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3 Status Expected in 1995 SOLAR AND HE[IOSPHERIC PHYSICS The subject areas of solar and heliospheric physics encompass a broad range of physical processes; by 1995 we may anticipate continued progress in a number of these areas: the structure and dynamics of the solar interior; variations In the solar luminous output; the emergence and evolution of solar surface magnetic fields, including solar flares; energy and momentum input to the solar corona; global structure and evolution of the heliosphere; and microscopic plasma processes. Helioseismological studies of the structure and dynamics of the convection zone, the temperature and molecular weight dis- tribution throughout the interior, as well as the radial and lati- tudinal variation of rotation should be initiated by ground-based networks of instruments as well as instruments on board the solar satellite of the ISTP. However, if our most optimistic expectations of the progress of these projects and their subsequent analysis were justified, we would be only beginning to investigate the possible variation of these properties through the activity cycle. The specification of the solar interior properties from helioseis- mological techniques will provide crucial new input into models of 17
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18 the solar dynamo. Indeed, progress in such modem fundamental to our understanding of basic solar cyclic variations will be stimu- lated by observational progress. The role of dispersal and diffusion of small-scale magnetic fields in the dynamo process (at least as reflected by surface fields) awaits clues from extended temporal observations of the successor to the SOT (see below). Solar irradiance will have been monitored on an episodic basis and the solar luminosity will have been monitored on a more con- tinuous basis over more than a complete solar cycle. However, it is unlikely that the solar radius will have been monitored accurately; this, together with the lack of accurate specification of the solar interior properties makes it unlikely that the issues concerning the storage and release of luminous energy (essential in understanding the transport of energy through the solar atmosphere) will have been resolved in a quantitative way. Extraordinary progress in our view of the fine structure of the solar photosphere and chromosphere will be made by Spacelab 2, SunIab, and the successor to SOT. It may be anticipated that fundamental new observations of the interaction of solar plasmas and magnetic fields on the spatial and temporal scales at which the basic physical processes occur will have been obtained. To study and understand these processes (for example, changes in magnetic field strength, waves, single pulses, and systematic mass flows), it will be necessary to resolve spatial scales over which significant gradients occur in the local magnetic and nonthermal velocity fields, as well as in the local densities and temperatures. Studies of the plasma-magnetic field interactions on these spatial scales will be directed toward understanding these most fundamental physical processes that can be observed in the solar atmosphere. The Pinhole Occulter Facility (POF) may be operational by 1995. If so, it will allow hard x-ray observations with a similar spatial resolution as that of SOT. However, no similar capability is anticipated for XUV and EUV spectral observations, or for the crucially -important high-energy flare particle- signatures. As a result, one may expect limited progress in our understanding of the flare process, which seems to take place also on spatial scales of a few hundred kilometers. In coronal physics, by 1995, several views of the global proton temperature distribution will have been obtained by new instru- mentation, from SPARTAN and SOHO, at least in the region be- yond 1.5 solar radii. Inferred coronal outflow speeds necessarily
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19 less reliable wiD follow. Thus, our understanding of the physics of the near-solar-w~nd acceleration wait remain largely Incomplete, as will our understanding of the origin of the energy flux from the lower solar atmosphere required to drive the wind. Unfortu- nately, little new information on the three-dimensional structure of the corona wiD be available by 1995, so all of the long-standing issues concerning the solar longitudinal density distribution of the corona and also the newer temperature and outflow speed distribution~will remain unresolved. Despite intensive efforts, there wiD remain a paucity of observations of the initiation of coronal mass ejection phenomena, and suitable diagnostic infor- mation on the structure of these events will remain unobserved. Hence, most of these fundamental issues will be unresolved. Our understanding of the three-dimensional structure of the heliosphere will be improved by the flight of the ISPM, but a second passage of the ISPM will be critical to gaining a reason- able knowledge of latitudinal variations of heliospheric structure during the period of the solar cycle when these variations are