Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

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1 SPACE WEATHER, VOL. 5, S11002, doi: /2007sw000341, 2007 Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting Geoffrey D. Reeves Published 2 November Citation: Reeves, G. D. (2007), Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting, Space Weather, 5, S11002, doi: /2007sw During the last solar cycle a dramatic change in our understanding of the Earth s radiation belts took place. Rather than the slowly varying, well-understood region described in most textbooks, the radiation belts were found to be highly dynamic and full of new surprises. For example, for reasons that are still not well understood, the fluxes of relativistic electrons throughout the belts will suddenly increase or decrease by factors of 100 or more. In addition, new, transient radiation belts were observed to form in regions where radiation belt particles are normally scarce. Such observations challenged earlier theories of radiation belt physics, prompting scientists to reevaluate existing space weather models and search for new data. An exciting new opportunity to explore radiation belt phenomena and to develop new space weather models will come in 2012 with the launch of NASA s Radiation Belt Storm Probes (RBSP) mission, which is currently in conceptual phase A development ( html). The mission objective for RBSP is to understand, ideally to the point of predictability, how populations of relativistic electrons and ions in space are formed or changed in response to the variable energy inputs from the Sun. RBSP will be studying conditions and phenomena that directly affect Earth-orbiting satellites that serve humanity in diverse ways, explained George Withbroe, former director of NASA s Sun-Earth Connections program. The Earth s radiation belts are an ideal laboratory for developing a detailed understanding of charged particle dynamics in space, both because of the diversity of processes that operate there and because of the direct practical application to satellites in Earth orbit. The high-energy electrons and ions that make up the terrestrial radiation belts can produce a number of effects that range in severity from minor disturbances to the catastrophic failure of entire satellites. Radiation belt effects include increased radiation dose to astronauts and satellites, internal charging and discharging in spacecraft materials, anomalous operation of electronic components, Copyright 2007 by the American Geophysical Union atomic displacement damage, and even nuclear activation of materials. As space systems become increasingly sophisticated and costly, the success of new space-based enterprises will hinge on improved reliability and better, more cost-effective design. Under-designing systems leads to increased risk but overdesigning leads to reduced capability or performance when resources are limited and resources are always limited. Just as current theoretical understanding is captured in state-ofthe-art physics models, best practices for space systems engineering are captured in radiation belt specification models. The current standard for specifying the radiation belts Figure 1. Relativistic electron fluxes ( MeV) from NASA s Polar satellite binned according to L-shell (a measure of the equatorial altitude of a magnetic field line) and time for the year Also shown is the daily minimum Dst value (a measure of storm intensity, with negative values being more intense). The dynamic nature of the radiation belts is evident as is the association with geomagnetic storms, but the nature of the radiation belt response is variable and (as yet) unpredictable. Indicated are several examples of storms that produced net flux enhancements in the radiation belt fluxes (red arrows), examples of net flux reductions (blue), and events that produced clear changes in the radiation belts but different net effects at different magnetic L-shells. Adapted from Reeves et al. [2003]. 1 of 5

2 for spacecraft design and operations is the AE/AP-8 model [Vette, 1991a, 1991b], which describes the average radiation environment as a function of energy, altitude, and latitude. AE/AP-8 has produced incredible return on investment for NASA and the aerospace industry, but we now know that future specification models must include new data to improve the overall accuracy and must also include time-dependence to represent the broad range of intensities, which can be many times higher or lower than the averages. Such improvements require new observations across a broad range of particle energies and geomagnetic conditions. The types and quality of measurements that are needed to develop the next generation of specification models will be obtained by RBSP in the process of developing a deeper understanding of radiation belt physics. Thus, the RBSP mission will produce more than purely scientific results. Beyond the obvious intellectual benefit this mission will bring, there is also economic reward to be gained said Barbara Giles, NASA s RBSP program scientist. RBSP Science Objectives The discovery of the Earth s radiation belts by James Van Allen s measurements on NASA s Explorer 1 satellite nearly a half century ago was one of the