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4 New Initiatives: 1995 to 2015 Many of the most novel and exciting initiatives in solar and space plasma physics for the period 1995 to 2015 will involve com- bining remote sensing or imaging techniques with in situ observa- tions for the study of the Earth's magnetosphere and the analysis of the solar corona, as well as other solar phenomena. These new initiatives fad into four groups, corresponding to the following subdisciplines of solar and space physics: heliospheric physics, ter- restrial magnetospheric physics, terrestrial atmospheric physics, and planetary science. SOLAR AND HE[IO SPHERIC PHYSICS Local Measurements in the Solar Atmosphere At present, our understanding of the origin of the solar wind is based entirely on theory and remote sensing. Direct measure- ments of the solar wind plasma, the interplanetary magnetic field, the energetic particle population, and associated wave-particle in- teractions are available, but only at distances greater than the 0.3 AU perihelion distances of Helios ~ and 2. The task group recom- mends a Solar Probe mission whose primary objective ~ to carry out the first in situ observations of the solar wind plasma and fields (electric and magnetic) near the source of the wind in the 33 I

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34 r Extends to ~ 10 Ret 1\ E - ~ / , 1 me' 1i,/i,,>: ~ FIGURE 4.1 A trajectory for the Solar Probe. , w solar atmosphere. Included will be a detailed study of energetic particles, which will yield import ant diagnostic data on particle acceleration processes and coronal structure. The spacecraft must be placed in an orbit that will bring it as close to the Sun as possible and still survive to provide useful data near closest approach. A perihelion distance of 4 solar radii is anticipated, with a local wind speed of about 50 km/s, electron and ion plasma temperatures of about 106K, and plasma density and magnetic field strength of less than 106 electrons/cm3 and 105 gamma, respectively. A drawing of the Solar Probe trajectory is shown in Figure 4.1. Theories of solar wind origin place the transition region from subsonic plasma flow to supersonic flow somewhere between 1 and

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35 10 solar radn. Radio scattering experunents on Viking during superior conjunction suggest a critical point closer to 10 solar radii. In situ measurements should clarify this msue. The location of the critical point and the plasma proper- ties (speed and temperature) of the supersonic wind will depend greatly on the physical processes that heat the corona. Theoret- ical studies suggest that the proton temperature profile is very sensitive to these heating processes. It is not clear whether the corona contains an extended region of heating (out to as far as 20 solar radii) or undergoes adiabatic expansion beyond the solar sur- face. Plasma temperature data and observations of the wave types and amplitudes should lead to the identification of the important heating and acceleration mechanisms. Many other important problems can be studied with Solar Probe, including a detailed! characterization of coronal streamers, the place of origin and the boundaries of high-speed and low-speed flows close to the Sun, the extent of heavy element fractionation and elemental abundance variations, and the scale sizes of inho- mogeneities and the development of the magnetohydrodynamic turbulence that characterizes the solar wind near ~ AU and be- yond. The Solar Probe mission can also study the solar spin down rate through measurements of solar wind angular momentum flux. Further study needs to be carried out to determine the best method of designing detectors that are required to look in the direction of the Sun. ~ In the original study, it was assumed that the spacecraft would go to Jupiter, where a gravity assist would send it on course to the inner corona. Our task group learned of a possible alternate trajectory involving a hypersonic flyby in the upper atmosphere of Venus; the two possibilities are sketched in Figure 4.2. It should be possible to add low-thrust propulsion In order to attain an ecliptic orbit around} the Sun with a 1-year periodicity so that the probe enters the vicinity of the Sun several times. As shown in Figure 4.2, the Venus flyby technique also yields a very short orbital period. High-[atitude Solar Studies The heliosphere is known to have a complicated three-dimen- sional structure. The magnetic field is a tight spiral near the solar equatorial plane, but is expected to be essentially radial over the

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36 JUPITER GRAVITY ASSIST EARTH ORBIT VENUS AERO-GRAVITY ASSIST EARTH ORBIT / VENUS ORBIT \ \ \ > JUPITER FLIGHT TIME: 4 years FINAL PERIOD: 4 years JUPITER RADIUS: 10 R /~~\ \ FLIGHT TIME: 1/2 year ~ FINAL PERIOD: 1/5 year FIGURE 4.2 Two concepts for the trajectory of a Solar Probe mission. solar poles. Coronal holes, one of the sources of high-speed solar wind, are expected to produce quasi-steady high-speed flows over the solar poles during much of the solar cycle, whereas at low latitudes interacting high- and low-speed flows predominate. To understand heliospheric conditions at low solar latitudes has required numerous missions, e.g., Explorers, Pioneers, Mari- ners, and Voyagers. To understand heliospheric conditions at high latitudes will similarly require repeated minions. NASA and ESA will fly the first exploratory mission over the solar poles (Ulysses). However, as with most exploratory missions, Ulysses will probably uncover more questions than it will answer, and follow-on missions will be required. The objective of the Solar Polar Orbiter (SPO) would be to provide a detailed, repeated study of conditions at all heliographic latitudes. In circular orbit, SPO will observe the heliosphere at constant radius and thus will distinguish latitude from radial ef- fects. With a circular orbit at less than or equal to ~ AU, and thus