ex- pected to be significant. We can also expect some improvement in our knowledge of the latitudinal variation of solar wind mass flux and flow speed (away from solar maximum) from EUV and radio observations that remotely sense the interplanetary medium. OF servations of the Mutant solar wind will be extended to beyond 60 AU, and our present view of distant stream evolution is likely to be confirmed, but it is not clear whether the termination of super- sonic. solar wind flow will be observed by the Voyager 1 spacecraft, which will have gone some 50 AU in the direction of the solar apex. Significant progress can be expected in two areas of study in- volving rrucroscopic plasma processes in the interplanetary medi- um. Continued theoretical work on the problem of heat conduc- tion and viscosity In a thermally driven stellar wind not domi- nated by Coulomb collisions should advance to the point where lack of in situ observations of the inner solar wind would pre- vent further progress. A combined observational and theoretical effort toward understanding the acceleration of particles at inter- planetary shocks should meet with some success and provide an improved basis for understanding energetic particle populations in interplanetary space and other astrophysical systems. We can expect to see some progress in the study of the role of corotating interaction regions on the interplanetary modulation of galactic cosmic rays. There ~ no reason to believe that we
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20 will learn much more about any other modulation mechanisms by 1995. MAGNETOSPHERIC PHYSICS The solar system offers a variety of magnetospheres for study. These magnetospheres have sufficiently different internal parame- ters and boundary conditions to allow testing of the universality of basic magnetospheric concepts. The solar wind interacts with planetary magnetic fields to produce diverse phenomena that in- volve the storage and energ~zation of charged particles. Also, a small fraction of the energy within a magnetosphere is converted into a broad assortment of radio emissions. Differences and simi- larities between magnetospheres, if properly un~lerstood and for- mulated, yield physical principles that can be applied throughout the universe. Sources of Plasma The solar wind, anmediately after its existence was estate fished, was thought to be the source of plasma, particularly en- ergetic plasma, within the Earth's magnetosphere. Nearly two decades later it was discovered that the ionosphere was a source of quantitatively significant flows of plasma into the magnetosphere. Then, consistent with the discovery that the solar wmd is a negligi- ble source of plasma for the magnetospheres of Jupiter and Saturn, experimenters have interpreted recent data from the DE satellite as indicating that perhaps most of the Earth's magnetospheric plasma may be supplied by the ionosphere. This question may be partially resolved by AMPTE, which can give a quantitative indication of the ability of solar wind, magnetosheath, and tad] plasma to enter the inner magnetosphere, and ISTP, which will make simultaneous, coordinated measurements in the solar wind and the magnetosphere. Sources of Power Phenomena within the Earth's magnetosphere are driven by power extracted from the solar wind. The solar wind, as it passes the Earth, drives plasma within the magnetosphere in a circula- tion pattern that energizes some of the magnetospheric plasma
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21 and drives strong magnetically field-aligned currents that connect to the auroral zones. Magnetic merging, in which topologically separate magnetic regions are connected and magnetic energy is released, is still a poorly understood process that is central to most of the discussion of magnetospheric substorrns and power delivery to the Earth's magnetosphere and other dynamic pro- cesses. It is expected that ISTP will provide data that is necessary to more fully understand this phenomenon. In contrast to the solar wind power source for the Earth's magnetosphere, Jupiter and probably Saturn draw power for their magnetospheres pri- marily from the kinetic energy of planetary rotation. The flybys of Uranus and Neptune by Voyager will give us two more examples of magnetospheric energ~zation from which to derive general phys- ical principles governing the transfer of power to magnetospheric regions. Generation of Plasma Waves and Radio Emissions Cosm~c-scale plasma and magnetic field regions are not quies- cent. One universal form of activity they exhibit is the generation and amplification of radio waves. Aside from the interest in the radio emissions themselves, they serve as a means of exchanging energy between particles in a low-density plasma where Coulomb collisions are ineffective. Because we can get so close to (even within) the emitting regions of a planetary magnetosphere, we hear all of the symphony of electromagnetic emissions they pro- duce: hms, chorus, narrow- and broad-band electrostatic waves, and decnnetric, decametric, and kilometric radiations. A common theoretical framework has been developed so that when account is taken of differing conditions within the various magnetospheres, radio emissions in one planetary magnetosphere can be related to that in another as different manifestations of the same physical process, although the frequency ranges of the emissions might be quite different. The Terrestrial Magnetosphere as a System The International Solar Terrestrial Physics Program will ac- quire simultaneous measurements throughout key regions in order to understand the behavior of the system as a whole. A coor- dinated network of spacecraft will permit us to investigate the
