solar neutron events as a tool to study particle acceleration at the sun

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Advances in Space Research 43 (2009) 565–572

Solar neutron events as a tool to study particle acceleration at the Sun

J.F. Valdes-Galicia a,*, Y. Muraki b,c, K. Watanabe b, Y. Matsubara b, T. Sako b,L.X. Gonzalez a, O. Musalem a, A. Hurtado a

a Instituto de Geofısica, Universidad Nacional Autonoma de Mexico, 04510 Mexico, D.F., Mexicob Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan

c Department of Physics, Konan University, Okamto 8-9-1, Kobe 658-8501, Japan

Received 23 March 2008; received in revised form 5 August 2008; accepted 27 September 2008

Abstract

The Sun provides unique opportunities to study particle acceleration mechanisms using data from detectors placed on the Earth’ssurface and on board spacecrafts. Particles may gain high energies by several physical mechanisms. Differentiating between these pos-sibilities is a fundamental problem of cosmic ray physics. Energetic neutrons provide us with information that keeps the signatures ofthe acceleration site. A summary of some representative solar neutron events observed on the Earth’s surface, including associated Xand c-ray observations from spacecrafts is presented. We discuss evidence of acceleration of particles by the Sun to energies up to severaltens of GeV. In addition, a recent solar neutron event that occurred on September 7th 2005 and detected by several observatories atEarth is analyzed in detail.� 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar activity Particle acceleration at the Sun Solar Neutrons

1. Introduction

Crucial information of the acceleration processes insolar flares is contained in solar neutrons as they are theproduct of intense high energy proton fluxes. As early as1951 Biermann et al. pointed out that neutrons were pro-duced in extreme solar flares and envisaged that they couldbe detected at Earth (Biermann et al., 1951). However, thedetection of solar neutrons was not made until June 21,1980 when instruments on board the SMM spacecraftrecorded a signal that could clearly be ascribed to neutrons(Chupp et al., 1982). As a further proof, neutron decayelectrons were observed at the ISEE-3/ICE spacecraftbefore the flare electrons arrived (Droge et al., 1995).Thefirst detection of solar neutrons by ground level instrumen-tation was made with neutron monitors operating at Euro-pean observatories during the solar flare of June 3, 1982

0273-1177/$34.00 � 2008 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2008.09.023

* Corresponding author.E-mail address: [email protected] (J.F. Valdes-Galicia).

(Efimov et al., 1983; Chupp et al., 1987) Protons from neu-tron decay were also observed as a consequence of this flare(Evenson et al., 1983). Only two solar neutron events wereobserved during solar cycle 21. Therefore, for a long timemany scientists believed that the detection of solar neu-trons was very difficult, comparable to other rare processesoccurring in cosmic ray physics.

Due to interplanetary magnetic effects, charged particlesfrom the Sun usually arrive at the Earth from tens of minutesto a few hours later than the X or c-rays that take onlyaround 500 s to travel the Sun–Earth distance. Additionally,the solar and interplanetary fields modulate the charged par-ticle fluxes; therefore their characteristics at the source aremodified. Observations of neutral particles do not sufferfrom these disadvantages. The times and loci when andwhere charged particles are accelerated are in close correla-tion with X and c-ray emissions. It is therefore importantto include X- and c-ray observations in our studies of neutralparticles. Neutral pion decay produces c-rays with energiesabout 70 MeV, nuclear deexcitation lines at 4.4 MeV from

rved.

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566 J.F. Valdes-Galicia et al. / Advances in Space Research 43 (2009) 565–572

Carbon and/or 6.1 MeV from Oxygen, the 2.223 MeV neu-tron capture line of deuterium formation or bremsstrahlungfrom accelerated electrons are also in the c-ray range. On theother hand, specific information on ion acceleration pro-cesses, unclouded by other effects, may be obtained throughthe neutron channel, because neutrons are produced solelyby ions collisions in the solar photosphere.

