Science Objectives and Rationale for the Radiation Belt Storm Probes Mission
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The NASA Radiation Belt Storm Probes (RBSP) mission addresses how populations of high energy charged particles are created, vary, and evolve in space environments, and specifically within Earth’s magnetically trapped radiation belts. RBSP, with a nominal launch date of August 2012, comprises two spacecraft making in situ measurements for at least 2 years in nearly the same highly elliptical, low inclination orbits (1.1×5.8 RE, 10∘). The orbits are slightly different so that 1 spacecraft laps the other spacecraft about every 2.5 months, allowing separation of spatial from temporal effects over spatial scales ranging from ∼0.1 to 5 RE. The uniquely comprehensive suite of instruments, identical on the two spacecraft, measures all of the particle (electrons, ions, ion composition), fields (E and B), and wave distributions (d E and d B) that are needed to resolve the most critical science questions. Here we summarize the high level science objectives for the RBSP mission, provide historical background on studies of Earth and planetary radiation belts, present examples of the most compelling scientific mysteries of the radiation belts, present the mission design of the RBSP mission that targets these mysteries and objectives, present the observation and measurement requirements for the mission, and introduce the instrumentation that will deliver these measurements. This paper references and is followed by a number of companion papers that describe the details of the RBSP mission, spacecraft, and instruments.
KeywordsRadiation belt Magnetosphere Geomagnetic storms NASA mission
The science objectives for the Radiation Belt Storm Probes Mission (RBSP) were first articulated by the NASA-sponsored Geospace Mission Definition Team (GMDT) report published in 2002, refined within the NASA RBSP Payload Announcement of Opportunity issued in 2005, and finalized in the RBSP Program Level (Level 1) requirements document signed by NASA’s Associate Administer for Science in 2008. The fundamental objective of the RBSP mission is to:
Provide understanding, ideally to the point of predictability, of how populations of relativistic electrons and penetrating ions in space form or change in response to variable inputs of energy from the Sun.
Which physical processes produce radiation belt enhancements?
What are the dominant mechanisms for relativistic electron loss?
How do ring current and other geomagnetic processes affect radiation belt behavior?
2 Background and Context
The work that was performed in conjunction with and following the CRRES and SAMPEX missions has convinced the scientific community that we are far from having a predictive understanding of the behavior of the Earth’s radiation belts, as discussed below. Present understanding of aspects of radiation belt physics is captured in several monographs and reviews. Lemaire et al. (1996) document the mid-1990’s understanding of the belts; and Hudson et al. (2008), Thorne (2010), and a series of papers in the Journal of Atmospheric and Solar-Terrestrial Physics edited by Ukhorskiy et al. (2008), review more recent understanding.
In parallel with the new findings and interest in the radiation belts of Earth, extraterrestrial planetary probes have revealed robust radiation belts at all of strongly magnetized planets, despite the huge differences between the respective planets and despite the huge differences in how the space environments of these different planets are powered (Mauk and Fox 2010, and references therein). The creation of trapped populations of relativistic and penetrating charged particles is clearly a universal characteristic of strongly magnetized space environments and not just a characteristic of the special conditions that prevail at Earth. For example, the solar wind, thought to be the overwhelming driver for energization of Earth’s radiation belts, has only a marginal influence at Jupiter on the creation of Jupiter’s dramatic, and much more energetic, radiation belts (Ibid).
3 Radiation Belt Science Mysteries
Sample Question 1
Why do the radiation belts respond so differently to different dynamic magnetic storm events?
It has long been conventional wisdom that the radiation belts dramatically intensify in association with geomagnetic storms. Such storms are often created by the impact of solar coronal mass ejections with the Earth’s magnetosphere and also the passage of high speed solar wind streams. Storms last for 1 to several days, occur roughly a dozen times a year, and cause dramatic increases in the flux of hot ion populations at geocentric distances between 2 and 6 R E . Currents associated with these ‘ring current’ ion populations distort inner magnetospheric magnetic fields and depress equatorial magnetic fields on the surface of the Earth. The so-called storm time disturbance (Dst) index, a measure of these depressions, is generally taken to provide a direct measurement of the ring current energy content according to the Dessler-Parker-Sckopke relationship (Dessler and Parker 1959; Sckopke 1966; however, there are caveats—Liemohn 2003).
Sample Question 2
Why do observed global electric field patterns behave so differently than expected?
A critical element in the control of the radiation belts is the distribution of other plasma populations relative to the radiation belt populations. Cold, warm, and hot plasma populations provide both the free energy needed for the generation and growth of various plasma waves and the media through which these waves propagate. The plasma waves can scatter and energize radiation belt particles. To a substantial degree, it is thought that large scale global electric field patterns within the inner and middle magnetosphere control the locations where the cold, warm, and hot plasma populations occur within the radiation belts. Here we are making a distinction between the quasi-steady (hours) global electric fields and the transient electric fields (minutes) associated with injections and other fast processes.
Classical models for inner and middle magnetospheric global electric fields often employ a so-called Volland-Stern type configuration (e.g. reviewed by Burke et al. 2007) with an electric potential: Φ=Φ 0 L γ cos[LT], where Φ 0 is the electric potential at some outer boundary position, L is the standard magnetospheric distance parameter (equatorial radial position in R E for a magnetic dipole field), LT is the angle that corresponds to local time, and γ is the so-called shielding parameter. The idea of this configuration is that the global electric field is applied “externally” by the interaction between the solar wind and the outer boundaries of the magnetosphere, and that the trapped inner region populations respond to partially shield out that electric field from the inner regions.
Clearly some fundamental issues concerning the generation and configuration of the global electric field patterns remain to be solved.
Sample Question 3
How are such large intensities of radiation belt electrons energized to multi-MeV energies?
The ultimate sources of radiation belt electrons are the ionosphere and the solar wind. Ionospheric electron temperatures are less than 0.1 eV. Temperatures of the core population in the solar wind are on the order 10 eV, while temperatures of the halo (heated) population in the solar wind are on the order of 60 eV (Feldman et al. 1975; Lin 1998). Auroral and related magnetospheric interaction processes extract and energize ionospheric electrons, providing them to the outer magnetosphere (generally at distances beyond 9 R E ) at energies ranging from 1 to 10’s of keV. Processes occurring at the Earth’s bow shock and magnetopause both energize and transport electrons into the magnetosphere. Reconnection and other processes within the Earth’s dynamic magnetotail magnetic current sheet then accelerate electrons of both ionospheric and solar wind origins still further. The resulting plasmasheet populations have temperatures of order 5 keV but often exhibit very substantial high energy tails (Christon et al. 1991).
One might then assume that Earth’s radiation belts result from the transport of these plasmasheet electrons into the inner magnetosphere in a fashion that conserves the first and possibly the second adiabatic invariants, those associated with gyration and bounce motion. Conservation of the first adiabatic invariant requires the energies of core and tail populations to increase by a factor of perhaps 40 as particles are transported Earthward from regions in the magnetotail where magnetic field strengths are on the order of 5 nT to regions of the inner magnetosphere where field strengths are on the order of 200 nT.
Sample Question 4
What causes “microbursts” and how important are they for the loss of particles from the radiation belts?
Since microbursts occur in the dawn-morning quadrant (O’Brien et al. 2004), where chorus/whistler waves are active (Fig. 13), it seems natural to assume that the bursts correspond to strong whistler-mode wave-particle interactions (Thorne et al. 2005). Strong wave phase trapping of the particles could be involved, again, given the now-recognized presence of very large amplitude whistler waves (Kersten et al. 2011; again see Fig. 12). We anticipate that the RBSP mission will resolve the uncertainties.
Sample Question 5
What causes the dramatic, sudden, large-scale dropout of radiation belt particles as near to Earth as L=4 R E ?
Sample Question 6
How important is the role of substorm injections in generating the radiation belts?
On <1 hour time scales of substorm injections themselves, injections are thought to only modestly perturb the distribution of MeV class electrons in the outer radiation belts. Their importance has traditionally been viewed as helping in the transport of the source populations, specifically by providing a “seed” population for the subsequent transport and energization that occurs during the generation of the radiation belts (Baker et al. 1979, 1981; Fok et al. 2001b). The uncertainties about the configuration of the global electric field configuration, and whether or not enhanced global electric fields move magnetotail plasma sheet particles Earthward during geomagnetic storms (Question 2) raises the importance of establishing the fundamental role that substorm injections may play in the transport of particles to the middle and inner magnetosphere. The relative importance of that role needs to be explored and resolved.
Evidence has been presented that substorms are critical to the fundamental processes that energize radiation belt electrons (Meredith et al. 2002, 2003). It is even suggested that substorms increase radiation belt intensities while storms reduce intensities (Li et al. 2009). Substorm injections disturb the structure of medium energy electron pitch angle distributions, making them highly conducive to the generation of strong whistler/chorus mode emissions. The waves in turn can accelerate the higher energy electrons in the manner described in the discussion of Question 3 (Fig. 11). The evidence in favor of this scenario is based on observed correlations between magnetic storms and substorms as diagnosed with magnetic indices, observations of whistler/chorus mode emissions, and observations of radiation belt intensities over a wide range of energies and extended periods of time. It is of interest that a similar scenario has been proposed for Jupiter’s dramatic radiation belt (Horne et al. 2007). Despite the absence of solar wind forcing, injection-like processes occur at Jupiter, associated with the shedding by Jupiter’s magnetosphere of the materials dumped into the magnetosphere by the volcanic moon Io. These Jovian injections are observed to be correlated with the generation of strong whistler mode emissions.
Because we are so uncertain as to the role of substorms in the processes of transporting particles from the magnetotail to the middle and inner magnetosphere, much work remains to be done in testing the ideas discussed above and in generally understanding the role of substorms in the generation of Earth’s radiation belts.
The sample science questions discussed in this section are intended to give a sense of the many fundamental scientific mysteries that presently pervade our understanding of the behavior of Earth’s radiation belts. Their purpose is specifically to confront the longstanding notion that developing a predictive understanding of Earth’s radiation belts is simply one of characterization or modeling, and to emphasize the need for comprehensive measurements of both particles and waves.
4 Science Implementation
There are two aspects of the RBSP Mission design that are critical to resolving the science issues illustrated above. RBSP must first deliver simultaneous multipoint sampling at various spatial and temporal scales. Secondly, RBSP must deliver very high quality, integrated in situ measurements with identical instrumentation on the multiple spacecraft.
4.1 RBSP Mission Design
It comprises two identically instrumented spacecraft.
The lines of apogee for the two spacecraft precess in local time at a rate of about 210∘ per year in the clockwise direction (looking down from the north). The 2 year nominal mission lifetime (∼4 years of expendables are available) allows all local times to be studied. By starting the mission with lines of apogee at dawn (a Program Level mission requirement), the nightside hemisphere will be accessed twice within the nominal 2 year mission lifetime.
Slightly different (∼130 km) orbital apogees cause one spacecraft to lap the other every ∼75 days, corresponding to about twice for every quadrant of the magnetosphere visited by the lines of apogee during the two year mission.
Because the spacecraft lap each other, their radial spacing varies periodically between ∼100 km and ∼5 R E ; and resampling times for specific positions vary from minutes to 4.5 hours.
The orbital cadence (9 hour periods; an average of 4.5 hours between inbound and outbound sampling for each spacecraft) is faster than the relevant magnetic storm time scales (day).
- (7)The low inclination (10∘) allows for the measurements of most of the magnetically trapped particles; while the precession of the line of apogee and the tilt of the Earth’s magnetic axis enables nominal sampling to magnetic latitudes of 0±21∘ (Fig. 20).
Spacecraft spin axes point roughly Sunward. Due to orbit precession, the spin axis must be re-aligned with respect to the sun periodically once each ∼21 days. The spin axis is always maintained to lie within 27∘ of the sun’s direction.
The 5 RPM spin period of the spacecraft, the nominal sunward orientation of the spin axis, and the positioning of the spacecraft near the magnetic equator of the quasi-dipolar magnetic configuration, combine to enable the particle detectors to obtain fairly complete pitch angle distributions twice for every spin of the spacecraft and the electric field instrument to make excellent measurements of the crucial dawn/dusk electric field.
RBSP is expected to see perhaps 2 dozen magnetic storms during its nominal 2-year lifetime. During critical events (e.g. the several hours that comprise “main phase” periods of magnetic storms), the two spacecraft will perform radial cuts through the inner regions with separation times that vary from minutes to several hours. For each quadrant of Earth’s magnetosphere, perhaps 6 storms will be observed within the first 20 months, and again specific features will be sampled with a distribution of separation distances and times. In this way, a range of spatial and temporal scales will be examined by the RBSP mission. To the extent that features such as the “bump” displayed in Fig. 17 characterize radiation belt responses to storms and other processes, as we know they do (Fig. 10), the RBSP mission will definitively distinguish the spatial from temporal structures and establish how they are generated.
