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NASA’s MESSENGER Spacecraft Observes Solar Neutrons

July 9, 2014
Tucson, Ariz. -- Scientists using data from NASA’s MESSENGER spacecraft have discovered part of the outer layer of the Sun that is sending out modestly energetic subatomic particles called fast neutrons.
MESSENGER’s close proximity to the Sun – the spacecraft orbits Mercury at a distance as close as 28 million miles from the Sun, compared to Earth’s 93 million miles – allowed instruments to detect solar neutrons as they flowed past Mercury into space, according to a paper in Journal of Geophysical Research: Space Physics.
"To understand all the processes on the Sun we look at as many different particles coming from the Sun as we can – photons, electrons, protons, neutrons, gamma rays – to gather different kinds of information," said David Lawrence of the Johns Hopkins Applied Physics Lab, the first author on the paper. "Closer to Earth we can observe charged particles from the Sun, but analyzing them can be a challenge as their journey is affected by magnetic fields." 
Such charged particles twirl and gyrate around the magnetic field lines created by the vast magnetic systems that surround the Sun and Earth. Neutrons, however, not being electrically charged, travel in straight lines from the flaring region and can carry information about flare processes unperturbed by the environment through which they move.  Information provided by neutrons can be used by scientists to decipher one aspect of the complicated acceleration processes that are responsible for the creation of solar energetic particles.
Solar neutrons have an average lifetime of about 15 minutes before they decay into a proton, an electron, and an electron-type antineutrino, so that modest-energy fast neutrons don’t travel far enough to be seen by solar telescopes in orbit around Earth.
“The observation of solar neutrons made using MESSENGER’s neutron spectrometer could not have been made near Earth because they decay before they get here,” said Planetary Science Institute Senior Scientist and co-author William Feldman, the MESSENGER mission Co-Investigator responsible for the neutron spectrometer. “However, this is not true for an orbit around Mercury, which is about three times closer to the Sun than is the Earth.”
These neutrons form an exosphere above the corona that decay to a large swarm of equally energetic protons that in turn, expand along the magnetic field that is carried by the expanding solar atmosphere into interplanetary space, said Feldman.
MESSENGER was launched in August 2004 and was inserted into orbit about Mercury in March 2011.  The spacecraft’s Neutron Spectrometer observed several neutron enhancements from solar eruptive events. 
Visit to see an image of a solar flare eruption.
Feldman’s research is funded under a subcontract to PSI from the Carnegie Institution of Washington as part of NASA’s MESSENGER mission.
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