first discoveries of the space age. A variety of radiation belt measurements were made in the following years, and fundamental theories of the radiation belt environment were developed. For many years, textbooks and courses that discussed the radiation belts treated them as a relatively well understood system. A dramatic change in that perspective occurred when the Combined Radiation Release and Effects Satellite (CRRES) observed an entirely new belt of extremely high energy electrons that was produced in the magnetosphere in a matter of minutes [Blake et al., 1992]. Around the same time, observations by geosynchronous satellites, by CRRES, the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX), and NASA s Polar satellite (among others), showed that the radiation belts are highly structured and highly dynamic, exhibiting variability on timescales of minutes, days, seasons, and solar cycles [Li et al., 2001; Baker et al., 2004]. Similarly, although fluxes of radiation belt electrons and ions have long been known to vary in response to geomagnetic storms and solar wind conditions (velocity and interplanetary magnetic field Bz), new studies showed that the magnitude and even the sign of the radiation belt responses are poorly understood and unpredictable. While the majority of radiation belt enhancements are observed during high-speed solar wind-driven storms, any given storm might either increase or decrease radiation belt fluxes (Figure 1). Scientists do not know why this happens or what controls the changes. This lack of predictability reflects a lack of quantitative understanding of the processes that transport, accelerate, or remove particles from the system. Therefore, developing this predictive capability is one of the foremost goals of NASA s Living With a Star (LWS) program. The structure and dynamics of the radiation belts are controlled by a host of variable processes whose effects must be understood both singly and collectively if dramatic improvements in space weather capability are to be achieved. A particular challenge is that many of those processes act simultaneously, sometimes in the same region of space and sometimes in vastly different parts of the inner magnetosphere [Friedel et al., 2002]. Acceleration processes compete with losses to determine the net effect of geomagnetic activity. The final intensity of the belts depends on the state of the lowerenergy source population and the magnetospheric transport processes as well as the effects of acceleration and loss. Furthermore, the whole chain of events takes place in a system of electric and magnetic fields that change dramatically in response to solar activity, solar wind energy inputs, the effects of the storm-time ring current (an electric current that encircles the Earth during geomagnetic storms), substorms, ionospheric coupling, and the whole host of interconnected parts of the Sun-Earth system. To study the inner magnetospheric processes that most strongly affect the Earth s radiation belts, the RBSP mission has identified eight primary science objectives (Table 1) that cover radiation belt acceleration and loss, the dynamics of transient and extreme radiation belt structures, the effects of the ring current and other storm phenomena, and the controlling influences of the magnetospheric plasma environ- 2 of 5

3 Figure 2. Many processes act upon radiation belt particles to produce acceleration, transport, and losses. These include radial diffusion, a variety of wave-particle interactions, injections of variable source populations, global electric and magnetic field configurations, and interactions of different plasma and energetic particle populations in the inner magnetosphere (e.g., plasma sheet, ring current, plasmasphere, etc.). Many of these processes are most intense and act in combination during geomagnetic storms when most large radiation belt events are observed. ment, electric fields, and magnetic fields on the whole coupled system. The physical equations that describe processes in the inner magnetosphere are, by necessity, kinetic equations that require knowledge of the phase space densities (not just flux at a given energy and location) that drive many of the measurement requirements for the RBSP instruments. Once particle phase space density distributions are determined, one can quantitatively investigate the interaction of different particle populations and the effects of electric and magnetic field and waves on those populations. A brief (and incomplete) list of processes targeted for detailed investigation by RBSP includes (see Figure 2) electron acceleration by local resonant interactions with very low frequency (VLF) waves such as the chorus and magnetosonic waves that are produced by unstable plasma distributions that arise during storms; acceleration and transport of radiation belt electrons and ions by radial diffusion that is driven by ultralow-frequency (ULF) electric and magnetic field fluctuations; the role of interplanetary shocks from solar eruptions that hit the magnetosphere and produce sudden, intense electron and ion acceleration events; losses of electrons and ions both by precipitation into the atmosphere and escape from the magnetosphere into the solar wind losses that are