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37 SOLAR POLAR $/ ORBITER 1 AU IMAGING STARPRO8E ~ C(~| R 4Re rat A\ - FROM ;\ JUPITER L 1 PLATFORM (AR-0.0 1 AU) \ HELIOSYNCHRONOUS 25 DAY ORBIT, 33 Re FIGURE 4.3 The orbit of the Solar Polar Orbiter shown together with lo- cations of other solar measurement platforms (Starprobe, the 1-AU observing network, and the Heliosynchronous Orbiter). an orbital period less than or equal to 1 year, SPO should be able to make several passes over the solar poles in a nominal mission lifetime, and thus distinguish spatial from temporal effects. No detailed study of an SPO mission has yet been done. However, the required orbit should be achievable through the use of a low-thrust, continuous acceleration propulsion system such as solar-electric propulsion with a final orbit as shown In Figure 4.3. The SPO spacecraft should carry a full complement of plasma, energetic particle, magnetic field, and radio wave instruments, similar to what is to be flown on ISPM. In addition, SPO should have pointing capability, through the use of a despun platform on a spinning spacecraft, or as a three-axis stabilized spacecraft, for detailed solar observations using a coronagraph, x-ray telescope, and similar photon observing instruments. The principal technical development required for SPO is a solar-electric propulsion system, or its equivalent, for low-thrust, continuous acceleration. In the cost projections for SPO it is assumed that such development will not be charged against the mission costs, because the need is common to several proposed

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38 programs. Also, studies need to be conducted on the impact of a continuous propulsion system on particle, field, and photon in- strumentation, and on the measurements these instruments make. Outer Coronal Physics The Heliosynchronous Orbiter (HSO), as described in the ESA document Horizon 2000, is an instrumented probe orbiting the Sun at about 30 solar radii with a 26-day period, synchronous with the rotation of the Sun (see Figure 4.3~. This mission will be able to address a very broad range of sci- entific objectives from solar physics, physics of the interplanetary medium, and high-energy astrophysics to relativity: . Investigation of the morphology and dynamical develop- ment of all solar structures from the photosphere to the outer corona from a vantage point close to the Sun (0.15 AU) over a large range of solar latitudes, with frequent access to the solar po- lar regions. Stereoscopic viewing of structures through motion of the spacecraft. The understanding of the relationship between the thermal structure and heating of the solar corona will ultimately permit the identification of the physical nature of the solar wind acceleration. Imaging of the coronal structures could be achieved by observations at 1 AU. Investigation of the three-dimensional structure of the in- ner heliosphere near or even outside the region where the wind is accelerated. . Measurements of solar wind particle fields and waves; stud- ies of the heating and acceleration of the solar wind (thermally or wave-driven wind?) with the advantage of a wide latitude cover- age. Studies of the propagation, acceleration, and modulation of solar energetic particles including the significant reduction of propagation effects with respect to 1 AU. Study of shock wave acceleration. . Radio sounding of the solar corona as the spacecraft passes behind the Sun. ~ Correlative studies of expanding and traveling solar struc- tures and their manifestation in interplanetary space. ~ Investigation of the three-dimensional distribution of mass and velocity of interplanetary dust in the inner heliosphere. .

