Ground-Based Solar Research: An Assessment and Strategy for the Future (1998)

The million-degree solar corona is the basis for the solar wind and the Sun's X-ray emission. Two decades ago space observations ruled out what was then the established explanation for coronal heating: that it was a consequence of acoustic waves generated in the photospheric convection and propagating up into the corona where they were damped by viscosity, thermal conductivity, and shock formation. Newer theoretical calculations indicated that the waves would damp before reaching the corona, and space observations of the widths and shifts of ultraviolet (UV) emission lines showed conclusively that there is insufficient energy in acoustic waves in the frequency range to explain coronal heating. X-ray observations from space have shown that coronal heating is closely related to magnetic structure in the corona. In particular, heating in the weak diverging magnetic fields of coronal holes is inferred to be a consequence of microflaring within and between the small photospheric bipoles. This is a different process from the more intense heating in the strong bipolar magnetic fields of the X-ray-emitting active corona.

The basic cause of the X-ray emission from coronal loops is the heat input and the magnetic confinement that allow the gas density to reach sufficient levels for effective emission. The heating is inferred to be a consequence of small-scale flaring deep within the bipolar magnetic fields of the active regions. In both circumstances the scientific investigation leads to the microstructure of the fields and fluids, together with the need for infrared (IR) observations to study the magnetic fields that extend from the photosphere up into the corona.

The mechanism of the solar cycle is another challenge, not only for solar physics, but for the whole of astrophysics, with most stars showing cyclic magnetic activity. There is even speculation that the magnetic fields in quasar accretion disks are generated by a dynamo process similar to the dynamo process believed to operate in the Sun. There is at this time an uneasy consensus that the solar cycle is a consequence of an oscillatory dynamo process, along the general lines of the αω dynamo.

The uneasiness arises from the inability to construct so far an internally consistent theoretical dynamo model that also fits the general facts of the observed 11- and 22-year solar cycle. One of the outstanding difficulties is the lack of a theoretical justification of the customarily assumed turbulent diffusion of strong vector magnetic fields that is so essential for getting the right scale and period of the magnetic fields produced by the dynamo. There is plenty of work for theoreticians here to search for alternative diffusion mechanisms or alternatives to the dynamo altogether. Relevant observational information will come from ground-based magnetograms of the solar surface, both

ground and space high-resolution studies of helioseismology, and space observations of the global magnetic activity of the Sun.

The Yohkoh spacecraft has provided a dramatic demonstration that the solar magnetic cyle affects not only the low-latitude sunspot zone, but the entire Sun. Comparison of the full-disk Yohkoh X-ray images from 1991 to 1998 shows that the entire corona, from the equator to the poles, waxes and wanes with the solar cycle. Interpretation of this important fact requires extensive ground-based follow-up. We need to know how the magnetic field and the velocity fields at the surface of the Sun and beneath the surface (insofar as helioseismology can infer them) vary with the solar cycle. This includes the torsional oscillations discovered nearly two decades ago and closely related to the solar cycle. A combination of synoptic data and high-resolution exploratory studies may turn up further clues to the nature of the solar dynamo, or perhaps upset the present picture altogether. Whatever the result, the solar studies will profoundly affect other areas of astrophysics.

The acceleration of electrons and ions to high energies in the magnetic activity of the Sun, in blast waves in interplanetary space, and in the terrestrial magnetosphere when impacted by a solar blast wave, is all part of the general solar outreach. The high efficiency of the particle acceleration is such that it is estimated that 10 to 50% of the total flare energy appears as fast particles in some cases. The isotopic abundances of the escaping fast particles can be directly determined with state-of-the-art spaceborne instruments, providing information of the matter density at the acceleration sites. The elemental abundances and total particle intensities define the hazards to humans and electronics in space.

The universal presence of fast particles, all the way to extreme relativistic energies, in active astronomical objects and throughout interstellar space illustrates the importance of the acceleration physics. The nuclear interactions initiated by fast ions in the solar chromosphere and corona provide otherwise rare isotopes, and it has been speculated that this may be a major source of the anomalous surface abundances in some active stars. Ground-based neutron monitors indirectly detect the larger outbursts of energetic solar particles by the prompt arrival of the neutrons along straight-line paths, while the energetic nuclei come along the spiral interplanetary magnetic field. The relative number of neutrons and the various exotic nuclear isotopes provides a direct measure of the density of matter at the acceleration site.

Helioseismology is the tool for exploring the basic hydrostatic structure and internal rotation of the Sun. In particular, it has established that the standard solar model is basically correct and has provided the curious internal rotation profile shown in the illustration on the front cover of this report.

