The COMPTEL Imaging Compton Telescope on‐board the Compton Gamma Ray Observatory measured significant neutron and γ‐ray fluxes from the solar flare of 9 June 1991. The γ‐ray flux had an integrated intensity (≳1 MeV) of ∼30 cm−2, extending in time from 0136 UT to 0143 UT, while the time of energetic neutron emission extended approximately 10 minutes longer, indicating either extended proton acceleration to high energies or trapping and precipitation of energetic protons. The production of neutrons without accompanying γ‐rays in the proper proportion indicates a significant hardening of the precipitating proton spectrum through either the trapping or extended acceleration process

Bennett, K

Debrunner, H

Forrest, D J

Hanlon, L O

Lockwood, J

Loomis, M

McConnell, Mark L

Morris, D

Rank, G

Ryan, James M.

Schonfelder, V

Swanenburg, B

Webber, W

Winkler, C

English

University of New Hampshire

University of New Hampshire
University of New Hampshire Scholars' Repository
Space Science Center Institute for the Study of Earth, Oceans, and Space(EOS)
1994
Neutron and gamma‐ray measurements of the solar
flare of 1991 June 9
James M. Ryan
University of New Hampshire, James.Ryan@unh.edu
D J. Forrest
University of New Hampshire - Main Campus
J Lockwood
University of New Hampshire - Main Campus
M Loomis
University of New Hampshire - Main Campus
Mark L. McConnell
University of New Hampshire - Main Campus, mark.mcconnell@unh.edu
See next page for additional authors
Follow this and additional works at: https://scholars.unh.edu/ssc
Part of the Astrophysics and Astronomy Commons
This Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University of
New Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of New
Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.
Recommended Citation
Neutron and gamma‐ray measurements of the solar flare of 1991 June 9 Ryan, J. and Forrest, D. and Lockwood, J. and Loomis, M. and
McConnell, M. and Morris, D. and Webber, W. and Bennett, K. and Hanlon, L. and Winkler, C. and Debrunner, H. and Rank, G. and
Schönfelder, V. and Swanenburg, B. N., AIP Conference Proceedings, 294, 89-93 (1994), DOI:http://dx.doi.org/10.1063/1.45205
Authors
James M. Ryan, D J. Forrest, J Lockwood, M Loomis, Mark L. McConnell, D Morris, W Webber, K Bennett, L
O. Hanlon, C Winkler, H Debrunner, G Rank, V Schonfelder, and B Swanenburg
This conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/169
Neutron and gammaray measurements of the solar flare of 1991
June 9
J. Ryan, D. Forrest, J. Lockwood, M. Loomis, M. McConnell, D. Morris, W. Webber
, K. Bennett, L. Hanlon, C. Winkler, H. Debrunner, G. Rank, V. Schönfelder, and
B. N. Swanenburg 
 
Citation: AIP Conference Proceedings 294, 89 (1994); doi: 10.1063/1.45205 
View online: http://dx.doi.org/10.1063/1.45205 
View Table of Contents:
http://scitation.aip.org/content/aip/proceeding/aipcp/294?ver=pdfcov 
Published by the AIP Publishing 
 
