The discipline of solar and space physics has made remarkable advances over the last decade, many of which have come from the implementation of the program recommended in the 2003 solar and space physics decadal survey. New missions, observations, models, and fundamental research have yielded new insights in every subdiscipline of heliophysics.
Many important scientific questions remain, and new ones have arisen. The answers to some will emerge from the research of the next decade. To guide NASA, NSF, and the other U.S. government agencies that fund this research, the decadal survey committee developed major research recommendations. After considering scientific priorities, rough estimates of project costs, and the long-term health of the research community, the committee prioritized the recommendations, indicating which programs the agencies should implement first. Ideally, all of the recommendations can be implemented in the coming decade. However, if agencies encounter limiting budgets or schedules, the prioritized recommendations provide tools with which to make tough decisions and preserve the most important research.
Most importantly, the decadal survey recommends that NASA and NSF maintain and complete the current research programs in solar and space physics. The currently active or developing programs—many of which were recommended in the 2003 decadal survey—should be considered a baseline priority. For NASA, current programs include recently launched missions (Van Allen Probes and the Interface Region Imaging Spectrograph) and missions under development (Magnetospheric Multiscale Mission, Solar Orbiter, and Solar Probe Plus). For NSF, current programs include the recently deployed Advanced Modular Incoherent Scatter Radar, the Daniel K. Inouye Solar Telescope that is currently under construction (formerly the Advanced Technology Solar Telescope), and the continued operation of essential ground-based instruments.
Taken together, the NSF and NASA missions and observatories currently observing the Sun-Earth system form the “Heliophysics System Observatory” (HSO). The decadal survey recommends continued support in the near term for the key existing program elements that constitute the HSO. This diverse collection of space- and ground-based instruments allows simultaneous observations from distributed vantage points of a highly interconnected system. An invaluable legacy of decades of strategic investment in solar and space physics research, the HSO is an enabling tool for conducting new research, interpreting new observations, and observing space weather events.
Artist’s depiction of Solar Probe Plus, solar panels folded into the shadows of its protective shield, as it gathers data on its approach to the Sun in 2024. SPP will plunge to within just 6 million km of the solar surface, 25 times closer to the Sun than Earth.
NSF and NASA’s current and near-future (indicated by yellow text) program of solar and space physics facilities and missions, which form the Heliophysics Systems Observatory (HSO).
A successful scientific program in solar and space physics over the next decade will balance spaceflight missions of various sizes, midscale ground-based instruments, and supporting programs and infrastructure investments. Relatively low-cost activities that maximize the scientific return of ongoing projects and enable new ones are both essential and cost effective. However, it is all too easy to forget these small-scale activities in planning discussions. To ensure a solid groundwork is developed for the coming decade and beyond, the decadal survey recommends as its highest priority after completion of the current program an array of actions collectively known as the DRIVE initiative. DRIVE—Diversify, Realize, Integrate, Venture, Educate—is an initiative unified not by a central management structure, but through a comprehensive set of multi-agency recommendations that will facilitate scientific discovery. System science requires new types and configurations of observations, as well as a new cadre of researchers who can cross disciplinary boundaries seamlessly and develop theoretical and computational models that extract the essential physics from measurements made using multiple observing platforms.
The DRIVE initiative.
THE COMPONENTS OF DRIVE ARE:
DIVERSIFY observing platforms with microsatellites and midscale ground-based assets
By diversifying observation platforms and facilities, a wider array of unique and complementary data can be collected over greater temporal and spatial ranges, often at relatively low cost. Additionally, more frequent, lower-cost observations provide broader opportunities for training the next generation of researchers. To achieve these goals, the decadal survey recommends that NSF create a new, competitively selected, midscale funding line to enable midscale projects and instruments that currently have no funding mechanism. Compelling examples of midscale projects include the Frequency Agile Solar Radiotelescope (FASR), designed to dynamically image space weather drivers in the Sun’s atmosphere at radio wavelengths; and the Coronal Solar Magnetism Observatory (COSMO), designed to conduct synoptic observations of coronal magnetic fields. NASA should fly more sounding rockets and research balloons, and both agencies should increase funds for newly developed “cube” satellites.
REALIZE scientific potential by sufficiently funding operations and data analysis
NASA and NSF invest heavily in complex missions and projects and should ensure that the value of these efforts is fully realized once operations begin. NASA should permanently establish funding for Heliophysics System Observatory mission extensions and set aside a small budget portion of all future missions to fund guest investigators. NSF has committed to construction of the powerful 4-meter Daniel K. Inouye Solar Telescope (formerly the Advanced Technology Solar Telescope) and should maximize the investment by adequately funding its operations and instrument development.
