- Email: [email protected]
Production of γ-ray lines by energetic ion interactions (determination of solar atmospheric abundances)
New Astronomy Reviews 48 (2004) 113–118 www.elsevier.com/locate/newastrev
Production of c-ray lines by energetic ion interactions (determination of solar atmospheric abundances) Edward L. Chupp *, Philip P. Dunphy Department of Physics and Space Science Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA
Abstract Acceleration of energetic ions in solar flares produces narrow and broad nuclear c-ray de-excitation lines, by nuclear interactions with the ambient solar atmosphere. Observations of several solar flare c-ray spectra have been made since 1972 by satellite-borne c-ray spectrometers on OSO 7, HEAO 3, SMM, HINOTORI, GRANAT, YOHKOH and CGRO. The line intensities depend directly on the ambient and accelerated particle abundances. In addition, production of neutrons in several reactions leads to emission of the neutron–proton capture c-ray line, from the photosphere, at an energy of 2.223 MeV. The rate of decay in intensity of this line depends on the photospheric 3 He/H abundance ratio. Current analysis gives a photospheric 3 He/H abundance ratio of ð2:3 1:2Þ 105 . In addition, a study of narrow c-ray line fluxes from several flares suggests that the ratio of (Mg + Si + Fe) to (C + N + O) abundances changes with time during the flare. Since this ratio measures the abundance ratio of low first ionization potential (FIP) elements to high FIP elements, the c-ray production site may start in the lower chromosphere and move, as the flare progresses, into the corona where the low/high FIP abundance ratio increases. A study of broad c-ray line fluxes also permits a determination of the abundance of flare-accelerated heavy ions. Ó 2003 Elsevier B.V. All rights reserved. PACS: 96.60.Rd Keywords: Solar flares; c-ray lines
1. History Biermann et al. (1951) suggested that the four large solar cosmic ray events, known by then, were due to a flux of relativistic neutrons from nuclear reactions of protons accelerated and interacting at
*
Corresponding author. E-mail address: [email protected] (E.L. Chupp).
the flare site. Morrison (1958) predicted a strong emission of the neutron–proton capture c-ray at 2.223 MeV during a solar flare. Later, comprehensive studies of nuclear processes that might occur during flares were carried out by Lingenfelter and Ramaty (1967). See also a review by Lingenfelter (1988). During the 1960s, several spectrometers studied flare continuum emissions up to photon energies of several million electron volts. The OSO-7
1387-6473/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2003.11.016
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E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
satellite, launched in 1971 September, carried a NaI (Tl) c-ray spectrometer (GRS) designed for the study of nuclear line emissions from solar flares. In early August 1972, a large active region produced several major flares with a major event occurring on August 4, 1972, which was observed by the GRS. During the rising phase of the flare, for 10 min, before satellite day/night transition, significant c-ray lines at 2.223 (from n/p capture) and 0.51 MeV (from e =eþ annihilation) were detected with faint evidence for nuclear de-excitation lines from C and O nuclei at 4.4 and 6.1 MeV and a strong underlying continuum to nearly 10 MeV. Further c-ray line and continuum detections were made during the August 7, 1972 flare from the same active region (see Chupp et al., 1973, 1975). Two further flare observations of the neutron/ proton capture line were made on the HEAO 1 satellite, July 11, 1978 (Hudson et al., 1980); and on the HEAO 3 satellite, November 9, 1979 (Prince et al., 1982). In 1972, a NASA panel, meeting at Woods Hole, MA, recommended a satellite mission for sunspot cycle 21 dedicated to a multi-wavelength study of solar activity. This led to the development of the Solar Maximum Mission/Gamma-Ray Spectrometer (SMM/GRS). This mission, launched on February 14, 1980 operated until late 1989 November, obtained c-ray spectra from more than 185 events (Vestrand et al., 1999). Other satellite missions with spectrometers which have obtained several solar flare c-ray spectra and their launch dates are: Hinotori/GRS (1981), GRANAT/PHEBUS (1989), CGRO/COMPTEL/OSSE (1991) and Yohkoh/GRS (1991). In February 2002, The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched and on July 23, 2002 obtained the first high energy resolution c-ray spectrum of an intense solar flare.
