in Table 1. All required on two platforms.

Table 4. Van Allen Probes investigations

Instrument/Suites Principal Investigator Key Partners Science Investigation Energetic Particle, Com- Harlan Spence, Los Alamos National Laboratory, Measure near-Earth space radiation position, and Thermal University of New Southwest Research Institute, Aero- belt particles to determine the physi- Plasma Suite (ECT) Hampshire space Corp., Laboratory for Atmo- cal processes that produce enhance- spheric and Space Physics (LASP) ments and loss Electric and Magnetic Craig Kletzing, NASA Goddard Space Flight Center, Understand plasma waves that Field Instrument Suite University of Iowa University of New Hampshire energize charged particles to very and Integrated Science high energies; measure distortions to (EMFISIS) Earth’s magnetic field that control the structure of the radiation belts Electric Field and Waves John Wygant, University of California, Berkeley; Study electric fields and waves that Instrument (EFW) University of LASP energize charged particles and modify Minnesota the inner magnetosphere Radiation Belt Storm Louis Lanzerotti, New Johns Hopkins University Applied Understand the creation of the Probes Ion Composition Jersey Institute of Physics Laboratory (APL), Fundamen- “storm time ring current” and the Experiment (RBSPICE) Technology tal Technologies role of the ring current in the creation of radiation belt populations Proton Spectrometer David Byers, National Aerospace Corp., MIT Lincoln Lab Create specification models of the Belt Research (PSBR) Reconnaissance Office high-energy particles in the inner radiation belt Relativistic Proton Joseph Mazur, Spectrometer (RPS) Aerospace Corp. Balloon Array for RBSP Robyn Millan, Measure, study, and understand elec- Relativistic Electron Dartmouth College tron loss processes from Earth’s outer Losses (BARREL) electron belt

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2. What are the dominant mechanisms for relativistic Particle energization mechanisms have been a key electron loss? focus of the Van Allen Probes mission. The Probes have provided the first definitive evidence that peaks in the 3. How do ring current and other geomagnetic pro- radial profiles of electron PSD result from local accel- cesses affect radiation belt behavior? eration (Fig. 5; Ref. 6). Simultaneous determinations of These questions were answered through a series of PSD by the two spacecraft, one traveling Earthward and observations, listed in Table 1. These observations were one traveling outward, were critical. In addition, sophis- used to derive level-1 requirements for the mission. ticated modeling using detailed measurements of plasma Tables 2 and 3 show the measurement requirements parameters and whistler-wave mode intensity spectra derived from the level-1 observations. The instruments demonstrated that quasilinear whistler wave diffusive and instrument suites are summarized in Table 4. Fig- scattering suffices to locally accelerate the electrons over ures 1 and 2 show the overlap between various investiga- the ~12-h period of observed acceleration (Fig. 6; Ref. 2). tion measurements.

Particle sensors SCIENCE HIGHLIGHTS PSBR/RPS Electrons HOPE ECT/REPT As mentioned earlier, all MagEIS ECT/MagEIS of the level-1 objectives have RBSPICE L1 requirement RBSPICE ECT/HOPE been achieved. The mission REPTREPT has contributed immensely to knowledge of the radiation belts and resulted in Protons HOPE MagEIS numerous discoveries. This MRD requirement section highlights findings RBSPICE to date. Capability REPT The Probes enabled dis- RPS covery of several new and Ion unanticipated structures composition HOPE within the radiation belts. RBSPICE The first is the so-called 1 eV 1 keV 1 MeV 1 GeV third belt or electron stor- Energy age ring (Fig. 3; Ref. 1), last- ing for months for electron Figure 1. Comparison of the energetic particle observations from the science investigations. The energies greater or equal to observations overlap each other to ensure calibration across the board. about 2 MeV. In retrospect, the belt is a predictable MRD requirement consequence of the weak DC magnetic Capability L1 requirement whistler-wave scattering EMFISIS FGM

losses that occur for those AC magnetic Fields and waves sensors portions of the outer belt EMFISIS SCM that overlap the dense plas- EMFISIS/Mag 2,3 DC electric EMFISIS/Waves masphere. The surprising EFW cause of a second new struc- EFW Perp 2D ture, the so-called zebra EFW Par 1D stripes within the inner electron belt, is drift reso- AC electric nance with the electric field EMFISIS Waves induced by Earth’s rotation EMFISIS Waves (Fig. 4; Ref. 4). The third unanticipated, and as-yet EFW E- d spectra unexplained, structure is ~DC 10 Hz 1 kHz 1 MHz the unusually sharp inner Frequency boundary for ultrarelativis- 5 tic electrons at L ~ 2.8 RE. Figure 2. Comparison of the fields and waves observations across the frequency spectra.

