Solar and Space Physics: Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988)

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2 Scientific Objectives i: The following summary of the scientific objectives of solar and space physics is taken from the NRC report An Implementation Plan for Priorities in Solar and Space Physics (1985), which is adapted from the Kennel report (Solar-System Space Physics in the 1980~: A Research Strategy, 1980) with appropriate changes and updates. SOLAR PHYSICS Major advances in our understanding of the Sun were made .n the 1970s and the early 1980s (see Figure 2.la). Most, if not all, of the magnetic flux that emerges from the convective zone is subsequently compressed into small regions of strong (1200 to 2000 G) field, a fact that is still not understood theoretically. Obser- vations confirmed earlier predictions that the 5-min photospheric oscillation, discovered in the early 1960s, is a global phenomenon. This discovery has made ~helioseismology" possible, by which the depth of the convective zone and the rotation below the photo- sphere have been inferred. In addition, by ruling out the classical model of coronal heating by acoustic waves, observations from the ground and from OSO-8 raised anew the question of what main- tains the corona's high temperature. Coronal holes were among 7

8 the major discoveries of the 1970s. White-light, extreme ultravim let, and x-ray observations suggested how coronal holes are related to the convective zone and to the solar wind. Skylab observations unambiguously identified magnetic arches as the basic structure of coronal flares. This perception altered our theoretical picture of solar flares and clarified the need for a coordinated multiin- strument attack, which was initiated with the Solar Maximum Mission. To better understand all the processes linking the solar interior to the corona, we need to study (see Figure 2.Ib) the following: ~ The Sun's global circulation how it reflects interior dy- namics, is linked to luminosity modifications, and is related to the solar cycle. ~ The interactions of solar plasma with strong magnetic fields—active regions, sunspots, and fine-scale magnetic knots- and how solar flare energy is released to the heliosphere. ~ The energy sources of the solar atmosphere and corona and the physics of the Sun's large-scale weak magnetic field. PHYSICS OF THE HE[IOSPHERE The Sun is the only stellar exosphere where complex phe- nomena common to aD stars can be studied in situ. Observations of the Sun and the heliosphere (the plasma envelope of the Sun extending from the corona to the interstellar medium) provide the bash for interpreting a variety of phenomena ranging from x-ray and gamma-ray radiation to cyclical activity and long-term evolution. The Sun, together with the heliosphere and planetary magnetospheres and atmospheres, makes up an immense labora- tory that exhibits complex magnetohydrodynam~cal and plasma physical phenomena whose study enhances our understanding of basic physical laws as well as our understanding of the influences of the Sun on our terrestrial environment.: The solar wind has been studied near the Earth since 1961. At the present time, measurements of the solar wind have primarily been extended to within Mercury's orbit (0.3 AU) and past Pluto's orbit but have been confined to near the ecliptic plane. Quantita- tive models of high-speed solar wind streams and flare-produced shocks have been developed and tested against data obtained near the ecliptic. The realization that high-speed streams originate in

9 A SO LAR PtlYSICS--R ECENT ACCOh/lPLISHMENTS IMPORTANCE OF STRONG MAGN ETI C F I E LDS ~ ELEMENTAL AND ISOTOPIC ~ / ALES DlFFERENTiATION /~ 1C _~< CORONAL HEATING At-_ ~ in_ / I D E NTI F ICAT 10 N OF ~ H E Ll OSE I SMOLOG Y SO LAR F LA R E G EOM ET R Y B SOLAR PHYSICS—OBJECTIVES How Do Global Circulation and Surface Oscillations Reflect Interior Dynamics? What Are the Corona's Energy Sources? - ~ ~q:_ ~ - How Does Solar Plasma / Interact with Strong Magnetic Fields7 How Is Solar Flare Energy Released ? C SOLAR PHYSICS—R ECU I R ED MEASU R EMENTS How Is Solar Wind Generated? - What Is the Physics of the La rge Scale Weak Magnetic Field? / V - 2. IN SITU MEASUREMENTS OF CORONAL PLASMA PROCESSES NEAR SOLAR / A) \ WIND CRITICAL SURFACE . HIGH RESOLUTION (0.1") SH UTTL E OBSE RVAT IONS OF ACTIVE REGIONS, SM A L L SCAL E F I E LDS AN D F LAR ES 3. SURVEY OBSERVATIONS OF SURFACE OSCILLATIONS, LARGE SCALE MAGNETIC F I E L D, LU M I NOSITY VAR I AT 10 N FIGURE 2.1 Solar physics: status, objectives, and recommendations. In this series of sketches of the Sun and its coronal magnetic field some recent accomplishments in solar physics are illustrated. (A) Questions that can be fruitfully attacked in the 1980s and l990s. (B) The principal research programs needed to answer these questions. (C) The Sun and solar corona within 5 solar radii. The influence of the processes occurring within this region extends throughout interplanetary space via the solar wind.

