fig. 5.8 The new (a) and old (b) pictures of how rotation speeds (or angular momenta) vary in the Sun's convection zone.
fields in an eleven-year cycle is at the base of the zone. This clue has been followed up vigorously, as we will see in chapter 9.
Then there is the whole question of just why the Sun chooses to rotate in this complicated fashion. Later on we'll look at recent simulations that try to answer that question, but in 1989 Gilman and company sketched a possible scenario, which figure 5.8 illustrates. They contrasted the new picture of constant rotation on cylindrical surfaces (5.8a) with the old one of radial surfaces (5.8b). They postulated that giant convection cells, which carry heat to the surface, also transport angular momentum from high to low latitudes. (How this is done would require a lengthy explanation.) The real mystery, for which nobody had a suggestion, is how the angular momentum is returned to high latitudes under the base of the convection zone (see the dark solid arrows) to complete a cycle. All this has become somewhat clearer in time, but still remains an open question.
AND WHAT ABOUT THE GORE?
In some ways the core of the Sun (inside a radius of about 0.3R) is the most important and least accessible part of the interior. It is important because only there is solar energy generated by thermonuclear conversion of hydrogen to helium. A beautiful theory had been constructed to describe this process, but although the theory accounted successfully for the Sun's luminosity, it predicted less than half of the observed flux of neutrinos (we will discuss all this in detail in chapter 7). Was the nuclear theory that far off? Or was something wrong with our ideas about the core of the Sun? It was a critical question for astrophysicists. Helio-seismologists were therefore eager to pin down the properties of the core.
How best to do that? The ideal way would be to observe the oscillations of gravity (that is, buoyancy) waves because they spend a lot of time in the core. However, they are so well confined that hardly any sign of them is expected at the surface. The next best approach is to observe sound waves (p-modes) of low degree (say L = 0,1,2, 3) because only these waves pass through the core. In chapter 7, we will see what we have learned about the core and how it bears on the missing neutrino problem.
The rotation of the core is also important, because it bears on the question of how an interstellar cloud redistributes its original angular momentum as it contracts into a star. Measuring the core's rotation is especially difficult because, as noted earlier, the frequency splitting patterns of the low L-modes are very narrow and require very long continuous data strings to resolve.
By the end of the 1980s the need for long, uninterrupted data strings became critical. Astronomers found ways to obtain them, as we will see in the following chapter.
BANISHING THE NIGHT
by the MID-19BDS, everyone recognized that future progress in he-lioseismology would depend on obtaining more precise oscillation frequencies. All the good things one wanted to learn about the solar interior (its rotation, composition, temperature profile, internal magnetic fields, for example) depended on resolving individual (N, L, M) oscillation modes. That meant that the splitting of frequencies had to be determined to within a gnat's eyebrow. And that, as we have seen, requires long, continuous observations, the longer the better, without any interruption from the setting of the Sun. In short, astronomers had to banish the night. And to do that, they would have to learn to organize and cooperate. A new era was arriving in which the lone researcher, using his own instrument, would become the exception.
Astronomers pursued three main routes to their goal. First, they ventured to the South Pole in the balmy austral summer, when the Sun skims above the horizon for six months. Second, they combined the observations from two or more well-separated locations—a network in which at least one site would always see daylight. And third, they launched their instruments into space, far from the nighttime shadow of Earth.
In this chapter we will follow their pursuit of long, long strings of data.
THE SOUTH POLE: PLUSES AND MINUSES
In principle, the Sun should be visible continuously for six months at the South Pole. In practice, a variety of factors limit the intervals of uninterrupted sunlight. High winds, snow squalls, cirrus clouds, and ice fog are the most serious obstacles. But even in clear weather, the low temperatures, high altitude, and difficulty of working in heavy clothing hamper the struggling astronomer. And if a crucial piece of equipment fails, there is little chance to repair or replace it in time.
We have already recounted the trials and tribulations of Gerard Grec and Eric Fossat, who had an unbroken run of five days. In their first attempt, Tom Duvall and Jack Harvey were less lucky and came away with only 50 hours (see fig. 5.5) at the pole. In 1987 they returned with Stuart Jefferies and obtained three runs of about 50 hours each over a period of 325 hours. Then in 1989, they won the brass ring: a run of 343 hours, with only 55 hours missing. Altogether, Duvall visited the pole five times but never exceeded this record. (As an award for his perseverance and his important scientific contributions, a mountain in Antarctica was named after him. That's almost as good as a Nobel Prize.)
Given the slim chances of exceeding even a five-day run and the difficulties of observing at the pole, only a few astronomers were willing to invest the time and energy. A recent example is David Rust, from the Johns Hopkins University. He conceived the Flare Genesis Experiment, a balloon-borne observatory that circled the South Pole in 19 days at an altitude of 38 km in order to observe the evolution of solar magnetic fields. But he is an exception. In the mid-1980s, most other astronomers were ready for another path to long-observing runs. They would create networks of ground-based observatories.
The Birmingham group blazed the trail. They first set up identical resonance cells at Izana in Tenerife and the Pic-du-Midi in France and observed intermittently from 1976 to 1979. These two sites are separated by only one hour of longitude but have different weather patterns. Then in 1980, they moved from Izana to Mount Haleakala, Hawaii, a 3000-meter extinct volcano. Their sites were then nine hours apart, with excellent summertime weather. Once, they were able to observe up to twenty-one hours a day for eighty-eight days with this setup.
The Stanford Solar Observatory followed soon afterward by cooperating with the Crimean Astrophysical Observatory, ten hours of longitude away. Each observatory used a modified magnetograph to detect long-period solar oscillations, particularly the puzzling 160-minute period, for several years. Ed Rhodes and Roger Ulrich, whom we have met before, were observing oscillations with a magneto-optical filter at the sixty-foot tower at Mount Wilson Observatory. Impressed with the success of Stanford, they too formed a partnership with the Crimean Observatory. Their two-station net is called the High Degree Helio-seismology Network, or HiDHN. Not to be outdone, Fossat and Grec of the Nice Observatory set up resonance cells at Izana and at the Pic-du-Midi observatory to improve their weather prospects.
These pioneers soon recognized, however, that to obtain complete twenty-four-hour coverage, more stations would be needed. The U.S. Air Force had learned that lesson long ago. It was interested in avoiding disruption of critical radio communications and damage to its satellites by solar flares. So in the late 1970s it built the Solar Optical Observing Network, with six stations distributed around the globe, to observe and report flares in real time. The facility cost millions of dollars and required a large staff of military personnel. Astronomers might dream about a similar network for helioseismology, but despite the public's growing interest and the high regard among physicists for the quality of science being produced, the prospects were dim.
Their networks could be developed only incrementally, as private funds or government grants became available. To attract funds they had to offer a special talent, technique, or idea. The Birmingham and Nice groups, for example, specialized in whole-Sun observations of low-degree oscillations; the Big Bear group in the interpretation of mode amplitudes and excitation. To reduce operating costs, these groups relied on the cooperation of host universities. For example, the Birmingham group enlisted the aid of Barry LaBonte and his students at the University of Hawaii to operate one of their resonance cells and return the data.
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