New observations help explain why sun's upper atmosphere is hotter than its surface

Several observatories, both on Earth's surface and in space, are dedicated to solving the mysteries of the sun's heating mechanisms.

One of them is the Big Bear Solar Observatory at the New Jersey Institute of Technology, and new observations by the observatory have offered fresh insights into source of extreme temperatures measured in the sun's upper atmosphere.

The sun's upper atmosphere and corona feature temperatures far exceeding those measured on the sun's surface, and scientists have struggled to understand why.

The latest findings, published in the journal Science, suggest geyser-like jets of magnetized plasma called spicules facilitate the transfer of energy through the sun's outer layers.

Spicules are relatively small by solar standards, measuring between 125 and 300 miles in diameter. With the help of Big Bear, scientists at the New Jersey institute captured detailed images of the enigmatic jets, revealing their tremendous speeds -- up to 60 miles per second -- and the paths they take from the upper atmosphere to the corona.

"Unprecedented high-resolution observations from BBSO's Goode Solar Telescope clearly show that when magnetic fields with opposite polarities reconnect in the Sun's lower atmosphere these jets of plasma are powerfully ejected," study author Wenda Cao, the Big Bear director and a solar physicist at the New Jersey institute, said in a news release.

"This is the first time we've seen direct evidence of how spicules are generated. We have tracked these dynamic features in the H-alpha spectral line down to their foot points, measured the magnetic fields at their foot point, captured the migration of the emerging magnetic elements and verified their interaction with existing magnetic fields of the opposite polarity."

Beginning roughly 1,300 miles from the star's surface, temperatures in the sun's upper atmosphere increase dramatically. As the upper atmosphere bleeds into the corona, the sun's outermost layer, temperatures exceed 1 million degrees Kelvin -- more than 100 times hotter than the sun's surface.

Using images captured in the extreme ultraviolet spectrum NASA's Solar Dynamics Observatory, scientists have previously confirmed that temperatures inside spicules match the temperatures in the corona. Now, thanks to Big Bear's 1.6-meter telescope, the largest working solar telescope in the world, scientists are beginning to understand the behavior of these ubiquitous jets.

"Using the Goode Solar Telescope at the Big Bear Solar Observatory, we observed spicules emerging within minutes of the appearance of opposite-polarity magnetic flux around dominant-polarity magnetic field concentrations," researchers wrote in their paper. "Data from the Solar Dynamics Observatory showed subsequent heating of the adjacent corona."

Big Bear previously observed a variety of subsurface solar heating mechanisms, including magnetic fields and plasma flows. Researchers hope ongoing observations will help them understand how different heating mechanisms combine to account for the discrepancy between the temperatures on the sun's surface and in the sun's upper atmosphere.

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Images from solar observatory peel away layers of a stellar mystery
Newark NJ (SPX) Nov 18, 2019
An international team of scientists, including three researchers from New Jersey Institute of Technology (NJIT), has shed new light on one of the central mysteries of solar physics: how energy from the Sun is transferred to the star's upper atmosphere, heating it to 1 million degrees Fahrenheit and higher in some regions, temperatures that are vastly hotter than the Sun's surface. With new images from NJIT's Big Bear Solar Observatory, the researchers have revealed in groundbreaking, granular de ... read more