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Solar flare neutrons and gamma rays 
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NEUTRON AND GAMMA-RAY MEASUREMENTS OF THE SOLAR 
FLARE OF 1991 JUNE 9 
J. Ryan, D. Forrest, J. Lockwood, M. Loomis, M. McConnell, D. Morris, 
W. Webber 
Space Science Center, Morse Hall, University of New Hampshire, 
Durham, NH 03824 
K. Bennett, L. Hanlon, C. Winlder 
Astrophysics Division, European Space Research and Technology Center, Noordwijk, 
The Netherlands 
H. Debrunner 
Physikalisches Institut, University of Bern, 3012 Bern, Switzerland 
G. Rank, V. Schrnfelder 
Max-Planck Institut f'tir Extraterrestrische Physik, 8046 Garching, Germany 
B.N. Swanenburg, 
SRON-Leiden, 2300 RA Leiden, The Netherlands 
ABSTRACT 
The COMPTEL Imaging Compton Telescope on-board the Compton Gamma Ray 
Observatory measured significant neutron and "/-ray fluxes from the solar flare of 9 
June 1991. The "/-ray flux had an integrated intensity (> 1 MeV) of -30 cm -2, 
extending in time from 0136 UT to 0143 UT, while the time of energetic neutron 
emission extended approximately 10 minutes longer, indicating either extended proton 
acceleration to high energies or trapping and precipitation of energetic protons. The 
production of neutrons without accompanying "/-rays in the proper proportion indicates 
a significant hardening of the precipitating proton spectrum through either the trapping 
or extended acceleration process. 
INTRODUCTION 
Active Region 6659 produced powerful flares throughout its transit across the disk 
from 2 June 1991 to 16 June 1991. On 9 June the region produced an X10/3B class 
flare that was observed by all instruments on the Compton Gamma Ray Observatory. 
At this time the active region was located at N34E04 in heliographic coordinates. The 
flare was impulsive in nature, with a duration on the order of 8 minutes and exhibiting 
two intense spikes of "/-ray emission > 600 keV. The Sun was positioned 
approximately 15 ~ off the telescope axis, well within the instrument field-of-view 
!FOV);COMPTEL measures T-rays by two means. These are the "telescope" and 
' ,1  burst modes. The two modes of operation are described elsewhere . The 
"telescope" mode relies on the Compton scattering process, whereby the `/-ray 
Compton scatters in a D 1 detector (liquid scintillator) and then scatters again in a 
separate D2 (NaI) detector. In measuring the solar "/-ray flux, the scattering angle, 
9 1994 American Institute of Physics 89 
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90 Neut ron  and Gamma-Ray Measurements  
deduced from the Compton formula and the direction of the scattered '/-ray must be 
consistent with a solar origin for subsequent analysis. The "burst" mode utilizes two of 
the D2 detectors as omnidirectional spectrometers. Neutrons are only measured in the 
"telescope" mode, but here the Compton scatter is replaced by an elastic neutron-proton 
scatter in the organic liquid scintillator in the D 1 detector. The energy estimate of the 
recoil neutron is performed by measuring the time-of-flight of the neutron from D1 to 
D2. The solar "/-ray and neutron measurement capabilities of the instrument are 
described in greater detail by Ryan et al. 2. 
OBSERVATIONS 
The raw omnidirectional count rates of individual detector types are shown in Figure 1. 
This X10/3B flare was impulsive in nature with a duration on the order of 8 minutes 
and exhibiting two intense spikes of 7-ray emission. The GOES X-ray flare started at 
0134 UT peaking at 0143 UT. The y-ray onset occurred at 0136 LIT and the impulsive 
phase lasted until ~ 0142 UT. Satellite sunrise precedes the flare only by a few 
minutes, thereby allowing a long observation of any extended flare emissions, such as 
neutrons. The count rates in the D 1 and D2 detecting systems with thresholds of - 60 
and - 350 keV, respectively, show how the flare behaved in hard X-rays and 't-rays. 
The slow rise and fall of the D2 counts rate after the impulsive phase arises from 
excursions in geomagnetic latitude and is not related to the flare. Also shown is the 
count rate in the Anticoincidence detection system (threshold ~100 keV). These 
