July 09, 2013

from PHYS Website






Drs. Michael Hahn and Daniel Wolf Savin, research scientists at Columbia University's Astrophysics Laboratory in New York, NY, found evidence that magnetic waves in a polar coronal hole contain enough energy to heat the corona and deposit most of their energy at sufficiently low heights for the heat to spread throughout the corona.


The observations help to answer a 60-year-old solar physics conundrum about the unexplained extreme temperature of the Sun's corona - known as the coronal heating problem.

Hahn and Savin analyzed data from the Extreme Ultraviolet Imaging Spectrometer onboard the Japanese satellite Hinode.


They used observations of a polar coronal hole, a region of the Sun where the magnetic fields lines stretch from the solar surface far into interplanetary space.


They have posted their results at "Observational Quantification of the Energy Dissipated by Alfvén Waves in a Polar Coronal Hole - Evidence that Waves Drive the Fast Solar Wind" and are presenting them at the 44th meeting of the Solar Physics Division (SPD) of the American Astronomical Society, now in progress in Bozeman, Montana.

To understand the coronal heating problem, imagine a flame coming out of an ice cube.


A similar effect occurs on the surface of the Sun. Nuclear fusion in the center of the Sun heats the solar core to 15 million degrees. Moving away from this furnace, by the time one arrives at the surface of the Sun the gas has cooled to a relatively refreshing 6000 degrees.


But the temperature of the gas in the corona, above the solar surface, soars back up to over a searing 1 million degrees. What causes this unexpected temperature inversion has puzzled scientists since 1939.

Two dominant theories exist to explain this mystery.

  • One attributes the heating to the loops of magnetic field which stretch across the solar surface and can snap and release energy.

  • Another ascribes the heating to waves emanating from below the solar surface, which carry magnetic energy and deposit it in the corona.

Observations show both of these processes continually occur on the Sun.


But until now scientists have been unable to determine if either one of these mechanisms releases sufficient energy to heat the corona to such blisteringly high temperatures.

Hahn and Savin's recent observations show that magnetic waves are the answer. The advance opens up a realm of further questions; chief among them is what causes the waves to damp.


Hahn and Savin are planning new observations to try to address this issue.





Heating The Solar Wind
April 03, 2013
from PHYS Website




A picture of the solar corona as seen during an eclipse.

A new paper presents a model that is able

to explain previously puzzling aspects of the coronal wind.



The Sun glows with a surface temperature of about 5500 degrees Celsius.


Meanwhile its hot outer layer (the corona) has a temperature of over a million degrees, and ejects a wind of charged particles at a rate equivalent to about one-millionth of the moon's mass each year.


Some of these wind particles bombard the Earth, producing radio static, auroral glows, and (in extreme cases) disrupted global communications.

There are two important, longstanding, and related questions about the corona that astronomers are working to answer:

  • how is it heated to temperatures that are so much hotter than the surface?

  • how does the corona produce the wind?

A related puzzle of the solar wind is why certain of its ions are hotter than others; one might naively expect uniform heating of the gas, but the temperature of helium ions, for example, is on average five times higher than that of the much lighter mass hydrogen ions.

The answers to these questions appear to involve turbulence and magnetic fields in the Sun's atmosphere.


Writing in the latest issue of Physical Review Letters, CfA astronomers Justin Kasper and Michael Stevens, with their collaborators, present a new model that demonstrates how waves in the hot ionized gas, called "cyclotron waves," will heat the gas, and do so preferentially for heavier ions.


These waves correspond to oscillations in the circular motion of ions as they twist around the magnetic fields present.


The new model explains how energy from these waves is transferred to the particles, thereby heating them. The astronomers were able to use their new model to explain successfully the measured ion temperatures over 17 years of data accumulated from the Wind spacecraft.


The new results represent a major advance in our understanding of how the solar wind works.












Solar Wind Energy Source Discovered
by Dr. Tony Phillips

March 11, 2013
from PHYS Website

Spanish version



Solar wind flows away from the sun

at speeds up to and exceeding 500 km/s (a million mph).

Using data from an aging NASA spacecraft, researchers have found signs of an energy source in the solar wind that has caught the attention of fusion researchers.


NASA will be able to test the theory later this decade when it sends a new probe into the sun for a closer look.

The discovery was made by a group of astronomers trying to solve a decades-old mystery:

What heats and accelerates the solar wind?

The solar wind is a hot and fast flow of magnetized gas that streams away from the sun's upper atmosphere. It is made of hydrogen and helium ions with a sprinkling of heavier elements.


Researchers liken it to the steam from a pot of water boiling on a stove; the sun is literally boiling itself away.

