On 31 December 2007 NASA's MESSENGER spacecraft made the first detection of solar neutrons at less than one astronomical unit from the Sun.

MESSENGER was just 0.5 AU from the Sun when it was bathed in high energy neutrons ejected in a solar flare, giving scientists a first close up look at neutron production from a solar flare. Previously, only the neutron bursts from the most powerful solar flares have been recorded on neutron spectrometers on Earth or in near-Earth orbit, which typically last for up to a minute 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,” says William Feldman, Co-Investigator for MESSENGER's Neutron Spectrometer, one of two sensors on the Gamma-Ray and Neutron Spectrometer instrument. “From this fact, we inferred the continuous production of protons in the 30-100 million electron volt (MeV) range due to the flare.”

Around 90 percent of all ions produced in a solar flare remain locked to the Sun on closed magnetic field lines, but a second population results from the decay of neutrons near the Sun which can be accelerated through interplanetary space by the shock waves produced by flares.

“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,” explains Feldman. “When coronal mass ejections (nuclear explosions in the corona) send shock waves into space, these feedstock protons are accelerated into interplanetary space.”

There has always been some debate as to why some coronal mass ejections produce almost no energetic protons that reach the Earth, while others produce huge amounts, and now it seems that the answer lies within the seed populations of energetic protons residing near the Sun. “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),” adds Feldman.

The seed populations are not evenly distributed, however, and it is pot luck as to whether they are in the right place for shock waves to send them careering towards the Earth or in a different direction entirely.

Studying the effects of solar flare radiation is of high level interest to spaceflight programs since flares can damage satellites and of course endanger astronauts working on the International Space Station, the Moon or Mars. Fortunately MESSENGER is in an ideal location – between 0.3 and 0.6 AU – to build up a picture of this type of activity over the coming years.

“What we saw and published is what we hope will be the first of many flares we’ll be able to follow through 2012,” says Feldman. “The beauty of MESSENGER is that it’s going to be active from the minimum to the maximum solar activity during Solar Cycle 24, allowing us to observe the rise of a solar cycle much closer to the Sun than ever before.”

MESSENGER will reach orbit around Mercury in March 2011 where it will be within 0.45 AU of the Sun for one year. The results from the December 2007 flare are discussed in a forthcoming paper to be published in the Journal of Geophysical Research.