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39 ~ Investigations of the Hermean magnetosphere and remote sensing of Mercury during flybys early in the mission (the last in situ measurements date from Mariner 10 in 1974-1975~. Establishment of a reference observatory for other missions in the heliosphere, in particular for solar optical remote sensing missions near 1 AU. Baseline observations of galactic gamma-ray bursts. ~ Performance of relativity experiments (if possible), e.g., determination of 32 (the second gravitational moment) in the case of a highly elliptic orbit; frame dragging experiments. The technological problems (propulsion, thermal design, data transmissions pose a considerable challenge. The mission concept is certainly not only attractive to a large scientific community, but it would also be appealing to the general public and from the technological point of view. 1-AU Obeying Network The Sun ~ the only star that we can observe from different directions, i.e., from any position within the heliosphere. This pro- vides a stereoscopic view of structures whose geometry and energy content cannot be determined because they are either optically thin or because parts of them are not entirely visible from one sin- gle viewing condition. In addition), simultaneous observations at different positions inside the heliosphere provide three-dimensional snapshots of the magnetic field and the solar wind, important oh servations that will give new insight into the mechanisms that govern the wind generation, acceleration, and propagation. Sim- ilarly, simultaneous measurements of the irradiance with a set of several spacecraft would allow us to infer what mechanisms induce variations in the solar constant, whether they are due to sunspot luminosity deficiencies compensated by equivalent increases on the hidden solar hemisphere or whether they are in phase over the whole surface and due to global variation of the solar volume. It should also be noted that a 360 network for ecliptic monitoring of flare events might become an indispensable element in any manned mission to another planet. A set of 4 1-AU spacecraft positioned at 90 in the eclip- tic plane and augmented by another one in a solar polar orbit should (see Figure 4.3) provide the necessary means to conduct

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40 these measurements. They should be equipped with coronagraphs, XUV and x-ray telescopes, particle detectors, magnetometers, and radiometers. The 1-AU spacecraft near Earth could be at Ll; the Space Station could service an Ll platform essentially as well as a geosynchronous one. Additional Solar and Helio~heric Studies There is now a considerable body of evidence to suggest that all scales of structure on the Sun, as well as other astrophysically interesting objects, are ultimately governed by small-scale pro- cesses associated with interrn~ttent magnetic fields and turbulent stresses. The understanding of the physics of the creation and decay of these dynamical structures is essential to a proper de- scription of large-scale structures (such as coronal active regions, flares, and the solar wind) and their effects on interplanetary space and the near-Earth environment. The interplay between processes occurring on vastly different spatial scales is ubiquitous in astrophysics. Whether in accretion disks feeding black holes at the center of active galaxies or quasars, in the magnetospheres of neutron stars, or in the x-ray coronae now known to surround a wide range of stars, small-scale magne- tohydrodynamic processes are thought to influence and sometimes control the behavior of the object. In these astrophysical situations, observations using even the most advanced technology currently conceivable will not allow us to directly observe the controlling small-scale processes. Using the Sun, however, we can indeed ~rnagine direct observations. The Sun is therefore a unique too! for advancing our understanding of a broad class of astrophysical phenomena, if we can penetrate to the domain of underlying processes that often operate on spatial scales of 1 to 100 km. An orderly progression of goals that could realize much of this promise would include the following: 1. Development of the successor to the Solar Optical Telescope and its integration into the Advanced Solar Observatory on the Space Station, along with the development of 0.1-arcsec ultraviolet and x-ray solar instruments on the Space Station; 2. Interferometric experiments in the ultraviolet and extreme ultraviolet, aimed at a preliminary reconnaissance of solar features at angular sizes much less than 0.1 arcsec.

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41 AT PERIHEUON,, av3s blVS, _~7~ , Anti tARTl1 Db~ - ~ _ _ _ _ _ 1 - - CROSS I~EPTUNES ORBIT IN -1.9 yr. :2 Yew Fit VOYAGER WRIER SWIRLY FIGURE 4.4 An interstellar probe using Jupiter gravity assist and solar swingby to escape the solar system. It would reach a distance of the orbit of Neptune in 1.9 years, in contrast to the 12 years required for Voyager 2 to reach the same distance. 3. Development of new I-m class facilities, utilizing the emerg- ing multilayer coating technologies, designed to obtain resolution in the O.Ol-arcsec regime at extreme ultraviolet or soft x-ray wave- lengths. 4. Improvement of the angular resolution of I-m cI - s tele- scopes by the use of multiaperture arrays to achieve baselines of order 10 m. further detain are contained in Appendix D. Interstellar Probe An Interstellar Probe, that could be launched about the year 2000, should reach beyond about 100 AU in a time interval of less than 10 years, preferably about 5 years. Several possible schemes, including Jupiter gravity assist and swingby of the Sun at 4 solar radii (see Figure 4.4) as well as use of megawatt nuclear electric propulsion, could provide the necessary acceleration for spacecraft velocities varying from about 50 to 100 km/s (11 to 21 AU/yr). The spacecraft should be instrumented redundantly with plasma, field, particle, and wave instruments, depending on detailed definition of science objectives and spacecraft capabilities. The fully instrumented spacecraft mass is likely to be in the range of 500 to 1000 kg. The objectives of the mission are to determine the characteris- tics of the heliopause, interstellar medium, low-energy cosmic rays excluded from the heliosphere, and global interplanetary gas and mass distribution of the solar system, and possibly, a much more