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22 physical behavior of each key region involved in the analysis of solar-terrestrial plasma physics. WIND will be stationed in the upstream solar wind to observe the interplanetary input function and to observe the escape of energetic magnetosphere particles back into the solar wind. POLAR will (1) observe directly the en- try of magnetosheath plasma via the dayside cusp, (2) measure the flow of hot plasma into and out of the ionosphere on auroral field lines, and (3) observe the deposition of particle energy into the ionosphere/atmosphere. CRRES will permit us to observe solar wind-magnetosphere coupling near the magnetic equator, and it will directly measure the interaction of ionosphere and tail plasmas in the ring current and the plasma-sheet storage and transport regions when the apogee ~ situated over the night hemisphere. GEOTAIL will provide extensive simultaneous measurements of entry, storage, acceleration, and transport in the geomagnetic tad! and the tad! plasma sheet; and it will also measure plasma entry and transport in magnetopause boundary layers along the dawn and dusk flanks of the magnetosphere. The four CLUSTER space- craft will provide detailed information on localized current systems and magnetohydrodynamic processes, and SOHO will yield addi- tional information on interplanetary and solar phenomena that can affect the terrestrial magnetosphere. These ISTP measurements will allow us to develop an under- standing of the physical processes occurring in the solar-terrestrial environment. In addition, theoretical studies will provide the framework upon which the empirical understanding from the oh servations can be both systematized and used to further our basic understanding of other plasma systems. Solar-Terrestrial Research In parallel with the major spacecraft efforts addressed above, progress in understanding the solar-terrestri~1 system as a whole will also occur through the use of existing and planned ground- based research programs in the United States and in nations throughout the world. These include magnetic field and iono- spheric arrays established for monitoring and/or campaign pur- poses and large radar systems for studies of high-altitude plasma convection and transport. During the ISTP program, the inter- national ground arrays will be considerably enhanced and sum plemented by more sensitive instruments and, particularly, by
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23 considerably advanced data acquisition, storage, and transrnis- sion systems. All of these advances will contribute crucially to understanding the global terrestrial magnetosphere system in the context of the ISTP program. In the United States the ongoing Solar-Terrestrial Theory Program and other theoretical support will be crucial in order to provide the predictive theoretical models for the global system. Input data required for the theory and mode} advances will occur from both the spacecraft and the ground-based experiments. UPPI:R ATMOSPHE:RE SCIENCE The upper atmosphere is defined here as the region of the neu- tral atmosphere above the tropopause (12 km) extending all the way to the exosphere. During the 1970s a great deal of progress was achieved in our understanding of mid- and low-latitude ther- ~nospheric photochemistry, mainly as a result of measurements obtained by the Atmosphere Explorer satellites. At the present time the dynamics of the thermosphere is being studied extensively by in situ and remote sensing instruments, which were carried by the Dynamics Explorer 2 satellite, by ground-based optical and radar methods, and by large-scale mode} studies. The dynam- ic~ of the magnetosphere and thermosphere are coupled through field-aligned currents, electric fields, Joule heating, and particle precipitation, which both heats the upper atmosphere and alters the electrical conductivity of the ionosphere. Before the end of this decade, this multipronged study of thermospheric dynamics is ex- pected to lead to significant advances in our understanding of the energy and momentum sources controlling atmospheric motions in these altitude regions. However, gaps will still remain in our understanding of how specific ionospheric and auroral phenomena correspond to magnetospheric sources and/or consequences. Significant progress in our understanding of the stratosphere, mesosphere, and lower thermosphere (often referred to as the m~d- dIe atmosphere) is expected as the result of measurements made by the instrument complement of the Upper Atmosphere Research Satellite (UARS), to be launched in the late 1980s. This mission will carry out a comprehensive set of measurements, particularly from the viewpoint of atmospheric chemistry. Global measure- ments of O3 and many of the radical species that destroy it (e.g.,