10 the rapidly diverging magnetic-flux tubes of coronal holes has re- oriented much solar wind research. The sector structure of the solar wind has been unambiguously related to a magnetic neu- tral sheet of solar system scale that connects to the large-scale magnetic field of the rotating Sun. Finally, microscopic plasma processes have been shown to regulate solar wind thermal conduc- tion and diffusion and, possibly, local acceleration of particles in solar wmd structures. To understand better the transport of energy, momentum, energetic particles, plasma, and magnetic field through interplan- etary space, we need to study the following: ~ First and foremost, the solar processes that govern the generation, structure, and variability of the solar wind. The three-dimensional properties of the solar wind and heliosphere. ~ The plasma processes that regulate solar wind transport and accelerate energetic particles throughout the heliosphere. MAGNETOSPHERIC PHYSICS New processes regulating Earth's magnetic interactions with the solar wind were Recovered in the 1970s and early 1980s (see Figure 2.2a). For example, unsteady plasma flows that apparently originate deep in the geomagnetic tad! en c] deposit their energy in the inner magnetosphere and polar atmosphere were observed. Observations of impulsive energetic particle acceleration suggested that the cros~tai! electric field is also highly unsteady. The dis- covery of energetic ionospheric ions in the near-tai! and inner magnetosphere forced a reevaluation of our ideas concerning the origin and circulation of magnetospheric plasma. The 1982-1983 ISE~3 reconnaissance of the geotai} out to 150 Re provided sig- nificant first-order information on tad! dynamics during quiescent and substorm conditions. Our understanding of many individual processes became more quantitative. The coupling of magnetospheric motions and energy fluxes to the thermosphere was observed and modeled. Currents flowing along the Earth's magnetic field and connecting the polar ionosphere to the magnetosphere were found to create strong lo- calized electric fields at high altitudes. These fields may accelerate the electrons responsible for intense terrestrial radio bursts and

11 A MAGNETOSPHERIC PHYSICS—RECENT ACCOMPLISHMENTS ELECTR IC Fl ELDS ACCELERATING AURORAL PARTICLES DISCOVERY OF ENERGETIC / IONS OF IONOSPHERIC ORIGIN /~ IN MAGNETOSPHERE. '/ ~> \N ~ / magnetopause \\/~= ~ MOC)ELLING OF THERMOSPHERIC ~ WINE) GENERATION / ~ X> shocked solar wind UNSTEADY PLASMA FLOWS, So// ELECTR IC F I E LDS AND '$~°n/ess ~ PARTICLE ACCELERATION IN °iVs~Ock GEOMAGNETIC TAIL E] MAGNETOSPHERIC PHYSICS—OBJECTIVES What is Origin and Fate of Magnetospheric Plasmas? How Does Solar Wind Couple to Magnetosphere? magne~opause - How Does Magnetosphere c /' Couple with Atmosphere Sail _ and ionosphere? °~s,,Ock C SIX CRITICAL REGIONS OF MAGNETOSPHERIC PHYSICS AURORAL FIELD An' ON THE LINES An' GROUND /, ~ ~ magnetopause How is Energy Stored and Released in Magnetic Tail? DEEP IN SOLAR 'I=---- GEOMAGNETIC W I ND (,~ ~ ~ TA t Is / ~ ~ ~ shocked solar wind Ml DMAGNETOSPHE R E \ cO// ~ ECU ATO R I A L P LA N E \ °~/e`S O ~ IN THE POLAR Elk UPPER ATMOSPHERE FIGURE 2.2 MagDetospheric physics: status, objectives, and recommenda- tions. Shown here is the Earth's magnetosphere—the cavity formed by the interaction of the solar wind with the Earth's magnetic field. A collisionless bow shock stands upstream of the magnetopause, the boundary separating shocked solar wind from the magnetosphere proper. The Moon is 60 earth radii from the Earth; the Earth's magnetic tail is thought to extend some thousand earth radii downstream. (A) Some recent achievements in mag- netospheric physics. (B) Objectives that can motivate research programs in the 1980s and 1990~. (C) The six critical regions where simultaneous studies are needed to help construct a global picture of magnetospheric dynamics.