"But," says Adam Szabo of the NASA Goddard Space Flight Center, "solar wind does something that steam in your kitchen never does. As steam rises from a pot, it slows and cools. As solar wind leaves the sun, it accelerates, tripling in speed as it passes through the corona. Furthermore, something inside the solar wind continues to add heat even as it blows into the cold of space."

Finding that "something" has been a goal of researchers for decades.


In the 1970s and 80s, observations by two German/US Helios spacecraft set the stage for early theories, which usually included some mixture of plasma instabilities, magnetohydrodynamic waves, and turbulent heating. Narrowing down the possibilities was a challenge.


The answer, it turns out, has been hiding in a dataset from one of NASA's oldest active spacecraft, a solar probe named Wind. Using Wind to unravel the mystery was, to Justin Kasper of the Harvard-Smithsonian Center for Astrophysics, a "no brainer."


He and his team processed the spacecraft's entire 19-year record of solar wind temperatures, magnetic field and energy readings and...

"I think we found it," he says. "The source of the heating in the solar wind is ion cyclotron waves."

Ion cyclotron waves are made of protons that circle in wavelike-rhythms around the sun's magnetic field.


According to a theory developed by Phil Isenberg (University of New Hampshire) and expanded by Vitaly Galinsky and Valentin Shevchenko (UC San Diego), ion cyclotron waves emanate from the sun; coursing through the solar wind, they heat the gas to millions of degrees and accelerate its flow to millions of miles per hour.


Kasper's findings confirm that ion cyclotron waves are indeed active, at least in the vicinity of Earth where the Wind probe operates.


An artist's concept of the Wind spacecraft

sampling the solar wind.

Justin Kasper's science result is inset.


Ion cyclotron waves can do much more than heat and accelerate the solar wind, notes Kasper.

"They also account for some of the wind's very strange properties."

The solar wind is not like wind on Earth.


Here on Earth, atmospheric winds carry nitrogen, oxygen, water vapor along together; all species move with the same speed and they have the same temperature. The solar wind, however, is much stranger.


Chemical elements of the solar wind such as hydrogen, helium, and heavier ions, blow at different speeds; they have different temperatures; and, strangest of all, the temperatures change with direction.

"We have long wondered why heavier elements in the solar wind move faster and have higher temperatures than the lighter elements," says Kasper. "This is completely counterintuitive."

The ion cyclotron theory explains it: Heavy ions resonate well with ion cyclotron waves. Compared to their lighter counterparts, they gain more energy and heat as they surf.

The behavior of heavy ions in the solar wind is what intrigues fusion researchers. Kasper explains:

"When you look at fusion reactors on Earth, one of the big challenges is contamination. Heavy ions that sputter off the metal walls of the fusion chamber get into the plasma where the fusion takes place. Heavy ions radiate heat. This can cool the plasma so much that it shuts down the fusion reaction."

Ion cyclotron waves of the type Kasper has found in the solar wind might provide a way to reverse this process.


Theoretically, they could be used to heat and/or remove the heavy ions, restoring thermal balance to the fusing plasma.

"I have been invited to several fusion conferences to talk about our work with the solar wind," he says.

The next step, agree Kasper and Szabo, is to find out if ion cyclotron waves work the same way deep inside the sun's atmosphere where the solar wind begins its journey.


To find out, NASA is planning to send a spacecraft into the sun itself.

Solar Probe Plus, scheduled for launch in 2018, will plunge so far into the sun's atmosphere that the sun will appear as much as 23 times wider than it does in the skies of Earth. At closest approach, about 7 million km from the sun's surface, Solar Probe Plus must withstand temperatures greater than 1400 deg. C and survive blasts of radiation at levels not experienced by any previous spacecraft.


The mission's goal is to sample the sun's plasma and magnetic field at the very source of the solar wind.

"With Solar Probe Plus we'll be able to conduct specific tests of the ion cyclotron theory using sensors far more advanced than the ones on the Wind spacecraft," says Kasper.


"This should give us a much deeper understanding of the solar wind's energy source."

The research described in this story was published in the Physical Review Letters on February 28, 2013: "Sensitive Test for Ion-Cyclotron Resonant Heating in the Solar Wind" by Justin Kasper et al.

Launched in 1994, Wind is so old that it uses magnetic tapes similar to old-fashioned 8-track tapes to record and play back its data.


Equipped with heavy shielding and double-redundant systems to safeguard against failure, the spacecraft was built to last; at least one researcher at NASA calls it the "Battlestar Gallactica" of the heliophysics fleet.


Wind has survived almost two complete solar cycles and innumerable solar flares.

"After all these years, Wind is still sending us excellent data," says Szabo, the mission's project scientist, "and it still has 60 years' worth of fuel left in its tanks."