How can astronomers detect neutrinos from the sun

A comparison of the fluxes detected at the various experiments supports a theory known as neutrino oscillations: that electron-neutrinos from the sun are transformed into other kinds of neutrinos muon-neutrinos or tau-neutrinos that are more difficult to detect. If the fusion were simply switching off--that is, if the sun were dying--the counts of all neutrinos low-, medium- and high-energy ones would be reduced by about the same amount, but that is not what is observed.

Instead we seem to be discovering that neutrinos are more complex particles than we initially thought. Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. John G. Learned, professor of physics in the department of physics and astronomy at the University of Hawaii, relays this answer: "The nuclear reactions that power the sun produce neutrinos, uncharged subatomic particles that fly freely from the center of the sun to Earth and beyond.

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Figure 2. Sunspot Structure: This drawing shows our new understanding, from helioseismology, of what lies beneath a sunspot. The black arrows show the direction of the flow of material. The intense magnetic field associated with the sunspot stops the upward flow of hot material and creates a kind of plug that blocks the hot gas.

As the material above the plug cools shown in blue , it becomes denser and plunges inward, drawing more gas and more magnetic field behind it into the spot. The concentrated magnetic field causes more cooling, thereby setting up a self-perpetuating cycle that allows a spot to survive for several weeks. Since the plug keeps hot material from flowing up into the sunspot, the region below the plug, represented by red in this picture, becomes hotter.

This material flows sideways and then upward, eventually reaching the solar surface in the area surrounding the sunspot. Below the convection zone, however, the Sun, even though it is gaseous throughout, rotates as if it were a solid body like a bowling ball. Another finding from helioseismology is that the abundance of helium inside the Sun, except in the center where nuclear reactions have converted hydrogen into helium, is about the same as at its surface.

That result is important to astronomers because it means we are correct when we use the abundance of the elements measured in the solar atmosphere to construct models of the solar interior.

Helioseismology also allows scientists to look beneath a sunspot and see how it works. In The Sun: A Garden-Variety Star , we said that sunspots are cool because strong magnetic fields block the outward flow of energy. Figure 2 shows how gas moves around underneath a sunspot. Cool material from the sunspot flows downward, and material surrounding the sunspot is pulled inward, carrying magnetic field with it and thus maintaining the strong field that is necessary to form a sunspot.

As the new material enters the sunspot region, it too cools, becomes denser, and sinks, thus setting up a self-perpetuating cycle that can last for weeks. The downward-flowing cool material acts as a kind of plug that block the upward flow of hot material, which is then diverted sideways and eventually reaches the solar surface in the region around the sunspot. This outward flow of hot material accounts for the paradox that we described in The Sun: A Garden-Variety Star —namely, that the Sun emits slightly more energy when more of its surface is covered by cool sunspots.

Helioseismology has become an important tool for predicting solar storms that might impact Earth. Active regions can appear and grow large in only a few days. The solar rotation period is about 28 days. Fortunately, we now have space telescopes monitoring the Sun from all angles, so we know if there are sunspots forming on the opposite side of the Sun.

Moreover, sound waves travel slightly faster in regions of high magnetic field, and waves generated in active regions traverse the Sun about 6 seconds faster than waves generated in quiet regions. By detecting this subtle difference, scientists can provide warnings of a week or more to operators of electric utilities and satellites about when a potentially dangerous active region might rotate into view.

With this warning, it is possible to plan for disruptions, put key instruments into safe mode, or reschedule spacewalks in order to protect astronauts. Recall from our earlier discussion that neutrinos created in the center of the Sun make their way directly out of the Sun and travel to Earth at nearly the speed of light.

As far as neutrinos are concerned, the Sun is transparent. If we can devise a way to detect even a few of these solar neutrinos, then we can obtain information directly about what is going on in the center of the Sun.

On very, very rare occasions, however, one of the billions and billions of solar neutrinos will interact with another atom. It was housed a mile underground in the caverns of the Homestake Gold Mine in South Dakota, which was then an active mine and is now used for science experiments, including further neutrino research in the Deep Underground Neutrino Experiment.

But only one third of the neutrinos seemed to arrive. Some scientists, including Bruno Pontecorvo, proposed that the neutrino model was the error, but many were skeptical.

In , the Kamiokande experiment in Japan added to the confusion. But there was still the question of all those missing neutrinos.

As measurements of the sun improved and the solar model was validated, researchers looked more and more to new physics beyond the Standard Model to explain the neutrino deficit. The breakthrough came with data from two newer experiments. Super-Kamiokande , an improved version of the Kamiokande experiment, began observations in , and the Sudbury Neutrino Observatory in Canada joined in Leaders of these two projects would go on to receive the Nobel Prize in physics for discovering the solution to the solar neutrino problem: neutrino oscillations.

Roughly two-thirds of the electron neutrinos coming from the sun were changing their flavor as they traveled, arriving as muon or tau neutrinos. Evidence that neutrinos changed type also proved that they have mass, a shocking discovery not predicted by the Standard Model.

Solar neutrinos still have much to teach us. For example, scientists can compare how solar neutrinos traveling through the vacuum of space differ from neutrinos traveling through denser areas such as Earth. Such investigations bring information about the neutrino oscillation phenomenon. Solar neutrinos can also provide direct insight about the core of our sun.