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42 precise determination of the stellar and galactic distance scale through parallax measurements of the distance to nearby stars. Some detailed plans for the science measurements were pro- vided in a NASA report An Interstellar Precursor Mission (3PL 77-70, October 1977~. A summary of the reco~nmendations con- tained in that report follows. Scientific Measurements Heliopause and Intersteliar Medium: Determination is needed of the characteristics of the solar wind just inside the heliopause, of the heliopause itself, of the accompanying shock (if one exists), and of the region between the heliopause and the shock. The location of the heliopause is not known; estunates now tend to center at about 100 AU from the Sun, but recent observations of low- frequency radio waves by the Voyager plasma wave investigation suggest even smaller distances. Key measurements to be made include magnetic field, plasma properties (density, velocity, temperature, composition, plasma waves), and electric field. Similar measurements, extending to low energy levels, are needed in the interstellar medium, together with measurements of the properties of the neutral gas (density, tem- perature, composition of atorn~c and molecular species, velocity) and of the interstellar dust (particle concentration, particle mass distribution, composition, velocity). The radiation temperature should also be measured. The magnetic, electric, and plasma measurements would re- quire only conventional instrumentation, but high sensitivity would be needed. In situ measurements of neutral gas composition might require development of a mass spectrometer with greater sensitiv- ity and signal-to-noise ratio than present instruments. Remote measurements of gas composition could be made by absorption spectroscopy, looking back toward the Sun. Of particular interest in the gas measurements are the ratios D/H, H/H2/H+, He/H, He3/He4; the contents of C, N. O. and if possible of I,i, Be, B; and the flow velocity. Dust within some size range could be observed remotely by changes In the continuum intensity. Cosmic Rays: Measurements should be made of low-energy cosmic rays, which the solar magnetic field excludes from the heliosphere. Properties to be measured include flux, spectrum,

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43 composition, and direction. Measurements should be made at energies below 10 MeV and perhaps down to 10 keV or lower. Conventional instrumentation should be satisfactory. Pluto: If a Pluto flyby is contemplated, measurements should include optical observations of the planet to deterrn~ne its diam- eter, surface and atmosphere features, and an optical search for and observations of any satellites or rings. Atmospheric density, temperature, and composition should be measured, along with charged particles and magnetic fields. Surface temperature and composition should also be observed. Suitable instruments include a TV camera, infrared radiometer, ultraviolet/visible spectrome- ter, particles and fields instruments, and an infrared spectrometer. For atmospheric properties, ultraviolet observations during solar occultation (especially for H and He) and radio observations of earth occultation should be useful. The mass of Pluto should be measured: radio tracking should provide this. TERRESTRL\[ MAGNETO SPHERIC PHYSICS Imaging of the Earth's Magnetosphere The In situ and remote sensing observations of the ISTP Pro- gram are designed to provide information that can be used to con- struct a global model of the Earth's magnetosphere, and follow-on programs must be designed to test these models. The concept of imaging the terrestrial magnetosphere is currently being investi- gated, and the results seem very promising (see Appendix A). It appears that from a platform as far away as the Moon (~4, L5, or a lunar base) or L1 (250 earth radu), ultraviolet emissions from He+ at 304 ~ will be sufficiently intense from the high-density, low-temperature plasma regions of the plasmasphere, the magne- tosphere, the geotai} plasma sheet (during storms), and even the region of the bow shock (with long integration tunes) so that im- ages could be constructed with reasonable resolution in time and space (approximately 100 km, approximately 10 min). Figure 4.5 shows a sketch of the magnetosphere, and it indicates the locations for the terrestrial unaging instruments. As noted on the figure, earth unaging and solar imaging can be carried out on low-altitude platforms (Space Station) as well as on these more distant bases. Within about 10 earth radii it is possible to "images the more