detectors reject charged cosmic ray particles but are also sensitive to large thermal X-ray 
fluxes. At the X10 level, the X-ray flux falling upon the large Anticoincidence 
detectors results in pulse pile-up and large dead time effects for the whole instrument. 
The Anticoincidence rate qualitatively traces the magnitude of the dead time effect, 
which reaches a value of approximately 75% during the impulsive phase. The dead time 
effect improves to the ~ 50% level by 0155 UT. 
5 A 
m 6 x 1 0  .,., 
c 
" 5 
>.,  
am 
W 
4 
~ 3 
Q 
" 2 W 
c- 
O 
0 0 
| |  i i  i i i  i I I i  | 1 |  i i  
m 
i 
01:35  
" 1  . . . . .  L ' ~  . . . . . . . . .  I . . . . . . . . .  I . . . . . . . . .  
1991 June 9 
COMPTEL Count Rates _ 
v e , o  
, ~ ! ,  . . . . . . . .  i . . . . . . . . .  
01:40  01:45  01:50  01:55  02 :00  
UT 
FIG. 1: Raw COMPTEL housekeeping data. 
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J. Ryan et al. 91 
During the impulsive phase the ~ray emission spectrum is obtained by selecting 
the telescope events consistent with the Sun's location, as shown in Figure 2. From 
the ~/-ray events in the telescope mode, although not shown here, an image of the sun 
can be constructed 3. The strong line from deuterium formation is seen at 2.2 MeV. 
Other lines are also present with less statistical significance. No background spectrum 
has been subtracted, but few photons are expected, since the background is suppressed 
by the large signal-to-noise ratio large. 
Solar neutron events from 0155 UT to 0222 UT were selected from the data and 
reprocessed. This time selection avoids the troublesome period around the impulsive 
phase with large dead times and other instrumental effects. The time 0155 UT 
corresponds to a 50 MeV neutron produced at the flare start 0136 UT or a 60 MeV 
neutron from the flare maximum at 0139 UT. Some selection bias, therefore, exists for 
neutrons detected shortly after 0155 UT (real time). The selected data were subjected to 
similar geometric constraints as were the ~/-rays, i.e., the origination direction of the 
neutron event must be consistent with the solar direction (+ 10~ The scatter angle 0P) 
of the individual neutron events was restricted to 20 ~ to improve the signal-to-noise 
ratio. The energy of each neutron is used to compute its production time at the Sun. 
Shown in Figure 3 in addition to the flux > 600 keV is the live time corrected and 
background subtracted intensity-time profile of the neutrons as produced at the Sun but 
plotted at the production time plus the light travel time over 1 AU (507 s). Therefore, 
photons and neutrons produced concurrently are plotted at the same time value even 
though the neutrons arrive later. 
20 
== 
o 10 
5 
0 
0 2000 4000 6000 8000 
ENERGY (keY) 
FIG. 2: The raw telescope mode count spectrum from the 9 June 1991 
solar flare. 
The background was estimated from the measured neutron flux ~ 24 hours later 
when the same orbital-geophysical conditions were reproduced. The on-board 
instrument software, however, was configured differently from the flare observation, 
being less restrictive in accepting neutron events. This resulted in a uniform 36% 
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92 Neutron and Gamma-Ray Measurements  
increase in the neutron background count rate. The background rate was scaled 
downward by this factor in order to provide a representative background for the flare 
orbit on 9 June. The background corrections were successfully tested on similar 
background orbits to ensure that a null result is obtained. Identical data cuts then were 
made on both the flare data and the prescaled background data. Live time fractions of 
60% to 100% were applied to the data as a function of real time from 0155 - 0220 UT. 
2 0  '1 . . . . . . . . . . . . . .  I . . . . . . . .  | . . . .  I . . . . . .  I . . . . . .  I . . . .  
1 
- 800 
= 9 June 1991 
r i s -  : ', Solar Flare Neutrons 
'- - e o o  . ,  - i  
o)  
c L. ' <  
:3 1 0 -  N e u t r o n "  9 y - r a y s  O o I - - - I  
0 =I- I > 600 keV - 400 o 
e-  . . .  
O 5 Ill 
- 200 
! | 
q)  ' : ' ' . t  - - I  
z o .... 4 1  
, ~ 0 
01:15 01:30 01:45 02:00 02:15 
U T  
FIG. 3: Neutron and ~-ray emission-time profiles for the 9 June 1991 