2. c-ray line observations and solar composition This presentation has the goal of reviewing what can be learned about solar isotopic and elemental composition in the flaring region from cray line observations.
The interaction of accelerated ions and electrons, from sub-MeV to several hundred MeV or even GeV energies, with the ambient solar atmosphere at the flare site produces c-ray line and continuum emissions from: (1) the de-excitation of nuclear energy levels populated by inelastic scattering of ions on target nuclei giving lines typically with energies up to 15.5 MeV, (2) the decay of neutral and charged mesons which are produced primarily by protons and a particles with energies above 300 or 185 MeV/nucleon. The former give a broad line at 70 MeV and the latter continuum electron bremsstrahlung from the charged meson decay electrons, and (3) direct relativistic electron bremsstrahlung from accelerated electrons. In addition; the neutrons produced in several nuclear reactions can thermalize in the solar photosphere and be captured by protons giving the characteristic c-ray line at 2.223 MeV. Also, the production of radioactive positron emitters and decay of positive mesons can lead to positron singlet and triplet annihilation with ambient electrons giving a line at 0.511 MeV or continuum cray photons, respectively. The emission of these lines is ‘‘delayed’’ from the time of production of the neutrons or the positrons. In the case of ‘‘prompt’’ spectral lines from nuclear de-excitation, valuable information is obtained from the study of their width. In particular: ‘‘narrow’’ spectral lines are produced by the inelastic scattering of protons or a particles on the heavier ambient nuclei, the line width depending primarily on the spectrum of the incoming ions and the mass of the target nuclei and, ‘‘broad’’ spectral lines which are produced by accelerated heavier ions (A > 4), interacting with the ambient hydrogen and helium nuclei. Ramaty (1986), Ramaty et al. (1995), Murphy et al. (1991) and others have prepared detailed models, giving the prompt c-ray line intensities in terms of the target abundances, the excitation cross-sections and various accelerated particle energy spectra. Such model calculations, using energy-dependent excitation cross-sections, have been carried out for both thin and thick target interaction conditions. In principle, absolute isotopic abundances can be found by comparing the observed and calculated line intensities, while rel-
E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
ative abundances can be found from line intensity ratios.
profile of the 2.223 MeV line emission is usually modelled by F ðtÞ ¼ A
3. Results on solar atmospheric composition A study of the time behavior of both the 2.223 MeV capture line and the prompt narrow c-ray lines enable a determination of the 3 He/H abundance ratio in the photosphere, while a study of the yield of individual c-ray lines gives the ambient chromosphere and coronal abundances where the lines are produced. In addition, the ratio of the line fluxes of elements with greatly different values of the first ionization potential (FIP) possibly indicates the location of the c-ray production region. 3.1. Photospheric 3 He/H Accurate determination of this isotopic ratio is of great interest since it relates to the composition of the early sun, to stellar evolution and to BigBang nucleosynthesis of the light elements. Here, we focus only on how the c-ray measurements are used to obtain this ratio. The scenario follows the production of neutrons by nuclear reactions of accelerated ions with the ambient atmosphere in magnetic loops. The downward directed neutrons slow down quickly in the photosphere and can be captured by protons giving a narrow line at 2.223 MeV, or by non-radiative capture on 3 He, or be back-scattered to space and decay. The 2.223 MeV line flux from an instantaneous production of neutrons can be approximated by exponential decay with a time constant hsi, where 1=hsi ¼ 1=sH þ 1=s3He þ 1=sd ; with sH the mean capture time on H, s3He the mean capture time on 3 He and sd the mean time for decay (918 s). sH and s3He depend on the reciprocal of the number densities of H and 3 He, hnH i and hn3He i are approximately 1:4 1019 =hnH i and 8:6 1014 =ð3 He=HÞhnH i, respectively (Hua and Lingenfelter, 1987). Since the production rate of neutrons follows the production rate of excited nuclear levels which give prompt emission of MeV c-ray lines, the time
115
Z
t
½Sðt0 Þ=s exp½ðt t0 Þ=s dt0 ; 1
