Solar flare neutrons observed on the ground and in space
From RHESSI Wiki
Contents |
Introduction
Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a complementary window on solar flare 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 by Muraki et al. combining ground-based and space neutron measurements associated with an M-class solar flare.
Neutron detection on the ground
Despite their name, neutron monitors (NM) normally detect not neutrons from space, but the secondary neutrons produced in the lead surround of the NM by cosmic ray particles. On only a handful of occasions has the signature of genuinely solar neutrons been diagnosed in the NM network. Unlike the NM signal produced by solar energetic particles (SEP's), the count rate due to neutrons depends on distance from the sub-solar point, on altitude, and not on the geomagnetic location - or cutoff rigidity - of the NM station.
Neutron monitors simply count neutrons - they have more or less thermalised (no apology for UK spelling!) when they capture in the NM tubes. In the last three decades neutron telescopes for medium- to high-energy neutrons have been established at a variety of high altitude locations. These combine scintillator crystals and proportional counters in an anti-coincidence arrangement that rejects charged particles and in some cases also gives a directional capability. Sensitive to neutrons in the 100 MeV to few GeV energy range, they will be most effective at low to mid latitudes. Muraki et al. employ measurements from the neutron telescopes on Mount Sierra Negra in Mexico (4580 m above sea level) and on Mount Chacaltaya in Bolivia (5250 m above sea level). Chacaltaya is the highest cosmic ray station in the world but both these mountaintop observatories sit below a column depth of atmosphere substantially less than at sea level - typically 500 - 600 g.cm-2 as opposed to > 1000 g.cm-2.
Neutrons in space
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, SEP's can impact the body of the spacecraft to generate energetic neutrons, temporally associated with flares 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.
The SEDA-NEM instrument on the International Space Station (ISS) employs a related approach rooted in the kinematics of neutron secondary production. It is composed of many strips of organic scintillator, each 3 x 6 x 96 mm. Each layer of the instrument includes 16 such strips and alternate layers are oriented perpendicular to one another. 16 layers make a cubic detector. Energetic neutrons enter the instrument and generate proton secondaries, whose track through several of the scintillator strips reveals their energies and directions. The energy and direction of the primary neutron follow from kinematics. The outermost detector layer serves for anti-coincidence rejection of incident charged particles. Neutrons inconsistent with a solar origin may be rejected and the background greatly reduced (Muraki et al. state that background is reduced by a factor of 36).
SOL2014-07-08
Muraki et al. searched the data from these instruments for possible enhancements around the times of M- or X-class solar flares (the precise periods and flare classes are in their paper). They found just one event with statistically significant enhancements in all three instruments, coinciding roughly with a M6.5 flare at ~1608 UT on 8 July, 2014. As seen by SDO this is a beautiful eruptive event:
RHESSI was annealing at this time. Also Fermi chose to hide in the SAA for about 10 minutes just as the flare was at its peak so there is a lack of corroborating, high-energy observations. Also there was no LAT enhancement above background around the time of the flare. This was an eruptive event with an associated CME, however, and a strong Type II radio burst provides evidence of at least some nonthermal activity in the corona.