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24 NO2, C10) will be acquired sunultaneously, allowing for anal- ysis of their interrelationships. Further, the "reservoirs species that sequester these radicals in relatively inert forms will also be measured (e.g., HNO3, HC1), along with the long lived molecules that are the sources of all these constituents (e.g., N2O, CHIT. In addition to these chemical measurements, atmospheric winds will be measured s~rnultaneously from WARS, allowing for a more complete analysis of transport processes than ever before possi- ble. An explorer-class missions MELTER, is also planned for the early 1990s for a focused study of the energetic and dynamic cou- pling processes between the mesosphere and lower thermosphere. Finally, atmospheric measurements need to be complemented by continued laboratory measurements of the rate coefficients of the important reactions as weD as the atomic and molecular parame- ters of the relevant absorbing and emitting atmospheric species. These data will be analyzed and interpreted with the aid of multidimensional, chemical-dynamical-radiative models that will include much more complete descriptions of many of the relevant physical and chemical processes and their coupling. The progress in all the various numerical modeling studies that are relevant to atmospheric sciences will be greatly accelerated by advances in computer technology. A critical aspect of our understanding of the middle atmo- sphere ~ the question ot what physical and temporal scales are involved in the various photochem~cal and transport processes. The spatial resolution achievable, currently as well as in the near future, by both satellite observations and numerical models is rel- atively large and represents a significant barrier to a thorough understanding of many of the smaller scales on which significant processes may well occur. Another important area of study ~ the l990s will continue to be the question of interactions between the middle atmosphere and its neighbors, the troposphere and the thermosphere. Ther- mospheric coupling includes studies of the particle and solar inputs and corresponding middle atmospheric responses on a global scale. Finally, the important question of the coupling of the troposphere and the middle atmosphere will be a central component of the field as a whole; anthropogenic perturbations to the atmosphere in general and the ozone layer specifically will continue to be an important central theme for middle atmospheric studies.
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25 Global Electric Circuit The existence of a global electric circuit has been known for many years, and yet we still do not understand the basic processes that drive and control the behavior of this system. According to the classical picture, thunderstorms are the sole generators within the circuit, and, acting together, they maintain a potential dif- ference of about 200 to 600 kV between the highly conducting ionosphere and the surface of the Earth. This potential differ- ence causes a downward conduction current of about ~ to 2 kA from the ionosphere to the ground in the fair-weather dissipative portion of the circuit. The global circuit Is believed to be closed through an upward current flow from the Earth's surface, beneath a thunderstorm, to the negative charge at the cloud base. Within a cloud, updrafts and downdrafts and ~rucrophysical and electri- fication processes maintain the charge separation. Although this picture is generally the accepted one today, considerable doubts and uncertainties are associated with many of the macrophysical and rn~crophysical concepts that have been advocated (e.g., are thunderstorms the main generators, and what controb charge sep- aration?~. Simple models of the circuit assume that the ionosphere is a highly conductive, equipotential upper boundary; however, in reality, there are significant horizontal potential differences of tens of kilovolts generated by both the ionospheric neutral wind and the solar wind/magnetosphere dynamos. The details of the telluric currents flowing in both the solid earth and oceans are complex and require comprehensive experimental and theoretical investi- gations. The problems associated with the global electric circuit cut across numerous disciplines, from magnetospheric convective pro- cesses at one end to soil and ocean conductivity issues at the other encI. Some of the more "relevantly ways in which atmospheric elec- tricity may play a potentially important role are as follows: ~ Causing interference in man-made systems such as com- munication cables, power lines, and pipe lines. ~ Influencing the spatial distribution and effectiveness of con- densation nuclei in the atmosphere. ~ Acting as a possible mechanism for the generation of odd nitrogen compounds through lightning processes.