where Sðt0 Þ is proportional to the time history of the prompt c-ray line emission. Using this approach, several events have provided data adequate for obtaining estimates for the photospheric 3 He/H abundance ratio (Prince et al., 1983; Trottet et al., 1994; Murphy et al., 1997; Dunphy et al., 1999; Yoshimori et al., 1999). For the June 3, 1982 flare, Hua and Lingenfelter (1987) [HL87] carried out detailed Monte Carlo calculations which treated the energy and angular distributions of the neutrons to compare with direct neutron observations in this flare. They, therefore, determined the accelerated ion angular distribution and energy spectrum, leading to a more accurate determination of the behavior of neutrons in the photosphere. For neutron production by a mirroring distribution of ions, they found a value of ð2:3 1:2Þ 105 for the 3 He/H abundance ratio. As discussed by HL87, this new photospheric value is ‘‘slightly’’ lower than the estimate, of ð3:4 1:7Þ 105 by Geiss (1984) for the outer convective zone of the sun, which was based on measurements of 3 He/4 He in meteorites and protosolar estimates for the D/H and 4 He/H ratios. This new value is also close to the primordial value (see Yang et al., 1984). It is clear that the method described here for getting the 3 He/H ratio is model-dependent, assuming a neutron production time history and requiring knowledge of the geometry of the accelerated ions and the angular distribution of the resulting neutrons. Also, the assumption that the neutron production rate is proportional to the c-ray line time history may not be strictly valid because the neutrons are produced by ions with higher energy (30–100 MeV/nucleon) than those producing the narrow lines (5–30 MeV for protons), and the accelerated ion spectrum may change with time. The 3 He/H measurement for the photosphere can be compared with the spectroscopic 3 He/He value of ð4 2Þ 104 in a solar prominence obtained by Hall (1975). If the ‘‘standard’’ value of 0.1 for
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photospheric He/H is appropriate, then the two ratios are in agreement. 3.2. Ambient chromosphere and coronal abundances Early in the SMM operations, the GRS recorded an intense, long-duration flare on April 27, 1981 with several narrow c-ray lines (see Fig. 1(a)). Forrest (1983) was the first to show that a theoretical c-ray spectrum (Ramaty et al., 1979), which assumed that the ambient gas and accelerated ion abundances were both photospheric, gave an unacceptable fit to the data. A better fit was obtained by enhancing the ambient abundances of Ne, Mg, Si and Fe relative to the photospheric abundance of C and O. Murphy et al. (1991) have carried out further detailed analysis of this spectrum and were able to determine ‘‘best fit’’ abundances for the ambient gas and the accelerated ions, the latter contributing broad lines to an underlying continuum also due to electron bremsstrahlung. Their analysis gives the best-fit incident photon spectrum shown in Fig. 1(b). This spectrum is folded through the SMM/GRS response function and overlayed with the observed count spectrum shown in Fig. 1(a), which shows how the moderate energy resolution NaI(Tl) spectrometer degrades the spectrum. As far as ambient abundances are
concerned, Mg/C, Si/C and Fe/C are enhanced relative to the photosphere, consistent with the earlier conclusion by Forrest (1983). The results also show that Ne/C is enhanced relative to both photosphere and corona. They also suggest that the ambient abundance ratios Mg/C, Si/C and Fe/ C are similar to coronal and about three times higher than in the photosphere. We have shown here how the c-ray spectral data can be used to obtain solar atmospheric abundances in the case of one flare. However, a broader picture of such data was obtained by Share and Murphy (1995) who measured the fluxes of 10 narrow c-ray lines in 19 intense solar flares observed by the SMM/GRS during its nearly 10-year mission. From a study of the flare-to-flare variations of the narrow line fluxes, they found that the elemental abundances are grouped in accordance with their FIP as discussed in the following section. Share and Murphy (1999) have studied the ‘‘broad’’ c-ray line spectra from the 19 flares and suggest that the accelerated Fe may be enhanced by 5 times over its ambient coronal abundance. 3.3. Low FIP/high FIP isotope line flux ratios An important result of the study of the line fluxes for the 19 SMM/GRS flares, reported by
Fig. 1. (a) Observed count spectrum of the April 27, 1981 flare fitted with the calculated spectrum for ‘‘best fit’’ abundances (see text) with [4 He/1 H] ¼ 0.5. (b) Corresponding photon spectrum (see Murphy et al., 1991).
