The Sun to the Earth – and Beyond Panel Reports (2003) / Chapter Skim
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2 Report of the Panel on Solar Wind and Magnetosphere Interactions
2 Report of the Panel on Solar Wind and Magnetosphere Interactions
Pages 47-124
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From page 47... ...
SUMMARY 49 2.1 SOLAR WIND-MAGNETOSPHERE INTERACTIONS: THE REALM OF MAGNETIZED PLASMAS 53 Introduction 53 The Fourth State of Matter 53 Reconnection 54 Flowing Magnetized Plasmas 55 The Storage and Release of Energy Coupling in a Collisionless Plasma Impact and Relevance 58 Summary 58 57 57 2.2 MAG N ETOSPH ERES AN D TH El R PARTS 58 Overview 58 Bow Shock 59 Magnetosheath 62 Magnetopause, Cusp, Boundary Layers 63 Magnetotai 1 66 I n ner Magnetosphere 70 Plasmasphere 72 Sol ar Wi nd I Interactions with Weakly Magnetized Bod ies 73 Outer Planets 83 2.3 PROCESSES 87 Introduction 87 The Creation and Annihilation of Magnetic Fields 87 Magnetospheres as Shields and Accelerators 88 Magnetospheres as Complex Coupled Systems 89 2.4 CURRENT PROGRAM 90 Introduction 90 Programs 91 Critical Needs 98 47
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From page 48... ...
48 2.5 FUTURE PROJECTS 99 Introduction 99 Addressing the Major Themes 99 Project Summaries 100 Science Traceability 105 Prioritization: NASA and NSF 1 06 Prioritization of Other Agency and Interagency Initiatives 106 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS TECHNOLOGY 1 08 Introduction 1 08 Propulsion Technology 1 09 S pacec raft Tech n o l ogy 1 1 1 Science Instrumentation Technology 1 1 3 Information Architecture Technology 1 1 4 Technology for Ground Systems and Operations 1 14 Recommendations and Priorities 1 1 4 2.7 SOLAR Wl N D-MAGN ETOSPHERE I INTERACTIONS: POLICY ISSU ES 1 1 5 I Introduction 1 1 5 Interagency Coordination 1 15 Coordination Between Programs and Divisions Within Agencies: NSF and NASA 117 Opportunities for Space Measurements in Entities Other Than NASA's Office of Space Science 118 Science in the Structure of Project Management 1 19 I International Cooperation 1 1 9 Model ing, Theory, and Data Assimi ration 1 20 Technology Development 1 21 Data Analysis, Dissemination, and Archiving 121 Extended Missions 1 22 ADDITIONAL READI NG 1 22
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From page 49... ...
Partners in these processes are the plasmas, energetic particles, waves, and electromagnetic emissions from radio to x-ray wavelengths in the solar wind and the planetary magnetospheres. Solar and planetary magnetic fields organize space into normally well-separated regions.
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INTRINSIC AND INDUCED Magnetospheres can be divided into two types: induced, if any intrinsic magnetic field of the body is so weak that the ionosphere is directly exposed to the flowing solar wind plasma, and intrinsic, if the body has an internal magnetic field sufficiently strong to deflect the plasma that flows against it. Induced magnetospheres form around highly electrically conducting obstacles if the conductor, generally an ionosphere, can stave off the solar wind flow.
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From page 51... ...
UNIFYING THEMES The outstanding questions that need to be addressed in planetary magnetospheres can be divided into three themes: the creation and annihilation of magnetic fields; magnetospheres as shields and accelerators; and magnetospheres as complex, coupled systems. The first theme includes the formation of the major magnetospheric current systems: the magnetopause, the tail current, the ring current, and the field-aligned currents.
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From page 52... ...
· Understanding planetary magnetospheres. The exploration of planetary magnetospheres is in its infancy, yet comparisons between these magnetospheres and the terrestrial magnetosphere and with each other are critical to fully understanding the processes taking place.