Using a recently developed local time-distance analysis called helioseismic tomography (which compares the phases of surface oscillations at many different pairs of points), helioseismology is also becoming the means for exploring localized fluid motions, and perhaps some of the magnetic fields, beneath the surface of the Sun.¹ Long-term detailed helioseismic monitoring is essential for tracking the interior solar variability, while precision long-term records of the magnetic fields and fluid streaming at the surface are essential for understanding the outward expression of these changes.

Solar physics has been confronted for two decades by the challenge of understanding measurements of the solar neutrino flux. The measurements at the Homestake site have yielded a flux of electron neutrinos that is about one-third of the best theoretical estimates of the electron neutrinos expected from the standard model of the solar interior. More recent measurements by the SAGE, GALLEX, and Kamiokande experiments lead to discrepancies that are similar but not exactly the same. It appears now that the neutrinos from ⁷ Be are missing entirely.

Careful theoretical analysis makes it clear that no readjustment of the standard solar model can explain the neutrino discrepancy. The ground-based helioseismology observations and the MDI instrument on the SOHO spacecraft have defined the standard solar model within very close limits, forcing the conclusion that the discrepancy in the calculated neutrino emission lies in an incomplete understanding of the neutrino physics. The conjecture is that the electron neutrinos emitted by the thermonuclear reactions in the core of the Sun, and carrying away about 2% of the total thermonuclear energy, are lost through spontaneous or resonant neutrino oscillations during their outward passage through the Sun, converting to mu and tau neutrinos, to which none of the existing neutrino detectors is sensitive. The recent determination of a nonzero neutrino rest mass by the Super Kamiokande detector gives strong support to this hypothesis, as already noted.

The next generation of neutrino detectors, including Super Kamiokande, Sudbury Neutrino Observatory, and the Borexino experiment presently coming online, should go a long way toward clearing up the puzzle. However, further ground-based observations are required. There is the far-out theoretical possibility that megagauss magnetic fields in the solar interior play a role in exciting the neutrino oscillations. Then there is the statistical evidence that the solar neutrino flux at Earth varies with time, possibly in connection with the solar cycle and with heliocentric latitude. If correct, this is an important clue to the puzzle, and it will require helioseismic and perhaps other observations over a complete solar cycle to follow it up.

This long-term challenge can be handled only from the ground. Depending on what we learn from the ground-based neutrino detectors and helioseismic observations, it may be possible in the next decade to learn a lot more about the physics of neutrinos. If this occurs, solar physics will once again have led to an advance in a field far removed from solar research itself. There is the possibility, too, that the stellar lithium problem,

involving the enormously suppressed lithium abundance in the Sun and some other stars (while beryllium and boron are not so suppressed) may be illuminated by what is learned of the solar interior in the process of investigating the neutrino problem.

The Sun is best described as a central core more than 10 times brighter than a supernova and as enclosed within an opaque shroud that reflects back the radiation from the core. Most important for the active character of the Sun is that the opacity of the shroud is so great that the tiny heat leak (one part in 2 × 10¹¹ ) from the supernova-bright core escapes by causing the outer 2 × 10⁵ km (solar radius = 7 × 10⁵ km) to overturn so as to transport the heat by convection, mixing the hot gas from below up toward the surface.

The hot electrically conducting gas carries the magnetic field along with it in the convective streaming and mixing. The nonuniform rotation and the convection (which is cyclonic because of the rotation of the Sun) evidently work together to produce the cyclic magnetic field of the Sun. It is here that the process becomes baffling to understand, indicating that we still have much to learn about hydrodynamics and magnetohydrodynamic dynamo effects. For example, it has not yet been possible to simulate the internal rotation profile of the Sun by starting with the basic principles of hydrodynamic convection. The principal difficulty seems to be the extreme density stratification of the convective zone (the density is 10⁶ times larger at the base than at the top) followed by the strong turbulence (Reynolds numbers of 10⁸), neither of which can be handled by existing computers. In addition, there is the problem presented by the evident “turbulent” diffusion of the magnetic field, described in the section above titled “The Magnetic Cycle.”

As already noted, the magnetic field does not appear at the visible surface in a continuum form but, instead, in an intensely fibril state with almost all of the individual magnetic flux bundles of small diameter (~100 km) below the limit of resolution of existing ground-based telescopes.² The fibril state of the field seems to contradict the notion of turbulent diffusion of magnetic field that plays an essential role in present-day theoretical models of the solar dynamo. Indeed, one wonders if the fibril structure of the field may be the basis for the rapid diffusion and easy dissipation of fields required for the successful operation of a solar dynamo.