E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
Share and Murphy (1995), is the strong evidence that the FIP abundance bias seen in coronal versus photospheric abundances and in SEPs and in the solar wind (Meyer, 1992 and Reames et al., 1994) is also present in the c-ray line source, although it can be highly variable. Murphy et al. (1997) continued this study using spectra obtained by CGRO/OSSE during the June 4, 1991 flare and compared them with the SMM/ GRS results. In this study low FIP elements are Mg, Si and Fe and the high-FIP elements are C, O and Ne. Fig. 2 shows the low-FIP to high-FIP summed line flux ratio ½RðlF=hFÞ for 19 SMM/ GRS flares, illustrating the variability in this quantity from flare to flare. Also shown is this ratio for two CGRO/OSSE orbits during the 1991 flare. Fig. 3 shows the time dependence of this flux ratio in more detail, suggesting the trend of an increasing low-FIP to high-FIP flux ratio with time during the flare. This behavior could suggest that the ion interaction region is moving to the coronal region as the flare progresses since it has been established (Meyer, 1985) that low-FIP coronal elements are enhanced relative to high-FIP elements, as compared to photospheric abundances. Similar behavior of RðlF=hFÞ was seen during the November 6, 1997 solar flare observed by the Yohkoh/GRS (Yoshimori et al., 1999). Evolution of the nuclear interaction region in
117
Fig. 3. Time profile of the summed low-FIP to 1.634 MeV Ne line flux ratio derived from off-pointed (filled diamonds) and sun-pointed (open diamonds) detector data (from Murphy et al., 1997).
flares to the coronal region has also been seen in SMM/GRS flares by studying the time correlation of c-ray flux with meter-wave emissions (Chupp et al., 1993 and Trottet et al., 1994). 4. The future The report at this workshop that RHESSI obtained an image of the 2.223 MeV line during the July 23, 2002 solar flare promises that the 3 He/H solar abundance ratio will be further constrained. Confidence in this outcome is supported by the availability of new Monte Carlo calculations of the angular and energy distributions of flare-produced neutrons (Hua et al., 2002) as well as consideration of the Compton-scattered continuum below the line, which can reveal the photospheric depth where the line originates.
Acknowledgements We thank an anonymous reviewer for valuable suggestions.
Fig. 2. Low FIP-to-high FIP line flux ratio for the 19 SMM/ GRS flares (Share, 1996, private communication) compared with the OSSE June 4 ratios obtained for the first and second orbits and the mean for both orbits (from Murphy et al., 1997).
References Biermann, L. et al., 1951. Natur. 6a, 47. Chupp, E.L. et al., 1973. Natur. 241, 333.
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Chupp, E.L. et al., 1975. In: Kane, S.R. (Ed.), Solar Gamma-, X- and EUV Radiation. IAU Symposium, vol. 68. D. Reidel Publishing Co., p. 341 (used by permission). Chupp, E.L. et al., 1993. A&A 275, 602. Dunphy, P.P. et al., 1999. 26th International Cosmic Ray Conference Paper, vol. 6, Salt Lake City, UT, p. 9. Forrest, D.J., 1983. In: Burns, M.L., Harding, A.K., Ramaty, R. (Eds.), Positron and Electron Pairs in Astrophysics. AIP, New York, p. 3. Geiss, J., 1984. SSRv 33, 201. Hall, D.N.B., 1975. ApJ 197, 509. Hua, X.-M., Lingenfelter, R.E., 1987. ApJ 319, 555. Hua, X.-M. et al., 2002. ApJ Suppl. Ser. 140, 563. Hudson, H. et al., 1980. ApJ (Lett.) 236, L91. Lingenfelter, R., Ramaty, R., 1967. In: Shen, B.S.P. (Ed.), High Energy Nuclear Reactions in Astrophysics. Benjamin, New York, p. 99. Lingenfelter, R.E., 1988. In: Gehrels, N., Share, G. (Eds.), Nuclear Spectroscopy of Astrophysical Sources. AIP Conference Proceedings, vol. 170, New York, p. 17. Meyer, J.-P., 1985. ApJ Suppl. Ser. 57, 151.
