Connections Between Solar and Space Physics and Other Disciplines
Solar physics and space physics have as their principal objects of study phenomena that occur in fully or partially ionized matter in the plasma state. These fields are related on a fundamental level to laboratory plasma physics, which directly investigates basic plasma physical processes, and to astrophysics, a discipline that relies heavily on understanding the physics unique to the plasma state. Solar physics and space physics share with laboratory plasma physics and astrophysics an interest in a variety of phenomena, including magnetic dynamo action, magnetic reconnection, turbulence, collisionless shocks, energetic particle transport and acceleration, and plasma instabilities. Understanding developed in one of these fields is thus in principle applicable to the others, and productive cross-fertilization between disciplines has occurred in a number of instances, for the reason that any fundamental principle, regardless of where it is discovered, is applicable throughout the universe.
At one time exploratory fields, solar and space physics and laboratory plasma physics have become mature disciplines probing fundamental scientific questions at levels of depth and sophistication that allow them to provide reality checks on one another and on astrophysics. Laboratory experiments have been developed to model particular processes occurring in space and astrophysical plasmas and to test the results of theoretical and computational studies. In situ and remote-sensing observations in the solar system have furnished a basis for the development of theoretical insights that in turn have been applied to astrophysical systems, while structures and processes observed in solar system plasmas have served as analogues for phenomena that may be occurring in astrophysical environments inaccessible to detailed study.
The various plasma regimes that are the focus of inquiry for solar and space physics, laboratory plasma physics, and astrophysics are characterized by a vast range of densities, temperatures, magnetic field strengths, and
scale lengths. A key factor in applying insights from one discipline to another is the degree to which appropriate scaling relations can be established between the plasma regimes in question. Here, depending on the particular phenomena under investigation, commonly used dimensionless quantities or other ratios between relevant physical parameters may be of greater importance than dimensional quantities such as density or temperature, and meaningful scaling may link apparently quite different plasma environments, even ones that differ in scale by the orders of magnitude that separate solar system plasmas from astrophysical plasmas. For some astrophysical plasmas, such as pulsar magnetospheres or the degenerate plasmas of white dwarf interiors, temperatures or densities are so extreme that descriptions of the systems must include relativistic or quantum effects, and simple scaling from the regimes explored within the solar system may not be possible.
Although solar physics and space physics are concerned primarily with electromagnetic effects and matter in the plasma state, their relevance also extends into the domain of atmospheric science and climatology. Some of the points of contact with these fields are therefore also reviewed here. Finally, the committee considers briefly the role played by theoretical and laboratory studies of atomic and molecular processes in solar and space physics research.
LABORATORY PLASMA PHYSICS
Laboratory plasma experiments represent a valuable tool for advancing our understanding of the physical processes underlying phenomena observed in both solar system plasmas and remote astrophysical systems. The utility of such experiments is described in Plasma Science, a 1995 report by the National Research Council, which recommends establishment of an initiative to support laboratory studies relevant to space plasmas.1 Benefits noted in the report include controllability, repeatability, precision, and the capacity for multipoint measurements and for observation of the temporal evolution of the process being studied. Laboratory plasma investigations are particularly important in that they provide a means of testing experimentally, under well-controlled conditions not achievable in space, the results of theoretical and computational studies. Such experiments can thus help to validate our theoretical understanding of space plasma processes. For example, the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory has demonstrated good agreement between the reconnection rate observed in a laboratory plasma and that predicted by the
Sweet-Parker model of slow reconnection. The experiment has also demonstrated the occurrence of faster reconnection in circumstances reminiscent of the rapid reconnection observed in collisionless space plasmas.