The global scientific challenge is to determine precisely how the convection creates the quasi-periodic magnetic fields and why these fields are broken up into the remarkable concentrated fibril form that appears at the visible surface. We also need to know how those fibrils behave as they rise to the surface and emerge into view where their active dissipation produces the variety of suprathermal phenomena of solar activity. The photosphere, the patchy chromosphere, the transition layer, the corona, and the clumps of magnetic field all are involved, and this creates a very complex problem in understanding the three-dimensional radiative transfer, with the transport and dissipation of convective-driven wave energy evidently playing a leading role in faculae and plages.

The very high observational resolution of 0.1″ or better is required for scientific investigation.

Over time, the dynamical state of the convection evidently changes at least slightly because the total brightness of the Sun changes by small amounts as the magnetic activity sometimes fades away for several decades, or rises to heights well above anything observed since the telescope was invented nearly four centuries ago. One assumes that the variations in the general level of activity over decades and centuries are associated with the long-term trends in the state of the convection, meridional circulation, and nonuniform rotation within the Sun; hence, the need to sustain helioseismology over extended periods. In addition, informed decisions on global change issues require a reliable knowledge of solar variability, such as the general trend toward higher mean activity levels and the presumed associated solar brightening throughout the 20th century.

At the visible surface the typical magnetic fibril has a diameter of the order of 100 km and a magnetic field of 1 to 2 kilogauss. The large-scale mean field of 10 to 500 gauss is determined largely by the separation of the individual fibrils. Only very recently, taking advantage of the best observing sites with the best telescopes, has it been possible to detect (but not resolve) the individual fibrils and follow them briefly to see their continual motion and exchange of magnetic flux. The same observational advances show that the surface of the Sun is densely sprinkled with small magnetic bipoles, representing O and Ω-loops of magnetic flux bundles bulging upward through the visible surface. These small bipoles appear to interact vigorously where they meet, producing microflaring on scales that are not resolved in the telescope.

Microflares are detected down to the instrumental threshold of about 10²⁴ ergs, appearing as small localized fluctuations in brightness and sometimes as brief X-ray bursts in space. The microflares need quantitative study to determine their intrinsic properties and to establish whether they are a major player in coronal heating. Improving spatial resolution to 0.1″ or better, combined with high-dispersion spectroscopy and rapid cadence to match the transient weak microflaring and small-scale flux motions, requires the photon-gathering ability of a 3- to 4-meter aperture. Without such an aperture, these small events and the subtleties of the larger events cannot be studied decisively, nor is the spatial resolution in the IR adequate to resolve any of the smaller events.

The traditional flares, arising in the large magnetic bipoles that define the active regions on the Sun, are evidently a consequence of rapid reconnection of magnetic fields on a large scale, although the reconnection itself is confined to thin surfaces and filaments. The total energies can be enormous, up to 10³² ergs in a single outburst and 10³³ to 10³⁴ ergs in an occasional prolonged flare event. The suprathermal effects associated with the magnetic energy release include plasma heated to 10⁷ to 10⁸ K,

intense X-ray emission, mobile populations of 1 to 10⁴ kilo-electron volt (keV) electrons and ions, and relativistic protons of up to 30 giga-electron volts (GeV). Nuclear reactions in and around the flare site produce gamma rays and also neutrons, recognized by their prompt arrival at Earth, while the fast protons come by a longer path along the spiral interplanetary magnetic field.

Many distinct plasma effects, as well as intense electric fields, are associated with magnetic reconnection. Radio observations of the flare, combined with visible, UV, X-rays, gamma rays, and neutrons, are increasingly able to define the heterogeneous and transient conditions through the life of a flare, requiring coordination of ground-based and space observations. Radio observations are now being contemplated with frequency coverage and agility sufficient to accomplish detailed three-dimensional mapping of flares.³ There is reason to expect microstructure in flares, with the hope of studying it with high resolution in both visible and IR.

The coronal mass ejection is a result of the forces in a strongly deformed magnetic field, but much remains to be learned before the phenomenon is fully understood. As already mentioned, the million-degree temperature in the open magnetic regions provides the high-speed solar wind, and the corona appears to be heated by the dissipation of short-period waves in the open magnetic regions, presumably from the magnetic and acoustic “noise” of the microflaring of the small bipoles. However, so far the idea is based more on the observational exclusion of other possibilities than on direct quantitative measures of the microflaring. The slower, denser solar wind appears to come from the periphery of active regions, under circumstances that are not yet clearly defined. That is to say, the Sun has a complex supersonic wind and mass loss because of ill-understood coronal heating mechanisms. The first step in solving this problem would be to resolve the magnetic activity that is suspected of being the driver.