Meyer, J.-P., 1992. In: Prantzos, N. et al. (Eds.), Origin and Evolution of the Elements. Cambridge. Morrison, P., 1958. NCim 7, 858. Murphy, R.J. et al., 1991. ApJ 371, 793. Murphy, R.J. et al., 1997. ApJ 490, 883. Prince, T. et al., 1982. ApJ (Lett.) 255, L81. Prince, T. et al., 1983. 18th International Cosmic Ray Conference Papers, vol. 4, Bangalore, India, p. 79. Ramaty, R. et al., 1979. ApJ (Suppl.) 40, 487. Ramaty, R., 1986. In: Sturrock, P. et al. (Eds.), Physics of The Sun II. D. Reidel, p. 292. Ramaty, R. et al., 1995. ApJ (Lett.) 455, L193. Share, G., Murphy, R.J., 1995. ApJ 452, 933. Share, G., Murphy, R.J., 1999. 26th International Cosmic Ray Conference Paper, vol. 6, Salt Lake City, UT, p. 13. Reames, D.V. et al., 1994. ApJ (Suppl.) 90, 649. Trottet, G. et al., 1994. A&A 288, 647. Vestrand, W.T. et al., 1999. ApJ (Suppl.), 120,409. Yang, J. et al., 1984. ApJ 281, 493. Yoshimori, M. et al., 1999. 26th International Cosmic Ray Conference, vol. 6, p. 5.
Production of c-ray lines by energetic ion interactions (determination of solar atmospheric abundances) Edward L. Chupp *, Philip P. Dunphy Department of Physics and Space Science Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA
Abstract Acceleration of energetic ions in solar flares produces narrow and broad nuclear c-ray de-excitation lines, by nuclear interactions with the ambient solar atmosphere. Observations of several solar flare c-ray spectra have been made since 1972 by satellite-borne c-ray spectrometers on OSO 7, HEAO 3, SMM, HINOTORI, GRANAT, YOHKOH and CGRO. The line intensities depend directly on the ambient and accelerated particle abundances. In addition, production of neutrons in several reactions leads to emission of the neutron–proton capture c-ray line, from the photosphere, at an energy of 2.223 MeV. The rate of decay in intensity of this line depends on the photospheric 3 He/H abundance ratio. Current analysis gives a photospheric 3 He/H abundance ratio of ð2:3 1:2Þ 105 . In addition, a study of narrow c-ray line fluxes from several flares suggests that the ratio of (Mg + Si + Fe) to (C + N + O) abundances changes with time during the flare. Since this ratio measures the abundance ratio of low first ionization potential (FIP) elements to high FIP elements, the c-ray production site may start in the lower chromosphere and move, as the flare progresses, into the corona where the low/high FIP abundance ratio increases. A study of broad c-ray line fluxes also permits a determination of the abundance of flare-accelerated heavy ions. Ó 2003 Elsevier B.V. All rights reserved. PACS: 96.60.Rd Keywords: Solar flares; c-ray lines
1. History Biermann et al. (1951) suggested that the four large solar cosmic ray events, known by then, were due to a flux of relativistic neutrons from nuclear reactions of protons accelerated and interacting at
*
Corresponding author. E-mail address: [email protected] (E.L. Chupp).
