CLUSTER and IMAGE: New Ways to Study the Earth’s Plasmasphere

Ground-based instruments and a number of space missions have contributed to our knowledge of the plasmasphere since its discovery half a century ago, but it is fair to say that many questions have remained unanswered. Recently, NASA’s Image and ESA’s Cluster probes have introduced new observational concepts, thereby providing a non-local view of the plasmasphere. Image carried an extreme ultraviolet imager producing global pictures of the plasmasphere. Its instrumentation also included a radio sounder for remotely sensing the spacecraft environment. The Cluster mission provides observations at four nearby points as the four-spacecraft configuration crosses the outer plasmasphere on every perigee pass, thereby giving an idea of field and plasma gradients and of electric current density. This paper starts with a historical overview of classical single-spacecraft data interpretation, discusses the non-local nature of the Image and Cluster measurements, and emphasizes the importance of the new data interpretation tools that have been developed to extract non-local information from these observations. The paper reviews these innovative techniques and highlights some of them to give an idea of the flavor of these methods. In doing so, it is shown how the non-local perspective opens new avenues for plasmaspheric research.     

Plasmaspheric Density Structures and Dynamics: Properties Observed by the CLUSTER and IMAGE Missions

Plasmaspheric density structures have been studied since the discovery of the plasmasphere in the late 1950s. But the advent of the Cluster and Image missions in 2000 has added substantially to our knowledge of density structures, thanks to the new capabilities of those missions: global imaging with Image and four-point in situ measurements with Cluster. The study of plasma sources and losses has given new results on refilling rates and erosion processes. Two-dimensional density images of the plasmasphere have been obtained. The spatial gradient of plasmaspheric density has been computed. The ratios between H⁺, He⁺ and O⁺ have been deduced from different ion measurements. Plasmaspheric plumes have been studied in detail with new tools, which provide information on their morphology, dynamics and occurrence. Density structures at smaller scales have been revealed with those missions, structures that could not be clearly distinguished before the global images from Image and the four-point measurements by Cluster became available. New terms have been given to these structures, like “shoulders”, “channels”, “fingers” and “crenulations”. This paper reviews the most relevant new results about the plasmaspheric plasma obtained since the start of the Cluster and Image missions.     

Chemical Composition of Icy Satellite Surfaces

Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous geochemistry. The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev., 2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table 1 from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our understanding (cf. Table 2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon. In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces, and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and discovery associated with our exploration of our planetary neighborhood.     

Observations of low energy magnetospheric plasma outside the plasmasphere

After some introductory discussions about morphological concepts and limitations of various measurement techniques, existing low energy plasma data, orginating primarily from the GEOS, Dynamics Explorer, and Prognoz spacecraft, is described and discussed. The plasmasphere measurements are not included (but for some observations of plasmasphere refilling). It is finally concluded that we are very far from a complete picture of the low-energy plasma component in the magnetosphere and that this problem has to be given high priority in planning payloads of future space plasma physics missions.     

Advances in Plasmaspheric Wave Research with CLUSTER and IMAGE Observations

This paper highlights significant advances in plasmaspheric wave research with Cluster and Image observations. This leap forward was made possible thanks to the new observational capabilities of these space missions. On one hand, the multipoint view of the four Cluster satellites, a unique capability, has enabled the estimation of wave characteristics impossible to derive from single spacecraft measurements. On the other hand, the Image experiments have enabled to relate large-scale plasmaspheric density structures with wave observations and provide radio soundings of the plasmasphere with unprecedented details. After a brief introduction on Cluster and Image wave instrumentation, a series of sections, each dedicated to a specific type of plasmaspheric wave, put into context the recent advances obtained by these two revolutionary missions.     