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44 L1 PLATFORM IN HALO ORBIT ~1 ( CONTINUOUS SOLAR IMAGING CONTINUOUS SOLAR WIND MEASUREMENTS CONTINUOUS IMAGING OF EARTH'S MAGNETOSPHERE ~ EARTH 1(~ \ - ~ \ LUNAR BASE EARTH MAGNETOSPHERE IMbGING Locations of instruments for imaging the terrestrial magneto- - SPACE STATION STO/ASO MAX RESOLUTION EUV, XUV SOLAR TELESCOPE FIGURE 4.5 ~ sphere: an L1 platform, a lunar base, and the Space Station. energetic plasmas (e.g., ring current, plasma cusp) by observing energetic neutrals produced by charge exchange. Plasmas et Interactions Cosmic plasmas are often "dusty," by which it is meant that they contain solid particles, some of which are very small dust grains. These are usually electrically charged, and if their charge- to-mass ratio is large enough their motion may be dominated by electromagnetic forces so that they can be considered to be one component of a Trusty plasma.n Charged dust In the solar system is known to be important for the origin of Jupiter's dust belt, for the fine structure of some Saturn's rings, and for the motion of both interplanetary and cometary dust. The population of dust grains in the Earth's environment appears to be rapidly increasing because of the injection of large amounts of exhaust in particulate form (mainly aluminum oxide) from solid fuel thrusters. Interstellar gas clouds contain large quantities of dust, which plays an important role in determining the physical properties of the clouds. Photoelectron emission from grains by ultraviolet radiation can be a heating mechanism for the gas; conversely, dark

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45 clouds can be cooled by radiation from grains. In addition, the electron and ion densities in a cloud can be depleted by surface recombination on grains. Thus the gas pressure in a dusty cloud can be affected by the dust, tenth implications for the evolutionary collapse of a cloud and subsequent star formation. The processes responsible for the production, growth, disper- sion, and destruction of dust grains are poorly understood. It is difficult to study grain growth and the processes of coagulation and fragmentation and their dependence on grain charge in labo- ratories on Earth because of the dominance of gravity. Controlled experiments in space, either on dust in a spacecraft plasma cham- ber or on dust ejected from a spacecraft into a plasma environ- ment, would greatly increase our understanding of these processes. A rendezvous mission of a spacecraft with a cometary dust cloud or planetary ring where the differential velocity of the spacecraft with the cloud was minimized would provide the opportunity for a detailed in situ investigation of both the dynamics and the physi- cal processes that occur in such environments. It would be useful to develop spacecraft-carried remote sensing techniques such as radar or lidar scattering to obtain information on a macroscopic scale. Several technology advances will be needed to study plasma- dust interactions. These include methods for manufacturing dust with the desired properties; methods for injecting dust clouds with the desired space, velocity, and charge distributions; methods for measuring the velocity, ma - , and charge distributions of dust particles; and methods for the remote sensing of dust clouds. Actne E"erim~te: Ga~Plasma Interactions Following the execution of gas releaser in the solar wind, magnetosheath, and distant magnetotai} by the Active Magne- tospheric Particle Tracer Explorers (AMPTE) program and in the near-Earth magnetosphere from Combined Release and Radiation Ejects Satellite (CARES) in the middle and late 1980s, it is ex- pected that studies of gas-plasma interactions will continue to expand in scope and complexity into the mid-199Os and beyond the year 2000. The objectives of this initiative are as follows: 1. To investigate the interaction between an artificially in- jected neutral gas and a cosmical plasma in the upstream solar wind, magnetosheath, plasma sheet, and auroral magnetosphere.

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46 2. To trace the flow of plasma from the solar wmd en c] the ionosphere through the entire magnetospheric system using plasma tracer techniques. 3. To modify the magnetosphere/plasma environment by means of plasma seedling in the equatorial magnetosphere and inducement of artificial auroras and magnetic substorms. The objectives outlined above can be readily addressed by using gas-carrying spacecraft for injection/modification of the plasma environment in various areas of the near-Earth system. Spacecraft already developed for the AMPTE and CRRES pro- grarns al could be utilized with appropriate propulsion systems to make releases at different locations. Also, spacecraft carrying appropriate diagnostic instrumentation (Explorer classy should be distributed in various orbits throughout the magnetosphere for observing the tracer elements and charting their flow through the system. The choice of appropriate tracer elements may make pos- sible remote sensing at very low levels of intensity, and thus provide an "image" of plasma motions in some regions. The techniques for gas releases in space are well developed both in the United States and abroad. Similarly, sensor instrumen- tation necessary for determining the results of releases ~ currently in existence and reasonably adequate. Potential development of remote sensing instrumentation (visible, ultraviolet) for imaging the tracer plasma would substantially enhance the utility of these techniques in the future. Ejections of Plasma Waves and Particle Beame All astrophysical plasma systems stellar coronae and winds, planetary magnetospheres, interstellar mediasupport plasma waves and nonthermal particle beam. The frequencies of the plasma waves range from the Alfven (hydromagnetic) regime (which In many solar system plasmas can have periods as low as a few milliseconds), to ion and electron cyclotron waves (a few hertz to many kilohertz in planetary magnetospheres), to synchrotron radiation (a few tens of megahertz Jupiter's magne- tosphere). Plasma waves are also produced in planetary mag- netospheres by collective effects, including electrostatic plasma oscillations, and by lightning in planetary atmospheres. Nonthermal particle populations can be produced by large- and small-scale electrostatic potentials and by energy conversion

