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MESSENGER Mission News
July 9, 2014

MESSENGER and STEREO Measurements Open New Window Into High-Energy Processes on the Sun
Understanding the Sun from afar isn't easy. How do you figure out what powers solar flares -- the intense bursts of radiation coming from the release of magnetic energy associated with sunspots -- when you must rely on observing only the light and particles that make their way to Earth's orbit? One answer: you get closer.

NASA's MESSENGER spacecraft -- which orbits Mercury, and so is as close as 28 million miles from the Sun versus Earth's 93 million miles -- is close enough to the Sun to detect solar neutrons that are created in solar flares. The average lifetime for one of these neutrons is only 15 minutes. How far they travel into space depends on their speed, and slower neutrons don't travel far enough to be seen by particle detectors in orbit around Earth. Results showing that MESSENGER has likely observed solar neutrons appeared in the Journal of Geophysical Research: Space Physics today.

"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, first author of the paper at The Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "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, because they are not 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.

Lawrence and his team looked at MESSENGER data from June 4 and 5, 2011, corresponding to flares that were also accompanied by energetic charged particles. The flare occurred on the far side of the Sun so that Earth-based views of the flare region could not be obtained. However, a solar telescope on the STEREO spacecraft did have a clear view of the far-side flare region and provided critical observations of the flare. This combined use of NASA mission data makes each individual mission more effective in addressing unsolved science questions.

The scientists observed an increase in the number of neutrons at Mercury's orbit hours before the large number of accelerated charged particles reached the spacecraft. This neutron arrival time combined with other information indicated that the neutrons were most likely from the Sun and produced by accelerated flare particles striking the lower solar atmosphere. Such high-energy collisions can break apart heavy ions and release neutrons. MESSENGER and STEREO data can thus provide new information about how particles are accelerated in solar flares.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft was launched on August 3, 2004, and entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to begin a yearlong study of its target planet. MESSENGER's first extended mission began on March 18, 2012, and ended one year later. MESSENGER is now in a second extended mission, which is scheduled to conclude in March 2015. Dr. Sean C. Solomon, the Director of Columbia University's Lamont-Doherty Earth Observatory, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

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