To be sure, laboratory plasmas are critically affected by boundary effects and by initial conditions, and the densities and temperatures characteristic of space plasmas cannot be reproduced in the laboratory. For certain processes, however, the properties of a physically interesting space plasma can be modeled in a laboratory by selecting plasma parameters that approximate the dimensionless ratios that characterize the physics of the space plasma. Laboratory experiments have recently made particularly noteworthy contributions on two topics—magnetic reconnection and magnetic dynamo action.2
Reconnection is being studied in several dedicated laboratory experiments around the world, including MRX and the Swarthmore Spheromak Experiment (SSX) (see sidebar, “Reconnection”). In each experiment, magnetized plasma loops are generated and merged. Magnetic energy is rapidly annihilated and converted to plasma flows, energetic particles, and heat. At present, the physical scale of such experiments is 0.1 to 1 m, and the key dimensionless parameter, the ratio of the time for the magnetic field to diffuse through a volume of plasma to the time for changes in field and plasma properties to propagate through the plasma, can be as large as ~1,000. Although this is much smaller than the corresponding ratio in magnetospheric plasmas, it is large enough to capture some of the qualitative aspects of the important physical processes. Diffusion is facilitated by particle collisions, and laboratory experiments can explore the transition from the collisional to the collisionless regime. Recent key results include the observation of basic two- and three-dimensional reconnection geometry, the observation of Alfvénic jets and super-Alfvénic particles, evidence that the thickness of the reconnection boundary layer is imposed by ion dynamics, and the demonstration of heating through the dissipation of magnetic energy in the reconnection layer.
In a dynamo, the motions of an electrically conducting fluid generate magnetic fields, thereby converting kinetic energy of motion into magnetic energy. Magnetic dynamos are critical elements of both solar and space
Magnetic reconnection is the fundamental mechanism by which magnetic energy is dissipated in the universe. Observationally, energy is released in bursts rather than in a continuous manner, driving phenomena such as solar flares and magnetospheric substorms.
The basic process of reconnection has been understood from the late 1950s. If two parcels of magnetized plasma have oppositely directed magnetic fields and there is a region of weaker or zero field between them, then under the right circumstances the parcels can approach each other. The oppositely directed magnetic flux can cancel out (annihilate), and the plasma can jet outward along the weaker field directions at a characteristic speed called the Alfvén speed.
Magnetic flux is lost from the structures on the “inflow” sides, while a new magnetic structure, formed from “reconnected” magnetic field lines, grows on the “outflow” sides. As the plasma approaches the central region, the magnetic field may change direction very quickly, producing intense channels of electric current density that can heat the plasma. The current typically takes the form of filaments or sheets, and the magnetic field often takes on a characteristic X-point shape (as illustrated on the facing page). This process is called magnetic reconnection or merging, and it is thought to be the main means by which magnetic fields in space plasmas change the way they are linked with one another.
Recent observations in the magnetosphere and in the solar corona provide mounting evidence of the key role of reconnection in space plasmas. There are important outstanding questions concerning the small-scale structure of the reconnection region, in which plasma kinetic effects are dominant. In these crucial small-scale regions, particles can be accelerated to high energies, magnetic field lines break and reconnect, plasma jets are formed, heat is released, and energy can be transferred from one region to another. In reconnection, large-scale dynamics and small-scale plasma physics come face to face: This is an essential feature of multi-scale, nonlinear space plasma physics.
plasmas, but there is no fully satisfactory theoretical understanding of solar and stellar dynamos.3 Several laboratory experiments around the world are investigating the dynamo problem, and the results of such investigations afford insight into the process. In each, the kinetic energy of an electrically conducting fluid is converted (in part) to large-scale magnetic field energy. Typically, the working fluid is sodium (a fluid at 100°C). Currently, the physical scale of such experiments is 0.1 to 1 m, and the key dimensionless parameter, which in this case is the ratio of the time for the field to diffuse
Schematic illustrating the reconnection process. Oppositely directed magnetic fields are “frozen” into the plasma flow and carried toward the reconnection site. Here, in a localized region of space known as the diffusion region (gray box), the magnetic flux and the plasma become decoupled, allowing the magnetic field lines to break and reconnect. The kinetic processes that occur in the diffusion region to facilitate reconnection are the subject of ongoing theoretical and modeling studies and will be studied in the geospace environment by the Magnetospheric Multiscale mission.
through a volume of plasma to the time for the fluid to flow across it, is about 10 to 50. Here, as in the reconnection studies, the laboratory plasma is more dissipative than are most typical space plasmas, but the laboratory experiments demonstrate and explore the physical effects that are relevant to the physical systems of interest to solar and space physics. Progress has been made in identifying the natural oscillations of the system, referred to as eigenmodes, observing the decay of excited eigenmodes, and measuring the effects of turbulence on electrical conductivity.