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From page 53... ...
This overview is followed in Section 2.2 by a detailed discussion of current understanding of the processes in the terrestrial magnetosphere and the environments of the planetary magnetospheres. This description is needed to understand why the panel has chosen the paths it recommends, but it may be skipped by those seeking only to learn the recommendations.
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From page 54... ...
This process in which charged particles lose their ability to define a magnetic field line is called reconnection. If they are antiparallel, two neighboring magnetic field lines, say one that starts and ends on Earth and another that starts and ends on the Sun, can become connected so that two new field lines are created, both of which have one end on Earth and one end on the Sun.
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From page 55... ...
wave that could deflect it around the planetary obstacle. The momentum flux of the solar wind represents a dynamic pressure that con fines the planetary magnetic field, but in order for it to be applied to the magnetosphere, the flow must pass through a bow shock that slows, deflects, and heats the flow, making it subsonic.
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From page 56... ...
This pileup region deflects the solar wind particles around the ionospheric obstacle while the magnetic field begins to diffuse into the ionosphere. However, the solar wind magnetic field is quite variable in direction, and the long-term (days)
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From page 57... ...
This force is the macroscale manifestation of the Lorentz force, which maintains charged particles in their orbits around magnetic field lines. It can also transfer stress from the ionosphere to the magnetosphere such as to enforce co-rotation of the cold magnetospheric ions.
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Three important aspects of such plasmas are that the magnetic fields can act as both shields and accelerators, that they can generate and annihilate magnetic fields, storing and releasing energy in the process, and that the coupling between the various plasma regimes occurring in planetary magnetospheres is complex. Understanding the physics of these magnetospheres is important to planetary scientists, to astrophysicists, to laboratory physicists, and to the inhabitants of this planet.
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From page 59... ...
The more than 40 years of research on planetary magnetospheres has yielded a wealth of information about the general properties of the solar wind-magnetospheric i Interaction. The characteristics of Earth's magnetosphere serve as well-documented examples of the complex interaction between the solar wind and the planet's magnetic field.
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From page 60... ...
Ions and electrons can reflect off the shock or escape from the downstream region near the region where the convecting solar wind magnetic field is nearly tangent to the shock interface. These particles can "surf" or drift along the shock front, in the direction of the tangential electric field seen in the shock frame, gaining (or losing)
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From page 61... ...
, these reflected ions execute approximately one half of a gyro-orbit in the upstream region, gain energy in the solar wind electric field, and return to the shock. Computer simulations showed that the reflection required both electric and magnetic forces at the shock and led to distinctive signatures in the magnetic field and phase space distributions of the particles near the shock.These features were conclusively identified in high-time-resolution, in situ observations near Earth's bow shock.Thus, by the mid 1 980s,the dissipation at Earth's quasi-perpendicular bow shock (i.e., the part of the bow shock where Ban > 45°)
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From page 62... ...
In particular, similar magnetic field turbulence is observed, as are similar plasma transitions. The magnetosheaths of active comets are interesting in that a strong region of piled up magnetic flux is formed outside the contact surface (the region where the solar wind magnetic field is excluded)
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From page 63... ...
The structure of Earth's magnetopause is complicated because the plasmas on both sides of the discontinuity are magnetized, so that when the fields on opposite sides of the boundary are nearly antiparallel they can reconnect. This connection spoils the simple topological distinction between solar wind and magnetospheric plasma by creating a region that contains both plasmas on the same magnetic field lines.
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From page 64... ...
In 1961 J.W. Dungey suggested that the solar wind and terrestrial magnetic fields would interconnect at a neutral point when the magnetospheric and solar wind magnetic fields were nearly antiparallel (see Box 2.3~.
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From page 65... ...
Dungey predicted that there would be reconnection at the high-latitude magnetopause during periods of northward IMF. Since magnetic reconnection provides a means to interconnect solar wind magnetic field lines with those in the magnetosphere, these two predictions indicate that there could be a nearly continuous connection between the magnetosphere and Earth's bow shock.