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Outer e-belt (4.5 MeV) “3rd” e-belt (4.5 MeV) However, the Probes have also observed very coherent and possibly nonlinear electron-whistler mode interactions.8 It is a key theme of the extended mission to sort out the relative importance of quasilinear and nonlinear interactions for a variety of external conditions. Another theme for the extended mission will be to determine the relative importance of local and often more global transport-related acceleration. A critical process associated with transport-related acceleration is drift resonance between ULF waves and azimuthally Inner belt particles drifting electrons. The Probes have directly observed such drift resonant interactions for the first time (Fig. 7; Figure 3. Van Allen Probes Relativistic Electron Proton Telescope Ref. 9), enabling the team to establish which of several (REPT) measurements demonstrated the existence of a third radi- possible mechanisms generate some ULF waves.10 These ation belt.7 Here, measurements from a single radial cut through waves also play a key role in particle energization when the belts and an assumption of adiabatic transport were used to distribute intensities around Earth and along the field lines. interplanetary shocks strike the Earth’s magnetosphere, as shown by the unique two-spacecraft measurements of the magnetospheric response to such shocks (Fig. 8; 250 Observations Ref. 11). A critical task of the extended mission is to distinguish between and quantify the energization associated with shock-generated wave fronts and ULF waves. 200 1 – The discovery of double-layer-like electric field 105 spikes within the heart of the radiation belts was com- pletely unexpected.12 Such structures, now thought to

150 s.sr.keV) 2 be a highly nonlinear evolution of strong whistler mode waves,13 may locally accelerate very low-energy elec- 100 trons up to the kiloelectronvolt range along field lines, thereby providing a seed population for subsequent Electron energy (keV) Electron Intensity (cm acceleration to the megaelectronvolt range of energies. 104 50 The importance of this seed population relative to that provided by injections and the inward radial transport 1.0 1.5 2.0 2.5 3.0 of low-energy magnetotail populations (see, e.g., Ref. 14) Earth radii remains unresolved and will be addressed during the extended mission. Particle injections appear to be much more common 250 Model in the inner (<5 RE) region than previously recognized, and they play an unanticipated prominent role in creat- ing ring current populations (Fig. 9; Refs. 15 and 16).

200 –1 105 These injections generate an array of different wave s.sr.keV)

150 2 Van Allen Probes–derived PSD

100 Electron energy (keV) Electron 104 Intensity (cm Radial accelerationdistanceLocal 50

1.0 1.5 2.0 2.5 3.0 Phase space density Earth radii

Figure 4. Top, Van Allen Probes RBSPICE measurements of inner belt electrons showing surprising “zebra stripes.” Bottom, Figure 5. ECT measurements of a relativistic electron PSD radial A model invoking quasi-resonant interactions between drift- profile as compared to expectations for local acceleration of the ing electrons and the electric field induced by Earth’s rotation electrons.6 Adiabatic radial transport would show a very different explains the observations. (Adapted from Ref. 4.) profile without a peak.