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26 PLANETARY SPACE PHYSICS There are basically four different types of interactions of plas- mas with bodies in the solar system. The plasmas can collide with solid bodies directly. For example, the solar wind strikes the lunar surface and is absorbed. Energetic particles are similarly absorbed by the jovian moons, albeit with sputtering of material that in turn supplies material to the jovian magnetosphere. The rings of Saturn are also subject to such processes. A second interaction is the deflection of a flowing plasma by a planetary magnetic field. The smallest scale on which such deflection is known to occur is the deflection of solar wind above the lunar terminators when magnetized regions of the lunar surface occur at the lunar limbs. Such deflection occurs on a global scale at Mercury, the Earth, Jupiter, and Saturn and perhaps other planets as well. If a body has no strong magnetic field but does have an atmosphere, two other interactions can occur, whose effects are sometimes difficult to separate. First, if an ionosphere can form due to strong ionization of the neutral atmosphere, a cold plasma region may form whose pressure is sufficient to exclude the external nonplanetary plasma. This process occurs at Venus, where the gravitationally bound ionosphere usually has sufficient pressure to stand off the solar wind. Fresh comets, strongly outgassing near the Sun, are also thought to have ionospheres. If the ionospheric pressure is insufficient to stand off the solar wind, it is essentially pushed back toward lower altitudes where the atmospheric density is greater. The resulting ion-neutral coupling due to the relative drift of the ions and neutrals transmits pressure to the plasma and aids In the support of the ionosphere. At comets, the outflow of neutral gas assists in this pressure balance. At the top of the ionosphere, the ionopause, a tangential discontinuity, separates the external plasma, the magnetosheath in the case of Venus, from the ionospheric plasma. The second process that occurs is the direct interaction of the neutral gas and the external plasma without the intermediary of an ionosphere. The neutral atmosphere at a planet such as Venus extends well above the ionopause. This gas can be photoionized by solar extreme ultraviolet or charge exchange with the magne- tosheath or solar wind plasma. At a satellite such as To, the neutral gas can be ionized also by impact ionization. This new source of
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27 plasma ma" loads the external plasma, slowing it down if it flowing.- Charge exchange can also lead to the creation of fast neutrals that remove momentum from the plasma. This process is thought to create the long tails seen behind Venus and comets and cause the field-aligned currents and distorted magnetic fields seen at To and Titan. Finally, it is noted that the solar wind flows much more rapidly than the speed of compressional waves in the solar wind plasma. Hence, when it interacts with a conducting planetary object, shock waves form to deflect the flow around the object. These collision- less shocks are very interesting objects that hare been intensively studied at the Earth. However, planetary studies have much to add to this investigation because the properties of the plasmas in the solar system change greatly with heliocentric distance. Interaction with Unn~agnetized' Atmo~hereless Bodies The interaction of the solar wind with the Moon ~ understood only on the most elementary level. Basically we know that the solar wind is absorbed by the forward hemisphere of the Moon, leaving a wake behind. Because of the limited plasma instrumentation on Explorer 35 and the Apollo subsatellites, we know little about the closure of plasma behind the Moon. Some work has been done with plasma instruments on the lunar surface and much empirical understanding obtained. The interaction of the radiation belt of Jupiter with its satel- lites ~ understood on an elementary level, although much more needs to be done on the role of sputtering in providing a source of jovian plasma. Galileo should contribute greatly in this area. We also have an elementary understanding of the interaction of Saturn's radiation belts with its satellites and rings. We expect new progress in this area through laboratory investigations, the- ory, and the Saturn orbiter of the Cassini my - ion. Studies of the interaction of the solar wind with cometary dust are also impor- tant. Here, we expect some near-term progress through laboratory data and theoretical treatment. Deflection by Planetary Magnetospheres We know little about the solar wind interaction with Mercury except that Mercury has a magnetosphere, bow shock, and tran- sient energetic particle population, suggesting that subetorm-like