Members of the RBSP team will employ modeling and partnerships with other missions to infer details concerning some crucial processes. For example, some strong whistler mode interactions that may energize electrons can occur at relatively high magnetic latitudes, particularly on the dayside (Horne et al. 2005a, 2005b; Bortnik et al. 2008). In the absence of other assets, RBSP will infer the characteristics of such interactions by observing the low-latitude consequences of such interactions and combining those observations with the sophisticated models that are now being brought to bear on the problem (e.g. Bortnik et al. 2008). Additionally, although the RBSP instruments do not have the pitch angle resolution to measure particle fluxes within the atmospheric loss cone, such particles are precisely those that will be measured by the Mission of Opportunity BARREL mission, which focuses upon the radiation belt particles precipitating into the atmosphere (Millan et al. this issue). BARREL will launch a series of balloon-borne X-ray sensors from the Antarctic during two month-long phases of the RBSP mission. Sensors on the SAMPEX, DMSP, and POES spacecraft can also be used to address this particle population. Third, the RBSP team will work with other missions such as THEMIS and geosynchronous spacecraft capable of measuring source populations outside the 5.8 R E apogee of the RBSP mission. Finally, ACE and other missions will supply information concerning the interplanetary drivers such as the interplanetary magnetic field, and prevailing solar wind conditions.
4.2 RBSP Observations and Instruments
RBSP program level (Level-1) observations
Determine spatial/temporal variations of medium & high energy electron & proton angle & energy distributions, faster than drift times, interior & exterior to acceleration events
Determine time history of energization, loss, & transport for hazardous particles. Understand/quantify source of these particles & source paths. Enable improved particle models
Derive electron & proton radial phase space density profiles for medium & high-energy electrons & protons on timescales short compared to storm times
Distinguish between candidate processes of acceleration, transport, & loss, & statistically characterize these processes versus solar input conditions
Determine spatial/temporal variations of charged particle partial pressures & their gradients within the inner magnetosphere with fidelity to calculate pressure-driven currents
Understand how large-scale magnetic & electric fields in the inner magnetosphere are generated & evolve, their role in the dynamics of radiation belt particles, & their role in the creation & evolution of the plasma environments for other processes
Determine spatial/temporal variations of low-to-medium energy electron & ion energy, composition, & angle distributions on timescales short compared to drift periods
Understand/quantify the conditions that control the production & propagation of waves (e.g. EMIC, whistler-mode chorus and hiss); & determine the wave propagation medium
Determine the local steady & impulsive electric & magnetic fields with fidelity to determine the amplitude, vector direction, and time history of variations on a timescale short compared times required for particle measurements
Determine convective & impulsive flows causing particle transport & energization; determine propagation properties of shock-generated propagation fronts; & inferring total plasma densities
Determine spatial/temporal variations of electrostatic & electromagnetic field amplitudes, frequencies, intensities, directions & temporal evolutions with fidelity to calculate wave energy, polarization, saturation, coherence, wave angle, and phase velocity for (A) VLF, and ELF waves, & (B) random, ULF, and quasi-periodic fluctuations
Determine the types/characteristics of plasma waves causing particle energization & loss: including wave growth rates; energization & loss mechanisms; diffusion coefficients & loss rates; plasma densities; ULF waves versus irregular fluctuations effects on radial transport; and statistical maps of wave fields versus position and conditions
Provide concurrent, multipoint measurements sufficient to constrain global convective electric field & storm-time electric and magnetic field models
Covert particle measurements to invariant coordinate systems; infer loss cone sizes; & model effects of global electric & magnetic field variations on particle distributions
Track/characterize transient structures propagating through the inner magnetosphere with fidelity to determine amplitude, arrival times, and propagation directions
Determine which shock-related pressure pulses produce significant acceleration, & provide estimate of their significance relative to other energization mechanisms
Energetic Particle, Composition and Thermal Plasma Suite (ECT)
Harlan Spence, PI
University of New Hampshire
Key partners: LANL, SwRI, Aerospace, LASP
Measure near-Earth space radiation belt particles to determine the physical processes that produce enhancements and loss
Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS)
Dr. Craig Kletzing, PI
University of Iowa
Key partners: NASA/GSFC, University of New Hampshire
Understand plasma waves that energize charged particles to very high energies; measure distortions to Earth’s magnetic field that control the structure of the radiation belts
Electric Field and Waves Instrument (EFW)
John Wygant, PI
University of Minnesota
Key partners: University of California, Berkeley, LASP
Study electric fields and waves that energize charged particles and modify the inner magnetosphere
Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE)
Louis Lanzerotti, PI
New Jersey Institute of Technology
Key partners: APL, Fundamental Technologies
Understand the creation of the “storm time ring current” and the role of the ring current in the creation of radiation belt populations
Proton Spectrometer Belt Research (PSBR)
David Byers, PSBR PI
National Reconnaissance Office
Key partners: Aerospace Corp. MIT Lincoln Lab.