Other plasma processes important to space physics such as the generation and propagation of waves unique to magnetized plasmas (e.g., whistler and shear Alfvén waves) have been investigated in laboratory experiments. In one experiment using the Large Plasma Research Device of the University of California at Los Angeles, shear Alfvén waves were generated within field-aligned density depletions under plasma conditions comparable, in terms of the relevant parameters, to those in the ionosphere. Interesting changes in wave properties have been observed as the waves propagate into regions of changing plasma properties, and these experiments are helping to elucidate processes such as particle heating and acceleration in the auroral ionosphere.
Global magnetospheric processes have also been studied in the laboratory. In particular, terrella experiments have been carried out at the University of California, Riverside to investigate the effects of the solar wind/ magnetosphere interaction on the distant magnetotail. Although the scaling between the laboratory magnetosphere and Earth’s magnetosphere is not as consistent as desired, the terrella experiments have provided qualitative support for the predicted development of X-type (Y-type) neutral lines under conditions of a southward (northward) interplanetary magnetic field and for the occurrence of high-latitude reconnection when the magnetic field in the incident plasma (representing the solar wind) points northward. Other plasma physical phenomena that have been the subjects of recent laboratory experiments include waves in dusty plasmas and solar prominences.
Recommendation: In collaboration with other interested agencies, the NSF and NASA should take the lead in initiating a program in laboratory plasma science that can provide new understanding of fundamental processes important to solar and space physics.
As noted above, the establishment of such a laboratory initiative was previously recommended in the 1995 National Research Council report Plasma Science.
The solar system is home to a rich variety of plasmas, ranging in density from the collisional plasma (~1026 cm-3) of the Sun’s interior to the attenuated, collisionless medium of the outer heliosphere (~0.002 cm-3 at 50 AU) and in energy from the cold ~1-eV plasma of Earth’s plasmasphere to the
MeV trapped particles of Jupiter’s synchrotron-emitting radiation belts. In addition to the highly ionized plasmas of the interplanetary medium and planetary magnetospheres, the inventory of solar system plasmas includes partially ionized gases, as in the terrestrial ionosphere, where ion-neutral collisions strongly influence the electrodynamics, and dusty plasmas in comet dust tails and in planetary ring systems, where the effects of gravity and/or radiation pressure must be considered as well as electromagnetic influences. In contrast to remote astrophysical plasma systems, solar system plasmas are accessible both to direct, in situ measurement and to systematic, highly resolved remote sensing, and, in contrast to laboratory plasmas, boundaries can be remote in solar and space plasmas. Further, the physical properties of solar system plasmas span an enormous range, with analogies in numerous other astrophysical environments. The solar system thus represents a laboratory for astrophysical studies in which fundamental plasma physical processes such as reconnection, turbulence, particle acceleration, and shocks can be investigated at both the micro- and macroscales and in ways not possible through numerical simulations or laboratory plasma experiments.4 The physical understanding derived from observational studies of solar system plasmas provides a basis for theoretical extrapolation to more extreme astrophysical systems. Fruitful cross-fertilization between solar-system plasma physics and astrophysics is particularly well exemplified in the study of collisionless shocks, magnetohydrodynamic turbulence, and magnetic reconnection.
The Panel on Opportunities in Plasma Science reported in 1995 that the “study of collisionless shocks is arguably the area in which the most significant advances have been achieved during the past decade and where the impact of space on basic plasma physics has been most profound.”5 Substantial advances in this area were based on in situ observations of Earth’s bow shock, of bow shocks at other planets and comets, and of interplanetary shocks. The observational and associated theoretical studies of shocks in the solar system established critical properties relevant to collisionless shocks in the interstellar medium and have illuminated the mechanisms through which such shocks produce particle acceleration. Diffusive shock acceleration, in which charged particles gain energy through repeated cycles of scattering and shock crossing, is a robust mechanism now thought to be responsible for energetic particles throughout the universe. Some astro-
physical shocks such as those driven by young supernova remnants have Mach numbers greater than 100. They are stronger than the Uranian bow shock, which, with a magnetosonic Mach number of 17, is the strongest shock encountered in the solar system. The character of a colli-sionless shock can change with increasing Mach number, and in extrapolating beyond the range of observations, phenomena not present in weaker shocks (turbulence, for example) may modify the form of shock-controlled particle acceleration. In seeking to understand astrophysical shocks stronger than those encountered in the solar system, the results of analytical theory and numerical simulations will be of critical value.