A scheme of the most popular production mechanismfor solar neutrons among solar physicists is depicted inFig. 1 (see e.g., Dennis and Schwartz, 1989). The originof particle acceleration is the collision of magnetic loopsproducing a reconnection that heats up the solar plasma.The hot plasma blows downward constituting a high speedplasma stream. The stream hits the top of a magnetic loop.Particles within an inverse U-shape magnetic loop will beaccelerated by collisions with the plasma jet. If we assumethe speed of the plasma jet to be around 3000 km/s, everymirroring reflection their energies will change by anamount DE where DE/E = 2v/c = 0.02.

After some 400 mirroring collisions, the energy may beboosted from 10 MeV to about 30 GeV (1.02400 = 2755). Arealistic assumption for the size of the magnetic loop maybe around 10,000 km. With these numbers the time neededfor acceleration is of the order of one minute. Of course, thisis only a rough estimate; the actual acceleration process iscertainly more complex. Nevertheless, this simple calcula-tion shows that particle acceleration may occur passingthrough the plasma mirror repeatedly back and forth (Tsun-eta and Naito, 1998). Testing this or other plausible scenar-ios requires extensive observations of the particle andradiation products of the solar flares. In this paper we willdiscuss observations of solar neutrons at the Earth’s surfaceand assess its contributions to the knowledge of the solarflare phenomenon.

Fig. 1. Standard particle acceleration model near the solar surface.Interaction and reconnection processes of magnetic loops, heat up theplasma in the loop. The hot plasma then forms a high speed downwardstream that hits the top of the lower magnetic loop. Particles are mirroredin the lower magnetic loop. As a consequence of this back and forthprocesses particles increase their energies with every pass.

2. Detectors to observe solar neutron events

From the Sun to the Earth, one GeV neutrons aredelayed 1 min with respect to photons, if they were emittedat the same time, this lag increases to 11 min for neutronsof 100 MeV. Therefore, even in the case of a simultaneousemission at the Sun, their arrival times on Earth differ con-siderably. As a consequence, in order to establish the neu-tron emission profile at the Sun, both the energies andarrival times at Earth need to be measured. The distinctionbetween continuous and impulsive production of solar neu-trons is complicated due to the velocity dispersion.

There is a neutron monitor (NM) world network in usesince the decade of the 1960s. The NM detects both neu-trons and protons. These interact with a lead target inthe detector producing more neutrons. The neutronsundergo successive collisions with protons in paraffin orpolyethylene sheets, losing momentum to be graduallythermalized, thus all knowledge of their original energiesand directions are lost. The neutron monitor has no direc-tional capabilities either.

To understand the dynamics of acceleration processesoccurring at the Sun, Solar Neutron Telescopes (SNT)were developed, they are able to measure both arrivaldirections and neutron energies. A scheme of the SNT inSierra Negra, Mexico, is shown in Fig. 2. The basic ideais to use a combination of proportional counters (PRCs)and scintillators, in the upper layer and surrounding thescintillators, PRCs working in anticoincidence to separatethe neutral and charged components of solar energetic par-ticles. To measure the energies of neutrons based on theintensity of the light they produce, 30 cm thick plastic scin-tillators in a total area of 4 m2 are used. The arrays ofPRCs underneath the plastic scintillators are located thereto measure the directions of incoming neutrons.

The detection efficiency of neutrons of the SNT is about30%, the arrival directions of neutrons are determined to anaccuracy of ±15� approximately. Iron plates of 2 cm thick-ness cover the four sides of the anti-coincidence counter toconvert c-rays into charged particles. At the top of the anti-coincidence counter a 5 mm thick lead plate is installed fora similar purpose. However, the efficiency of the anti-coinci-dence counter is <100%. It is estimated to be �80% due tothe conversion rate of photons to charged particles in the leadand iron plates. Thicker plates might be a better shielding atthe expense of converting a fraction of incoming neutronsto charged particles and hence being effectively not counted.