An Overview of Results From rpi on Image

The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft was designed as a long-range magnetospheric radio sounder, relaxation sounder, and a passive plasma wave instrument. The RPI is a highly flexible instrument that can be programmed to perform these types of measurements at times when IMAGE is located in key regions of the magnetosphere. RPI is the first radio sounder ever flown to large radial distances into the magnetosphere. The long-range sounder echoes from RPI allow remote sensing of a variety of plasmas structures and boundaries in the magnetosphere. A profile inversion technique for RPI echo traces has been developed and provides a method for determining the density distribution of the plasma from either direct or field-aligned echoes. This technique has enabled the determination of the evolving density structure of the polar cap and the plasmasphere under a variety of geomagnetic conditions. New results from RPI show that the plasmasphere refills in slightly greater than a day at L values of 2.8 and that ion heating is probably playing a major role in the overall density distribution along the field-line. In addition, RPI's plasma resonance observations at large radial distances over the polar cap provided in situ measurements of the plasma density with an accuracy of a few percent. For the first time in the magnetosphere, RPI has also observed the plasma D resonances. RPI's long antennas and its very low noise receivers provide excellent observations in the passive receive-only mode when the instrument measures the thermal plasma noise as well as natural emissions such as the continuum radiation and auroral kilometric radiation (AKR). Recent passive measurements from RPI have been compared extensively with images from the Extreme Ultraviolet (EUV) imager on IMAGE resulting in a number of new discoveries. For instance, these combined observations show that kilometric continuum can be generated at the plasmapause from sources in or very near the magnetic equator, within a bite-out region of the plasmasphere. The process by which plasmaspheric bite-out structures are produced is not completely understood at this time. Finally, RPI has been used to successfully test the feasibility of magnetospheric tomography. During perigee passages of the Wind spacecraft, RPI radio transmissions at one and two frequencies have been observed by the Waves instrument. The received electric field vector was observed to rotate with time due to the changing density of plasma, and thus Faraday rotation was measured. Many future multi-spacecraft missions propose to use Faraday rotation to obtain global density pictures of the magnetosphere.     

Comparative Study of the Dynamics of the Outer Planets

The two classes of outer planets, Gas Giants and Ice Giants, have distinctly different global circulation patterns and internal structure. Ongoing ground-based observations of the Ice Giants provide clues to better understanding and Galileo and Cassini data will generate constraints for Gas Giant modeling. The composition below the cloud levels, the depths to which the winds penetrate and the processes that sustain the zonal winds and weather systems are not understood. Basic questions concerning the structure, composition and atmospheric dynamics that are sustained on the four giants could be answered by a combination of orbiters and probes. Future missions that could answer these questions are not currently under development.     

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.     

Cometary Materials: Progress Toward Understanding the Composition in the Outer Solar Nebula

A major objective of the workshop was to learn about the chemical composition, physical structure, and thermodynamic conditions of the outer parts of the solar nebula where comets formed. Here we sum up what we have learned from years of research about the molecular constituents of comet comae primarily from in situ measurements of Comet 1P/Halley and remote sensing of Comets 1P/Halley, Hale-Bopp (C/1995 O1), and Hyakutake (C/1996 B2). These three bright comets are presumably captured Oort cloud comets. We summarize the analyses of these data to predict the composition of comet nuclei and project them further to the composition, structure, and thermodynamic conditions in the nebula. Near-future comet missions are directed toward less active short-period Jupiter-family comets. Thus, future analyses will afford a better understanding of the diversity of these two major groups of comets and their respective regions of origin in the solar or presolar nebula. We conclude with recommendations for determining critical data needed to aid in further analyses. Results of the workshop provide new guidelines and constraints for modeling the solar nebula. This revised version was published online in June 2006 with corrections to the Cover Date.     

Radio Wave Emission from the Outer Planets Before Cassini

We review observations and theories of radio wave emissions from the outer planets. These include radio emissions from the auroral regions and from the radiation belts, low-frequency electromagnetic emissions, and atmospheric lightning. For each of these emissions, we present in more details our knowledge of the Saturn counterpart, as well as expectations for Cassini. We summarize the capabilities of the radio instrument onboard Cassini, observations performed during the Jupiter flyby, and first (remote) observations of Saturn. Open questions are listed along with the specific observations that may bring responses to them. The coordinated observations (from the ground and from space) that would be valuable to perform in parallel to Cassini measurements are briefly discussed. Finally, we outline future missions and perspectives.     