the flare site. Morrison (1958) predicted a strong emission of the neutron–proton capture c-ray at 2.223 MeV during a solar flare. Later, comprehensive studies of nuclear processes that might occur during flares were carried out by Lingenfelter and Ramaty (1967). See also a review by Lingenfelter (1988). During the 1960s, several spectrometers studied flare continuum emissions up to photon energies of several million electron volts. The OSO-7
1387-6473/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2003.11.016
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satellite, launched in 1971 September, carried a NaI (Tl) c-ray spectrometer (GRS) designed for the study of nuclear line emissions from solar flares. In early August 1972, a large active region produced several major flares with a major event occurring on August 4, 1972, which was observed by the GRS. During the rising phase of the flare, for 10 min, before satellite day/night transition, significant c-ray lines at 2.223 (from n/p capture) and 0.51 MeV (from e =eþ annihilation) were detected with faint evidence for nuclear de-excitation lines from C and O nuclei at 4.4 and 6.1 MeV and a strong underlying continuum to nearly 10 MeV. Further c-ray line and continuum detections were made during the August 7, 1972 flare from the same active region (see Chupp et al., 1973, 1975). Two further flare observations of the neutron/ proton capture line were made on the HEAO 1 satellite, July 11, 1978 (Hudson et al., 1980); and on the HEAO 3 satellite, November 9, 1979 (Prince et al., 1982). In 1972, a NASA panel, meeting at Woods Hole, MA, recommended a satellite mission for sunspot cycle 21 dedicated to a multi-wavelength study of solar activity. This led to the development of the Solar Maximum Mission/Gamma-Ray Spectrometer (SMM/GRS). This mission, launched on February 14, 1980 operated until late 1989 November, obtained c-ray spectra from more than 185 events (Vestrand et al., 1999). Other satellite missions with spectrometers which have obtained several solar flare c-ray spectra and their launch dates are: Hinotori/GRS (1981), GRANAT/PHEBUS (1989), CGRO/COMPTEL/OSSE (1991) and Yohkoh/GRS (1991). In February 2002, The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched and on July 23, 2002 obtained the first high energy resolution c-ray spectrum of an intense solar flare.
2. c-ray line observations and solar composition This presentation has the goal of reviewing what can be learned about solar isotopic and elemental composition in the flaring region from cray line observations.
The interaction of accelerated ions and electrons, from sub-MeV to several hundred MeV or even GeV energies, with the ambient solar atmosphere at the flare site produces c-ray line and continuum emissions from: (1) the de-excitation of nuclear energy levels populated by inelastic scattering of ions on target nuclei giving lines typically with energies up to 15.5 MeV, (2) the decay of neutral and charged mesons which are produced primarily by protons and a particles with energies above 300 or 185 MeV/nucleon. The former give a broad line at 70 MeV and the latter continuum electron bremsstrahlung from the charged meson decay electrons, and (3) direct relativistic electron bremsstrahlung from accelerated electrons. In addition; the neutrons produced in several nuclear reactions can thermalize in the solar photosphere and be captured by protons giving the characteristic c-ray line at 2.223 MeV. Also, the production of radioactive positron emitters and decay of positive mesons can lead to positron singlet and triplet annihilation with ambient electrons giving a line at 0.511 MeV or continuum cray photons, respectively. The emission of these lines is ‘‘delayed’’ from the time of production of the neutrons or the positrons. In the case of ‘‘prompt’’ spectral lines from nuclear de-excitation, valuable information is obtained from the study of their width. In particular: ‘‘narrow’’ spectral lines are produced by the inelastic scattering of protons or a particles on the heavier ambient nuclei, the line width depending primarily on the spectrum of the incoming ions and the mass of the target nuclei and, ‘‘broad’’ spectral lines which are produced by accelerated heavier ions (A > 4), interacting with the ambient hydrogen and helium nuclei. Ramaty (1986), Ramaty et al. (1995), Murphy et al. (1991) and others have prepared detailed models, giving the prompt c-ray line intensities in terms of the target abundances, the excitation cross-sections and various accelerated particle energy spectra. Such model calculations, using energy-dependent excitation cross-sections, have been carried out for both thin and thick target interaction conditions. In principle, absolute isotopic abundances can be found by comparing the observed and calculated line intensities, while rel-
E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
ative abundances can be found from line intensity ratios.