The detector records energy deposits by neutrons in fourranges, >30, >60, >90 and >120 MeV. This sets the limit totest the models of solar neutron production. Details of theSNT operating principles may be found elsewhere [Murakiet al., 1991].

3. First solar neutrons detected near and at earth

The time profile of the gamma ray telescope on boardSMM for the event observed on June 21, 1980 is shown

Fig. 2. The Solar Neutron Telescope at the summit of the Sierra Negra volcano, Mexico, at 97.3W, 19.0N; 4580 m a.s.l. (from Valdes-Galicia et al., 2004).Five gondolas of PRCs are located at the top and sides of the central scintillators to detect and discriminate charged particles. The lead and iron platesserve the purpose of converting c-rays into charged particles and prevent being counted as neutrons in the scintillators. The four gondolas underneath thescintillators serve to identify the neutron arrival directions as they detect the protons produced by neutron nuclear interactions inside the scintillator.

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in Fig. 3 [Chupp et al., 1982]. The first peak is due togamma-rays, whilst the second broad enhancementcorresponds to solar neutrons. Even if the neutrons wereimpulsively injected from the Sun (within 1 min), their arri-

Fig. 3. The first solar neutron event detected onboard the SMM satellite(based on Chupp et al., 1982). The initial spike was produced by photons,the broad hump is due to neutrons. The time profile of neutrons may beexplained by a simple impulsive production model of neutrons in the MeVrange.

val times would be spread over a time span of 20 min, withhigh-energy neutrons arriving first and low energy neutronsarriving later. The flux increased in the first 10 min, reacheda maximum and decreased afterwards due to the decay oflow-energy neutrons in transit from the Sun to the Earth,but also because instrument efficiency decreases at lowerenergies. The differential energy spectrum may be expressedby a power index law (dN/dE / E�c) where c = 3.5, assum-ing impulsive production at the Sun. The alternative expla-nation that neutrons with high energies (say J 1 GeV)were emitted for around 20 min cannot be excluded bythe observations.

Fig. 4 shows the data for the event on June 3, 1982, thesituation was more complex then (Chupp et al., 1987).The ground level neutron monitor at Jungfraujochshowed an enhancement that continued for around 12–15 min. The structure of the gamma ray detector timeprofile is very similar to that of the 21 june 1980 event.Based on this evidence, one could ascribe a similar inter-pretation to this event; namely the first spike is due togammas and neutrons cause the broader hump after-wards. However, the data of this event show the arrivalof neutrons for an extended time span as indicated bythe Jungfraujoch count rates in the bottom panel ofFig. 4. Thus, in this event at least some gradual produc-tion of high energy neutrons might have occurred since,according to a detailed Monte Carlo simulation (Shibata,1994), neutrons with energy less than 70 MeV cannotreach the ground due to strong attenuation in the atmo-sphere, because the interaction cross-section of neutronswith the air molecules increases substantially below100 MeV. The association of high-energy neutrons with

Fig. 4. First solar neutron event detected by a neutron monitor (Chuppet al., 1987). The first spike is due to photons, the following hump areneutrons (middle panel). The data of the ground level event on theJungfraujoch neutron monitor (bottom panel) shows an enhancementlasting some 15 min.

Fig. 5. The solar neutron event observed on May 24, 1990 (Debrunner et al.,1997). The first event where solar neutrons preceded a solar proton GLE.The production spectrum of neutrons can be fit by a power law with an indexc = �2.9, consistent with an impulsive production at the Sun (see text).


the gradual phase establishes that the gradual phase had asignificant hadronic component. Murphy et al. (1987) pro-posed that the protons responsible for the time-extendedphase are not the same as those responsible for the impul-sive phase. In any case, as we have seen, evidence mayhave several interpretations.