INTEGRATE observing platforms and strengthen ties between agencies and disciplines
The study of solar and space physics is an inherently multidisciplinary and multi-agency endeavor, combining disciplines as diverse as climatology and plasma physics to study complex systems. The decadal survey recommends that NSF ensure that funding is available for basic research that falls between its divisions. NASA and NSF should coordinate and take full advantage of their numerous ground- and space-based solar-terrestrial observational and technology programs.
VENTURE forward with science centers and with instrument and technology development
Future progress in heliophysics hinges on new observational capabilities in state-of-the-art instrumentation, access to unique vantage points in space, and new methods of collaboration. NSF and NASA should jointly establish heliophysics science centers where multidisciplinary teams can tackle key science problems. To enable future missions, NASA should consolidate and increase funding for solar and space physics instrument and technology development.
EDUCATE, empower, and inspire the next generation of space researchers
Continuing the scientific advances in solar and space physics requires support for future researchers through outreach and recruitment, education, and employment. The decadal survey recommends that NASA and NSF continue outreach efforts and funding for undergraduate and graduate research in the space sciences. NSF should also continue its faculty development program and recognize solar and space physics as a specific subdiscipline of physics and astronomy.
By implementing the recommended DRIVE components, NASA and NSF can ensure that the next decade will be rich in new observations from diverse platforms, new science harvested from missions and projects, new synergisms between disciplines and platforms, new technologies and theories to enable future missions and projects, and talented new students to form the future workforce.
Since 1958, when the first U.S. satellite—Explorer 1—discovered Earth’s radiation belts, the Explorer program has produced a wealth of information about the nature of our space environment and properties of the universe. The data returned from Explorer missions have contributed to three of the Nobel Prizes awarded for NASA-directed space science. Over the years, Explorer missions have operated in different management modes, but the common feature is that a principal investigator (PI) in partnership with the NASA Explorer Program Office is tasked to ensure the overall success of the mission and is given the authority to make critical decisions to control cost and schedule. Because many of these missions have continued to provide data beyond their design lifetime, they have become cornerstones of the Heliophysics System Observatory, indispensable to basic research as well as to space weather operations.
The Explorer program’s strength lies in its ability to respond rapidly to new concepts and developments in science, as well as in the program’s synergistic relationship with larger-class strategic missions. Heliophysics Explorers launched since 2000 have made fundamental discoveries in space and solar physics from the edge of the heliosphere (IBEX) to flare and reconnection physics on the Sun (RHESSI) to the explosive releases of energy taking place in Earth’s magnetosphere (THEMIS) to the enigmatic formation of ice clouds in Earth’s polar regions (AIM). In addition, the Explorer Program is the home for missions of opportunity, including SNOE, CINDI and TWINS, instruments that have produced science benefits far beyond their cost.
The decadal survey does not recommend specific science targets for the Heliophysics Explorer program. Investigations are competitively selected to address the highest-priority science that can be accomplished with small-class missions. The Explorer Program has repeatedly proven to be one of the most cost-effective and best cost-controlled avenues for implementing space science missions.
The decadal survey recommends increasing the cadence of the Heliophysics Explorer program to one mission every two to three years. The decadal survey also recommends regular selections of missions of opportunity, which allow the research community to respond quickly and to leverage limited resources with interagency, international, and commercial flight partnerships. For a relatively modest investment, a renewed Explorer program can address many of the science challenges listed in the decadal survey.
Astrophysics and Heliophysics Explorers Missions
Astrophysics and Heliophysics Explorers Missions of the past 2 decades from http://explorers.gsfc.nasa.gov/missions.html.
Initially conceived as a program to implement medium-class missions, the Solar-Terrestrial Probe (STP) program has evolved into a large-scale mission program, dominated by NASA centers, with cost growth over the past decade that threatens its future viability. The decadal survey recommends that NASA’s future Solar Terrestrial Probes be restructured as moderate-scale competed, principal investigator-led (PI-led) mission line that is cost-capped at $520 million per mission. NASA’s Planetary Science Division uses this structure for its Discovery and New Frontiers programs, and these medium-class missions have a history of superior cost performance relative to larger flagship missions. STP missions should be managed likewise, with each mission’s principal investigator empowered to make the scientific and mission design trade-offs necessary to remain within the cost cap. This should enable the STP program to achieve the recommended minimum cadence of one mission every four years. The decadal survey identified top scientific priorities for the program and estimated for each the cost of a notional mission. The survey recommends the scientific targets for the STP program, not the specific implementation.
