Solar flare neutrons observed on the ground and in space
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|1st Author:||Alexander MacKinnon|
|Published:||13 August 2016|
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Trickier to detect than gamma-rays, energetic neutrons nonetheless have useful stories to tell. 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. They give information on flare accelerated ions, complementary to that obtained from gamma-rays.
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 energy range, they will be most effective at low to mid latitudes. Muraki et al. employ measurements from the neutron telescopes on Sierra Negra peak 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 of these mountaintop observatories sit below a column depth of atmosphere substantially less than that 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.
To the data from the mountaintop observatories, Muraki et al. add measurements from the SEDA-NEM instrument on the International Space Station (ISS). SEDA-NEM 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 when one selects events within a cone of 2π/3 steradians around the solar direction.
Muraki et al. searched the data from these instruments for possible enhancements around the times of GOES 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. Muraki et al. identify features of its time development coinciding with the episodes of neutron production.
There are statistically significant enhancements in both the Chacaltaya and Sierra Negra neutron telescopes at ~16:07 and ~16:35,the former at the 6 - 7 sigma level, in all of the instruments' neutron energy channels. The similarity of the time profiles, from geographically widely separated experiments, argues strongly for the reality of the event. The presence of a signal in the highest energy channel indicates the presence of ~GeV energy neutrons at the top of the Earth's atmosphere. Unfortunately the ISS was in spacecraft night at this time but a search of the SEDA-NEM data from later time intervals found 30 candidate neutrons at later times; less energetic neutrons that arrive later at Earth. Because the energies of these neutrons are known the time profile of their production at the Sun can be reconstructed. Evidently neutron production continued for some tens of minutes.
There is an associated CME in the SoHO LASCO Catalog. Events observed by Fermi LAT at > 100 MeV are often associated with fast CME's. At ~ 700 km.s-1 this CME was not exceptionally fast, however.
Corroborating high-energy observations are lacking. RHESSI was annealing at this time, hence unavailable. Fermi GBM saw the onset of a hard X-ray flare but there are no data for about 7 critical minutes around the peak of the event - presumably a spectacularly badly timed SAA passage. Fermi LAT events sometimes continue for tens of minutes or hours but there is no significant enhancement above background around the time of this flare. A strong Type II radio burst was seen by WIND/WAVES and by several stations in the e-Callisto network, providing at least some evidence of nonthermal phenomena associated with a shock in the corona.
A few comments
Without the neutron observations we would not have suspected this M-class flare of ion acceleration, far less of ~GeV energy ions. The absence of GBM data at the peak of the impulsive phase is a stroke of bad luck. Clearly the neutron observations have something useful to say about flare ion acceleration. The ability to determine the neutron energy distribution, which in turn could be used to deduce the energetic ion distribution at the Sun, is a further strength.
It's interesting to think about the relative merits of detectors on the ground and in space. A spaceborne instrument must meet stringent constraints of mass, power, etc. Nonetheless the SEDA-NEM detector yields information on interacting ions at the Sun. Even at high altitudes, ground-based neutron telescopes view solar neutrons through the complication of the terrestrial atmosphere. Neutrons have scattered and generated secondaries en route to the telescope. On the other hand one can easily build a much larger instrument, for instance 4 x 1m2, as in this case.
It seems clear that flares do not all accelerate ions to >MeV energies. When present, however, accelerated ions may embody a total energy comparable to that found in X-ray emitting electrons. We would like to understand the trigger that results in effective ion acceleration. It will be particularly interesting to combine such neutron observations with gamma-rays, and indeed to locate ion acceleration within the details of flare development revealed by rich, modern observations spanning several wavelength ranges.