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47 processes involving plasma instabilities. The propagation of such beanLs through an ambient plasma population can result in beam- plasma instabilities, which can radiate various plasma wave modes. The physical interpretation of the plasma waves and nonther- mal particle beamm measured by spacecraft in situ in solar shy tem plasmas requires thorough diagnostics of the existing plasma environment. interpretations of the electromagnetic plasma ra- diations measured from remote astrophysical sources rely heavily upon the physical understanding achieved from detailed studies of solar system plasmas. An entirely new method of gaining under- stand~ng of naturally occurring plasma waves and particle beams is through the injection of man-made waves and beams into the natural plasma media. Wave injection experiments into the magnetosphere have been conducted for some time from the ground. Recently, wave and beam injection experiments have begun using opportunities avail- able with the Space Transportation System. Extended STS oper- ations and, ultunately, the Space Station will provide the opportu- nities for more considered and extended Ejection experiments taking into account the different ambient plasma conditions pre- sensed by different levels of geomagnetic activity. Higher power leveb available from a Space Station for the injected beams and waves will permit study of highly nonlinear beam-plasma and wave-plasma systems, representative of a wide variety of natural systems. An electrodynam~c tether will be flown on a future STS mid sign. Again, a Space Station will provide the means for further and extended tether flights to study the injection of waves into an astrophysical plasma, waves with frequencies ranging from the Alfven regime to the very low frequencies regime. The results of such experiments will be of importance ~ their own right, from a basic plasma physics vantage point, and will also be of consid- erable relevance to astrophysical plasma systems containing large conducting, moving bodies, such as lo in Jupiter's magnetosphere. TE1tRESTRLAl ATMOSPHERIC PHYSICS Upper Atmosphere Science The Earth's mesosphere and lower thermosphere are the least explored regions of the Earth's atmosphere. They are influenced by

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48 varying solar extreme ultraviolet (EUV), ultraviolet (UV), and x- ray radiation, auroral particles and fields, and upward propagating waves and tides from the lower atmosphere. There are strong in- teractions between the chemistry, dynamics, and radiation of both the neutral and the ionized constituents of this region. It ~ known that the global structure of this region of the atmosphere can be perturbed during stratospheric warm~ngs and solar-terrestrial events (e.g., magnetospheric substorms, solar flares), but the over- aD structure and dynamic responses to these effects and even the basic controlling physical and chemical processes of these effects are not understood. A comprehensive multisatellite mission Is needed to measure the thermal, compositional, radiational, and dynamic structure of the mesosphere-ionosphere-thermosphere system. Remote probing of winds, temperature, and composition, combined with in situ measurements of thermospheric and ionospheric properties, will provide information on the dynamic processes, the depth of pene- tration into the atmosphere of solar effects, and the dissipation and transmission of waves and tides from the lower atmosphere. Two polar-orbiting spacecraft are needed to define interhem~spheric dif- ferences in the response to solar-terrestrial events, and an elliptic orbiter ~ needed to probe the lower thermosphere and define in- teractions with the magnetosphere through auroral imaging and other measurements. In order to be able to make in situ measure- ment at the lowest possible altitudes (about 120 km), the tether facilities of the Space Station also need to be taken advantage of. An extensive network of ground-based radar and optical in- terferometers and spectrometers wall determine time variations of atmospheric and ionospheric properties at given locations. Large numerical models of the general circulation, energetice, and chem- istry of the thermosphere, mesosphere, and ionosphere will be used in the analysis and interpretation of data. Space Station Atmospheric Studies The Space Station and the co-orbiting and polar-orbiting plat- form~ provide opportunities for extensive probing of the upper at- mosphere and ionosphere. High-resolution interferometers, spec- trometers, and radiometers can be used to determine detailed

