The chromosphere is important because it is largely responsible for the deep ultraviolet radiation that bathes the Earth, producing our atmosphere's ozone layer, and it has the strongest solar connection to climate variability. The new result also helps explain a mystery that's existed since the middle of the last century -- why the chromosphere (and the tenuous corona above) is much hotter than the visible surface of the star. "It's like getting warmer as you move away from the fire instead of cooler, certainly not what you expect," said Scott McIntosh, a researcher at Southwest Research Institute, Boulder, Colo.

"This work finds the missing piece of the puzzle that has fascinated many generations of solar astronomers. When you fit this piece in place, our vision of the chromosphere becomes clear," said Alexei Pevtsov, Program Scientist NASA Headquarters, Washington.

Using spacecraft, ground-based telescopes, and computer simulations, these new results show that the Sun's magnetic field allows the release of wave energy from its interior, permitting the sound waves to travel through thin fountains upward into the solar chromosphere. These magnetic fountains form the mold for the chromosphere.

"Scientists have long realized that solar magnetic fields hold the key to tapping the vast energy reservoir locked in the Sun's interior," said Paul Bellaire, program director in NSF's division of atmospheric sciences. "These researchers have found the ingenious way that the Sun uses magnetic keys to pick those locks."

Over the past twenty years, helioseismologists have studied energetic sound waves as probes of the Sun's interior structure because they are largely trapped by the Sun's visible surface -- the photosphere. The new research found that some of these waves can escape the photosphere into the chromosphere and corona.

To make the new discovery, the team used observations from the SOHO and TRACE spacecrafts combined with those from the Magneto-Optical filters at Two Heights (MOTH) instrument stationed in Antarctica, and the Swedish 1 meter (3 foot) Solar Telescope on the Canary Islands. The observations gave detailed insight into how some of these trapped waves manage to leak out through magnetic "cracks" in the photosphere, sending mass and energy shooting upwards into the atmosphere above. "The Sun's interior vibrates with the peal of millions of bells, but the bells are all on the inside of the building. We have been able to show how the sound can escape the building and travel a long way using the magnetic field as a guide," continued McIntosh.

By analyzing motions of structures in the solar atmosphere in detail, the scientists observed that near strong knots of magnetic field, sound waves from the interior of the Sun can leak out and propagate upward into its atmosphere. "The constantly evolving magnetic field above the solar surface acts like a doorman opening and closing the door for the waves that are constantly passing by," said Bart De Pontieu, a researcher Lockheed Martin Solar and Astrophysics Lab, Palo Alto, Calif.

These results were confirmed by state-of-the-art computer simulations that show how the leaking waves continually propel fountains of hot gas upward into the Sun's atmosphere, which fall back to its surface a few minutes later.

The scientists were able to independently demonstrate that the magnetic field controls the release of mass and wave energy into the solar atmosphere. The combination of these results demonstrates that a lot more energy can be pumped into the chromosphere by wave motions than researchers had previously thought. This wouldn't be possible without the relentlessly changing magnetic field at the surface.

earlier related report
Screaming CMEs Warn of Radiation Storms
A CME (Coronal Mass Ejection) is a solar body slam to our high-tech civilization. CMEs begin when the sun launches a billion tons of electrically conducting gas (plasma) into space at millions of miles per hour. A CME cloud is laced with magnetic fields, and CMEs directed our way smash into Earth's magnetic field. If the CME magnetic fields have the correct orientation, they dump energy into Earth's magnetic field, causing magnetic storms. These storms can cause widespread blackouts by overloading power line equipment with extra electric current. But wait; there's more.

Some CMEs also bring intense radiation storms that can disable satellites or cause cancer in unprotected astronauts. As the CME blasts through space, it plows into a slower stream of plasma blown constantly from the sun in all directions, called the solar wind. The CME causes a shock wave in the solar wind. If the shock is strong enough, it accelerates electrically charged particles that make up the solar wind to high speeds, forming the radiation storm.

"Some CMEs produce radiation storms, and some don't, or at least the level of radiation is significantly lower," said Dr. Natchimuthuk Gopalswamy of NASA's Goddard Space Flight Center in Greenbelt, Md. "The trick is to identify the ones that can produce dangerous radiation, so we can warn astronauts and satellite operators."

Gopalswamy and his team may have found a way to do just that. Like a pro wrestler roaring before an attack, CMEs with powerful shocks capable of causing radiation storms "scream" in radio waves as they slam through the solar wind, according to the team.

They made the connection by analyzing observations of CMEs from the SOHO (Solar and Heliospheric Observatory) and Wind spacecraft. SOHO has an instrument that can see CMEs (the Large Angle and Spectrometric Coronagraph), and an instrument that detects their radiation (the Energetic and Relativistic Nucleon and Electron experiment). Wind has an instrument that can pick up a CME's radio signal (The Radio and Plasma Wave experiment). The team compared observations from both spacecraft and looked at 472 CMEs between 1996 and 2005 that were fast and covered a large area of the sky.

They discovered that those CMEs which generated a radio signal also produced radiation storms, but CMEs without a radio signal did not.

Strong CME shocks accelerate electrons (the particles that carry electricity) in the solar wind, which produce the radio signal. The same strong shock must also accelerate atomic nuclei (the central part of atoms) in the solar wind, which produce the radiation storm, according to the team.

