Solar flare neutrons observed on the ground and in space

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== Introduction ==
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Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a complementary window on solar ion acceleration. Free neutrons are produced when accelerated ions in the MeV energy range and above collide with ambient nuclei. Some slow down and thermalise in the solar atmosphere, contributing to the observed flux in the 2.223 MeV deuterium formation line. Others escape completely from the solar atmosphere, potentially to be detected in space or even on Earth.   
Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a complementary window on solar ion acceleration. Free neutrons are produced when accelerated ions in the MeV energy range and above collide with ambient nuclei. Some slow down and thermalise in the solar atmosphere, contributing to the observed flux in the 2.223 MeV deuterium formation line. Others escape completely from the solar atmosphere, potentially to be detected in space or even on Earth.   

Revision as of 21:30, 3 August 2016

Introduction

Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a complementary window on solar ion acceleration. Free neutrons are produced when accelerated ions in the MeV energy range and above collide with ambient nuclei. Some slow down and thermalise in the solar atmosphere, contributing to the observed flux in the 2.223 MeV deuterium formation line. Others escape completely from the solar atmosphere, potentially to be detected in space or even on Earth.

The possibility of detecting energetic neutrons from flares was first aired at the very start of the 1950s by Ludwig Biermann, but the first detection was only achieved some decades later, using the Gamma-Ray Spectrometer instrument on the Solar Maximum Mission. Simultaneous detection in ground-based neutron monitors offered confirmation, and convincing evidence for neutron decay protons in space sealed the discovery (free neutrons are unstable, beta decaying with a mean lifetime of 880 s). A previous nugget looked at energetic neutron detections at 1 AU along with Fermi LAT gamma-ray data; here we discuss a recent report combining ground-based and space neutron measurements associated with an M-class solar flare.

<a href="https://heasarc.gsfc.nasa.gov/docs/cgro/cgro/comptel.html"><img src="https://heasarc.gsfc.nasa.gov/docs/cgro/images/cgro/comptel.gif" alt="Comptel graphic" align="left"></a>Spacecraft in low Earth orbit are continually bombarded by "albedo" neutrons, spat out in nuclear reactions of | galactic cosmic ray particles with atmospheric nuclei. Even worse for the phenomena we want to study, Solar Energetic Particles (SEPs) impacting the body of the spacecraft can generate energetic neutrons, temporally associated with flares, etc., but of local, as opposed to solar origin. To distinguish solar neutrons we really need a detector with some angular discrimination. In astrophysics, | Comptel showed the way: we need a multiple scatter instrument, in which energy deposits in two or more detectors allow the total energy and arrival directions of neutrons to be deduced. Then neutrons that scatter cleanly (i.e. elastically) in the instrument, in a way that is consistent with a solar origin, may be selected, and the background greatly reduced.

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