in turn controlled by movement of the magnetopause, strong stretching of the magnetic field, and scattering by a variety of waves such as whistler hiss, Alfvèn waves, and electromagnetic ion cyclotron (EMIC) waves; storm and substorm phenomena including buildup and decay of the ring current (which forms when electrons and ions injected by substorms encircle the Earth in the same region of space where the radiation belts are found), large-scale reconfiguration of the electric and magnetic fields, and development of unstable plasma distributions that enhance the waves that drive many radiation belt processes. The RBSP science goals were evaluated and vetted with two criteria in mind: (1) the potential to fundamentally change our understanding of energetic processes that occur in near-earth space, throughout the solar system, and in astrophysical systems, and (2) the potential to dramatically improve our ability to predict and avoid hazardous space weather conditions. To see RBSP become a reality will be very special to the team that created the LWS vision, commented George Withbroe. RBSP Observatories The RBSP mission will launch two satellites, with identical sets of instruments, to measure charged particle populations, fields, and waves in the inner magnetosphere. The spacecraft are designed to be Sun pointed spinners in near-equatorial elliptical orbits with apogees inside geosynchronous orbit. Nearequatorial orbits and spin orientation were chosen in order to maximize the coverage of the full range of particles and waves, particularly those that are confined to the portions of magnetic field lines near the equator. As with the CRRES mission [Johnson and Kierein, 1992], elliptical orbits are desirable so that two radial profiles of particle distributions are obtained for each satellite, each orbit. Apogees inside geosynchronous orbit optimize the coverage of the inner magnetosphere and the frequency of orbital passes through the belts. During the 2-year nominal mission lifetime, the two RBSP spacecraft will slowly change their position relative to the Sun and Earth (defined by local time ), moving through the night (antisunward) side of the Earth in the first year and across the day (sunward) side in the second year, enabling RBSP to measure the different processes that occur in different local time sectors. The two-spacecraft configuration is unique for the inner magnetosphere. The two spacecraft will have slightly different velocities in their orbits, and the two orbits will also drift in local time at slightly different rates. This produces two types of satellite configurations: one where the two satellites measure different regions simultaneously and another where the two satellites measure the same region one after the other. Together, these two configurations allow scientists to understand both the spatial extent and the time history of wave fields and particle populations and the processes that drive them. 3 of 5

4 With two spacecraft in the same orbit one following the other it will be possible to eliminate much of the temporal/spatial ambiguity that plagued previous missions, said Kile Baker, NSF s Magnetospheric Physics Program director. RBSP will host five science investigations that together will cover the broad range of particle energies from a few electron volts (ev) to over a gigaelectron volt (1 GeV = 109 ev) and the electromagnetic spectrum from quasi-static fields to nearly a megahertz (MHz; see Table 2). One investigation, the RBSP Energetic, Composition, and Thermal plasma suite (RBSP-ECT), consists of three instruments measuring electrons and ions over a broad range of energies. These instruments include the Helium, Oxygen, Proton, Electron (HOPE) spectrometer, which will measure the major ion species, He+, O+, and H+ along with electrons and will cover energies from a few ev to approximately 50 kev (1 kev = 1000 ev), and the Magnetic Electron and Ion Spectrometer (MagEIS), which will overlap with the HOPE energy range and extend the electron and ion composition measurements up to several million electron volts (MeV). The third instrument in the suite, the Relativistic Electron-Proton Telescope (REPT), will further extend the measurements above 10 MeV for electrons and above 75 MeV for ions. Another investigation, the RBSP Ion Composition Experiment (RBSPICE), is designed to measure the composition, energy, and characteristic distributions of the ions that make up the storm-time ring current: predominantly H+, He+, and O+ ions with energies from about 40 to 400 kev. To complete the particle measurements, the U.S. National Reconnaissance Office will provide the Relativistic Proton Spectrometer (RPS) as a third particle investigation on RBSP. The RPS instrument will measure the most energetic populations in the radiation belts radiation belt ions with energies of tens of MeV to over 1 GeV. The HOPE, MagEIS, REPT, RBSPICE, and RPS instruments all utilize the spacecraft spin and near-equatorial orbit to provide the most complete possible coverage of electron and ion distributions including the critical part of the distribution that is trapped near the magnetic equator. Together they also provide the continuous energy coverage and excellent energy and species identification