"Since the radio signal moves at the speed of light while the particles follow behind, we can use a CME's radio noise to give warning that it is generating a radiation storm that will hit us soon," said Gopalswamy. "This will give astronauts and satellite operators anywhere between a few tens of minutes to a couple hours to prepare, depending on how fast the particles are moving."

The team also noticed that most radio-loud CMEs came from parts of the sun in line with Earth (areas near the solar equator), while radio-quiet CMEs mostly came from the edges of the sun. Since all the CMEs studied were fast and could have produced strong shocks, detecting radio noise and radiation from some but not others might simply be due to geometry. CMEs near the edge of the sun only present a small section of their shock surface towards us as they expand through space, and therefore tend to be radio-quiet and radiation-free from our point of view, according to the team. However, it means that explorers on other worlds in our solar system, like Mars, will need a spacecraft positioned between them and the sun to take advantage of the radio warning.

"Radio is one of the strongest indicators of an impending radiation storm, so if astronauts are out at Mars off the edge of the Sun, our radio telescopes at or near Earth won't do much to protect them," said Dr. Bill Wagner of NASA Headquarters, Washington.

The research was funded by NASA and will be presented at the American Astronomical Society's 210th Meeting during 27-31 May 2007 in Honolulu, Hawaii. The team includes researchers from Goddard, the Catholic University of America, Washington, the Naval Research Laboratory, Washington, and the Observatory of Paris. SOHO is a cooperative project between the European Space Agency and NASA. NASA's Wind spacecraft was launched on November 1, 1994, and is operated from Goddard. The spacecraft is maintained at a position approximately one million miles from Earth along the Earth-sun line to measure the solar wind without any contamination from Earth's magnetic field.

earlier related report
Radio 'screams' from the Sun warn of radiation storms
ESA's SOHO has helped uncover radio screams that foretell dangerous Coronal Mass Ejections, or CMEs, which produce radiation storms harming infrastructure on ground, in space as well as humans in space.

Scientists made the connection by analyzing observations of CMEs from ESA/NASA's SOHO (Solar and Heliospheric Observatory) and NASA's Wind spacecraft. The team includes researchers from Goddard, the Catholic University of America, Washington, the Naval Research Laboratory, Washington, and the Observatory of Paris. A CME is a solar slam to our high-tech civilization. It begins when the sun launches a thousand million tons of electrically conducting gas (plasma) into space at millions of kilometres per hour.

A CME cloud is laced with magnetic fields and when directed our way, smashes into Earth's magnetic field. If the magnetic fields have the correct orientation, they dump energy into Earth's magnetic field, causing magnetic storms. These storms can cause widespread blackouts by overloading power line equipment with extra electric current.

Some CMEs also bring intense radiation storms that can disable satellites or cause cancer in unprotected astronauts. As the CME blasts through space, it plows into a slower stream of plasma blown constantly from the sun in all directions, called the solar wind. The CME causes a shock wave in the solar wind. If the shock is strong enough, it accelerates electrically charged particles that make up the solar wind to high speeds, forming the radiation storm.

"Some CMEs produce radiation storms, and some don't, or at least the level of radiation is significantly lower," said Dr. Natchimuthuk Gopalswamy of NASA's Goddard Space Flight Center in Greenbelt, Maryland, lead author of the results. "The trick is to identify the ones that can produce dangerous radiation, so we can warn astronauts and satellite operators."

Gopalswamy and his team may have found a way to do just that. CMEs with powerful shocks capable of causing radiation storms 'scream' in radio waves as they slam through the solar wind, according to the team.

SOHO's Large Angle and Spectrometric Coronagraph (LASCO) can see CMEs and the Energetic and Relativistic Nucleon and Electron experiment (ERNE) detects their radiation. Wind has an instrument that can pick up a CME's radio signal (The Radio and Plasma Wave experiment).

The team compared observations from both SOHO and Wind and looked at 472 CMEs between 1996 and 2005 that were fast and covered a large area of the sky. They discovered that those CMEs which generated a radio signal also produced radiation storms, but CMEs without a radio signal did not.

Strong CME shocks accelerate electrons in the solar wind, which in turn produce the radio signal. The same strong shock must also accelerate atomic nuclei in the solar wind, which produce the radiation storm, according to the team.

"Since the radio signal moves at the speed of light while the particles lag behind, we can use a CME's radio noise to give warning that it is generating a radiation storm that will hit us soon," said Gopalswamy. "This will give astronauts and satellite operators anywhere between a few tens of minutes to a couple of hours to prepare, depending on how fast the particles are moving."

The team also noticed that most radio-loud CMEs came from parts of the sun in line with Earth (areas near the solar equator), while radio-quiet CMEs mostly came from the edges of the sun. Since all the CMEs studied were fast and could have produced strong shocks, detecting radio noise and radiation from some but not others might simply be due to geometry.

CMEs near the edge of the sun only present a small section of their shock surface towards us as they expand through space, and therefore tend to be radio-quiet and radiation-free from our point of view, according to the team. However, it means that explorers on other worlds in our solar system, like Mars, will need a spacecraft positioned between them and the sun to take advantage of the radio warning.

The research team includes Stuart Jefferies, University of Hawaii, Maui, Hawaii; Scott McIntosh, Southwest Research Institute, Boulder, Colo.; Bart De Pontieu, Lockheed Martin, Palo Alto, Calif.; and Viggo Hansteen, University of Oslo, Norway and Lockheed Martin.

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