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From page 66... ...
Is Reconnection Patchy or Quasi-continuous, Inherently Unstable or Quasi-static? Does It Occur When Magnetic Fields Are Exactly Antiparallel or When Only One Component Is Oppositely Directed?
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In ideal MHD, with negligible parallel electric field, this potential is constant along magnetic field lines. However, average electric fields in the tail plasma sheet are considerably smaller than those obtained by mapping the ionospheric potential to the tail.
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From page 68... ...
First, the plasma conditions are different at the two locations; second, we need to learn how reconnection produces the observed dynamics of the magnetopause and the tai 1. For example, at the magnetopause the interaction geometry produces a current layer across which the magnetic field is generally not antiparallel, and there is no interconnecting magnetic component.
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From page 69... ...
Reconnection is thought to occur where the magnetic field configuration resembles an X If two field lines merge at a point, then the reconnection location can be referred to as a neutral point.
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From page 70... ...
Empi rical magnetic field and magnetospheric specification models that include observations made in the inner magnetosphere over a wide range of geomagnetic conditions have significantly improved the ability to describe the configuration of the magnetosphere during geomagnetically disturbed intervals. These models are aiding in the understanding of the relevant scale sizes of
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From page 71... ...
Several models have been proposed, from the classic radial diffusion picture, driven by stochastic electric and magnetic field fluctuations, to shock resonance acceleration, recirculation models, and ULF wave-electron interactions. Again, the major impediment to solving the electron acceleration and transport problem is the paucity of critical electron observations and poor knowledge of the background magnetic and electric fields during the acceleration events.
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From page 72... ...
Thus the multipoint magnetic field and energetic electron and ion observations can, in principle, be used to generate a self-consistent and dynamic field model on particle-drift-period time scales instead of satellite-orbital-period time scales. How Does the Electric Field (Convection PatternJ Dynamically Evolve During Storms?
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From page 73... ...
Hence both the similarities and the dissimilarities between different magnetospheres can provide deeper insight into the operation of such mechanisms. The basic parameters determining the features of the plasma interactions of weakly magnetized planets are the presence or absence of a substantial atmosphere and ionosphere; the presence or absence of significant remanent magnetization, or magnetic fields induced in
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From page 74... ...
PioneerVenus Orbiter observations showed that the ionosphere's upper boundary ranges from average altitudes of ~300 km subsolar to ~800-1,000 km at large solar zenith angles around solar maximum, a height and shape determined by pressure balance between the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS normal component of the incident solar wind pressure and the thermal pressure of the ionospheric plasma (see Box 2.8~. In the inner magnetosheath, at the boundary with the ionosphere, the initially dynamic solar wind pressure is transformed to magnetic pressure in a layer of enhanced magnetic field and depleted solar wind plasma known as the magnetic barrier.
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From page 75... ...
The subject of cometlike ion production in the solar wind interaction region, reinvigorated by PVO observations of the wake of escaping planetary O+ pickup ions, has proven particularly important for present and future space physics investigations at the weakly magnetized planets. As at comets, the induced magnetotail of Venus, made up of highly draped magnetosheath flux tubes that sink into the wake created by solar wind flow divergence around the dayside ionosphere, rotates with the IMP from which it is largely constructed.
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From page 76... ...
There were also a number of inferred "boundaries,"whose probable connection with the later-discovered crustal magnetic fields was not apparent at the time. In its final, nearly circular orbit at ~2.7 Mars radii,data were obtained that led to better understanding of the bow shock positions near the terminator and the induced character of the martian magnetotail.
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From page 77... ...
The study of this solar wind interaction is tremendously complicated by the fact that the oncoming solar wind encounters different crustal field configurations as Mars rotates,as well as bythe usual variations due to the changing solar wind and interplanetary magnetic field (with which the crustal fields reconnect)
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From page 78... ...
