A Positron Named Priscilla Scientific Discovery at the Frontier (1994) / Chapter Skim
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1 Shake, Rattle, and Shine: New Methods of Probing the Sun's Interior
1 Shake, Rattle, and Shine: New Methods of Probing the Sun's Interior
Pages 2-33
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From page 2... ...
And despite reports that dark splotches occasionally appeared on the face of the sun, this ancient conception of a flawless solar globe held firm even into the Middle Ages. This illusion, of course, was shattered in the early seventeenth century when Galileo in Italy, as well as observers in Holland, Germany, and England, pointed a newfangled instrument, called a telescope, at the sun and confirmed that the solar surface was indeed spotted.
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From page 3... ...
What is known about the sun essentially comes from examination of its outer features, although modernday instruments, both on the ground and in space, have revealed a solar surface more turbulent and varied than seventeenth-century astronomers could ever have imagined: High-speed streams of solar particles emanate from dark coronal "holes"; solar prominences, immense arches of glowing gas, soar for hundreds of thousands of kilometers above the solar surface; and solar flares, lightning-like cataclysmic explosions, can flash across a region of the sun in a matter of minutes. Nearly all these effects reflect complicated and tumultous activities inside the sun itself.
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From page 4... ...
provided accurate probes that have made seeing inside at least one star a reality." Although still in its infancy, helioseismology has already challenged and revised several long-held conceptions of the solar interior, such as the depth of the convection zone and the way in which the inner sun rotates. Astronomers expect additional revisions as an international helioseismological network, presently in the process of being established, attempts to measure the solar quivers more accurately than ever before.
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From page 5... ...
The lines were observed to Doppler shift, to move to higher or lower frequencies, as gases at the surface of the sun moved either toward or away from the observers. By measuring this shift the observing team hoped to discern the bobbing motions of individual solar granules, the cells of upwelling and sinking gases that cover the solar surface.
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From page 6... ...
The physics of these solar acoustic waves was already well understood from studies of the earth's atmosphere. Such waves propagate at the speed of sound by means of alternating compression and rarefaction of the solar gas, with pressure as the restoring force.
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From page 7... ...
Why should solar acoustic waves exist at all? That is not known with certainty, but helioseismologists have their suspicions.
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From page 8... ...
And with a wave's propagation dependent on the tempera ture, velocity, and density of the medium through which it is traveling, each mode offers valuable clues on the makeup and structure of the solar interior, much the way the resonant tone of a musical instrument provides hints as to the instrument's design for instance, whether it's shaped like a flute or clarinet. Such effects afford solar astronomers with enormous diagnostic capabilities.
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From page 9... ...
For the lowest modes, waves with values from zero to three, whose lengths are comparable to the size of the sun, observers look at the collective Doppler shift of a spectral line averaged over all or much of the solar disk. Since day/night gaps introduce spurious signals that make analysis difficult, investigators at the University of Birmingham In Great Britain and the Observatory of Nice In France established field stations around the globe to obtain an uninterrupted record of the sun's activity.
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From page 10... ...
Traditionally, researchers have constructed a set of solar models and then adjusted certain parameters, such as the temperature and density of various solar elements, until they best fit the p modes observed ringing through the sun. More recently, however, theorists have been developing mathematical techniques known collectively as inversion, which extract the solar parameters directly from the modes themselves.
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From page 11... ...
"This is poorly understood," notes Libbrecht, "but possibly linked to both convection processes and coriolis forces." Numerical simulations of this process had N an Cat E 0.2 c' 0.1 o 1 1 1 1 0 0.2 0.4 0.6 r/R FIGURE 1.5 This diagram, based on helioseismological data, shows how the speed of sound steadily increases {right to leg: from one solar radius to fractions thereof as temperatures increase toward the center of the sun. But the rise diminishes near the core, where hydrogen is being converted into helium, because the speed of sound decreases in denser materia/.
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From page 12... ...
Brown and Morrow's model was sustained and extended by a wealth of new data gathered by Ken Libbrecht. For 6 months in 1986 at Caltech's Big Bear Solar Observatory, located in the center of Southern California's Big Bear Lake, Libbrecht and his students took a Doppler image of the sun each minute, gathering a total of around 70,000 pictures.
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From page 13... ...
, the data do strongly suggest that the innermost core, a few percent of the sun's total volume, is not rotating fast enough to squish the sun and disrupt Einstein's theory. Also affected by the changing profile of the inner sun has been astronomers' understanding of the solar dynamo, the "engine" that drives the ebb and flow of activity over a solar cycle by inducing immense electrical currents and magnetic fields.
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From page 14... ...
A top priority is understanding the solar cycle, that 11-year period over which sunspot counts and solar flares wax and wane and solar magnetic field strengths build up and decline. Solar astronomers already suspect that the sun's magnetic fields interact with convection, indeed at times might suppress it, but helioseismology is needed to see such an effect below the solar surface.
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From page 15... ...
Some even speculate, although it's very controversial, that the very core of the sun, where thermonuclear reactions take place, may somehow participate in the solar cycle. The potential uses of helioseismology are legion.
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From page 16... ...
