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RAS PN 08/25 (NAM 16): "Focused" solar explosions get hotter

Last Updated on Tuesday, 06 April 2010 18:28
Published on Wednesday, 02 April 2008 00:01
Using data from the Hinode and RHESSI solar observatories, astronomers have discovered that solar flares - explosions in the atmosphere of the sun - get much hotter when they stay "focused".

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Date:  28 March 2008
Ref.: PN 08/25 (NAM 16)

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RAS PN 08/25 (NAM 16) (EMBARGOED): "FOCUSED" SOLAR EXPLOSIONS GET HOTTER

Using data from the Hinode and RHESSI solar observatories, astronomers have discovered that solar flares - explosions in the atmosphere of the sun - get much hotter when they stay "focused".

Dr Ryan Milligan of Oak Ridge Association of Universities, Tennessee, who is stationed at NASA’s Goddard Space Flight Center in the US, will present his result on Wednesday 2 April at RAS National Astronomy Meeting in Belfast.

Solar flares are caused by the sudden release of magnetic energy. The largest can release as much energy as a billion one-megaton nuclear bombs. However, the flare observed in this study was a much more common "micro" flare. Researchers at space agencies like NASA and ESA want to understand flares because they generate radiation that can be hazardous to unprotected astronauts, like those walking on the surface of the Moon.

"A flare typically divides its energy between directly heating the solar atmosphere and accelerating particles," said Dr. Milligan. "These unusual flares seem to focus on one task, devoting all their energy to heating. This allows them to become millions of degrees hotter than their multi-tasking cousins."

Flares normally occur above loops of electrically conducting gas, called plasma, in the Sun's atmosphere. When a typical flare goes off, it heats the plasma and sends beams of electrons racing down the sides of the loops. The electron beams evaporate more plasma from the Sun's visible surface, which expands back up the loops.

"This evaporated plasma has traditionally been believed to be the source of the hottest temperatures seen in solar flares," said Dr Milligan. "However, the flare in this new observation reached a temperature of 15 million degrees Celsius - some five million degrees hotter than expected for a flare of this size - without any evidence for beams of accelerated electrons."

Dr Milligan used the RHESSI and Hinode spacecraft to make his observation of the microflare on 7 June 2007. RHESSI revealed that the flare had a peak temperature of 15 million degrees, and also that the flare showed no evidence for high energy electrons. Hinode was able to show the effects of the energy released at various layers in the solar atmosphere. In particular, the Extreme ultraviolet Imaging Spectrometer (EIS) instrument was used to detect signatures of plasma evaporation from the Sun's surface. The low velocities observed confirmed the RHESSI observation that high energy electrons were not present.

"If our assumption is correct, then this result tells us that the energy released during a solar flare is more efficient at achieving a higher temperature if the energy is used to directly heat the plasma in the Sun's atmosphere, instead of being divided between heating and particle acceleration. This very effect has recently been shown in simulations of energy release during microflares," said Dr Milligan.

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NOTES FOR EDITORS

The research was funded by the NASA Postdoctoral Program administered by the Oak Ridge Association of Universities, Tennessee.

The Hinode (Japanese for sunrise) mission was launched in October 2006. It is used to study magnetic fields on the Sun and their role in powering the solar atmosphere and driving solar eruptions. Hinode was developed and launched by the Japanese Space Agency ISAS/JAXA, with the National Astronomical Observatory of Japan (NAOJ) as the domestic partner and NASA and the UK Science and Technology Facilities Council (STFC) as international partners. It is operated by these agencies in co-operation with the European Space Agency (ESA) and the Norwegian Space Centre (NSC). The Extreme Ultraviolet Imaging Spectrometer (EIS) was built by a consortium led by the Mullard Space Science Laboratory in the UK.

The RHESSI project is a NASA Small Explorer mission managed by the Space Sciences Laboratory of the University of California, Berkeley. The Explorers Program Office at Goddard provides management and technical oversight under the direction of the Heliophysics Division of Science Missions Directorate at NASA Headquarters in Washington, D.C.

The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen’s University Belfast. It is principally sponsored by the RAS and the Science and Technology Facilities Council (STFC). NAM 2008 is being held together with the UK Solar Physics (UKSP) and Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring meetings.

CONTACT

Dr. Ryan Milligan
Code 671
Solar Physics Laboratory
Heliophysics Science Division
NASA Goddard Space Flight Center
Greenbelt, MD
USA 20771
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