Solar flare neutrons observed on the ground and in space
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The possibility of detecting energetic neutrons from flares was first aired at the very start of the 1950s by [http://wwwmpa.mpa-garching.mpg.de/mpa/institute/news_archives/news1101_biermann/news1101_biermann-en-print.html Ludwig Biermann], but the first detection was [http://adsabs.harvard.edu/abs/1987ApJ...318..913C 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 [http://adsabs.harvard.edu/abs/1983ApJ...274..875E convincing evidence for neutron decay protons] in space sealed the discovery (free neutrons are unstable, beta decaying with a mean lifetime of 880 s). [[Solar Cosmic Rays, Neutrons, and Fermi Gamma-Rays | A previous nugget]] looked at energetic neutron detections at 1 AU along with Fermi LAT gamma-ray data; here we discuss [http://adsabs.harvard.edu/abs/2016SoPh..291.1241M| a recent report] combining ground-based and space neutron measurements associated with an M-class solar flare. | The possibility of detecting energetic neutrons from flares was first aired at the very start of the 1950s by [http://wwwmpa.mpa-garching.mpg.de/mpa/institute/news_archives/news1101_biermann/news1101_biermann-en-print.html Ludwig Biermann], but the first detection was [http://adsabs.harvard.edu/abs/1987ApJ...318..913C 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 [http://adsabs.harvard.edu/abs/1983ApJ...274..875E convincing evidence for neutron decay protons] in space sealed the discovery (free neutrons are unstable, beta decaying with a mean lifetime of 880 s). [[Solar Cosmic Rays, Neutrons, and Fermi Gamma-Rays | A previous nugget]] looked at energetic neutron detections at 1 AU along with Fermi LAT gamma-ray data; here we discuss [http://adsabs.harvard.edu/abs/2016SoPh..291.1241M| a recent report] combining ground-based and space neutron measurements associated with an M-class solar flare. | ||
| - | == | + | ==Neutrons in space== |
| - | <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 | + | <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 continually encounter "albedo" neutrons, spat out in nuclear reactions of [http://helios.gsfc.nasa.gov/gcr.html | galactic cosmic ray] particles with atmospheric nuclei; the same neutrons that [http://www.phy6.org/Education/winbelt.html | populate the inner radiation belt via the CRAND mechanism]. Even worse for the phenomena we want to study, Solar Energetic Particles (SEPs) can impact the body of the spacecraft to generate energetic neutrons, temporally associated with flares, etc., but of local, as opposed to solar origin. |
| + | |||
| + | To distinguish genuinely solar neutrons we really need a detector with some angular discrimination. In astrophysics, [http://imagine.gsfc.nasa.gov/features/satellites/archive/cgro_solarflares.html | Comptel showed the way]: in a <em>multiple scatter</em> instrument, 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. | ||
Revision as of 11:39, 4 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.
Neutrons in space
<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 continually encounter "albedo" neutrons, spat out in nuclear reactions of | galactic cosmic ray particles with atmospheric nuclei; the same neutrons that | populate the inner radiation belt via the CRAND mechanism. Even worse for the phenomena we want to study, Solar Energetic Particles (SEPs) can impact the body of the spacecraft to generate energetic neutrons, temporally associated with flares, etc., but of local, as opposed to solar origin.
To distinguish genuinely solar neutrons we really need a detector with some angular discrimination. In astrophysics, | Comptel showed the way: in a multiple scatter instrument, 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.