Magnetohydrodynamic (MHD) turbulence is characterized by nonlinear interactions among fluctuations of the magnetic field and flow velocity over a range of spatial and temporal scales. It plays an important role in plasma heating, the transport of energetic particles, and radiative transfer and is ubiquitous in space and astrophysical plasmas. The solar wind and the diffuse interstellar medium (ISM) are both examples of plasmas that exhibit turbulent behavior, as evidenced by the power spectra determined from radio propagation observations (ISM, solar wind) and in situ data (solar wind) (see sidebar, “Turbulence in the Solar Wind”). These observations have stimulated an ongoing effort to develop theoretical treatments of MHD turbulence appropriate to the two similar systems. Although this effort relies heavily on studies of the solar wind as the more fully characterized of the two plasmas, it is strongly interdisciplinary in character, drawing on insights from theoretical plasma physics and laboratory plasma physics as well as on observational and theoretical studies. The last 20 years have seen considerable progress in this area. However, a number of important theoretical issues remain to be resolved—for example, regarding scaling in MHD turbulence and compressibility effects.6 In addition, the mechanisms responsible for ISM turbulence are poorly understood, and heliospheric analogies are expected to provide useful insights. Supersonic turbulence is studied in the star-forming regions in dense molecular clouds, and it is noteworthy that turbulent energy decay rates are found to scale much the same way as in the heliosphere, suggesting a commonality of MHD turbulence principles. Random electric fields associated with turbulence can also play a role in the stochastic acceleration of charged particles, and this is a possible mechanism for reacceleration of cosmic rays in the Galaxy.
Magnetic Reconnection Theory
Reconnection theory is an especially compelling example of a profound contribution made by solar and space physics to astrophysics, as well as to basic plasma physics and fusion research. The concept of magnetic reconnection was initially developed principally in the context of efforts to explain the production of solar flares. Reconnection is now considered to offer a plausible explanation for a number of other solar phenomena as well, such as transition-region explosive events, surges and sprays, and x-ray bright points. Since the early 1960s, reconnection theory has been extensively and successfully applied, in the open model of the magnetosphere, to the related problems of energy, mass, and momentum transfer from the solar wind into the magnetosphere and the explosive release of stored magnetic energy in substorms. Recent theoretical studies and numerical simulations have yielded new insights into the microscopic aspects of the reconnection process. Finally, theoretical reconnection models are coming into correspondence with observations and experimental results on the subject, promising rapid progress in the coming decade.
The concept of reconnection has been invoked to address numerous problems in astrophysics. One of its earliest applications was to the problem of magnetic flux reduction in gravitationally collapsing protostellar clouds as part of the star formation process. Flux removal by reconnection has also been proposed as relevant to the production of magnetic viscosity in accretion disks. Magnetic reconnection is thought to be responsible for flarelike releases of energy in interactions between neutron star magnetospheres and accretion disk magnetic fields as well as for flares occurring in accretion disk coronae, and the solar flare model of reconnection has been applied to flares from dwarf stars, binaries, and T Tauri stars. Recent studies suggest that reconnection may plausibly play a role in rapid particle acceleration—e.g., in the reacceleration of high-energy electrons in extragalactic jets and in the acceleration of ultrahigh-energy cosmic rays. Such a mechanism would be similar to direct acceleration of charged particles by solar flares. Fast reconnection in a magnetized relativistic outflow has also been put forward as a candidate mechanism for the production of radiation in gamma-ray-burst fireballs. Numerous other examples of reconnection in astrophysical phenomena can be found in the recent literature, documenting the importance of this fundamental concept. Applications of reconnection theory—as well as of the knowledge obtained through the study of turbulence and shocks in the solar wind—to astrophysical phenomena exemplify the increasingly wide recognition within the astrophysics
TURBULENCE IN THE SOLAR WIND
Spacecraft venturing into the solar wind measure broadband fluctuations in all plasma variables, including the proton fluid velocity, the density, and the magnetic field. Such fluctuations are in many ways reminiscent of ordinary fluid turbulence; however, turbulence in the solar wind is also distinctly a plasma phenomenon, involving magnetic fields, kinetic effects, and interactions with charged particles. The observed turbulence is thought to enhance heating of the interplanetary medium, and it is responsible for scattering and transport of cosmic rays originating from both inside and outside the heliosphere.