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49 chemical, radiative, and dynamic properties of the middle and up- per atmosphere and ionosphere, and are part of the proposed in- strument complement of the Solar Terrestrial Observatory (STO) and the Earth Observation System (EOS). The tether provides an excellent opportunity to explore in situ the properties of the thermosphere and ionosphere from orbital altitude down to about 120 km, an altitude region that has been studied directly only interrn~ttently by sounding rockets. Global Current Minions In a laboratory plasma the electric and magnetic fields are usually generated by circuits external to the plasma. A space plasma differs from a laboratory plasma in that the magnetic and electric fields and currents in the space plasma are self-consistently determined by the Attribution of charged particles in the plasma. To understand the behavior of a space plasma, we must measure these fields and currents. Current systems in magnetospheric plasmas may be district uted over large volumes, concentrated in sheets, or confined to flux tubes. The ring current in the Earth's radiation belt is a volume current. Examples of sheet currents are found at the magnetopause and in the center of the geomagnetic tail. The phenomenon known as a flux transfer event is principally a line current. Similar current systems are seen in planetary magnetospheres and ionospheres, the solar corona, and astrophysical systems, but the most accessible of these regions ~ the terrestrial magnetosphere. Not only the distribution of current but also its temporal variation ~ important. Electric fields associated with the variation of electric currents and magnetic fields are responsible for the acceleration of particles to high energies. There ~ no way to measure these currents and fields remotely; in situ observation programs that probe the volume of interest are required. The existence of both large-scale and fine-scale current systems poses a programmatic challenge. A large-scale network ~ required with coarse resolution, and small clusters of probes with fine resolution that move In eccentric orbits through the lattice of the large-scale network are required. Possible configurations for these probes have been examined, and it is estimated that on the order of 300 probes are necessary for this investigation. These probes are identically configured to measure the magnetic and electric fields and electric

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50 current, except for a few that mclude selected charged particle measurements. This massive commonality of the systems used should enable the program to be undertaken for an acceptable cost. PLANETARY SCIENCE Mare's Aeronomy and Magnetosphere At present we have explored the solar wind interaction with each of the planets out to Saturn, with the single exception of Mars. Thus, the magnitude of any intrinsic martian magnetic field remains uncertain, and the nature of the solar wind inter- action is not known because the solar wind could be deflected by either a weak planetary magnetic field or the planetary ionosphere. Mass loading of the solar wind by the planetary ionosphere, the process responsible for the formation of cometary magnetotails, may or may not be ~rnportant. We do not know whether the plan- etary atmosphere is shielded from the solar wind or whether it interacts strongly with the solar wind. Hence, we do not know the sources and losses of the martian ionospheric plasma and upper atmosphere. While the Viking entry probes did carry retarding potential analyzers, those were insufficient to provide the global structure of the ionosphere and provided no data on the dynamics, energetics, or chemistry of the ionosphere and upper atmosphere. Thus, our understanding of the martian ionosphere lags far behind that of Venus. Because the state of magnetization of the martian iono- sphere is expected to be different from that of Venus, we cannot simply extrapolate Venus data to Mars. Currently approved missions do not address the problems of martian aeronomy and magnetospheric processes. The Mars Geo- science Climatology Observer is planned to carry a magnetometer. However, this one instrument is insufficient to address any of the outstanding problems of martian agronomy, except the existence of an intrinsic field. Further, the low-altitude circular polar orbit is inappropriate for addressing the majority of the questions out- . ~ _ ~ ~ ~ ~ ~' ~ ~ ''. ~ ~ ~ ~ - lmed above. The solutions require an elllptlca1 orbiter carrying a complement of ionospheric and magnetospheric instruments such as neutral, thermal ion, and suprathermal ion mass spectrome- ters, thermal and suprathermal electron detectors, and magnetic

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51 and electric field and wave detectors. There are no technological developments required to undertake this mission. It ~ a prime candidate for an early flight in the planetary observer series. Merc~'s Magnetosphere The dynamics of the terrestrial magnetosphere is strongly affected by field-aligned currents that close in the terrestrial iono- sphere. These currents transmit stress from the outer magneto- sphere to the ionosphere and thence to the neutral atmosphere. Mercury has no dynamically significant atmosphere or ionosphere and thus should respond very differently to the solar wind. In particular, ionosphere line-tying (which controls current systems in the Earth's magnetosphere) should be unimportant at Mercury and hence new classes of substorm mechanisms may develop there. We have few data on Mercury's magnetosphere. The existence of an intrinsic magnetic field was determined from two nighttime passes of Mariner 10. However, lack of knowledge of solar wind conditions during these passes limited the accuracy of the determi- nation of the intrinsic dipole moment. The magnitudes of higher order moments have been determined even less accurately. The na- ture of the plasma circulation in Mercury's magnetosphere and its variation due to changes In solar wind conditions remains unstud- ied. Although energetic particle transients were found, the spectra were not adequately measured. Comparative studies of Mercury's magnetosphere are crucial to understanding the role of the terres- trial ionosphere in magnetospheric processes and to bridging the gap between the solar wind interaction with the Moon, which has weak, localizes] magnetization and has no atmosphere, and with the Earth, which has a strong magnetic field. There are no currently approved missions to Mercury. A study of the intrinsic magnetic field of Mercury requires a low-periapsis polar orbiter and some knowledge of the strength of the solar wind. This could be provided by a single elliptical orbiter or multiple spacecraft. The plasma, energetic particle, wave, and magnetic field mea- surements needed on this mission could be carried on a rather modest spacecraft. However, other discipline objectives could be accommodated on a large spacecraft; the trajectories permitting