MHD processes in the outer heliosphere

Conclusion Much has been learned about the structure and dynamics of the outer heliosphere during the last decade as a result observations from the Voyager and Pioneer spacecraft. The large scale of the observations forces one to consider the heliosphere from a new perspective, to think of new dynamical processes, and to introduce new concepts. The early studies of isolated gas dynamic flows must be replaced by MHD dynamics of interacting flows and flow systems. The simple deterministic models that have been dominant in early studies of the solar wind are now seen to have limited applicability, and statistical approaches are being developed. New concepts that have been introduced, such as inverse cascades, filtering, entrainment, etc., must be further explored and clarified, to make them more precise and quantitative. MHD turbulence is probably very important in solar wind dynamics, but the subject is poorly developed from a theoretical point of view. The statistical analysis of solar wind parameters has scarcely begun, but it is clearly necessary for an understanding of complex, large-scale flows. The multitude of possible interactions among shocks and flows of various types needs to be explored systematically with observations, models and analytical theory. Voyagers 1 and 2 and Pioneers 10 and 11 are continuing to move through the outer heliosphere and gather data. The lengthy data reduction procedures require even more care in dealing with the low field strengths, densities and temperatures at large heliocentric distances, and the analysis of the complex flows and fields in the outer heliosphere becomes increasingly difficult. Thus one can expect continued growth of our knowledge of the heliosphere, but comprehensive understanding of the data will take some time. If this review stimulates the specialists in solar wind physics to think critically about the results presented and to remedy the deficiencies of current knowledge of the heliosphere, then it will have served its purpose. It is also hoped that this review will serve to encourage specialists in other fields to bring their talents to bear on heliospheric problems and to transfer results of heliospheric physics to their fields.     

Some contributions of the ISIS program towards advances in knowledge of low latitude ionospheric phenomena

The ISIS (International Satellites for Ionospheric Studies) program yielded four scientific satellites, Alouette 1 and 2, and ISIS 1 and 2. This review is limited to the scientific contribution of the program to advances in knowledge relating to the portion of our atmosphere and ionosphere that lie within the plasmasphere. Topside ionograms form the principal data base from which global distributions of electron density and other geophysical parameters can be obtained. Ionospheric ducts and bubbles have also been studied with the aid of ionograms. The utilization of other instruments has facilitated investigations of ion composition, electron temperature and airglow variations.     

Updated Review of Planetary Atmospheric Electricity

This paper reviews the progress achieved in planetary atmospheric electricity, with focus on lightning observations by present operational spacecraft, aiming to fill the hiatus from the latest review published by Desch et al. (Rep. Prog. Phys. 65:955–997, 2002). The information is organized according to solid surface bodies (Earth, Venus, Mars and Titan) and gaseous planets (Jupiter, Saturn, Uranus and Neptune), and each section presents the latest results from space-based and ground-based observations as well as laboratory experiments. Finally, we review planned future space missions to Earth and other planets that will address some of the existing gaps in our knowledge.     

The Global-Scale Observations of the Limb and Disk (GOLD) Mission

The Earth’s thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below. This system can exhibit a significant response within an hour to changes in those inputs, as plasma and fluid processes compete to control its temperature, composition, and structure. Within this system, short wavelength solar radiation and charged particles from the magnetosphere deposit energy, and waves propagating from the lower atmosphere dissipate. Understanding the global-scale response of the thermosphere-ionosphere (T-I) system to these drivers is essential to advancing our physical understanding of coupling between the space environment and the Earth’s atmosphere. Previous missions have successfully determined how the “climate” of the T-I system responds. The Global-scale Observations of the Limb and Disk (GOLD) mission will determine how the “weather” of the T-I responds, taking the next step in understanding the coupling between the space environment and the Earth’s atmosphere. Operating in geostationary orbit, the GOLD imaging spectrograph will measure the Earth’s emissions from 132 to 162 nm. These measurements will be used image two critical variables—thermospheric temperature and composition, near 160 km—on the dayside disk at half-hour time scales. At night they will be used to image the evolution of the low latitude ionosphere in the same regions that were observed earlier during the day. Due to the geostationary orbit being used the mission observes the same hemisphere repeatedly, allowing the unambiguous separation of spatial and temporal variability over the Americas.     