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modes that may play a significant role in particle accel- sometimes map to the positions of the Van Allen Probes eration and loss. Injections generate an unusual low-fre- spacecraft. Exceedingly close correlations between whis- quency hiss mode within the plasmasphere,17 coherent tler mode hiss waves and electron precipitation modula- whistler modes,8 double layers,13 and kinetic scale field- tions have been observed (Fig. 10; Ref. 21), verifying an line Alfvén resonances.18 The relative importance of all important theoretical expectation. Surprisingly, the cor- of these wave structures will be established during the relation is coherent over vast spatial scales, suggesting a extended mission. global extent for the losses. Particle scattering losses have been the particular The Probes mission has significantly advanced focus of joint measurements between the Van Allen our understanding of Earth’s inner radiation belt Probes and the associated Mission of Opportunity (R < 2.5 RE). East–west gradient measurements of BARREL,19 which comprises multiple high-altitude bal- inner belt energetic protons enabled by high-quality loons deployed within the Antarctic during two winter Probes instrumentation revealed the details of proton– seasons. The balloons carry X-ray spectrometers to mea- atmosphere interactions.22 Pitch angle distributions of sure the bremsstrahlung emissions from precipitating this population clarified the role of interplanetary solar relativistic electrons on radiation belt field lines that proton events as the source of the outer shoulder of the inner proton belt.23 The first truly clean measurements Whistler wave intensity 10–2 6 demonstrated that the ener-

10–3 Arb. units gies of inner radiation belt

) 5 E electrons never exceeded 10–4

L (R 4 900 keV during the Probes’ 10–5 prime mission.24,25 3 Finally, the Van Allen 10–6 Electron ux (2.5 MeV) Probes mission has enabled 2.5 MeV electrons 6 substantial improvement to cm 103 the modeling of radiation –2 ) 5 s E

–1 belt processes, and it repeat- 102 keV edly provided the measure-

L (R 4 –1 ments needed to constrain 1 3 10 these models; these advance- 7 8 9 10 11 12 ments are critical for space Days of Sept 2012 weather applications (see, e.g., Refs. 26 and 27).

Figure 6. Five days of REPT relativistic electron intensity measurements revealed a 12-h interval of strong electron acceleration on 9 Sept 2012. Modeling revealed that the simultaneous whistler PLANS FOR EXTENDED waves observed by EMFISIS and EFW performed the acceleration.20 MISSION

(a) The great successes of 2 the Van Allen Probes mission during its prime phase (nT) 0 and the subsequent bridge MFA

Br phases introduced new sci- –2 Radial magnetic perturbations ence questions that need to be addressed before we can fully attain predictive under- 80 keV (b) 4 ] standing of radiation belt

1 10 100 generation and dynamics. keV 80 keV electrons –1 r

s On 1 November 2015, the –1 s

–2 Van Allen Probes began the 3 Pitch angle (deg) [c m 10 mission that will bring us 0 15:45:00 16:00:00 16:15:00 16:30:00 much closer to meeting that objective. Figure 7. Magnetic Electron Ion Spectrometer (MagEIS) and EMFISIS made the first direct mea- The overarching objec- surement of a drift-resonant energy exchange between ULF waves (a) and energetic electrons (b). tive of the Van Allen Probes The energy dependence of phasing (not shown) is critical to the interpretation.9 extended mission is to quan-

178 Johns Hopkins APL Technical Digest, Volume 33, Number 3 (2016), www.jhuapl.edu/techdigest Van Allen Probes Mission Overview and Discoveries to Date tify the processes governing Drift echoes RBSP-B the Earth’s radiation belt Sun shock B RBSP-A arrival and ring current environ- shock ment as the solar cycle tran- 3 arrival Drift sitions from solar maximum A echoes through the declining phase. The sunspot number reached 8 Oct 2013 ~ 2022 a peak in April 2014, and this number will likely become 2 the official maximum of the 3.6 MeV current solar cycle (cycle 24). electrons Based on historical measurements, the radiation belt particle ux activity is expected to con- 10 RBSP-A RBSP-B tinue to intensify with the Log 1 decline of the solar cycle: the biggest radiation belt storms of two previous solar cycles occurred in their declining phase when the high-speed solar wind streams prevailed 0 1 2 3 4 5 6 over coronal mass ejec- L-Star (R ) tions. Not surprisingly, the E two biggest solar storms of Figure 8. REPT measurements of relativistic electrons accelerated by interplanetary shock per- this decade occurred within turbations transmitted into the radiation belts. The shock-generated wave arrived almost simulta- the last several months on neously at both spacecraft while they were outbound from Earth at different positions (top left).11 17 March and 24 June 2015. Radiation belts comprise particles trapped by the 100 (a) Earth’s geomagnetic field and 50 held in closed drift trajecto- 0 –50 ries around Earth. Particle SYMH (nT) –100 ASYMH (nT) 106 intensities are controlled by (b) the shifting balance among 105 ) –1

