Tiny Flares Responsible for Outsized Heat of Sun's Atmosphere

Aug 14, 2009 by Laura Layton
This false-color temperature map shows solar active region AR10923, observed close to center of the sun's disk. Blue regions indicate plasma near 10 million degrees K. Credit: Reale, et al. (2009)

(PhysOrg.com) -- Solar physicists at NASA have confirmed that small, sudden bursts of heat and energy, called nanoflares, cause temperatures in the thin, translucent gas of the sun's atmosphere to reach millions of degrees.

"Why is the sun's corona so darned hot?" asks James Klimchuk, an astrophysicist at the Goddard Space Flight Center's Solar Physics Laboratory in Greenbelt, Md.

The mystery of why temperatures in the solar corona, the sun's outer atmosphere, soar to several million degrees Kelvin (K) —much hotter than temperatures nearer the sun's surface—has puzzled scientists for decades. New observations made with instruments aboard Japan's Hinode satellite reveal the culprit to be nanoflares.

Nanoflares are small, sudden bursts of heat and energy. "They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop," says Klimchuk. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun's corona.

Scientists previously thought steady heating explained the corona's million degree temperatures. The steady heating model indicates that a coronal loop of a given length and temperature should have a specific density. However, observations showed that coronal loops have much higher density than the steady heating model predicts. Newer models based on nanoflares can explain the observed density. But no direct evidence of the nanoflares existed until now.

Observations from the NASA-funded X-Ray Telescope (XRT) and Extreme-ultraviolet Imaging Spectrometer (EIS) instruments aboard Hinode reveal that ultra-hot plasma is widespread in solar active regions. The XRT measured plasma at 10 million degrees K, and the EIS measured plasma at 5 million degrees K. "These temperatures can only be produced by impulsive energy bursts,"says Klimchuk, who presented the findings on August 6 at the International Astronomical Union General Assembly meeting in Rio de Janeiro, Brazil.

"Coronal loops are bundles of unresolved strands that are heated by storms of nanoflares."

Two active regions appear as bright areas on this full-disk image of the sun, taken with the Hinode spacecraft's X-Ray Telescope. Credit: NASA

Coronal heating is a dynamic process. The brightness of the observed X-ray and ultraviolet emission is strongly dependent on the density of the coronal plasma. Where there's low density, there isn't much brightness. Where there's high density, there's a lot of brightness. The corona is mostly bright at about 1 million degrees K.

Klimchuk and colleagues constructed a theoretical model to explain how plasma evolves within these coronal tubes and what causes temperatures to skyrocket. "We simulate a burst of heating and see how the corona responds," says Klimchuk. "Then we make predictions about how much emission we should see from plasma of different temperatures."

Klimchuk surmises that when a nanoflare suddenly releases its energy, the plasma in the low-temperature, low-density strands becomes very hot—around 10 million degrees K—very quickly. The density remains low, however, so the emission, or brightness, remains faint. Heat flows from up in the strand, where it's hot, down to the base of the coronal loop, where it's not as hot. This heats up the dense plasma at the loop’s base. Because it is so dense at the base, the temperature only reaches about 1 million degrees K. This dense plasma expands up into the strand. Thus, a coronal loop is a collection of 5-10 million degree K faint strands and 1 million degree K bright strands.

"What we see is 1 million degree K plasma that has received its energy from the heat flowing down from the superhot plasma," says Klimchuk. "For the first time, we have detected this 10 million degree plasma, which can only be produced by the impulsive energy bursts of nanoflares."

The observations and the scientists' analysis verify that nanoflares are occurring on the sun and that they explain much and perhaps most coronal heating. The observations also confirm "there is some nanoflare activity everywhere" in the sun's active regions, says Klimchuk.

Nanoflares are responsible for changes in the X-ray and ultraviolet (UV) radiation that happen as an active region evolves. X-ray and UV get absorbed by Earth's upper atmosphere, which heats up and expands. Changes in the upper atmosphere can affect the orbits of satellites and space debris by slowing them down, an effect known as "drag." It is important to know the changing orbits so that maneuvers can be made to avoid space collisions. The X-ray and UV also affect the propagation of radio signals and thereby adversely affect communication and navigation systems.

The discovery that nanoflares play an important and perhaps dominant role in coronal heating paves the way to understanding how the affects Earth, our place in the universe.