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s2 larger payloads should be determined. Further, studies of ther- mal input to the spacecraft as a function of orbital characteristics should be made. One addition to the Mercury orbiter concept that may be espe- cially valuable concerns relativity. The objectives are to determine the precession of perihelion of the planet with much unproved accu- racy; to determine whether G is constant with 10~~3/yr accuracy; to measure 32 for the Sun directly; to improve measurement of the relativistic time delay and check for preferred frame effects; to improve knowledge of the gravity field of Mercury. These could be accomplished with a small transponder satellite in an orbit with a mean altitude of about 2500 km and eccentricity of less than 0.25. Two-frequency tracking from the Earth could deterrn~ne the center-omass distance and provide unproved information on the gravity fielcI. The currently planned DSN Doppler tracking accu- racy of ~ x 10-~5 and unproved ranging accuracy of 10 cm are required. Tracking data are needed for about 24 hours per week over a 2- to Year mission lifetime. Jupiter's lonosphere/Magneto~here and Mail The dynamics of the Jovian magnetosphere and its ionospheric interactions are different from that of Earth because plasma from an internal source, the satellite to, is known to dominate much of the behavior of the outer magnetosphere of Jupiter. The so- lar wind, on the other kand, is known to modulate intensities of some long-wavelengtk radio emissions. However, it is not known whether the Jovian aurora is associated with the To plasma torus alone or with the Jovian cusp region, two logical possibilities that involve different energy transfer mechanisms. Practically no infor- mation is available on the structure of the Jovian ionosphere and its interaction with the magnetosphere; the four occultation mea- surements of electron concentration, necessarily made at differing latitudes and Jovian magnetic longitudes, are not mutually self- consistent. Furthermore, no direct information is available on the chemistry, energetics, and dynamics of the ionosphere and upper atmosphere. Meaningful advances in understanding the chemical and physical processes in a hydrogen/hydrocarbon-dominated up- per atmosphere have to await such measurements. Finally, the magnetospheric tail, knowledge of whose dynamical behavior Is important to understanding how the magnetosphere works, has

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53 not been explored at Al on the dusk side, nor wall it be explored during the Galileo prime mission. Without knowing how the tad] behaves, particularly in the pre-midnight sector, we will not be able to understand how the magnetosphere operates as a system. Without knowing the structure of the ionosphere as a function of latitude and magnetic longitude, we will not be able to understand either the ionosphere or how it couples to the magnetosphere. In order to address these questions, the task group suggests a Planetary Observer-cIass vehicle in an elliptical polar orbit about Jupiter, with a perijove of 500 km above the 1-mbar level, an apojove of 8 R3 from center of Jupiter, and a period of 28 hours. For such an orbit, the major axis processes approximately 1/day and thus alternately ~ in a position either to view one of the poles or to pass through the To torus. Measurement objectives include Unaging of the aurora and the torus, determination of the plasma composition and density, and observations of energetic particles, plasma waves, radio emissions, and field configuration, as well as several ionosphere parameters. Further details are con- tained in Appendix B. SUMMARY OF TECHNOLOGY DEVELOPMENT NEEDS The technology development needs identified by this task group can be summarized as follows: . Low-thrust propulsion (Solar Probe, Solar Polar Orbiter, Heliosynchronous Spacecraft, possibly Interstellar Probe) 4-solar radii heat shield (Solar Probe, Interstellar Probe) Perihelion thruster" (Solar Probe, Interstellar Probe) . Magnetospheric imaging techniques (~! Platform. Snace Station, Lunar Base) High-leve} radiation-resistant components (Jupiter Polar Orbiter) . Lidar System for active probing of the atmosphere (Space Station) . Ultra-Iow-cost current-measuring spacecraft (Terrestrial Magnetosphere) . Active plasma physics experunents (interactions of plasmas with beams, waves, gases, and dust) (Space Station) . High-reflectivity multilayer coating for EUV and XUV (Space Station) . Enhanced dust impact protection (Jupiter Polar Orbiter) . _ ~ _ _ _ ~ ~ ~ ~