multiple acceleration and sr 100 loss mechanisms that inter- 104 –1 keV act with particles at different 3 45 10 –1 40 s

parts of their drift trajecto- –2 Energy (keV) 35 2 ries. The lines of apsides of 10 (cm 30 the Van Allen Probes orbits 25 1 drift westward and complete 20 10 a full circle around Earth 14 (c) LOCALIZED INJECTIONS over ~2 years (Fig. 11). By 12 the end of the extended mis- 10 sion, the Van Allen Probes 8 will be the first inner mag- 6 netospheric mission to circle 4 around Earth four times, 2 allowing the team to quan- pressure (nPa) Perpendicular 0 Time (days) 17.0 17.5 18.0 tify how the relative role of L 1.12 5.42 5.56 1.12 5.43 5.35 1.12 5.42 5.36 1.11 various acceleration and loss MLT (hours) 14.0 23.6 1.85 13.3 3.69 1.76 12.7 23.5 1.73 12.0 mechanisms changes with the decline of the solar cycle. Figure 9. Three orbits of RBSPICE energetic ion data before and during a magnetospheric storm Particle acceleration show that localized injections are far more common within the heat of the ring current popula- mechanisms have been a tions than previously recognized. The color spectrogram shows angle-averaged proton intensi- key focus of the Van Allen ties, and panel c shows integrated particle pressure.15 (Adapted from Ref. 16.)

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Magnetic eld spectral power—Van Allen Probes A ties for taking such measure- 500 ments. The first opportunity 10.00 arises because the spacecraft orbits were adjusted by increasing the separation of 1.00 their apogees (i.e., the apogee 100 /(Hz)

2 of spacecraft A was raised

pT by 75 km, and the apogee 0.10 Frequency (Hz) Frequency of spacecraft B was lowered 30 by 75 km). This adjustment 0.01 increased the cadence of lap- 30 ping events from 67 to 35 days, Van Allen Probes A hiss 20-min detrend doubling the opportunities 20 for two-point sampling of wave–particle interactions 10 3000 along field lines. The second opportunity to sample wave 0 2000 interactions simultaneously at different magnetic lati-

Hiss intensity (pT) 1000 –10 tudes will arise via coordi- 0 nating with the Japanese –20 Exploration of energization –1000 and Radiation in Geospace BARREL Balloon 21 –2000 (ERG) spacecraft, planned

Peak X-ray counts/20 min for launch in the summer of –3000 2016. ERG, by design, will 2000 2100 2200 sample at higher magnetic hhmm of 3 Jan 2014 latitudes than the Probes. Using three-point measure- Figure 10. The close correlation of EFW measurements of whistler wave hiss emissions (top ments will provide a more panel and bottom black curve) and the intensity of magnetically conjugate electron precipita- global view of wave–particle tion into the atmosphere as diagnosed from bremsstrahlung X-rays measured with one of the interactions at different mag- Antarctic BARREL balloons led Breneman et al.21 to conclude that the hiss causes the precipitation netic latitudes, important for over very broad regions. (Adapted from Ref. 21.) quantifying nonlinear effects. Particle loss mechanisms Probes mission. The Probes have provided the first are another phenomenon that must be studied in pursuit definitive evidence of local peaks in the radial profiles of of understanding dynamic variability of radiation belt electron PSD,7 and these peaks were attributed to local intensities. Particle scattering losses into the atmosphere quasilinear acceleration by whistler waves.2 The Probes have been the particular focus of joint measurements of have also discovered highly unexpected nonlinear wave the Van Allen Probes and BARREL.19 Exceedingly close structures in the heart of the radiation belt.28 Such struc- correlations between whistler mode hiss waves and elec- tures can rapidly energize very low-energy electrons up tron precipitation modulations have been observed,21 to the kiloelectronvolt range, thereby providing a seed suggesting a global extent for the losses capable of population for subsequent acceleration to radiation belt depleting radiation belt intensities on timescales as energies. The Probes have also observed large-amplitude short as 1–20 min. Another process that can rapidly whistler mode waves (~100–500 mV/m) (see, e.g., Refs. 29 deplete radiation belt intensities during storms is particle and 30) that are likely to rapidly accelerate kiloelectron- streaming through the magnetopause boundary, caus- volt particles to megaelectronvolt energies in the process ing particles to escape into the interplanetary environ- on nonlinear wave–particle interactions along magnetic ment. A goal of the Van Allen Probes extended mission field lines. A key theme of the extended mission is to sort is to enable understanding of the relative importance out the relative importance of quasilinear and nonlinear of precipitation and magnetopause losses. The launch interactions for the buildup of radiation belt intensities. of NASA Magnetospheric Multiscale (MMS) mission To determine the relative importance of nonlinear in March 2015 offers an ideal opportunity to observe interactions, the evolution of the wave fields and particle directly these escaping electrons at the magnetopause distributions along field lines needs to be measured. The while the Van Allen Probes measure inner magneto- extended mission will provide two unique opportuni- spheric losses and the processes that drive these losses.