Specification models of the high-energy particles in the inner radiation belt
Relativistic Proton Spectrometer (RPS)
Joseph Mazur, RPS PI Aerospace Corp.
Balloon Array for RBSP Relativistic Electron Losses (BARREL)
Robyn Millan, PI
Measure, study, and understand electron loss processes from Earth’s outer electron belt
5 Closing Remarks
The high level objectives of the RBSP mission are articulated in Sect. 1. To achieve those objectives it is necessary to develop science questions, like those presented in Sect. 2, that are specific enough to invite the generation of testable hypotheses. The RBSP mission design has many of the capabilities that are needed to discriminate between and test these hypotheses. Most critical is the ability of RBSP to perform simultaneous multipoint sampling over a broad spectrum of spatial and temporal scales, combined with extremely capable and highly coordinated instrumentation. These capabilities will enable researchers to discriminate between time and space variations. With such capabilities one may compare the time scales for the generation of local particle acceleration features with the theoretical expectations based on the measurements of the static and dynamic fields. With such capabilities one may measure rather than just infer the gradients that generate currents and the gradients that reveal electric potential distributions. With the capabilities of the RBSP instrumentation, one may determine the detailed characteristics of resonant interactions between particle and waves.
An important element in achieving complete science closure for some of the science objectives is the utilization of sophisticated models and simulations to place the RBSP multipoint measurements into the broader 3-dimensional picture. Strong coordination between data analysts and model builders is described in each of the investigation reports in this special issue, and specifically in the articles by Spence et al., Kletzing et al., Lanzerotti et al., Wygant et al., and Ginet et al.
A distinction is made in the structure of this special issue on the RBSP mission between the instrument investigations and the instruments themselves. The papers cited at the end of the last paragraph describe the instrument investigation for the ECT, EMFISIS, RBSPICE, EFW, and PSBR investigations (see Table 2). These papers describe in various degrees the science objectives of the individual team investigations, the science teams involved, the data processing, analysis, and archiving plans, the role of theory and modeling in resolving the targeted science issues, and the role of modeling in synthesizing the limited two point measurements that are made by the RBSP instruments. The instrumentation associated with these instrument investigations are in some cases described within the same instrument investigation papers (EMFISIS: Kletzing et al.; RBSPICE: Lanzerotti et al.; and EFW: Wygant et al.). In other cases the instrumentation is described in separate papers (ECT-HOPE: Funsten et al.; ECT-MagEIS: Blake et al.; ECT-REPT: Baker et al.; PSBR-RPS: Mazur et al.; again see Table 2).
Other papers in this special issue describe engineering details of the RBSP mission (Stratton et al.), the RBSP spacecraft (Kirby et al.), the RBSP contributions to the practical issues of space weather (Kessel et al.), the overarching RBSP data processing, analysis, dissemination, and archiving plans (Science Operations: Fox et al.), and the RBSP Education and Public Outreach plan (EPO: Fox et al.). Additionally, Ukhorskiy and Sitnov review present understanding regarding the definitions and calculations of various parameters that order the radiation belts and the mathematical tools that are used to manipulate those parameters; and Millan et al. describe the Mission of Opportunity Antarctic high-altitude balloon program called BARREL that will make measurements of precipitated electrons in coordination with the RBSP mission. Finally, Goldsten et al. describe an engineering sub-system, the Environmental Radiation Monitor that measures total radiation dose under various shielding thickness and monitors the potential for deep dielectric discharge by measuring the penetrating electron current delivered to two deeply buried conductors.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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