NASA's SOHO Celebrates 20 Years of Space-based Science

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Scientists almost missed out on this and SOHO’s other discoveries. In 1998, the spacecraft was lost for four months because of a software error. A joint ESA/NASA team was finally able to recover the spacecraft in September 1998, in part using the giant Arecibo radio telescope to locate the spacecraft and reestablish command. This rescue was crucial for heliophysics, as much of SOHO’s scientific success can be attributed to its 20 years of near-constant observation.

“With SOHO, we found that the sun varies on every timescale we can measure,” said Gurman. “Whether it’s 20 years or just a few milliseconds, we discover new phenomena.”

Though it expanded our knowledge of every facet of heliophysics, SOHO was launched to answer three primary questions. First—what is the interior structure of the sun?

Though scientists had developed theories about the layers of ionized gas and complex magnetic field that compose our nearest star, they had no way of confirming their ideas other than by observing the sun’s surface. But SOHO carries onboard an instrument that can take a kind of solar sonogram, helping researchers understand the sun’s internal structure.

This helped to solve what was known as the solar neutrino problem, in which the number of a certain type of solar neutrino observed at Earth didn't jibe with the number predicted by our theories about the sun.

“Getting an accurate picture of the interior structure of the sun confirmed our theories about the number of neutrinos it emits,” said Fleck. “That proved the solar neutrino problem came from a misunderstanding of neutrinos themselves—not the sun.”

It was later discovered that neutrinos can undergo a change of type in their journey from the sun, accounting for the difference between predictions and observations. This research won the Nobel Prize in Physics in 2015.

The second question SOHO was designed to answer was that of solar wind acceleration. The sun is constantly losing material in all directions, but the speed of that flowing material—known as the solar wind—is much higher than one would expect from a relatively simple view of the sun. SOHO’s observations showed how some of the fastest solar wind streams are accelerated in coronal holes, areas on the sun where the magnetic field is open to interplanetary space.

As of yet, no one has managed to definitely answer SOHO’s third question—what causes the extraordinarily high temperatures in the sun’s atmosphere, the corona?

“The corona is incredibly hot, hundreds of times hotter than the layers below,” said Fleck. “Since the sun’s source of energy is at the center, on a simple level, we would expect the corona—the outermost layer—to be the coolest.”

Though SOHO’s observations have provided the basis for many possible explanations for the coronal heating problem, as it’s known, it still hasn’t been settled. However, NASA’s Solar Probe Plus mission, planned for launch in 2018, will fly closer to the sun than any other spacecraft in order to investigate this very question.

Solar Probe Plus is one of many missions that has been shaped by SOHO and its discoveries. Others include NASA’s Solar Dynamics Observatory, NASA’s Solar and Terrestrial Relations Observatory, and NASA’s Interface Region Imaging Spectrograph, and JAXA/NASA’s Hinode.

“Without SOHO, there would be no SDO, no STEREO, no IRIS, no Hinode,” said Young. “SOHO showed us things we’d never seen before, and then we realized we needed more eyes on the sun.”



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