180 Johns Hopkins APL Technical Digest, Volume 33, Number 3 (2016), www.jhuapl.edu/techdigest Van Allen Probes Mission Overview and Discoveries to Date CONCLUSION Over last 3 years, the Van Allen Probes mission has radically changed our understanding of Earth’s inner magnetosphere and radiation belts. As of March 2016, the Van Allen Probes bibliography contains over 225 publications, including a number of articles in presti- gious journals such as Nature and Science. (See http:// rbspgway.jhuapl.edu/biblio?s=year&o=desc&f[keyw ord]=10 for bibliographies.) With all instruments return- ing quality data, both spacecraft being healthy, and the remaining propellant sufficient to support spacecraft operations well into 2019, many more publications and science discoveries are expected during the extended mission. Stay tuned!

REFERENCES 1Baker, D. N., Kanekal, S. G., Hoxie, V. C., Batiste, S., Bolton, M., et al., “The Relativistic Electron-Proton Telescope (REPT) Instrument on Board the Radiation Belt Storm Probes (RBSP) Spacecraft: Char- acterization of Earth’s Radiation Belt High-Energy Particle Popula- tions,” Space Sci. Rev. 179(1), 337–381 (2013). Figure 11. As the spacecraft orbits precess around Earth, the 2Thorne, R. M., Li, W., Ni, B., Ma, Q., Bortniket, J., et al., “Rapid Probes sample different acceleration and loss mechanisms that Local Acceleration of Relativistic Radiation-Belt Electrons by Mag- sculpt global distributions of radiation belts and ring current netospheric Chorus,” Nature 504(7480), 411–414 (2013). 3Shprits, Y. Y., Subbotin, D., Drozdov, A., Usanova, M. E., Keller- populations. man, A., et al., “Unusual Stable Trapping of the Ultrarelativistic Elec- trons in the Van Allen Radiation Belts,” Nat. Phys. 9(11), 699–703 (2013). 4 During major loss events, the MMS spacecraft will skim Ukhorskiy, A. Y., Sitnov, M. I., Mitchell, D. G., Takahashi, K., Lan- zerotti, L. J., and Mauk, B. H., “Rotationally Driven ‘Zebra Stripes’ the dayside magnetopause region for extended intervals. in Earth’s Inner Radiation Belt,” Nature 507(7492), 338–340 (2014). With huge energetic electron sensor geometric factors 5Baker, D. N., Jaynes, A. N., Hoxie, V. C., Thorne, R. M., Foster, J. C., 2 31 et al., “An Impenetrable Barrier to Ultrarelativistic Electrons in the totaling more than G = 2 cm -sr for the cluster, MMS Van Allen Radiation Belts,” Nature 515(7528), 531–534 (2014). will directly measure escaping radiation belt electrons. 6Baker, D. N., Kanekal, S. G., Hoxie, V. C., Henderson, M. G., Li, X., The buildup of energetic (kiloelectronvolt) ion popu- et al., “A Long-Lived Relativistic Electron Storage Ring Embedded in Earth’s Outer Van Allen Belt,” Science 340(6129),186–190 (2013). lation during storms creates a source of hot plasma pres- 7Reeves, G. D., Spence, H. E., Henderson, M. G., Morley, S. K., Frie- sure in the inner magnetosphere that drives the global del, R. H. W., et al., “Electron Acceleration in the Heart of the Van ring current system. Ring current controls the distribu- Allen Radiation Belts,” Science 341(6149), 991–994 (2013). 8Fennell, J. F., Roeder, J. L., Kurth, W. S., Henderson, M. G., tion of the magnetic field that governs the motion of Larsen, B. A., et al., “Van Allen Probes Observations of Direct Wave- radiation belt particles. Energetic ions also provide the Particle Interactions,” Geophys. Res. Lett. 41(6), 1869–1875 (2014). 