12 aurora] arcs. Thus, the problem of auroral particle acceleration is nearing quantitative understanding. By contrast, the relationship between energy circulating in the magnetosphere, the energy dis- sipated in the atmosphere, and the concurrent state of the solar wind has not been unambiguously quantified even today. To understand better the tune-dependent interaction between the solar wind and Earth, we need to study (see Figure 2.2b) the following: The transport of energy, momentum, plasma, and magnetic and electric fields across the magnetopause, through the magneto- sphere and ionosphere, and into or out of the upper atmosphere. The storage and release of energy in the Earth's magnetic tail. The origin and fate of the plasmats) within the magneto- sphere. ~ How the Earth's magnetosphere, ionosphere, and atmo- sphere interact. UPPER ATMOSPHERIC PHYSICS The upper atmosphere has traditionally been divided into the stratosphere, mesosphere, thermosphere (and ionosphere), and exosphere, in order of increasing altitude. Recent research makes it clear that these layers and their chemistry, dynamics, and transport are coupled (see Figure 2.3a). For example, downward transport from the thermosphere can provide a source of nitrogen compounds to the mesosphere and possibly to the upper stratm sphere. The catalytic reactions of odd hydrogen, nitrogen, and chlorine compounds destroy ozone, thereby altering the absorb tion of solar ultraviolet radiation. Results from three Atmospheric Explorers, which largely quantified the photochemistry of the ther- mosphere and ionosphere, also illustrate the strength of the elec- trodynam~c coupling of the thermosphere to the magnetosphere. Finally, understanding how the upper "d lower atmospheres af- fect each other will be necessary to complete the description of the chain of solar-terrestrial interactions. This will require consider- able improvement in our understanding of the chemistry, dynam- ics, and radiation balance of the mesosphere and stratosphere, as well as of troposphere-stratosphere exchange processes. To understand better the entire upper atmosphere as one

13 A UPPER ATMOSPHERIC PHYSICS—RECENT ACCOMPLISHMENTS IMPORTANCE OF ELECTRODYNAM IC COUPLI NG GENERAL REALIZATION OF IMPORTANT COUPLINGS BETWEEN LAYERS AND THEIR CHEMISTRY, DYNAMICS AND TRANSPORT B UPPER ATMOSPHERIC PHYSICS—OBJECTIVES How Do Energetics, Chemistry, and Dynamics Interact to Establish the Structure and Variability of the Middle Atmosohere? " C sax - . USA` AL ATMOSPHERE EXPLORERS , ~ , ~ QUANTIFI~DTHERMOSPHERE A-o~a PHOTOCHEMISTRY ABOVE ~ 130 km _: ~ How Do Variable Photon and Particle Fluxes Affect the Thermosphere, Mesosphere, and Stratosphere? So. `*'a~ U~n~.c tl - 'C - S ~ ~ ~ ·~< '~ ~ ~ . ~/~ —~ Off; UPPER ATMOSPHERIC PHYSICS—REQUIRED MEASUREMENTS A SERIES OF SPACE OBSERVATIONS mesosphere and stratosphere SELECTED HIGH RESOLUTION OBSERVATIONS FROM FREE FLYERS, PLATFORMS, AND sHIJTTl F What Are the World-Wide Effects of the Magnetosphere's Interaction with the Upper Atmosphere? MONITOR SOLAR LUMINOSITY < as AND SPECTRAL IRRADIANCE, Ados !` LONG-WAVP RAD IAT ION, CH EM ICAL COMPOSITION, DYNAMICS, AND FNFR~.FTIC PARTICLE INPUT /~ := ~ `~.~ ^ FIGURE 2.3 Upper atmospheric physics: status, objectives, and recommen- dations. Sketched in these figures are the layers into which the atmosphere has traditionally been divided. Our studies of these layers, and the in- teracting processes occurring within them, are becoming more integrated. Solar ultraviolet photons deposit their energy largely in the stratosphere and above. The magnetosphere interacts with the upper atmosphere both through energetic plasma deposition and through electric fields, which are generated by magnetospheric motions. Plasma heating and electric fields both couple to upper atmosphere wince.