Source: NASA/Goddard Space Flight Center

Explore further: First potentially habitable Earth-sized planet confirmed: It may have liquid water

add to favorites email to friend print save as pdf

Related Stories

'Focused' Solar Explosions Get Hotter

Apr 02, 2008

A NASA-funded researcher has discovered that solar flares -- explosions in the atmosphere of the sun -- get much hotter when they stay "focused".

The Sun's X-file under the Spotlight

Sep 03, 2004

One of the Sun's greatest mysteries is about to be unravelled by UK solar astrophysicists hosting a major international workshop at the University of St Andrews from September 6-9th 2004. For years scientists have been baffled ...

Skeleton Of Sun's Atmosphere Reveals Its True Nature

Apr 16, 2007

The Sun's outer atmosphere or corona is incredibly complex, as shown in observations from space. It is also extremely hot, with a temperature of over a million degrees by comparison with that of the Sun's ...

The Spooky Sun

Oct 31, 2006

Just in time for Halloween, astronomers have taken a haunting new portrait of the sun. In this color-coded image from the Hinode spacecraft (formerly Solar-B), the sun glows eerily orange as though celebrating ...

Hinode's X-Ray Telescope Reveals the Sun's Secrets

Mar 21, 2007

Even though the sun is the closest star to Earth and has been studied for hundreds of years, it still holds surprises. The recently launched Hinode spacecraft is one of the latest observatories to probe the ...

Recommended for you

A sharp eye on Southern binary stars

20 hours ago

Unlike our sun, with its retinue of orbiting planets, many stars in the sky orbit around a second star. These binary stars, with orbital periods ranging from days to centuries, have long been the primary ...

Hubble image: A cross-section of the universe

20 hours ago

An image of a galaxy cluster taken by the NASA/ESA Hubble Space Telescope gives a remarkable cross-section of the Universe, showing objects at different distances and stages in cosmic history. They range ...

Cosmologists weigh cosmic filaments and voids

Apr 17, 2014

(Phys.org) —Cosmologists have established that much of the stuff of the universe is made of dark matter, a mysterious, invisible substance that can't be directly detected but which exerts a gravitational ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Slotin
2.3 / 5 (3) Aug 14, 2009
heres another explanation based on Alphven waves, which are driving X-ray spikes
http://www.iop.or...ext.html
http://www.scienc...856/1580

omatumr
1 / 5 (4) Aug 15, 2009
EARTH'S HEAT SOURCE - THE SUN

It is great that NASA has a renewed interest in the Sun, the object that heats planet Earth and sustains life here.

Welcome to reality!

For decades NASA has steadfastly refused to even consider experimental data that showed our Sun is not the ball of Hydrogen (H) described by the standard solar model (ssm).

Now an astrophysicist at NASA's Goddard Space Flight Center's Solar Physics Laboratory in Greenbelt, Md finally asks, [quote] Why is the sun's corona so darned hot? [end quote]

[quote] These temperatures can only be produced by impulsive energy bursts,[end quote] he answers.

Deep-seated magnetic fields that produce energy outbursts from the Iron Sun have already been explained:

1. Superfluidity in the solar interior: Implications for solar eruptions and climate, Journal of Fusion Energy 21 (2002) 193-198; http://arxiv.org/...501441v1

2. Observational confirmation of the Sun's CNO cycle, Journal of Fusion Energy 25 (2006) 107-114; http://arxiv.org/...512633v2

3. Earth's Heat Source - The Sun, Energy and Environment 20 (2009) 131-144 [SPECIAL ISSUE: Natural drivers of weather and climate] http://arxiv.org/pdf/0905.0704

With kind regards,
Oliver K. Manuel
http://www.omatumr.com

More news stories

Hubble image: A cross-section of the universe

An image of a galaxy cluster taken by the NASA/ESA Hubble Space Telescope gives a remarkable cross-section of the Universe, showing objects at different distances and stages in cosmic history. They range ...

Male monkey filmed caring for dying mate (w/ Video)

(Phys.org) —The incident was captured by Dr Bruna Bezerra and colleagues in the Atlantic Forest in the Northeast of Brazil.  Dr Bezerra is a Research Associate at the University of Bristol and a Professor ...

'Exotic' material is like a switch when super thin

(Phys.org) —Ever-shrinking electronic devices could get down to atomic dimensions with the help of transition metal oxides, a class of materials that seems to have it all: superconductivity, magnetoresistance ...