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Moon The lunar solar wind interaction was explored to a first level of overall understanding during the Explorer 35 and Apollo 15 and 16 subsatellite missions. Those observations consisted of basic magnetic field and plasma measurements, together with suprathermal electron measurements that could be used to diagnose the near-surface field via magnetic reflection signatures in their pitch-angle distributions.
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From page 79... ...
Our experiences from exploring the solar wind interactions with these other obstacles, together with the assumptions about Pluto's size and extended atmosphere derived from remote sensing, suggest that the Pluto interaction may vary from cometlike to Venus-like to Moon-like, depending on a number of factors. A cometlike interaction is indicated if and when the hypothesized outflow of ~1 o27 particles per second of atmospheric neutrals occurs, perhaps near Pluto's perihelion at ~30 AU.
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From page 80... ...
In addition, the natural variability of the interplanetary magnetic field makes identifications of weak signatures from these obstacles difficult without significant supporting plasma measurements in specifical Iy targeted regions such as the near-object wake. The largest asteroids, I i ke Ceres and Vesta, represent the best chance of observing a signature with limited space physics instrument capability.
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From page 81... ...
Solar wind interaction studies generally require a relatively close flyby, but ionization and pickup scale lengths are larger in the dim sunlight and weak magnetic field of the Kuiper Belt region. Comets Achievements In the mid-1980s, the Soviet VEGA 1 and 2, ISAS Sakigake and Suisei, and ESA Giotto spacecraft were sent toward Comet Halley, destined to pass upstream and observe the solar wind interaction in detail, while imaging the cometary nucleus at perihelion.
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From page 82... ...
The future observations of the comet solar wind interaction largely rest with ESA's Rosetta mission. Rosetta is capable of a fu I I complement of sol ar wi nd i Interaction measurements, including particles over a broad range of energy and composition, magnetic fields, and waves.
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From page 83... ...
We must aggressively take advantage of the current preference for limited missions or forever lose our place in this fruitful area of space exploration. OUTER PLANETS The detection of strong radio signals from Jupiter quickly led to the realization that Jupiter had a strong intrinsic magnetic field and an extensive magnetosphere and radiation belt.
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From page 84... ...
Jupiter Achievements The first pass of Pioneer 10 through the Jovian magnetosphere, whose noon-midnight cross section is shown in Figure 2.7, revealed a magnetosphere quite u n I i ke that of Earth. The i n nermost part of the magnetosphere, like Earth's, was dominated by the strong intrinsic magnetic field, but the radiation levels far exceeded those of Earth, limiting the time that spacecraft could survive in that environment.
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From page 85... ...
In 1996, the Galileo Orbiter discovered that Ganymede has a permanent internal magnetic field large enough to form a magnetosphere (Figure 2.12.11.The magnetosphere shields the Moon from direct interaction with the flowing plasma of the Jovian magnetosphere within which it is embedded.The discovery of an intrinsic magnetic field in a small planetary body that was thought to have solidified fully over its geological history is a surprise that has led to substantial rethinking of our ideas of planetary evolution.The small magnetosphere has been well characterized in multiple passes, at altitudes between 200 and 3,000 km. Analogies with and differences from the terrestrial magnetosphere have enabled us to test and extend our theories of magnetospheric processes.
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From page 86... ...
While the dipole magnetic field of the planet is not tilted much with respect to the rotation axis, the rotation axis itself has a substantial inclination to its orbital plane, much as has Earth's dipole axis. The magnetic field at the surface of Saturn is similar to but slightly less than the terrestrial THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS surface field.
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From page 87... ...
A major objective of space physics is to understand the creation of these magnetic fields and their causative currents. We understand some aspects of the currents well, but many not at all.
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From page 88... ...