Fig. 5 shows the event observed on May 24, 1990 by aset of neutron monitors at the Earth’s surface. A spike at20:40 UT was undoubtedly caused by neutrons (Debrunneret al., 1997). The significance of the peak is not ordered bythe energy cut-off of the stations, rather it is better orga-nized by a combination of atmospheric depth and the lineof sight to the Sun of the site. A proton GLE was observedafter 21:00 UT. The energy spectrum of solar neutrons fol-lows a power law with differential index c = �2.9, assum-ing an impulsive injection. This assumption is justifiedbased on observations of gamma rays by the GRANATsatellite (Muraki and Shibata, 1996).

The first solar neutron event to be observed by a SNTwas on June 4, 1991 (Fig. 6). It was detected also by a

neutron monitor, and a 36 m2 muon detector, all operat-ing at Mt. Norikura, Japan. A very soft spectral power


index was deduced for this event (c = �5.4) assumingimpulsive injection at the Sun (Muraki et al., 1992).Because the flare was near the east limb (N30 E70), noprompt protons were expected that could be mixed withneutrons at ground level stations. However, Struminskyet al. (1994) used the neutron monitor data of Mt. Norik-ura to model a time-extended neutron production. Theirmodel predicts neutron spectra at the Sun that are muchharder (around E�3.5) than those reported by Murakiet al. (1992). This spectrum is in better agreement withthe proton spectrum estimate (E�2.8) at the Sun by Mur-phy et al. (1994), based on the long duration of the c-ray neutron capture and carbon de-excitation lines(around 160s).

4. The big events of the solar cycle 23

A gigantic solar flare occurred on ‘‘Easter day” 15 April,2001; the X-ray measurements onboard GOES classified itas an X14 flare, located at S20 W85 at the solar surface.The onset was detected by the GOES satellite at13:19UT, it reached X14 at 13:50UT. This was one of thestrongest flares observed during solar cycle 23.

The detection of c-ray lines provides evidence for protonacceleration at the Sun. The Yohkoh satellite recorded c-ray lines between 13:45UT and 13:51UT. Therefore pro-

Fig. 6. The solar neutron event observed on June 4, 1991 (Muraki et al.,1992). The first solar neutron event to be detected by a SNT at Mt.Norikura (top panel). The enhancement induced by solar neutrons wasalso registered in two other detectors at the same site: the 36 m2 muondetector (middle) and the neutron monitor (bottom). The time profile wasalmost the same at the muon detector and the neutron monitor.

tons must have been accelerated impulsively to high ener-gies approximately between 13:43UT and 13:47UT(Bieber et al., 2001; Muraki et al., 2007). A large GLEwas observed at many stations as a consequence. TheGLE started shortly after 14:00UT. In some polar stationsthe five minute counting rates increased over 100% (Bieberet al., 2001; Bombardieri et al., 2007).

The neutron monitor at Chacaltaya (5250 m a.s.l.)recorded a significant increase after 13:51UT continuinguntil 14:15UT. Muraki et al. (2007) estimated the signifi-cance of the hump to be 8.2r. The 3 min time profile count-ing rates are shown in Fig. 7, a second peak can beobserved between 14:06 and 14:12, coincident with theonset of the GLE. Thus it is very probable that this secondpeak was produced by protons. The rigidity cut-off of theChacaltaya observatory is 12.1 GV, therefore protonsmight have been accelerated to very high energies in thisevent. An additional remarkable fact is that the flux maybe represented by a simple power law with an indexc = �2.75 ± 0.15, in the energy range between 650 MeVand 12 GeV, the range of the neutron monitors. However,the alternative approach of Bombardieri et al. (2007) pro-duced also good fittings to the data using a power law spec-trum truncated by an exponential (Ellison and Ramaty,1985). Fitting the Chacaltaya neutron monitor data exclud-ing second peak, implies a spectral index c = �4.0 for theneutrons.

The Sun was intensely active during October–November2003, 11 X-class flares were observed between 19 Octoberand 4 November, from those, two remarkable solar neu-tron events were observed by ground-based neutronmonitors.

