NASA's MESSENGER gets closest-ever look at solar-flare neutrons
27 Oct
2009, 1412 Hrs
Washington, October 27 The Neutron Spectrometer flying aboard NASA's MESSENGER spacecraft has achieved the first-ever, up-close look at neutron production from a solar flare.
In fact, it was the first time that scientists detected solar neutrons at less than 1 AU (Astronomical Unit) from the Sun.
When the flare erupted, MESSENGER was flying at about half an AU, according to William C. Feldman, a Senior Scientist at the Tucson-based Planetary Science Institute.
"For the first time, scientists were able to directly observe the neutron output from an average-sized solar flare," Feldman said.
Previously, only the neutron bursts from the most powerful solar flares have been recorded on neutron spectrometers on Earth or in near-Earth orbit, he added. hese bursts typically last about 50 to 60 seconds at the Sun.
"But we recorded neutrons from this flare over a period of six to ten hours, and what that's telling us is that at least some moderate-sized flares continuously produce high-energy neutrons in the solar corona," Feldman said.
"From this fact, we inferred the continuous production of protons in the 30-to-100-MeV (million electron volt) range due to the flare," he added.
About 90 percent of all ions produced by a solar flare remain locked to the Sun on closed magnetic lines, but another population results from the decay of the neutrons near the Sun.
This second population of decayed neutrons forms an extended seed population in interplanetary space that can be further accelerated by the massive shock waves produced by the flares, according to Feldman.
"So, the important results are that perhaps after many flare events, two things may occur: continuous production of neutrons over an extended period of time and creation of seed populations of neutrons near the Sun that have decayed into protons," he explained.
"When coronal mass ejections (nuclear explosions in the corona) send shock waves into space, these feedstock protons are accelerated into interplanetary space," said Feldman.
"There has always been the question of why some coronal mass ejections produce almost no energetic protons that reach the Earth, while others produce huge amounts," he added.
"It appears that these seed populations of energetic protons near the Sun could provide the answer, because it's easier to accelerate a proton that already has an energy of 1 MeV than a proton that is at 1 keV (the solar wind)," he said.
MESSENGER is currently orbiting the Sun between 0.3 and 0.6 AU on its way to orbit insertion around Mercury in March 2011. (ANI)
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