Solar wind turbulence also provides important clues about the nature of the lower solar corona. Fluctuations are observed over a wide range of spatial scales, from the system size (AUs in size) down to electron kinetic scales, 100,000 times smaller. Large-scale fluctuations can be described by fluid models, but the small-scale activity (less than the proton kinetic scales) requires a kinetic description.
The physics of the couplings between large-scale and kinetic-scale processes in the solar wind is mediated by properties of the turbulence. A substantial portion of the turbulence energy is distributed in a distinctive power-law spectral distribution over about three decades of spatial scale, and is highly reminiscent of ordinary fluid turbulence. Interacting waves originate in the corona; and in interplanetary space, turbulence is further enhanced by instability and interactions with shear layers near high-speed solar wind streams. So begins a cascade in which turbulent fluctuations, including eddies and waves, interact with one another to produce smaller-scale fluctuations. Eventually these are damped by kinetic processes and heat the plasma. With an explicitly turbulence-based model, the radial evolution of turbulence intensity and the temperature of the plasma can be computed.
Dissipative turbulence is driven by shear in solar wind streams and by pickup ion effects. Such a model can account for observed amplitudes and temperatures from 1 AU to beyond 60 AU (see figure on the facing page). Like its terrestrial counterparts, solar wind turbulence remains an incompletely understood topic but an important one, in that it mediates complex dynamical couplings between very large and very small scales, very slow fluid motions and very fast kinetic processes, and very-low-energy and very-high-energy plasma particles.
Proton temperatures from 1 AU to 60 AU as measured by Voyager 2. Temperatures are significantly hotter than those indicated by the adiabatic law (dotted) and can be described by a turbulence model (solid). By including pickup ions, the turbulence model captures the temperature increase from 20 AU to 60 AU, showing highly non-adiabatic behavior. Courtesy of W.H. Matthaeus. (This figure originally appeared in W.H. Matthaeus, G.P. Zank, C.W. Smith, and S. Oughton, Physical Review Letters 82:3444-3447, 1999. Copyright 1999 by the American Physical Society.)
community of the importance of magnetic fields and electrodynamics in the structure and dynamics of astrophysical plasmas.
Solar and Stellar Processes
As the only well-resolved star, the Sun plays a uniquely important role as a laboratory for stellar processes. Helioseismology has confirmed the theoretical model of the solar interior and revealed complex, three-dimensional flow patterns within the convective zone, presumably responsible for generating and sustaining the Sun’s magnetic fields. Precision confirmation of the theoretical model of the Sun gives us confidence in the theoretical models of the interior structure, dynamics, and evolution of other stars (see sidebar, “The Solar Laboratory”). Similarly, recent high-resolution observations of the Sun’s highly structured and dynamic corona, with its magnetic loops, arcades, filaments, and fibrils, contribute to our understanding of other stellar x-ray coronas, of which the Sun is the prototype.
Despite considerable progress in our knowledge of solar processes, many fundamental questions of importance to solar physics and astrophysics have yet to be answered. For example, the detailed workings of the Sun’s magnetic dynamo remain a major mystery—one whose eventual solution will have profound implications for our understanding of MHD dynamos not just in other solar-type stars but in other astrophysical settings as well. Still an open question some 40 years after the theory of the supersonic solar wind was formulated and confirmed by in situ measurement is how the corona is heated and the solar wind accelerated. In situ measurement of the near-Sun environment (inside 0.3 AU), combined with high-resolution remote sensing, promises to resolve this question and to yield insights into coronal processes at other late-main-sequence stars.
Finally, the committee notes that the cross-fertilization between solar physics and astrophysics is bidirectional. Studies of the activity and luminosity of other solar-type stars can validate our theories of the solar dynamo, provide a glimpse into the Sun’s past and a preview of its future, and contribute to our efforts to understand secular variations in solar activity such as the Maunder Minimum and Medieval Maximum.
ATMOSPHERIC SCIENCE AND CLIMATOLOGY
All solar-system-body atmospheres are exposed to and interact with either the solar wind (Venus, Mars, Pluto) or the plasma contained within a magnetosphere (Earth, the giant planets, the Galilean moons, and the moons
THE SOLAR LABORATORY
Helioseismic data from SOHO/MDI and GONG are fast becoming a scientific treasure house that connects several fields. In the 1980s, there were detectable discrepancies between the Sun’s observed seismic frequencies and those calculated from theoretical models of the Sun. With time, the measured seismic frequencies have become much more precise.