On October 28, 2003, in association with an X17.2 largeflare, solar neutrons were detected with high statistical sig-nificance (6.4r) by the neutron monitor at Tsumeb, Nami-bia. c-ray lines from neutron capture and excited ions of Cand O nuclei were clearly observed, these were quite differ-ent from the time profile of bremsstrahlung c-rays. Itappears that the time profile of electron acceleration wasdistinctly different from the time profile of ion acceleration.

Fig. 7. Three minute values of the Chacaltaya neutron monitor on April15, 2001(Muraki et al., 2007). The line at 13:45:00 UT is the estimated timeof particle acceleration and the red dots represent the expected curve forthe impulsive production of neutrons with c = �4.0. Around 14:06-14:12UT, there is another enhancement due to the GLE, caused by highenergy protons.


From the time profile of the neutron capture c-rays it seemsthat high energy neutrons were produced with the sametime profile as the c-ray lines of the de-excited ions. Thecorresponding spectrum for these solar neutrons(c = �2.9) is somewhat harder than other events (Watana-be, 2005).

In association with an X28 class flare, relativistic solarneutrons were observed by the neutron monitors at Halea-kala in Hawaii and Mexico City, and by the solar neutrontelescope at Mauna Kea in Hawaii simultaneously onNovember 4, 2003. Clear excesses are present in the timeprofiles of these detectors, with a significance of 7.5 r forHaleakala, and 5.2 r for Mexico City. The detectoronboard the INTEGRAL satellite observed a high flux ofhard X- and c-rays at the same time. Using the time pro-files of the c-ray lines, Watanabe (2005) explained thoseof the neutron monitors. From the time profile of the2.2 MeV neutron capture c-ray line, it appears that thetime profile of ion acceleration was approximately the sameas that of bremsstrahlung emissions. Assuming that solarneutrons were produced at the time when these c-rays wereemitted, the observed excesses are explained. The data maybe fitted with a propagation model with spectral indexc = 3.5.

A summary of spectral indices determined for solar neu-tron events observed by neutron monitors was done byWatanabe (2005), it is shown in Fig. 8. The power indexfor the 4 june 1991 event is that of Muraki et al. and notthe estimate given by Struminsky et al. (1994). If we usethis last result, all these solar neutron events spectra maybe described by power laws with power indexes�4 6 c 6 �3. In the two cases where we also have esti-mates of the proton spectra, these tend to be harder. Thismay be a systematic fact that would require further detailedinvestigations to determine whether the difference is due tothe solar source or to proton interplanetary modulation.Nevertheless, the most relevant fact here is that both, pro-ton and neutron spectra tend to adjust to power law fits, aspredicted by shock acceleration theory.

Fig. 8. The estimated spectral indexes of solar neutron events detected byneutron monitors with energy range around En = 70–700 MeV (Watan-abe, 2005). Most of the indices are between 3 and 4.

We have reserved the following section to discuss aremarkable solar neutron event registered on September7, 2005. The event was detected by the SNTs at SierraNegra and Chacaltaya, and by neutron monitors at Cha-caltaya and Mexico City. This example illustrates the capa-bilities of these types of detectors.

5. September 7, 2005 Neutron event at the american sector

SNT and NM world networks

The SNT world network has detectors located at Gorn-ergrat (Switzerland) [Butikofer et al., 2001], Tibet (China)(Katayose et al., 1999), Mt. Norikura (Japan) (Tsuchiya,2001), Mauna Kea (Hawaii) (Matsubara et al., 1997) andSierra Negra (Mexico). Although not identical, all theSNTs are similar; the SNT installed at Sierra Negra isshown in Fig. 2.

In September 2005, an extensive active region(NOAA10808) produced 10 X-class solar flares. The firstof them (17:17 UT, September 7) was the most energetic;it had a magnitude of X17. This event was at the East solarlimb (S06, E89). At that time, the Sun was above the Atlan-tic side of the American continent. The intensity of solarneutrons was extraordinarily strong, four detectors locatedat Sierra Negra, Mexico City and Chacaltaya registered theevent. This provided an opportunity to compare the detec-tion efficiency of the SNTs at Chacaltaya and at SierraNegra (Watanabe et al., 2007).