solar flare, plotted at the time corresponding to a photon arrival time. 
DISCUSSION 
The most important feature to note in Figure 3 is the time coincidence of the 
onsets of neutron and the y-ray production. The neutron intensity-time profile is 
expected to be smoother and somewhat broader than that of y-rays because of the 
neutron energy resolution which maps into an error (FWHM) in production time at the 
Sun (~ 1-3 minutes, dependent on energy). Although the y-rays and neutrons are 
observed to start simultaneously, there is evidence that the neutron emission persists for 
- 10 minutes after the > 600 keV y-ray flux has subsided. 
This extended emission is evidence for a significant evolution (hardening) of the 
progenitor proton spectrum, arising from either additional acceleration or differential 
trapping and precipitation of protons. As seen in Figure 2 a large fraction of the y-rays 
> 1 MeV are of nuclear origin. With no evolution of the proton spectrum, we would 
expect the > 600 keV T-ray flux to follow the neutron production profile. However, a 
hardening of the spectrum would enhance the neutron emissivity with respect to that of 
the nuclear "/-rays. It should be noted, though, that the primary electron 
bremsstrahlung component has not yet been separated from the nuclear component, so 
that part of the decay of the flux > 600 keV could result from the decay of the pure 
electron component of the y-ray flux. 
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J. Ryan et al. 93 
The trapping scenario is consistent with the > 50 MeV ~/-ray flux detected by 
EGRET after 0145 UT 4. The net neutron count rate from 0136 UT to 0150 UT is 
positive at the 4.2 t~ significance level. The absolute neutron flux is uncertain at this 
time due to the difficulty in computing the effective area of the instrument under the 
conditions of the flare and the data restriction used in the analysis. Work continues on 
this effort. The uncertainty of the instrument response manifests in the relative neutron 
flux among the three 5 minute intervals of neutron production. The assignment of a 
production time to an individual neutron is not affected by this uncertainty, only the 
intensity of the neutron emission. Thus, the intensity-time profile in Figure 3 may 
change shape when neutron fluxes are computed, but the duration of the event will 
remain unchanged. 
In all other respects the 9 June 1991 was purely impulsive in nature, but the 
prolonged energetic particle activity as manifest in neutrons and 't-rays > 50 MeV 
establishes this flare as one of a class of long duration events, similar to that of 3 June 
1982. Two models for describing this phenomenon have been put forth 5,6. Ryan and 
Lee 5 propose that a steadily hardening proton spectrum is produced by proton trapping 
in a turbulent coronal magnetic loop with second order stochastic acceleration, while 
Mandzhavidze and Ramaty ~ posit that the protons responsible for extended high energy 
emission such as this arise in the impulsive phase with differential trapping and 
precipitation in a relatively quiet coronal loop. In either scenario, it is the diffusion of 
protons out of the loop which precipitate and produce the high energy emission. The 
data of the 9 June 1991 flare do not distinguish between the two models. With the 
great sensitivity of the instruments on the Compton Gamma Ray Observatory it is likely 
that other flares exhibiting this behavior will be observed or discovered in the data. By 
studying these flares we hope to gain a greater understanding of the processes of proton 
acceleration and transport in solar flares. 
REFERENCES 
1. SchtJnfelder et al., Astrophys. J. (Suppl.), 86, 657. 
2. Ryan, J.M. et al., in Data Analysis in Astronomy IV, Ed. V. Di GesO et 
al.,New York, Plenum Press (1992). 
3. McConnell, M. et al., Adv. in Sp. Sci., to be published. 
4. Kanbach, G., priv. communication, (1992). 
5. Ryan, J.M., Lee, M.A,, Astrophys.J., 368, 316 (1991). 
6. Mandzhavidze, N., Ramaty, R., Astrophys. J. (Letters), 396, L111 (1992) 
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Neutron and gamma‐ray measurements of the solar flare of 1991 June 9