profile of the 2.223 MeV line emission is usually modelled by F ðtÞ ¼ A
3. Results on solar atmospheric composition A study of the time behavior of both the 2.223 MeV capture line and the prompt narrow c-ray lines enable a determination of the 3 He/H abundance ratio in the photosphere, while a study of the yield of individual c-ray lines gives the ambient chromosphere and coronal abundances where the lines are produced. In addition, the ratio of the line fluxes of elements with greatly different values of the first ionization potential (FIP) possibly indicates the location of the c-ray production region. 3.1. Photospheric 3 He/H Accurate determination of this isotopic ratio is of great interest since it relates to the composition of the early sun, to stellar evolution and to BigBang nucleosynthesis of the light elements. Here, we focus only on how the c-ray measurements are used to obtain this ratio. The scenario follows the production of neutrons by nuclear reactions of accelerated ions with the ambient atmosphere in magnetic loops. The downward directed neutrons slow down quickly in the photosphere and can be captured by protons giving a narrow line at 2.223 MeV, or by non-radiative capture on 3 He, or be back-scattered to space and decay. The 2.223 MeV line flux from an instantaneous production of neutrons can be approximated by exponential decay with a time constant hsi, where 1=hsi ¼ 1=sH þ 1=s3He þ 1=sd ; with sH the mean capture time on H, s3He the mean capture time on 3 He and sd the mean time for decay (918 s). sH and s3He depend on the reciprocal of the number densities of H and 3 He, hnH i and hn3He i are approximately 1:4 1019 =hnH i and 8:6 1014 =ð3 He=HÞhnH i, respectively (Hua and Lingenfelter, 1987). Since the production rate of neutrons follows the production rate of excited nuclear levels which give prompt emission of MeV c-ray lines, the time
115
Z
t
½Sðt0 Þ=s exp½ðt t0 Þ=s dt0 ; 1
where Sðt0 Þ is proportional to the time history of the prompt c-ray line emission. Using this approach, several events have provided data adequate for obtaining estimates for the photospheric 3 He/H abundance ratio (Prince et al., 1983; Trottet et al., 1994; Murphy et al., 1997; Dunphy et al., 1999; Yoshimori et al., 1999). For the June 3, 1982 flare, Hua and Lingenfelter (1987) [HL87] carried out detailed Monte Carlo calculations which treated the energy and angular distributions of the neutrons to compare with direct neutron observations in this flare. They, therefore, determined the accelerated ion angular distribution and energy spectrum, leading to a more accurate determination of the behavior of neutrons in the photosphere. For neutron production by a mirroring distribution of ions, they found a value of ð2:3 1:2Þ 105 for the 3 He/H abundance ratio. As discussed by HL87, this new photospheric value is ‘‘slightly’’ lower than the estimate, of ð3:4 1:7Þ 105 by Geiss (1984) for the outer convective zone of the sun, which was based on measurements of 3 He/4 He in meteorites and protosolar estimates for the D/H and 4 He/H ratios. This new value is also close to the primordial value (see Yang et al., 1984). It is clear that the method described here for getting the 3 He/H ratio is model-dependent, assuming a neutron production time history and requiring knowledge of the geometry of the accelerated ions and the angular distribution of the resulting neutrons. Also, the assumption that the neutron production rate is proportional to the c-ray line time history may not be strictly valid because the neutrons are produced by ions with higher energy (30–100 MeV/nucleon) than those producing the narrow lines (5–30 MeV for protons), and the accelerated ion spectrum may change with time. The 3 He/H measurement for the photosphere can be compared with the spectroscopic 3 He/He value of ð4 2Þ 104 in a solar prominence obtained by Hall (1975). If the ‘‘standard’’ value of 0.1 for
116
E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
photospheric He/H is appropriate, then the two ratios are in agreement. 3.2. Ambient chromosphere and coronal abundances Early in the SMM operations, the GRS recorded an intense, long-duration flare on April 27, 1981 with several narrow c-ray lines (see Fig. 1(a)). Forrest (1983) was the first to show that a theoretical c-ray spectrum (Ramaty et al., 1979), which assumed that the ambient gas and accelerated ion abundances were both photospheric, gave an unacceptable fit to the data. A better fit was obtained by enhancing the ambient abundances of Ne, Mg, Si and Fe relative to the photospheric abundance of C and O. Murphy et al. (1991) have carried out further detailed analysis of this spectrum and were able to determine ‘‘best fit’’ abundances for the ambient gas and the accelerated ions, the latter contributing broad lines to an underlying continuum also due to electron bremsstrahlung. Their analysis gives the best-fit incident photon spectrum shown in Fig. 1(b). This spectrum is folded through the SMM/GRS response function and overlayed with the observed count spectrum shown in Fig. 1(a), which shows how the moderate energy resolution NaI(Tl) spectrometer degrades the spectrum. As far as ambient abundances are