14 dynamic, radiating, and chemically active fluid, we should study (see Figure 2.3b) the following: The radiant energy balance, chemistry, and dynamics of the mesosphere and stratosphere and their interactions with at- mospheric layers above and below. . The worldwide effects of the magnetosphere's interaction with the polar thermosphere and mesosphere and the role of elec- tric fields in the Earth's atmosphere and space environment. ~ The effects of variable photon and energetic particle fluxes on the thermosphere and on chemically active minor constituents of the mesosphere and stratosphere. SO[AR-TERRESTRIAl COUPLING Solar-terrestria] coupling is concerned with the interaction of the Sun, the solar wind, and the Earth's magnetosphere, ions sphere, and atmosphere, with particular emphasis on the response of the system to solar variability. For example, a solar flare pros duces both a strong solar wind shock that initiates a magnetic storm when it passes over the magnetosphere and energetic pro- tons that penetrate deep into the polar atmosphere. Studies of such solar-terrestrial phenomena are of considerable practical im- portance. To understand better the effects of the solar cycle, solar ac- tivity, and solar wind disturbances upon Earth, we need to do the following: models of these processes. ~ Provide, to the extent possible, simultaneous measure- ments on many links in the chain of interactions coupling solar perturbations to their terrestrial response. Create and test increasingly comprehensive quantitative Whereas 10 years ago it was generally believed that signifi- cant effects of solar variability penetrate only as far as the upper atmosphere, some scientists now believe that they also reach the lower atmosphere and so affect weather and climate in ways not yet completely understood. For example, it has recently been sug- gested that the mean annual temperature in the north temperate zone followed long-term variations of solar activity over the past 70 centuries.

15 To clarify the possible solar-terrestrial influence on Earth's weather and climate, we need to do the following: ; · Determine if variations in solar luminosity and spectral irradiance sufficient to modify weather''and cInnate exist, and understand the solar physics that controb these variations. . Ascertain whether any processes involving solar and mag- netospheric variability can cause measurable changes in the Earth's lower atmosphere. . Strengthen correlation studies of solar-terrestrial, climato- logical, and meteorological data. COMPARATIVE PLANETARY STUDIES Comparative studies of the interaction of the solar wind with planets and comets highlight the physics pertinent to each and put solar-terrestrial interactions in a broader scientific context. The solar system has a variety of magnetospheres sufficient to make their comparative study fruitful. Because the planets and their satellites have different masses, magnetic fields, rotation peri- ods, surface properties, and atmospheric chemistry, dynamics, and transport, comparative atmospheric and magnetospheric studies can help us to understand these processes in general and possibly to identify terrestrial processes that might otherwise be missed. In the 1970s, Pioneer and Voyager spacecraft made flyby stud- ies of Jupiter's atmosphere and magnetosphere, the largest and most energetic in the solar system. Pioneer 11 and the Voyagers encountered Saturn in 1979, 1980, and 1981. Mariner 10 flybys discovered an unexpected, highly active magnetosphere at Mer- cury. Pioneer Venus results suggest that the strong interaction between the solar wind and the upper atmosphere of Venus plays a significant role In the evolution of the atmosphere. To understand better the interactions of the solar wind with solar system bodies other than Earth, and from their diversity to learn about astrophysical magnetospheres In general, we need to do the following: . Investigate in situ Mars's solar-wind interaction in order to fill an important gap in comparative magnetospheric studies; previous missions provided little such information. · Make the first in situ measurements of the plasma, mag- netic fields, and neutral gases near a comet.

16 ~ Secrete our underst~dlog of rapidly rotating magneto spheres Evolving strong stmospberic ~d sateDite interactions. ~ Determine the role of at~spberes in substor~ Id other magnetospber~ processes by orbits studies ~ ~e~ury-tbe only known m~ne~zed planet without ~ dyn~caDy d~c~t Tam sphere.