Fig. 9 shows data from the neutron monitors and SNTslocated in Bolivia and Mexico. Clear excesses wereobserved by all four detectors after 17:36:40 UT, the peakof the c-ray event. Solar neutron telescopes have energyresolution (Matsubara et al., 1993, 1997; Valdes-Galiciaet al., 2004), although we show here only the >120 MeVchannel, significant excess signals were observed in allchannels.

The highest neutron energy channel at Sierra Negra isE > 120 MeV. At those energies, neutrons will arrive tothe Earth nine minutes later than the electromagnetic radi-ation produced in the solar flare. Therefore a neutron sig-nal lasting longer than nine minutes is an indication ofan extended neutron emission at the Sun. An impulsiveemission will produce a signal of less than nine minutesduration. It is therefore not surprising that an attempt tofit the observed solar neutron profiles at Chacaltaya neu-tron monitor, assuming a simple d-function injection anda simple power law spectrum at the source was not success-ful (Watanabe et al., 2007). In contrast, a much betteragreement with the observed profile was obtained usingthe 4.4 MeV c-ray line emission from the carbon nuclearde-excitation line with counting rates above the back-ground level from 17:36 to 18:00UT, and a power law spec-trum with index of �3.1 (Watanabe et al., 2008).

A completely independent study using a comparison ofthe Sierra Negra SNT data, with Monte Carlo simulationsof the detector response, reached the conclusion that thesolar neutron spectrum was a power law whose index

Fig. 9. Five minutes counting rates of Bolivian and Mexican detectors for the flare on 7 September 2007. The neutron monitor at the Chacaltayaobservatory recorded 95,000 neutrons during 10 min, while the neutron telescope at Sierra Negra recorded 21,500 events for En > 30 MeV, 11,700 eventsfor En > 60 MeV, 3,000 events for En > 90 MeV and 820 events for En > 120 MeV.


was most likely around -3, in agreement with the previousdeterminations based on the Chacaltaya neutron monitordata (Sako et al., 2008).

There are, however, subtle differences between these lasttwo reported investigations of the event: an energy cut-offof 500 MeV was assumed by Watanabe et al. (2008), butno cut-off was included in the simulations of Sako et al.(2008). Further analyses that are currently under courseare necessary to have a more firmly based solar neutronspectra and time profile of the solar emission. These willundoubtedly shed light on the acceleration process andthe physical scenario of the flare occurrence.

6. Summary and conclusions

The lessons learnt from solar neutron observations maybe summarized as follows:

– Observation of solar neutron events may be an impor-tant tool to understand solar particle accelerationmechanisms during strong flares as solar neutronscarry unmodulated information from the solarsource.

– Simultaneous observations of X-rays and c-raystogether with ground located neutron detectors arenecessary to elucidate the time profiles of the solarparticle emissions and the particular conditions ofthe solar magnetic loops producing the flares. Withthe accumulation of more data, we may discriminatebetween various acceleration models for particles atthe Sun.

– Most solar neutron events may be fitted with animpulsive injection model. However the strongestsolar flares (rated X > 10) present important evidencein favor of extended injection. Examples are the

events on 15 April 2001, 28 October 2003 and 7 Sep-tember 2005.

– Further analyses of the available data are needed todetermine the injection profiles and energy spectrafor the flares where there is evidence of extendedinjection.

– Solar Neutron Telescopes have proved to be a usefultool to study solar neutrons as they separate the neu-tral and charged particle fluxes and provide spectraland directional information of the solar neutronfluxes. The improvement of the particle identificationability of the present solar neutron telescope worldnetwork is an important task for the solar cycle 24.

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solar neutron events as a tool to study particle acceleration at the sun

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