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28 processes occur. Mercury's magnetosphere is a very important one because of its lack of a dynamically important atmosphere. Thus field-aligned currents should not play a dominant role in the magnetosphere of Mercury. Our studies of the jovian magnetosphere have proceeded from the exploratory phase to the beginnings of intensive investigation. We expect further refinements from the Galileo mission. The jo- vian magnetosphere is important because of its rapid rotation and the large mass loading by To deep in the interior of the magneto- sphere. Saturn complements the jovian studies tenth its extensive ring system that both absorbs and supplies charged particles. Titan is also important as discu~ed below. We expect little change in our understanding of the saturnian magnetosphere until the data from the Cassini minion are obtained. Voyager has provided existence data on the magnetosphere of Uranus, leaving us with elementary concepts on the physics of these systems. Similar data will be available from Neptune in 1989. Plasma-Atmopphere Attractions The solar wind interaction with Venus is now understood to first order. The present and future data base from Pioneer Venus plus computer modeling should give us sufficient insight to ask more fundamental questions, but large gaps remain in our observational knowledge. The magnitude of the Mars intrinsic magnetic moment is not known. This will be addressed with the Mars Observer in the early l990s. However, we will still be ignorant of all the basic plasma processes on Mars, as well as of the processes In the upper neutral atmosphere. The interaction of the jovian plasma with To and the inter- action of the saturnian plasma with Titan are also key elements in our study of plasma/neutral gas interactions. The former will be addressed briefly by Galileo in one flyby. This may not be sufficient. The latter will be addressed by the Cassini mission to Saturn. Interest in the interaction of the solar wind with comets ~ undergoing a strong resurgence owing to the high interest in the Giacobini-Z~nner and Halley missions. Theoretical progress should
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29 be made by the end of the decade, and there should be some confirmatory data. SUMMARY Tables 3.1 and 3.2 provide a graphic summary of the expected status of research by 1995 on the science objectives defined in Chapter 2. Table 3.1 depicts in situ investigations; Table 3.2 depicts remote sensing investigations.
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30 TABLE 3.1 Leveb of In Situ Spacecraft Investigation Disolpline Awaiting Reconnals- Exploration Intensive Physical Recon- sance Study Under- naissance . standing Solar physics Sun, solar corona Hellospheric physics Generation of solar wind High-latitude solar wind In-ccliptic solar wind beyond Saturn In-ecilptle solar wind between Bllercury and Jupller Hellopause, Interstellar medium Terrestrial, magnetospheric physics Magnetosphere <60 Rig Earth magnetic tall, wake Terrestrial, atmospheric, ionospheric physics Thermosphere, ionosphere >150 km M exosphere _- Solar Probe 1~ - Solar Probe - ~0~ - ISPM
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31 TABLE S.1 (continued) Disclpllne =on~l~ ~p~-on licensee Phil Recon- sance Study under- -~-e amid Panda, _-ed nos~ed~ magn~osphedc phyla Comas Solar wind Oregon of Ears Atmosphere of Bars Soar and l~em~lon of Venus ~agna10sphere of ~ ercu ~ magnetosphere of Junker ~agne10spbere of Saturn ~agne10sphere of ursnus, Neptune upper atmosphere Ionosphere of Judged ~urn, Than Galileo
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32 TABLE 3.2 Levels of Remote Sensing Investigation Discipline Awaiting Preliminary Global Intensive Physical Preliminary Survey Survey Study Under- Survey standing Solar physics Coronal holes, large-scale magnetic fields Radio bursts Global oscillation i-fares Internal structure and dynamics solar dynamo Surface plasma- magnetic field interactions Energy storage and release Atmospheric heating Structure and dynamics of corona and solar wind Heliospheric physics Interstellar neutrals Terrestrial, magnetospheric physics Global auroral morphology Remote sensing of magnetospheric structure Terrestrial, atmospheric, ionospheric physics Middle atmosphere SOT/ASO SOT/ASO SOT/ASO 1' ISTP
Representative terms from entire chapter: