Longstanding Mystery of Sun's Hot Outer Atmosphere Solved

 One of the most enduring mysteries in solar physics is why the Sun's outer atmosphere, or corona, is millions of degrees hotter than its surface.

Now scientists believe they have discovered a major source of hot gas that replenishes the corona: jets of plasma shooting up from just above the Sun's surface.

The finding addresses a fundamental question in astrophysics: how energy is moved from the Sun's interior to create its hot outer atmosphere.

"It's always been quite a puzzle to figure out why the Sun's atmosphere is hotter than its surface," says Scott McIntosh, a solar physicist at the High Altitude Observatory of the National Center for Atmospheric Research (NCAR) in Boulder, Colo., who was involved in the study.

"By identifying that these jets insert heated plasma into the Sun's outer atmosphere, we can gain a much greater understanding of that region and possibly improve our knowledge of the Sun's subtle influence on the Earth's upper atmosphere."

The research, results of which are published in the journal Science, was conducted by scientists from Lockheed Martin's Solar and Astrophysics Laboratory (LMSAL), NCAR, and the University of Oslo. It was supported by NASA and the National Science Foundation (NSF), NCAR's sponsor.
"These observations are a significant step in understanding observed temperatures in the solar corona," says Rich Behnke of NSF's Division of Atmospheric and Geospace Sciences, which funded the research.

"They provide new insight about the energy output of the Sun and other stars. The results are also a great example of the power of collaboration among university, private industry and government scientists and organizations."
The research team focused on jets of plasma known as spicules, which are fountains of plasma propelled upward from near the surface of the Sun into the outer atmosphere.

For decades scientists believed spicules could send heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so the theory largely fell out of vogue.

"Heating of spicules to millions of degrees has never been directly observed, so their role in coronal heating had been dismissed as unlikely," says Bart De Pontieu, the lead researcher and a solar physicist at LMSAL.
In 2007, De Pontieu, McIntosh, and their colleagues identified a new class of spicules that moved much faster and were shorter-lived than the traditional spicules.

These "Type II" spicules shoot upward at high speeds, often in excess of 100 kilometers per second, before disappearing.

The rapid disappearance of these jets suggested that the plasma they carried
might get very hot, but direct observational evidence of this process was missing.

The researchers used new observations from the Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope (SOT) on the Japanese Hinode satellite to test their hypothesis.

"The high spatial and temporal resolution of the newer instruments was crucial in revealing this previously hidden coronal mass supply," says McIntosh.

"Our observations reveal, for the first time, the one-to-one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona."

The findings provide an observational challenge to the existing theories of coronal heating.

During the past few decades, scientists proposed a wide variety of theoretical models, but the lack of detailed observation significantly hampered progress.
"One of our biggest challenges is to understand what drives and heats the material in the spicules," says De Pontieu.

A key step, according to De Pontieu, will be to better understand the interface region between the Sun's visible surface, or photosphere, and its corona.

Another NASA mission, the Interface Region Imaging Spectrograph (IRIS), is scheduled for launch in 2012 to provide high-fidelity data on the complex processes and enormous contrasts of density, temperature and magnetic field between the photosphere and corona. Researchers hope this will reveal more about the spicule heating and launch mechanism.

The LMSAL is part of the Lockheed Martin Space Systems Company, which designs and develops, tests, manufactures and operates a full spectrum of advanced-technology systems for national security and military, civil government and commercial customers.

Narrow jets of material, called spicules, streak upward from the Sun's surface at high speeds. (Credit: NASA)

The finding has implications for understanding future climate change because dust plays a significant role in controlling the amount of solar energy in the atmosphere. Depending on their size and other characteristics, some dust particles reflect solar energy and cool the planet, while others trap energy as heat.

"As small as they are, conglomerates of dust particles in soils behave the same way on impact as a glass dropped on a kitchen floor," Kok says. "Knowing this pattern can help us put together a clearer picture of what our future climate will look like."

The study may also improve the accuracy of weather forecasting, especially in dust-prone regions. Dust particles affect clouds and precipitation, as well as temperatures.

The research was supported by the National Science Foundation, which sponsors NCAR.

Shattered soil
Kok's research focused on a type of airborne particle known as mineral dust. These particles are usually emitted when grains of sand are blown into soil, shattering dirt and sending fragments into the air. The fragments can be as large as about 50 microns in diameter, or about the thickness of a fine strand of human hair.

The smallest particles, which are classified as clay and are as tiny as 2 microns in diameter, remain in the atmosphere for about a week, circling much of the globe and exerting a cooling influence by reflecting heat from the Sun back into space. Larger particles, classified as silt, fall out of the atmosphere after a few days. The larger the particle, the more it will tend to have a heating effect on the atmosphere.

Kok's research indicates that the ratio of silt particles to clay particles is two to eight times greater than represented in climate models.

Since climate scientists carefully calibrate the models to simulate the actual number of clay particles in the atmosphere, the paper suggests that models most likely err when it comes to the number of silt particles. Most of these larger particles swirl in the atmosphere within about 1,000 miles of desert regions, so adjusting their quantity in computer models should generate better projections of future climate in desert regions, such as the southwestern United States and northern Africa.

Additional research will be needed to determine whether future temperatures in those regions will increase more or less than currently indicated by computer models.

The study results also suggest that marine ecosystems, which draw down carbon dioxide from the atmosphere, may receive substantially more iron from airborne particles than previously estimated. The iron enhances biological activity, benefiting ocean food chains, including plants that take up carbon during photosynthesis.

In addition to influencing the amount of solar heat in the atmosphere, dust particles also get deposited on mountain snowpacks, where they absorb heat and accelerate melt.

Glass and dust: Common fracture patterns
Physicists have long known that certain brittle objects, such as glass or rocks, and even atomic nuclei, fracture in predictable patterns. The resulting fragments follow a certain range of sizes, with a predictable distribution of small, medium, and large pieces. Scientists refer to this type of pattern as scale invariance or self-similarity.

Physicists have devised mathematical formulas for the process by which cracks propagate in predictable ways as a brittle object breaks. Kok theorized that it would be possible to use these formulas to estimate the range of dust particle sizes. He turned to a 1983 study by Guillaume d'Almeida and Lothar Schüth from the Institute for Meteorology at the University of Mainz in Germany that measured the particle size distribution of arid soil.

By applying the formulas for fracture patterns of brittle objects to the soil measurements, Kok determined the size distribution of emitted dust particles. To his surprise, the formulas described measurements of dust particle sizes almost exactly.

"The idea that all these objects shatter in the same way is a beautiful thing, actually," Kok says. "It's nature's way of creating order in chaos."

Dust particles in the atmosphere range from about 0.1 microns to 50 microns in diameter (microns are also known as micrometers, abbreviated as µm). The size of dust particles determines how they affect climate and weather, influencing the amount of solar energy in the global atmosphere as well as the formation of clouds and precipitation in more dust-prone regions. The NASA satellite image in this illustration shows a 1992 dust storm over the Red Sea and Saudi Arabia. (Credit: Copyright UCAR)