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)1994Neutron and gamma‐ray measurements of the solarflare of 1991 June 9James M. RyanUniversity of New Hampshire, James.Ryan@unh.eduD J. ForrestUniversity of New Hampshire - Main CampusJ LockwoodUniversity of New Hampshire - Main CampusM LoomisUniversity of New Hampshire - Main CampusMark L. McConnellUniversity of New Hampshire - Main Campus, mark.mcconnell@unh.eduSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/sscPart of the Astrophysics and Astronomy CommonsThis Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University ofNew Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationNeutron and gamma‐ray measurements of the solar flare of 1991 June 9 Ryan, J. and Forrest, D. and Lockwood, J. and Loomis, M. andMcConnell, M. and Morris, D. and Webber, W. and Bennett, K. and Hanlon, L. and Winkler, C. and Debrunner, H. and Rank, G. andSchönfelder, V. and Swanenburg, B. N., AIP Conference Proceedings, 294, 89-93 (1994), DOI:http://dx.doi.org/10.1063/1.45205AuthorsJames M. Ryan, D J. Forrest, J Lockwood, M Loomis, Mark L. McConnell, D Morris, W Webber, K Bennett, LO. Hanlon, C Winkler, H Debrunner, G Rank, V Schonfelder, and B SwanenburgThis conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/169Neutron and gammaray measurements of the solar flare of 1991June 9J. Ryan, D. Forrest, J. Lockwood, M. Loomis, M. McConnell, D. Morris, W. Webber, K. Bennett, L. Hanlon, C. Winkler, H. Debrunner, G. Rank, V. Schönfelder, andB. N. Swanenburg  Citation: AIP Conference Proceedings 294, 89 (1994); doi: 10.1063/1.45205 View online: http://dx.doi.org/10.1063/1.45205 View Table of Contents:http://scitation.aip.org/content/aip/proceeding/aipcp/294?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Observations of the 1991 June 11 solar flare with COMPTEL AIP Conf. Proc. 294, 100 (1994); 10.1063/1.45207  EGRET observations of extended highenergy emissions from the nuclear lineflares of June 1991 AIP Conf. Proc. 294, 94 (1994); 10.1063/1.45206  Solar flare neutrons and gamma rays AIP Conf. Proc. 294, 77 (1994); 10.1063/1.45204  COMPTEL observations of gammaray flares in October 1991 AIP Conf. Proc. 294, 55 (1994); 10.1063/1.45200  Highenergy gammaray signature of proton acceleration during 1991 June 15 solarflare AIP Conf. Proc. 294, 45 (1994); 10.1063/1.45198   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 17:45:24NEUTRON AND GAMMA-RAY MEASUREMENTS OF THE SOLAR FLARE OF 1991 JUNE 9 J. Ryan, D. Forrest, J. Lockwood, M. Loomis, M. McConnell, D. Morris, W. Webber Space Science Center, Morse Hall, University of New Hampshire, Durham, NH 03824 K. Bennett, L. Hanlon, C. Winlder Astrophysics Division, European Space Research and Technology Center, Noordwijk, The Netherlands H. Debrunner Physikalisches Institut, University of Bern, 3012 Bern, Switzerland G. Rank, V. Schrnfelder Max-Planck Institut f'tir Extraterrestrische P ysik, 8046 Garching, Germany B.N. Swanenburg, SRON-Leiden, 2300 RA Leiden, The Netherlands ABSTRACT The COMPTEL Imaging Compton Telescope on-board the Compton Gamma Ray Observatory measured significant neutron and "/-ray fluxes from the solar flare of 9 June 1991. The "/-ray flux had an integrated intensity (> 1 MeV) of -30 cm -2, extending in time from 0136 UT to 0143 UT, while the time of energetic neutron emission extended approximately 10minutes longer, indicating either extended proton acceleration tohigh energies or trapping and precipitation of energetic protons. The production of neutrons without accompanying "/-rays in the proper proportion indicates a significant hardening of the precipitating proton spectrum through either the trapping or extended acceleration process. INTRODUCTION Active Region 6659 produced powerful flares throughout its transit across the disk from 2 June 1991 to 16 June 1991. On 9 June the region produced an X10/3B class flare that was observed by all instruments on the Compton Gamma Ray Observatory. At this time the active region was located at N34E04 in heliographic coordinates. The flare was impulsive in nature, with a duration on the order of 8 minutes and exhibiting two intense spikes of "/-ray emission > 600 keV. The Sun was positioned approximately 15 ~ off the telescope axis, well within the instrument field-of-view !FOV);COMPTEL measures T-rays by two means. These are the "telescope" and ' ,1 burst modes. The two modes of operation are described elsewhere . The "telescope" mode relies on the Compton scattering process, whereby the ` /-ray Compton scatters in a D 1 detector (liquid scintillator) and then scatters again in a separate D2 (NaI) detector. In measuring the solar "/-ray flux, the scattering angle, 9 1994 American Institute of Physics 89  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 17:45:2490 Neutron and Gamma-Ray Measurements deduced from the Compton formula and the direction of the scattered '/-ray must be consistent with a