Imagine the Universe!

Solar Flares

The Sun Shines on Compton's Front Porch

Image of Solar Flare
SOON Telescope image of a solar flare.

Solar flares are explosions of energetic particles and electromagnetic radiation in the outer atmosphere of the sun. Lessons learned from solar explosions apply to much larger explosions that we see elsewhere in the Universe. Closer to home, these particles from the Sun can cause communications and electrical problems on Earth.

Compton was launched just after the last solar maximum, a period of increased solar activity. Fortunately the Sun was still active, and Compton got itself a tan with several large flares in June 1991. In this regard, Compton was following in the footsteps of OSO Orbiting Solar Observatory 7 (1971-'74) and the Gamma-Ray Spectrometer on the Solar Maximum Mission (1980-'81, '84-'89), the only other space missions to see gamma-ray emission lines in solar flares.

OSSE detected several nuclear emission lines from a solar flare on June 4, 1991, including lines from iron, magnesium, neon, silicon, carbon, oxygen and nitrogen. These give information about the abundances of elements in the ambient coronal gas. EGRET detected a high-energy afterglow from a solar flare on June 11, 1991. No high-energy cutoff was detected, so presumably higher-energy photons were also produced. The emission lasted for at least eight hours after the impulsive phase of the flare. This afterglow may have been due to either continuous acceleration or long-term plasma trapping.

COMPTEL detected neutrons from a solar flare on June 15, 1991. This resulted in the first particle image of any astrophysical source. The Sun may be the only astrophysical source that will ever be imaged in neutrons, since neutrons decay with a half-life of only five minutes. COMPTEL also detected a gamma-ray afterglow from the same flare. In this case, the particles were likely not just accelerated during the impulsive phase at the beginning of the flare, but were continuously accelerated over an extended period of time.

Neutron Image of June 15, 1991 Solar
CGRO Comptel's Neutron Image of the Sun
showing solar flare of June 15, 1991

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Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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