concerned, Mg/C, Si/C and Fe/C are enhanced relative to the photosphere, consistent with the earlier conclusion by Forrest (1983). The results also show that Ne/C is enhanced relative to both photosphere and corona. They also suggest that the ambient abundance ratios Mg/C, Si/C and Fe/ C are similar to coronal and about three times higher than in the photosphere. We have shown here how the c-ray spectral data can be used to obtain solar atmospheric abundances in the case of one flare. However, a broader picture of such data was obtained by Share and Murphy (1995) who measured the fluxes of 10 narrow c-ray lines in 19 intense solar flares observed by the SMM/GRS during its nearly 10-year mission. From a study of the flare-to-flare variations of the narrow line fluxes, they found that the elemental abundances are grouped in accordance with their FIP as discussed in the following section. Share and Murphy (1999) have studied the ‘‘broad’’ c-ray line spectra from the 19 flares and suggest that the accelerated Fe may be enhanced by 5 times over its ambient coronal abundance. 3.3. Low FIP/high FIP isotope line flux ratios An important result of the study of the line fluxes for the 19 SMM/GRS flares, reported by
Fig. 1. (a) Observed count spectrum of the April 27, 1981 flare fitted with the calculated spectrum for ‘‘best fit’’ abundances (see text) with [4 He/1 H] ¼ 0.5. (b) Corresponding photon spectrum (see Murphy et al., 1991).
E.L. Chupp, P.P. Dunphy / New Astronomy Reviews 48 (2004) 113–118
Share and Murphy (1995), is the strong evidence that the FIP abundance bias seen in coronal versus photospheric abundances and in SEPs and in the solar wind (Meyer, 1992 and Reames et al., 1994) is also present in the c-ray line source, although it can be highly variable. Murphy et al. (1997) continued this study using spectra obtained by CGRO/OSSE during the June 4, 1991 flare and compared them with the SMM/ GRS results. In this study low FIP elements are Mg, Si and Fe and the high-FIP elements are C, O and Ne. Fig. 2 shows the low-FIP to high-FIP summed line flux ratio ½RðlF=hFÞ for 19 SMM/ GRS flares, illustrating the variability in this quantity from flare to flare. Also shown is this ratio for two CGRO/OSSE orbits during the 1991 flare. Fig. 3 shows the time dependence of this flux ratio in more detail, suggesting the trend of an increasing low-FIP to high-FIP flux ratio with time during the flare. This behavior could suggest that the ion interaction region is moving to the coronal region as the flare progresses since it has been established (Meyer, 1985) that low-FIP coronal elements are enhanced relative to high-FIP elements, as compared to photospheric abundances. Similar behavior of RðlF=hFÞ was seen during the November 6, 1997 solar flare observed by the Yohkoh/GRS (Yoshimori et al., 1999). Evolution of the nuclear interaction region in
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Fig. 3. Time profile of the summed low-FIP to 1.634 MeV Ne line flux ratio derived from off-pointed (filled diamonds) and sun-pointed (open diamonds) detector data (from Murphy et al., 1997).
flares to the coronal region has also been seen in SMM/GRS flares by studying the time correlation of c-ray flux with meter-wave emissions (Chupp et al., 1993 and Trottet et al., 1994). 4. The future The report at this workshop that RHESSI obtained an image of the 2.223 MeV line during the July 23, 2002 solar flare promises that the 3 He/H solar abundance ratio will be further constrained. Confidence in this outcome is supported by the availability of new Monte Carlo calculations of the angular and energy distributions of flare-produced neutrons (Hua et al., 2002) as well as consideration of the Compton-scattered continuum below the line, which can reveal the photospheric depth where the line originates.
Acknowledgements We thank an anonymous reviewer for valuable suggestions.
Fig. 2. Low FIP-to-high FIP line flux ratio for the 19 SMM/ GRS flares (Share, 1996, private communication) compared with the OSSE June 4 ratios obtained for the first and second orbits and the mean for both orbits (from Murphy et al., 1997).
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