To obtain a good signal above the noise, a GONG instrument should then detect movements over the sun with a precision as small as a centimeter per second. Timothy Brown of NCAR pioneered Fourier tachometry, and the GONG instrument is a product of the evolution of that technology.
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From page 17... ...
, the two will combine to produce a dark image. The Fourier tachometer's keen ability to detect small Doppler shifts on the solar surface results from the makeup of the cube: one pathway or arm is solid glass; the other is air.
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From page 18... ...
But the payoff will be big if g modes are firmly discovered, for the detection will enable solar astronomers to peer directly into the sun's core. "The very core of the sun where nuclear reactions take place, that's the biggest prize in helioseismology," declares Cough.
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From page 19... ...
But to maintain that allegiance, Pauli took the rather radical step in 1930 of suggesting that an entirely new particle, invisible to ordinary instruments, had to exist to explain the discrepancies seen in the Cavendish experiments. Every time a nucleus undergoes beta decay, he proposed to some colleagues by letter, this neutral phantomlike particle is emitted and vanishes off into the night carrying off that extra bit of energy and momentum.
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From page 20... ...
The particle that Cowan and Reines detected is specifically known as the electron neutrino because of its appearance in nuclear reactions involving ordinary electrons. Since then, physicists have found a second type of neutrino, the muon neutrino, which is associated with interactions involving the muon, a more massive relative of the electron.
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From page 21... ...
To use the jargon of physics, neutrinos come in three "flavors": electron, muon, and taut In the standard model of particle physics, the rest mass of each neutrino type is arbitrarily set to zero. But theories that go beyond that standard model suggest that might not be Cue; neutrinos may have mass.
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From page 22... ...
Here was the perfect tool for looking into the sun, as physicists estimate that every second about 66 billion of these solar neutrinos rain down on each square centimeter of the earth's surface. There are additional nuclear reactions going on inside the sun, besides the proton-to-helium chain, and each of these side reactions gives off neutrinos as well neutrinos with characteristic energies.
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From page 23... ...
With this gargantuan apparatus, University of Pennsylvania radiochemist Raymond Davis, the founding father of neutrino astronomy, has been catching a few electron neutrinos out of the legions that are continually spewed into the solar system as the sun burns its nuclear fuel. For more than two decades now, the chlorine atoms in his cleaning fluid have been occasionally stopping some of the cagy particles.
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From page 24... ...
A POSITRON NAMED PRISCILLA 24 FIGURE 1.6 For more than two decades, this huge tank, filled with chlorine-rich cleaning fluid and situated 1 mile underground in the Homestake Gold Mine in South Dakota, has been capturing fewer solar neutrinos than theory predicts. (Courtesy of Brookhaven National laboratory.}
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From page 25... ...
Japan's Kamiokande detector, a huge vat of water originally built to look for the proton decays anticipated by physicists' grand unified theories, was reconfigured to search for solar neutrinos. The Kamiokande detector is fundamentally different than the radiochemical type used by Davis.
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From page 26... ...
New experiments are poised to discover whether this deficit is a problem in our understanding of how the sun works, is a hint of new neutrino properties beyond those predicted by the standard model of particle physics, or perhaps a combination of both."
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From page 27... ...
It is nearly impossible for solar physics models to account for this result and, when taken in conjunction with the Davis and Kamiokande data, offers the hint that the low solar neutrino flux may be due to new neutrino properties." A similar gallium experiment, called GALLEX, is under way in a laboratory set within Italy's Gran Sasso tunnel in the Apennines, a mountain range northeast of Rome. Operating since 1991, the GALLEX 27
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From page 28... ...
The neutrino deficiency, so long seen in South Dakota and later in Japan, Russia, and Italy, might be explained if electron neutrinos have mass and so are able to "oscillate" on their way out of the sun; with mass, a fraction of them might be able to transform themselves into the other two neutrino types, the muon neutrino and the tan neutrino. Since the perchloroethylene and gallium detectors can only "see" electron neutrinos and not the other two types, this might easily explain the shortfall, why only a fraction of the expected neutrino signal is detected.
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From page 29... ...
~ . Solar neutrinos producecl by thermonuclear reactions at the sun's core to Underground laboratory in Caucasus Mountains with tanks containing liquid gallium metal to _!
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From page 30... ...
Some theorists are suggesting that electron neutrinos must have energies greater than 500,000 electron volts to experience the conversion, which may explain why the Homestake and Kamiokande detectors are noticing the biggest shortfall; each is particularly sensitive to neutrinos in that energy range. "The MSW model is definitely the leading contender, and it's a beautiful theory," notes Wilkerson, "but it only takes a few ugly facts to kill a beautiful theory." Neutrino astronomers will have a better chance at testing the MSW model later this decade with the opening of the Sudbury Neutrino Observatory, a collaborative venture sponsored by Canada, Great Britain, and the United States.
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From page 31... ...
"It seems likely that the longstanding mystery of missing solar neutrino flux will be solved during the decade of the 90s," he predicts. "It will be ~ntereshng to see if Me answers will come from solar physics or particle physics.
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From page 32... ...
1988. Solar models, neutrino experiments, and helioseismology.
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From page 33... ...
1991. Seismic observations of the solar interior.
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