Perhaps the most startling observational discovery in recent decades, after the fibril structure of the magnetic fields, is the variation of the total brightness of the Sun by 1 or 2 parts in 1,000 with the 11-year sunspot cycle. Absolute radiometers on NASA, NOAA, and ESA spacecraft since 1978 have established this fact, and observations of other solar-type stars have now shown this variation to be a universal property of a

normal star. No more than about a third of the total variation can be accounted for with the varying UV and X-ray fluxes; most of the change is evidently in the visible and IR where the faculae and plages increase with the activity and account for half or more of the total. No one anticipated the variation in brightness, and the phenomenon remains without firm explanation to the present day. The origin of the variations lies somewhere in the dynamics of the convection below the visible surface.

Such questions are fundamental to all late main-sequence stars, of course, with their universal X-ray emission and inferred tenuous high-speed stellar winds, and their cyclic magnetic cycles and brightness variations. We will understand none of these stars until we understand the Sun.

Solar activity affects terrestrial climate and weather in the low troposphere, where humans reside, and in the upper atmosphere, where spacecraft are exposed to fast particles. The magnetic fluctuations and fast particle populations caused by coronal mass ejections, and the increased drag of the expanded atmosphere heated by enhanced solar UV, have resulted in the loss of both commercial and military satellites. In addition, geomagnetic currents induced on the ground are occasionally so violent as to overload regional power grids, causing power blackouts. Disruption of worldwide radio communication and of DOD satellite tracking has occurred on several occasions. Moreover, distortions in wave propagation can introduce errors into the signals of the Global Positionary System. For these and other reasons, DOD operations require specification of the variable solar energy inputs to Earth.

The Sun-Earth connection also includes fast particles from solar flares, which pose a hazard to excursions of humans into space, and the effect of varying ultraviolet emissions, which drive the chemistry of the upper atmosphere of Earth and can cause variations in ozone of the same magnitude as those changes attributed to man-made chlorofluorocarbons.

This brings us to the tendency, already noted, for the general level of magnetic activity to phase in and out over decades and centuries. The activity at the visible surface of the Sun is never the same in successive cycles and sometimes fades away almost altogether for several decades or more, as it did during the Maunder Minimum, 1645-1715 AD.⁴ On the other hand, the activity may leap to extraordinary levels, as it appears to have done during the 12th century, referred to as the Medieval Maximum. The ¹⁴C record shows that in 10 of the last 70 centuries there was little or no activity, and that in 8 centuries the activity was greatly enhanced.

The terrestrial climate appears to have responded strongly. It is estimated from the observed calibration of activity versus brightness of the Sun and other solar-type stars that the Sun was fainter by something of the order of 4 parts in 1,000 during the Maunder Minimum. A comparable brightening evidently occurred during the Medieval Maximum. The agricultural consequences were devastating at high latitudes in both Europe and Asia during the cold of the Maunder Minimum, and there was severe drought at arid low latitudes, for example in what is now the southwestern United States, during the

Medieval Maximum. The complicated climatic effects of varying IR, visible, and UV radiation are not properly understood in terms of a global climate model for Earth, nor do we understand the nature and cause of the variations at the Sun.

An immediate question concerns the substantial increase in the general level of activity of the Sun through the 20th century, with an increase in brightness inferred to be nearly 0.1%. Unfortunately, this has occurred on top of the observed increased abundance of greenhouse gases, particularly carbon dioxide, in the atmosphere, the observed warming of the ocean surface waters charged with dissolved carbon dioxide, and the burning of fossil fuels that releases carbon dioxide into the atmosphere. There is no evident way to disentangle the climatic consequences of these separate processes, nor to distinguish which is cause and which is effect. On the other hand, as already noted, U.S. policy making on global change requires an understanding of solar variability and its effects on climate, so the problem is urgent and baffling.

We clearly need observations of the extreme phases of solar activity as exemplified by the Maunder Minimum and the Medieval Maximum. We may reasonably expect the Sun to pass through such phases over the next 1,000 years, but millennial research is not timely. Therefore, we turn to the only source of information available—monitoring a large sample of distant solar-type stars.

Fortunately, the effort has already been started in a modest way by a dedicated group of nighttime observers who have carried on the monitoring of the chromospheric activity of other stars, begun over 30 years ago by O.C. Wilson. For the last decade they have monitored the brightness of several nearby solar-type stars, with the interesting discovery that one of the stars is resting in a Maunder Minimum state, while another slid through a weak activity maximum into the Maunder phase. In addition, they have found an approximate universal linear relation between changes in brightness and the activity level that applies to the Sun and other middle-aged late main-sequence stars. These observers have also found that the brightness of one of their stars declined by about 0.4 percent over a few years of sharply declining activity, suggesting a similar possibility for the Sun at some point in time, presumably accompanied by significant terrestrial climatic effects.⁵ The observations also show that early in its life (during the first 3 × 10⁸ years), a star like the Sun is fainter, rather than brighter, during times of high activity, suggesting a preponderance of sunspots over faculae. This is yet another dimension to the physics of the rotating convecting star.