In short, we have much to learn about both the creation and the annihilation of magnetic fields i n planetary magnetospheres. MAGNETOSPHERES AS SHIELDS AND ACCELERATORS One of the fascinating lessons from magnetospheric research is that magnetized bodies (intrinsic or induced)
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From page 89... ...
What consequences would these effects have had for the viability and evolution of life? The answers to these profound questions will lie in a better understanding of how the solar wind interacts with the atmosphere and its ionized upper reaches under conditions of weak or absent intrinsic magnetic fields.
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From page 90... ...
The coupled current systems also involve currents and electric fields that are aligned with the magnetic field and cause bright auroral features. However, we still do not know how a substorm is triggered.
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From page 91... ...
Table 2.4 1 ists NASA's missions relevant to planetary magnetospheres, both intrinsic and induced. The funding levels for these various programs are not given.
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From page 95... ...
PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 95 ~~: ~~ ; .~ ' , ~e' ~ in.
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From page 96... ...
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From page 97... ...
PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS o ~ E ~ · E o ° u ~ ·4- Q ~ u ~ ~ E _ o rc ~ ~ : ~ E E ~ ~ S ~ ~ ~ ~ 8, ~ ~ .
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From page 98... ...
Interplay Among Space- and Ground-Based Observations and Data Analysis, Modeling, and Theory In a program such as we have at present new observations can be incorporated into the existing paraTHE SUN TO THE EARTH AND BEYOND: PANEL REPORTS digms rapidly and progress is rapid. There are supporting data from space for ground-based observations, and supporting data from the ground are often available for space-based studies.
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From page 99... ...
While planetary magnetospheres display as much complexity and coupling as the terrestrial magnetosphere, the need to sample the volume of space under investigation as completely as possible means that complex, coupled processes will have to be studied mainly at the Earth. In order to study the complex, coupled system of the solar wind's interaction with Earth, we need both space-based and ground-based facilities.
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From page 100... ...
Even in a magnetosphere such as Jupiter's, where the energization of the plasma and its circulation are derived principally from the rotation of the planet, reconnection is a critical process in governing the plasma circulation. While the creation and annihilation of magnetic fields occurs in microscopic regions, these microscale processes control and are controlled by macroscale proTHE SUN TO THE EARTH AND BEYOND: PANEL REPORTS cesses.
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From page 101... ...
Three orthogonal spatial gradients of Magnetospheric Multiscale (MMS) — three-dimensional plasma and energetic particle distribution functions, vector electric and magnetic fields, and electrostatic and electromagnetic waves from ion cyclotron frequency to electron plasma frequency.
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From page 102... ...
Determine the relative contributions of planetary rotation and of the interaction with the IMP to jovian magnetospheric dynamics; determine how global electric and magnetic fields regulate magnetospheric processes; identify the particles responsible for the jovian aurora and their source region(s)
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From page 103... ...
It will also determine how global electric and magnetic fields regulate magnetospheric processes, as wel l as identify the particles responsible for the Jovian aurora and determine their source regions. Neptune provides a magnetosphere ordered by a dipole field that has a very large tilt angle.
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From page 104... ...
The Solar Wind Sentinels (SWS) mission will measure the upstream solar wind and interplanetary magnetic fields and their longitudinal variations both for normal conditions and also for conditions associated with coronal mass ejections and interplanetary shocks.
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From page 105... ...
Projects that address primarily a specific science question are denoted by a P Those that provide secondary support are denoted by an S
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From page 106... ...
In particular, they may help to provide critically needed solar wind and i nterpl anetary magnetic field man itori ng as wel I as
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From page 107... ...
What is the relative P S P importance of various sources for populating the terrestrial plasma sheet? NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support.
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From page 108... ...
What is the dynamic evolution of auroral acceleration? = SMI DBC SWS MagTom GSRI AMS p p p JPO NO IF VAP MAP RAG MSA S P P p p P P P P p D NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support.
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From page 109... ...
NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support. · Information architecture (e.g., data synthesis and assimilation, model development, multipoint data visualization)
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From page 110... ...