9Thorne, R. M., Li, W., Ma, Q., Ni, B., and Bortnik, J., “Radiation Belt energy source for an array of different wave modes that Electron Acceleration by Chorus Waves During the 17 March 2013 play a significant role in radiation belt acceleration and Storm,” in Proc. XXXIth URSI General Assembly and Scientific Symp., loss. A surprising discovery of the Van Allen Probes Beijing, p. 1 (2014). 10Claudepierre, S. G., Mann, I. R., Takahashi, K., Fennell, J. F., prime mission was that a substantial fraction of hot Hudson, M. K., et al., “Van Allen Probes Observation of Local- ized Drift-Resonance Between Poloidal Mode Ultra-Low Frequency plasma pressure is transported in mesoscale (~1–2 RE) dynamic ion injections that were previously considered Waves and 60 keV Electrons,” Geophys Res. Lett. 40(17), 4491–4497 (2013). infrequent in the inner magnetosphere. The structure 11Dai, L., Takahashi, K., Wygant, J. R., Chen, L., Bonnell, J. W., et al., and occurrence rates of the injections remain unknown, “Excitation of Poloidal Standing Alfven Waves Through the Drift Resonance Wave-Particle Interaction,” Geophys Res. Lett. 40(16), and the amount of hot plasma transported remains 4127–4132 (2013). poorly quantified. It is our goal for the extended mission 12Foster, J. C., Wygant, J. R., Hudson, M. K., Boyd, A. J., Baker, D. N., et to understand the relative roles of global-scale trans- al., “Shock-Induced Prompt Relativistic Electron Acceleration in the Inner Magnetosphere,” J. Geophys. Res. Space Phys. 120(3), 1661–1674 port processes and mesoscale injections in the buildup (2015). of hot plasma pressure during storms. This investiga- 13Mozer, F., Bale, S.., Bonnell, J. W., Chaston, C., Roth, I., et al, “Mega- tion will be greatly enabled by the recent adjustment volt Parallel Potentials Arising from Double-Layer Streams in the Earth’s Outer Radiation Belt,” Phys. Rev. Lett. 111(23), 235002 (2013). of the spacecraft orbit, which doubled the cadence of 14Mozer, F. S., Agapitov, O., Krasnoselskikh, V., Lejosne, S., simultaneous two-point measurements at the separa- Reeves, G. D., and Roth, I., “Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by tion of 1–2 RE, necessary to quantify the properties of Nonlinear Whistlers,” Phys. Rev. Lett. 113(3), 035001-1–035001-5 dynamic injections. (2014).

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THE AUTHORS

Elena Y. Adams is a systems engineer in APL’s Space Exploration Sector. She was a payload systems engineer for the Van Allen Probes, responsible for the ECT suite: MagEIS, HOPE, and REPT. Kristin A. Fretz is a systems engineer in APL’s Space Exploration Sector. She led the fault management team for the Van Allen Probes mission during the development, integration, and test phases of the project and is currently serving as the Mission Systems Engineer for the extended mission. Aleksandr Y. Ukhorskiy is a senior scientist in APL’s Space Exploration Sector. He is currently serving as the Project Scientist for the Van Allen Probes mission. Nicola (“Nicky”) J. Fox is the Chief Scientist for Space Weather in the Space Research Branch of APL’s Space Exploration Sector. She is the Deputy Project Scientist for the Van Allen Probes mission. For further information on the work reported here, contact Elena Adams. Her e-mail address is [email protected].

182 Johns Hopkins APL Technical Digest, Volume 33, Number 3 (2016), www.jhuapl.edu/techdigest