solar origin for subsequent analysis. The "burst" mode utilizes two of the D2 detectors as omnidirectional spectrometers. Neutrons are only measured in the "telescope" mode, but here the Compton scatter is replaced by an elastic neutron-proton scatter in the organic liquid scintillator in the D 1 detector. The energy estimate of the recoil neutron is performed by measuring the time-of-flight of the neutron from D1 to D2. The solar "/-ray and neutron measurement capabilities of the instrument are described in greater detail by Ryan et al. 2. OBSERVATIONS The raw omnidirectional count rates of individual detector types are shown in Figure 1. This X10/3B flare was impulsive in nature with a duration on the order of 8 minutes and exhibiting two intense spikes of 7-ray emission. The GOES X-ray flare started at 0134 UT peaking at 0143 UT. The y-ray onset occurred at 0136 LIT and the impulsive phase lasted until ~ 0142 UT. Satellite sunrise precedes the flare only by a few minutes, thereby allowing a long observation of any extended flare emissions, such as neutrons. The count rates in the D 1 and D2 detecting systems with thresholds of - 60 and - 350 keV, respectively, show how the flare behaved in hard X-rays and 't-rays. The slow rise and fall of the D2 counts rate after the impulsive phase arises from excursions in geomagnetic latitude and is not related to the flare. Also shown is the count rate in the Anticoincidence detection system (threshold ~100 keV). These detectors eject charged cosmic ray particles but are also sensitive to large thermal X-ray fluxes. At the X10 level, the X-ray flux falling upon the large Anticoincidence detectors esults in pulse pile-up and large dead time effects for the whole instrument. The Anticoincidence rate qualitatively traces the magnitude of the dead time effect, which reaches avalue of approximately 75% during the impulsive phase. The dead time effect improves to the ~ 50% level by 0155 UT. 5 A m 6x10 .,., c " 5 >., am W 4 ~ 3 Q " 2 W c- O 0 0 | |  i i  i i i  i I I i  |1 |  i i  m i 01:35 "1  . . . . .  L '~ . . . . . . . . .  I . . . . . . . . .  I . . . . . . . . .  1991 June 9 COMPTEL Count Rates _ ve ,o  , ~ ! ,  . . . . . . . .  i . . . . . . . . .  01:40 01:45 01:50 01:55 02:00 UT FIG. 1: Raw COMPTEL housekeeping data.  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 17:45:24J. Ryan et al. 91 During the impulsive phase the ~ray emission spectrum is obtained by selecting the telescope vents consistent with the Sun's location, as shown in Figure 2. From the ~/-ray events in the telescope mode, although not shown here, an image of the sun can be constructed 3. The strong line from deuterium formation is seen at 2.2 MeV. Other lines are also present with less statistical significance. No background spectrum has been subtracted, but few photons are expected, since the background is suppressed by the large signal-to-noise ratio large. Solar neutron events from 0155 UT to 0222 UT were selected from the data and reprocessed. This time selection avoids the troublesome p riod around the impulsive phase with large dead times and other instrumental effects. The time 0155 UT corresponds to a 50 MeV neutron produced at the flare start 0136 UT or a 60 MeV neutron from the flare maximum at 0139 UT. Some selection bias, therefore, exists for neutrons detected shortly after 0155 UT (real time). The selected ata were subjected to similar geometric onstraints as were the ~/-rays, i.e., the origination direction of the neutron event must be consistent with the solar direction (+ 10~ The scatter angle 0P) of the individual neutron events was restricted to 20 ~ to improve the signal-to-noise ratio. The energy of each neutron is used to compute its production time at the Sun. Shown in Figure 3 in addition to the flux > 600 keV is the live time corrected and background subtracted intensity-time profile of the neutrons as produced at the Sun but plotted at the production time plus the light travel time over 1 AU (507 s). Therefore, photons and neutrons produced concurrently are plotted at the same time value even though the neutrons arrive later. 20 == o 10 5 0 0 2000 4000 6000 8000 ENERGY (keY) FIG. 2: The raw telescope mode count spectrum from the 9 June 1991 solar flare. The background was estimated from the measured neutron flux ~ 24 hours later when the same orbital-geophysical conditions were reproduced. The on-board instrument software, however, was configured ifferently from the flare observation, being less restrictive in accepting neutron events. This resulted in a uniform 36%  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 17:45:2492 Neutron and Gamma-Ray Measurements increase in the neutron background count rate. The background rate was scaled downward by this factor in order to provide a representative background for the flare orbit on 9 June. The background corrections were successfully tested on similar background orbits to ensure that a null result is obtained. Identical data cuts hen were made on both the flare data and the prescaled background data. Live time fractions of 60% to 100% were applied to the data as a function freal time from 0155 - 0220 UT. 20 '1 . . . . . . . . . . . . . .  