Clouds and Hazes of Venus

More than three decades have passed since the publication of the last review of the Venus clouds and hazes. The paper published in 1983 in the Venus book summarized the discoveries and findings of the US Pioneer Venus and a series of Soviet Venera spacecraft (Esposito et al. in Venus, p. 484, 1983). Due to the emphasis on in-situ investigations from descent probes, those missions established the basic features of the Venus cloud system, its vertical structure, composition and microphysical properties. Since then, significant progress in understanding of the Venus clouds has been achieved due to exploitation of new observation techniques onboard Galileo and Messenger flyby spacecraft and Venus Express and Akatsuki orbiters. They included detailed investigation of the mesospheric hazes in solar and stellar occultation geometry applied in the broad spectral range from UV to thermal IR. Imaging spectroscopy in the near-IR transparency “windows” on the night side opened a new and very effective way of sounding the deep atmosphere. This technique together with near-simultaneous UV imaging enabled comprehensive study of the cloud morphology from the cloud top to its deep layers. Venus Express operated from April 2006 until December 2014 and provided a continuous data set characterizing Venus clouds and hazes over a time span of almost 14 Venus years thus enabling a detailed study of temporal and spatial variability. The polar orbit of Venus Express allowed complete latitudinal coverage. These studies are being complemented by JAXA Akatsuki orbiter that began observations in May 2016. This paper reviews the current status of our knowledge of the Venus cloud system focusing mainly on the results acquired after the Venera, Pioneer Venus and Vega missions.     

Primordial matter in the outer solar system: A study of its chemical composition from remote spectroscopic analysis

Our knowledge of the primordial matter from the objects of the outer solar system has made a considerable progress over the past years, in spite of the lack of any in situ measurements of these objects at the present time. The recent progress of ground-based instrumentation and the launch of the two Voyager fly-by missions have provided a huge amount of new informations about the origin and the evolution of the primitive Solar System objects.The most significant discoveries concerning the atmospheres of the Giant Planets can be summarized as follows: (1) there does not seem to be any differentiation in the internal structure of Jupiter during the planet's history; thus, the H₂/He ratio measured on Jupiter seems to be representative of the H/He ratio of the Primordial Nebula; (2) there is some evidence for a helium differentiation, relative to hydrogen, in Saturn's interior; (3) there seems to be a carbon enrichment on both Jupiter and Saturn by a factor about 2; this result is consistent with a model in which the planetary core is formed first, and the atmosphere accreted by this core in a second stage; (4) the D/H ratio measured on Jupiter should be representative of the D/H value in the Primordial Nebula, 4.5 billion years ago; this value is 2 to 5 times larger than the mean value measured in the local interstellar medium now; (5) Titan's atmosphere is dominated by nitrogen and contains traces of organic and prebiotic molecules (HCN, C₂N₂, HC₃N); the chemical composition of Titan's atmosphere could be favorable for the early stages of life development.The small bodies of the Solar System — asteroïds and comets — are still very poorly known. However they contain a key information about the physical and chemical properties of dust in the Primordial Nebula and the interstellar medium. With the launch of expected fly-by missions towards Comet Halley and, possibly, towards asteroïds, we may hope to know a new development of our understanding of these objects, comparable to the progress we have known on the Giant Planets over the past ten years.     