The frequency discrepancies led atomic physicists to improve the treatment of atomic physics of solarlike plasmas—opacities and the equation of state. This development, coupled with improved treatments of diffusion, has improved the theoretical model of the solar interior to the point that it predicts a sound speed that nowhere differs from that determined by helioseismology by more than one part in a thousand (the present uncertainty in the helioseimic speed of sound). The precision is now such that we can use the helio-seismic frequencies to test individual nuclear cross sections of the p-p chain, which are the source of the Sun’s energy. These reactions are nearly impossible to measure in the laboratory because they are so low in energy.
For years, it has been known that the models of the Sun predict far fewer solar neutrinos than are observed. With the agreement between the theoretical solar model and the precise results from helioseismology, it is now clear that there is no apparent astrophysical solution to the Sun’s neutrino deficit. Particle physicists have recently determined that the origin of the deficit is that standard electro-weak physics is wrong—that is, neutrinos have mass.
Left: An artist’s concept of the solar oscillations used in helioseismology to probe the solar interior. Right: The neutrino flux from the Sun, observed with the Super-Kamiokande neutrino detector and integrated over a period of 500 days. The cutaway view of the Sun courtesy of J.W. Harvey (GONG Project, National Solar Observatory/AURA/NSF). The neutrino image courtesy of R. Svoboda (University of Louisiana).
Titan and Triton). The atmospheres are also exposed to bombardment by galactic cosmic rays. Thus there are numerous points of contact between space physics and atmospheric science, particularly in the area of aeronomy, which represents the interface between the two disciplines and which can arguably be considered a subdiscipline of either field. It is not an exaggeration to say that the structure and evolution of a planetary atmosphere cannot be understood without considering the effects of solar system plasmas.
The interaction of a planet’s or satellite’s atmosphere with the plasma environment affects the upper atmosphere, with its ionized and neutral components, most strongly. At magnetized planets this interaction occurs primarily between the ionosphere and the magnetosphere, which are electrodynamically coupled by field-aligned currents. The deposition of electromagnetic and particle kinetic energy and momentum by auroral processes modifies atmospheric structure, circulation, and composition. Auroral energy input dominates the energetics of Jupiter’s high-latitude upper atmosphere and, during times of intense geomagnetic disturbance, the energetics of Earth’s upper atmosphere as well.
In the case of unmagnetized or weakly magnetized bodies such as Venus, Mars, and Titan (the moon with the most massive atmosphere), auroral processes play no role in upper atmosphere aeronomy. The interaction of the ionospheres of such bodies with the ambient plasma does, however, have a significant atmospheric effect owing to the removal of ionospheric ions that are picked up by the external plasma and swept away in its flow. Sputtering by returning pickup ions is another important loss process driven by the atmosphere-plasma interaction at unmagnetized bodies. Theoretical studies have shown that the loss of atmospheric material as a result of scavenging and sputtering may have been substantial over the age of the solar system. Moreover, sputtering may also have contributed to the loss of the primordial atmospheres of Mercury and the Moon.
Sputtering not only plays a role in atmospheric loss but also, through the sputtering of surfaces, produces tenuous atmospheres such as the recently discovered oxygen exospheres about the jovian moons Europa and Ganymede. Sputtering of the surface of Mercury by solar wind particles probably contributes, along with other processes such as photodesorption, to the creation of Mercury’s variable sodium exosphere. (Charged-particle bombardment can also affect the composition and properties of the impacted surfaces, a process known as space weathering. Investigation of this process is thus an important point of contact between space physics and
solid-body studies as well as between space physics and the study of planetary atmospheres.)