I . . . . . . . .  | . . . .  I . . . . . .  I . . . . . .  I . . . .  1 - 800 = 9 June 1991 r i s -  : ', Solar Flare Neutrons '- - eoo  . ,  - i  o) c L. '<  :3 1 0 -  Neutron"  9 y - rays  O o I--- I  0 =I- I > 600 keV - 400 o e- . . .  O 5 Ill - 200 ! | q) ' : ' ' . t  - - I  z o .... 41 , ~ 0 01:15 01:30 01:45 02:00 02:15 UT FIG. 3: Neutron and ~-ray emission-time profiles for the 9 June 1991 solar flare, plotted at the time corresponding to a photon arrival time. DISCUSSION The most important feature to note in Figure 3 is the time coincidence of the onsets of neutron and the y-ray production. The neutron i tensity-time profile is expected to be smoother and somewhat broader than that of y-rays because of the neutron energy resolution which maps into an error (FWHM) in production time at the Sun (~ 1-3 minutes, dependent on energy). Although the y-rays and neutrons are observed to start simultaneously, there is evidence that he neutron emission persists for - 10 minutes after the > 600 keV y-ray flux has subsided. This extended emission is evidence for a significant evolution (hardening) of the progenitor proton spectrum, arising from either additional acceleration or differential trapping and precipitation of protons. As seen in Figure 2 a large fraction of the y-rays > 1 MeV are of nuclear origin. With no evolution of the proton spectrum, we would expect the > 600 keV T-ray flux to follow the neutron production profile. However, ahardening of the spectrum would enhance the neutron emissivity with respect to that of the nuclear "/-rays. It should be noted, though, that the primary electron bremsstrahlung component has not yet been separated from the nuclear component, sothat part of the decay of the flux > 600 keV could result from the decay of the pure electron component ofthe y-ray flux.  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 17:45:24J. Ryan et al. 93 The trapping scenario is consistent with the > 50 MeV ~/-ray flux detected by EGRET after 0145 UT 4. The net neutron count rate from 0136 UT to 0150 UT is positive at the 4.2 t~ significance l vel. The absolute neutron flux is uncertain at this time due to the difficulty in computing the effective area of the instrument under the conditions of the flare and the data restriction used in the analysis. Work continues on this effort. The uncertainty ofthe instrument response manifests in the relative neutron flux among the three 5 minute intervals of neutron production. The assignment of a production time to an individual neutron is not affected by this uncertainty, only the intensity of the neutron emission. Thus, the intensity-time profile in Figure 3 may change shape when neutron fluxes are computed, but theduration of the event will remain unchanged. In all other espects the 9 June 1991 was purely impulsive in nature, but the prolonged energetic particle activity as manifest in neutrons and 't-rays > 50 MeV establishes this flare as one of a class of long duration events, similar to that of 3 June 1982. Two models for describing this phenomenon have been put forth 5,6. Ryan and Lee 5 propose that a steadily hardening proton spectrum isproduced by proton trapping in a turbulent coronal magnetic loop with second order stochastic acceleration, while Mandzhavidze and Ramaty ~posit that he protons responsible for extended high energy emission such as this arise in the impulsive phase with differential trapping and precipitation in a relatively quiet coronal loop. In either scenario, it is the diffusion ofprotons out of the loop which precipitate and produce the high energy emission. The data of the 9 June 1991 flare do not distinguish between the two m dels. With the great sensitivity of the instruments on the Compton Gamma Ray Observatory it is likely that other flares exhibiting this behavior will be observed or discovered in the data. By studying these flares we hope to gain a greater understanding of the processes of proton acceleration a d transport in solar flares. REFERENCES 1. SchtJnfelder t al., Astrophys. J (Suppl.), 86, 657. 2. Ryan, J.M. et al., in Data Analysis in Astronomy IV, Ed. V. Di GesO et al.,New York, Plenum Press (1992). 3. McConnell, M. et al., Adv. in Sp. Sci., to be published. 4. Kanbach, G., priv. communication, (1992). 5. Ryan, J.M., Lee, M.A,, Astrophys.J., 368, 316 (1991). 6. Mandzhavidze, N., Ramaty, R., Astrophys. J (Letters), 396, L111 (1992)  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. 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Neutron and gamma‐ray measurements of the solar flare of 1991 June 9

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