Discovery Mars Aeronomy Probe (MAP) Discovery or Mars Scout NOTE: Cost caps in FY 2003 for New Frontiers, Discovery, and Mars Scout were <$700 million, <$350 million, and <$350 million, respectively.
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From page 111... ...
and awaiting a global change studies. launch to L1 point Measure the far upstream solar wind and interplanetary magnetic fields associated with coronal mass ejections and interplanetary shocks.
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From page 112... ...
Inflatable antenna structures may be an enabling technology for such missions insofar as they reduce the ground or NASA Deep Space Network (DSN) system resources required to support the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS missions.
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From page 113... ...
This is true for optical, magnetic field, and plasma wave measurements also. There must be in place a process to identify the promising technologies and to support their migration into sensor systems.
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From page 114... ...
Their technologies should be studied and emulated where appropriate; otherwise they should be used as a starting point for the development of the operations and ground systems that will be required by MagCon and other mu Itisatel I ite science missions. A separate issue is the retrieval and handling of the science data from modern missions.
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From page 115... ...
Observations that are or should be planned for transition i ncl ude the fol lowi ng: · Interplanetary magnetic field and solar wind observations analogous to those provided in rea/ time from
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From page 116... ...
The panel recommends that these transitioning efforts be supported aggressively to meet the science and applications objectives of the space environment community. DOD-DOE: Coordinated Planning for Launch and Flight Opportunities and Access to Relevant Data Sets The Department of Defense and the Department of Energy conduct operational flight and observation programs that are directly relevant to the science objectives of magnetospheric-solar wind interactions.
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From page 117... ...
COORDINATION BETWEEN PROGRAMS AND DIVISIONS WITHIN AGENCIES: NSF AND NASA Because magnetospheric physics is one of a number of priorities in NSF and NASA space science programs, and because responsibility for it is split between NASA and NSF, it is important to recognize and eliminate unnecessary compartmentalization. The panel encourages cooperation and coordination between agencies and between programs within each agency.
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From page 118... ...
Space Station Attached Payloads The knowledge gained in studies of the interaction of the solar wind with the magnetosphere and the ensuing understanding of the entry of solar energetic particles into the magnetosphere is particularly beneficial to the ISS and its occupants.7 On the other hand, the ISS is not a natural or optimum platform for observing magnetosphere-solar wind interactions. It provides at best a limited opportunity for space physics research, owing to its orbit and facility configuration constraints.
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From page 119... ...
INTERNATIONAL COOPERATION Historically, research in space science, especially in solar wind-magnetosphere interactions, has had a strong international element. This international element arises first from the need for globally situated, ground-based measurements and then from the immensity of the task, which requires a cooperative effort to obtain the critical mass for its successful outcome.
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From page 120... ...
MODELING,THEORY, AND DATA ASSIMILATION Modeling and theory need to be integrated into ongoing research. Because the terrestrial magnetosphere's reconfiguration time scale is tens of minutes, far shorter than satellite orbit periods of hours to tens of hours, data sampl i ng i n Earth's magnetosphere wi 11 always be sparse THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS and i Incomplete.
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From page 121... ...
Because the redundancy is built into the constellation concept itself, one can accept single-string concepts in the spacecraft design. In a similar way, the science return is enhanced primarily by the large number of distributed measurements rather than by the high precision of the measurements, so that the requ i remeets for i nstru ment performance relative to that demanded for single-satellite missions should be critically examined.
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From page 122... ...
If the cost of extending missions could be significantly reduced and the pressure on mission operations and data analysis resources relieved to allow more simultaneous operations, a broader array of productive observatories could be maintained for magnetosphere-solar wind interaction science. ADDITIONAL READING A strategy for the conduct of space physics research has been set down in a number of reports by the NRC's Space Studies Board and its predecessor, the Space Science Board.
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From page 123... ...
1995. Introduction to Space Physics.
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