The space environment influences the middle and lower atmospheres of some solar system bodies as well as their upper atmospheres, although in these regions the effects are generally less extensive than at higher altitudes. For example, bombardment by energetic electrons from Saturn’s magnetosphere is a possible mechanism for producing hydrocarbon aerosols in Titan’s haze layers. Similarly, energetic auroral particle precipitation is thought to lead to the formation of the high-latitude stratospheric hazes observed at Jupiter and Saturn. Modeling studies indicate that heating by such hazes may play an important role in the large-scale meridional circulation of Jupiter’s stratosphere. Nucleation induced by ions created by galactic cosmic ray (GCR) bombardment has been shown to be a viable mechanism for the formation of hydrocarbon hazes and clouds in Neptune’s lower stratosphere and troposphere; if GCR-induced ionization does indeed play a role in aerosol production at Neptune, Neptune’s haze, and hence albedo, may vary in phase with the solar-cycle modulation of the cosmicray flux. In the case of Earth, the effect of solar energetic particle events on the ozone chemistry of the middle atmosphere is well known. Odd-hydrogen and odd-nitrogen production initiated by solar proton bombardment leads to the destruction of ozone in the polar mesosphere and upper (and sometimes middle) stratosphere. Most of the solar energetic particle-induced ozone loss is relatively short-lived; however, particularly intense events, such as those that occurred in August 1972, October 1989, and July 2000, can have longer-lasting (months to years) effects.
Other possible influences of the space environment on Earth’s lower atmosphere should be noted. Ionization produced by galactic cosmic rays could have an influence on the nucleation of cloud particles. An apparent correlation between globally averaged low cloud cover and the cosmic ray flux over solar cycle 22 has been adduced in support of this mechanism. Another postulated influence involves solar-wind-modulated variations in the flow of current density in the global electric circuit, variations that cause changes in cloud microphysics. Both mechanisms provide a means of coupling solar/solar wind activity to weather and climate. However, while observations have occasionally been found to support these mechanisms, they remain ill substantiated and controversial. If they are real, solar-induced changes in cloudiness are likely to produce regional rather than global changes, in contrast to the effects of small variations in irradiance. However, the societal impacts of regional climate changes are important
and justify continuing attention to understanding how, or indeed whether, the solar-cycle effects act as proposed.
In addition to such possible influences from space on Earth’s weather and climate, the influence of global climate change on the geospace environment—at least on its lower reaches—must also be considered. Studies using the thermosphere-ionosphere general circulation model developed at the National Center for Atmospheric Research have demonstrated that the ionosphere-thermosphere system may experience long-term changes resulting from the buildup of anthropogenic trace gases in the middle atmosphere. By analogy with the term “space weather,” these secular changes could appropriately be described collectively as “space climate.” The predicted effect is principally one of mesospheric and thermospheric cooling due to increased concentrations of carbon dioxide and methane. In a reasonable scenario of trace gas increases, the global average temperature could decrease during the 21st century by ~10 K in the mesosphere and by ~50 K in the thermosphere. The consequences of such a change in the thermal structure of these regions would include changes in the densities of the major and minor neutral species, in ionospheric plasma density and temperature, and in the dynamics of the upper atmosphere.
ATOMIC AND MOLECULAR PHYSICS AND CHEMISTRY
Knowledge of the properties of atoms and molecules, ranging from their spectra to their cross sections for excitation, ionization, and charge exchange, is critical for our understanding of a number of magnetospheric, ionospheric, solar, and heliospheric processes. Such data are needed, for example, to understand how energetic magnetospheric particles interact with satellites such as the Galilean moons of Jupiter to produce their tenuous atmospheres and the heavy ion plasmas that characterize the giant magnetospheres. The interpretation of observations of auroral and dayglow emissions from the outer planets requires accurate excitation and emission cross sections of molecular hydrogen. Accurate modeling of complex processes such as EUV and collisional ionization, recombination, charge exchange, and electron-stripping of high-energy particles is essential for interpreting the ionic charge state and elemental composition of the solar wind. The observational determination of velocities in the corona requires precise wavelengths for the UV and EUV emission lines, and the determination of abundances of the diverse ions requires precise knowledge of the line strengths.
As these examples demonstrate, laboratory and theoretical studies of atomic and molecular properties are central to our understanding of important aspects of solar system plasmas. In many cases, however, our current knowledge of the atomic and molecular processes for a particular problem is inadequate, and further studies are required. For instance, the recently discovered x-ray emission from comets has been attributed to charge-transfer collisions between highly charged solar wind ions and cometary neutrals. However, the cross section information needed to understand this phenomenon is currently inadequate for the relevant collision species and at solar wind energies. Fortunately, a number of new laboratory investigations have recently been initiated in this area.
Recommendation: The NSF and NASA should take the lead and other interested agencies should collaborate in supporting, via the proposal and funding processes, increased interactions between reseachers in solar and space physics and those in allied fields such as atomic and molecular physics, laboratory fusion physics, atmospheric science, and astrophysics.