in the heart of the radiation belts where penetrating radiation has overwhelmed or contaminated many previous observations. The fields and waves investigations provide similarly complete coverage of the electromagnetic spectrum from quasi-static fields to nearly a megahertz. One investigation, the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS), consists of two instruments: the Mag, a triaxial fluxgate magnetometer, and Waves, consisting of a triaxial search coil and waveform receivers. The other investigation, the Electric Fields and Waves experiment (EFW), consists of two pairs of spin plane wire antennas and a pair of rigid antennas aligned along the spacecraft spin axis. Together, the EMFISIS and EFW investigations will measure the quasi-static and slowly varying electric and magnetic fields that determine the convective and magnetic motion of charged particles in the inner magnetosphere, including the strong perturbations in those fields associated with geomagnetic storms. Particularly important is the measurement of a broad range of electromagnetic wave modes that transport, accelerate, and scatter energetic charged particles including VLF chorus, EMIC waves, plasmaspheric hiss, magnetosonic waves, magnetohydrodynamic (MHD) waves and injections fronts, interplanetary shock-generated waves, and waves at the electron cyclotron and upper hybrid resonance harmonics. The full RBSP fields and particles payload will provide the most complete set of observations of the radiation belts yet obtained, but, as important as these measurements are, it is also important that information gets to space weather forecasters in time to be useful for predictions. To this end it is envisioned that RBSP will implement a broadcast antenna to provide critical space weather parameters in real time to users in the operational space weather community. The Growing Space Weather Satellite Network The Radiation Belt Storm Probes mission will provide a unique new body of knowledge that will answer some of the most important questions in space weather. As Kile Baker noted, Clearly, anything that will lead to improved understanding and predictability of the radiation belts will help fulfill the goals of the National Space Weather program. However, the RBSP spacecraft do not have to answer all the questions alone (Figure 3). Many current and future space programs will also be essential: first to lay the groundwork for discovery in the years before the launch of RBSP and then to extend the science return from RBSP by providing understanding of the larger, global context. Notable among those are the other LWS program elements (Solar Dynamics Observatory, the Ionosphere-Thermosphere Storm Probes, and 4 of 5

5 Solar Wind Sentinels); proposed radiation belt missions from Europe, Canada, and Japan such as Energization and Radiation in Geospace (ERG) and Orbitals; and ongoing civilian and military programs such as GOES and GPS. Hence, RBSP will be an integral but unique part of an emerging network of satellites to monitor and predict space weather in the inner magnetosphere. References Figure 3. A representation of some of the satellite programs that provide continuous monitoring of the radiation belts and a more global context for interpretation of the RBSP observations. Shown schematically are the radiation belt electron fluxes as seen from above the North Pole, two geosynchronous NOAA/GOES satellites (dark blue), the Los Alamos National Laboratory geosynchronous systems (dark green), an Aerospace-HEO system (yellow) in Molniya orbit, the GPS satellite constellation consisting of 24 satellites in six orbital planes (gray), and the two RBSP satellites (black) in an equatorial elliptical orbit. Baker, D. N., S. G. Kanekal, X. Li, S. P. Monk, J. Goldstein, and J. L. Burch (2004), An extreme distortion of the Van Allen belt arising from the Halloween solar storm in 2003, Nature, 432(7019), Blake, J. B., W. A. Kolasinski, R. W. Fillius, and E. G. Mullen (1992), Injection of electrons and protons with energies of tens of MeV into L < 3 on 24 March 1991, Geophys. Res. Lett., 19(8), Friedel, R. H. W., et al. (2002), Relativistic electron dynamics in the inner magnetosphere: A review, J. Atmos. Sol. Terr. Phys., 64(2), Johnson, M. H., and J. Kierein (1992), Combined Release and Radiation Effects Satellite (CRRES): Spacecraft and mission, J. Spacecr. Rockets, 29(4), Li, X., D. N. Baker, S. G. Kanekal, M. Looper, and M. Temerin (2001), Long-term measurements of radiation belts by SAMPEX and their variations, Geophys. Res. Lett., 28(20), Reeves, G. D., K. L. McAdams, R. H. W. Friedel, and T. P. O Brien (2003), Acceleration and loss of relativistic electrons during geomagnetic storms, Geophys. Res. Lett., 30(10), 1529, doi: /2002gl Vette, J. I. (1991a), The AE-8 trapped electron model environment, NSSDC/ WDC-A-R&S 91-24, NASA Goddard Space Flight Cent., Green belt, Md. Vette, J. I. (1991b), The NASA/National Space Science Data Center Trapped Radiation Environment Model Program (TREMP) ( ), NSSDC/WDC-A-R&S 91-29, NASA Goddard Space Flight Cent., Greenbelt, Md. Geoffrey D. Reeves is a senior research scientist in the Space Science and Applications group at Los Alamos National Laboratory, Los Alamos, New Mexico. 5 of 5