The first recommended new STP science target is to understand the outer heliosphere and its interaction with the interstellar medium. A notional mission known as the Interstellar Mapping and Acceleration Probe (IMAP) would greatly extend the highly successful first heliospheric mapping mission, IBEX, to enable the discovery of the detailed processes and interactions between the heliosphere and the local interstellar medium. In addition, as the mission implementation requires local measurements of solar wind, magnetic fields, and energetic particles, IMAP inherently provides key observations relevant to understanding and predicting Earth's space weather. If launched on the schedule recommended in the survey, IMAP operations would overlap those of NASA’s Voyager spacecraft, which are poised on the boundaries of interstellar space, but nearing the end of their lifetime. Simultaneous remote and in situ observations from IMAP and the Voyagers would enable critical comparisons that are otherwise impossible. The survey report also recommends STP science targets to follow IMAP: First the notional Dynamical Neutral Atmosphere-Ionosphere Coupling (DYNAMIC) mission would provide a comprehensive understanding of the variability in space weather driven by lower atmospheric weather on Earth. Second, the notional Magnetosphere Energetics, Dynamics, and Ionospheric Coupling Investigation (MEDICI) mission would determine how the magnetosphere-ionosphere-thermosphere system is coupled and how it responds to solar and magnetospheric forcing.
The next STP mission, the notional Interstellar Mapping and Acceleration Probe (IMAP), will solve fundamental mysteries of the heliosphere’s interaction with the interstellar medium and particle acceleration in the solar wind. Projected onto the outer boundary of the solar system, measurements from the Interstellar Boundary Explorer (IBEX) mission, launched in 2009, revealed an unexpected source of high-energy particles, which are shown here in yellow and green. This enigmatic “ribbon” raises basic and profound questions about its unexplained origin, the nature of the outer boundaries of our solar system, and the surrounding galactic medium. With more than 20 times the resolution, IMAP will probe the detailed source of the ribbon. Shown in the figure inset is a representation of substructure that scientists can presently only hypothesize. The gray arrow indicates the direction the solar system moves through interstellar space. The dark lines suggest how interstellar magnetic field lines may be draped across the bow of the heliosphere. Icons show the locations of the two Voyager spacecraft.
Certain scientific problems can be addressed only by missions that are relatively complex and more costly. Solar Probe Plus, which will travel closer to the Sun than any previous spacecraft, is an example of this type of mission; future constellation missions that would utilize multiple spacecraft to provide simultaneous measurements from broad regions of space (in order to separate spatial from temporal effects and reveal the couplings between adjacent regions of space) are another. As research evolves naturally from the discovery-based mode to one focused increasingly on quantification and prediction, missions benefit strongly from an integrative approach, whereby the knowledge obtained from prior research can be combined with new, innovative measurements for the development of understanding of the global machinery of the system. This effort may naturally require a larger mission, and it also accords with the societal-relevance theme of NASA’s Living with a Star (LWS) program. In the survey committee’s plan, major missions are thus appropriately undertaken via NASA’s LWS program and would continue to be executed by NASA centers, whereas the STP program should be considered a community program, like the Explorer program.
For the next LWS mission, the survey committee recommends a comprehensive investigation of how Earth’s atmosphere absorbs solar wind energy. The notional Geospace Dynamics Constellation (GDC) mission would determine how solar wind energy is regulated throughout geospace. Geospace is the region surrounding Earth, including its upper atmosphere, that is influenced by the particles and fields coming from the Sun. GDC uses a constellation of satellites to differentiate between spatial and temporal changes in energy flow and mass transport. With simultaneous multipoint measurements, the six identical satellites of GDC would offer a comprehensive view of the global effects of geomagnetic storms on Earth’s atmosphere and reveal the links between the atmosphere, ionosphere, and magnetosphere. GDC would address fundamental physical processes and make comprehensive measurements needed to improve space weather forecasting and models of the ionosphere and upper atmosphere.
Better understanding of how Earth’s atmosphere interacts with the space plasma environment requires that a number of key measurements be made simultaneously all around the globe. GDC will directly measure these parameters where the plasma density is highest. This comprehensive view will enable prediction of complex behaviors that emerge under constantly varying conditions in space and reveal how Earth's atmosphere affects space weather.