Plasmasphere Response: Tutorial and Review of Recent Imaging Results

The plasmasphere is the cold, dense innermost region of the magnetosphere that is populated by upflow of ionospheric plasma along geomagnetic field lines. Driven directly by dayside magnetopause reconnection, enhanced sunward convection erodes the outer layers of the plasmasphere. Erosion causes the plasmasphere outer boundary, the plasmapause, to move inward on the nightside and outward on the dayside to form plumes of dense plasma extending sunward into the outer magnetosphere. Coupling between the inner magnetosphere and ionosphere can significantly modify the convection field, either enhancing sunward flows near dusk or shielding them on the night side. The plasmaspheric configuration plays a crucial role in the inner magnetosphere; wave-particle interactions inside the plasmasphere can cause scattering and loss of warmer space plasmas such as the ring current and radiation belts.     

Ultraviolet spectroscopy and remote sensing of the upper atmosphere

The Earth's ultraviolet airglow contains fundamental diagnostic information about the state of its upper atmosphere and ionosphere. Our understanding of the excitation and emission processes which are responsible for the airglow has undergone dramatic evolution from the earliest days of space research through the past several years during which a wealth of new information has been published from high-resolution spectroscopy and imaging experiments. This review of the field begins with an overview of the phenomenology: how the Earth looks in the ultraviolet. Next the basic processes leading to excitation of atomic and molecular energy states are discussed. These concepts are developed from first principles and applied to selected examples of day and night airglow; a detailed review of radiation transport theory is included. This is followed by a comprehensive examination of the current status of knowledge of individual emission features seen in the airglow, in which atomic physics issues as well as relevant atmospheric observations of major and minor neutral and ionic constituents are addressed. The use of airglow features as remote sensing observables is then examined for the purpose of selecting those species most useful as diagnostics of the state of the thermosphere and ionosphere. Imaging of the plasmasphere and magnetosphere is also briefly considered. A summary of upcoming UV remote sensing missions is provided.     

Structure of the Solar Wind and Compositional Variations

The composition of the solar wind is largely determined by the composition of the source material, i.e. the present-day composition of the outer convective zone. It is then modified by the processes which operate in the transition region and in the inner corona. In situ measurements of the solar wind composition give a unique opportunity to obtain information on the isotopic and elemental composition of the Sun. However, elemental — and to some degree also isotopic — fractionation can occur in the flow of matter from the outer convective zone into the interplanetary space. The most important examples of elemental fractionation are the well-known FIP/FIT effect (First Ionization Potential/Time) and the sometimes dramatic variations of the helium abundance relative to hydrogen in the solar wind. A thorough investigation of fractionation processes which cause compositional variations in different solar wind regimes is necessary to make inferences about the solar source composition from solar wind observations. Our understanding of these processes is presently improving thanks to the detailed diagnostics offered by the optical instrumentation on SOHO. Correlated observations of particle instruments on Ulysses, WIND, and SOHO, together with optical observations will help to make inferences for the solar composition. Continuous in situ observations of several isotopic species with the particle instruments on WIND and SOHO are currently incorporated into an experimental database to infer isotopic fractionation processes which operate in different solar wind regimes between the solar surface and the interplanetary medium. Except for the relatively minor effects of secular gravitational sedimentation which works at the boundary between the outer convective zone and the radiative zone, refractory elements such as Mg can be used as faithful witnesses to monitor the magnitude of these processes. With theoretical considerations it is possible to make inferences about the importance of isotopic fractionation in the solar wind from a comparison of optical and in situ observations of elemental fractionation with the corresponding models. Theoretical models and preliminary results from particle observations indicate that the combined isotope effects do not exceed a few percent per mass unit. In the worst case, which concerns the astrophysically important 3He/4He ratio, we expect an overall effect of at most several percent in the sense of a systematic depletion of the heavier isotope. Continued observations with WIND, SOHO, and ACE, and, with the revival of the foil technique, with the upcoming Genesis mission will further consolidate our knowledge about the relation between solar wind dynamics and solar wind composition. This revised version was published online in June 2006 with corrections to the Cover Date.