The solar system is constantly bathed by a sea of charged particles that emanate from the sun, and one research team spent 14 years chasing solar eclipses to learn more about this stellar spray.
The particles are known as solar wind, and they come from the sun's outermost region: the corona. If a person is lucky enough to observe a total solar eclipse, they will definitely see this pearly-looking region surrounding the sun. This wispy area has all the visual charm of a cosmic halo, but it is equal parts inferno. The corona is the hottest region of the sun, reaching temperatures upwards of 2 million degrees Fahrenheit (more than 1 million degrees Celsius). The extreme temperature found in this gaseous stellar cocoon has puzzled scientists because this is quite a different story to what is happening 1,000 miles (1,600 kilometers) below the corona, where the temperature is a significantly cooler 10,000 degrees F (5,500 degrees C), according to NASA.
Scientists want to understand the solar wind for many reasons. Solar wind defines the parameters of the solar system and sets its boundary: As the solar wind spreads itself thinner and thinner the farther it travels away from the sun, the particles can't resist the pushback from interstellar space. Much closer to Earth, solar wind protects us from dangerous cosmic radiation but can also disrupt space-based communication satellites and GPS systems.
Shaddia Habbal, a solar researcher at the University of Hawaii, led a team to Oregon, Indonesia, Argentina, the Sahara Desert in Libya, the Gobi Desert in Mongolia, and many more solar-eclipse sites over the last 14 years. Her goal was to observe the sun's corona over a slightly longer amount of time than it takes the sun to complete one cycle of solar activity, about 11 years. Total solar eclipses, which happen as the moon's disk blocks all sunlight except the corona, are a key opportunity for researchers to observe the source of the solar wind from Earth.
The team took high-resolution observations of 11 total solar eclipses using cameras equipped with specialized filters. This gear allowed the scientists to measure the temperatures of the particles from the innermost part of the corona, which is the birthplace of the solar wind. The researchers then compared their findings with data from NASA's Advanced Composition Explorer (ACE) mission, which launched in 1997 and observes the solar wind from a place in space located about 1/100 the distance from Earth to the sun.
During the sun's 11-year cycle, the star oscillates between dormant and volatile periods. When the sun is quiet, the face of the star sports few solar spots. That's quite different than during the sun's volatile season, when blemishes and solar flares are commonplace.
The sun's radically different appearance over the course of the solar cycle left these scientists surprised to find that, despite all the global solar changes that happen over the course of 11 years, their 14 years of observations showed no major temperature change for the inner-corona particles, which create the solar wind.
"Whatever is heating the majority of the corona and solar wind is not very dependent on the sun's activity cycle," Benjamin Boe, a solar researcher at the University of Hawaii involved in the new research, said in a NASA statement that describes the study.
"The temperature at the sources of the solar wind in the corona is almost constant throughout a solar cycle," Habbal said in the statement. "This finding is unexpected because coronal structures are driven by changes in the distribution of magnetized plasmas in the corona, which vary so much throughout the 11-year magnetic solar cycle."
To solve this new mystery, researchers plan to continue chasing total solar eclipses in the future to gain more observations, according to NASA. The next total solar eclipse will occur in December over Antarctica.
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