Production and propagation of cosmic ray H2 and He3 nuclei

The results of detailed calculations on the production of H² and He³ nuclei by cosmic ray protons and helium nuclei in interstellar medium are presented. The flux and energy spectra of these nuclei as well as those of cosmic ray H¹ and He⁴ nuclei in the vicinity of the Earth are calculated. For this purpose the source spectra are assumed to be in the form of a power law in total energy per nucleon with an additional velocity dependent term. This spectrum denoted as Fermi Spectrum, is about midway between the power law spectrum in rigidity and in total energy per nucleon. The fluxes are calculated taking into account: (1) energy dependent cross-sections of thirteen nuclear reactions of cosmic ray protons and helium nuclei with interstellar H¹ and He⁴ leading to the production of H² and He³ nuclei, (2) angular distributions and kinematics of these reactions, (3) ionization loss of the primary and secondary nuclei in interstellar medium, (4) elastic collisions of cosmic ray protons and helium nuclei, (5) distributions of cosmic ray path-lengths in in terstellar space as in gaussian and exponential forms, and (6) interplanetary modulation of cosmic rays from the numerical solution of the complete Fokker-Planck equation describing the diffusion, convection and adiabatic deceleration of cosmic ray nuclei in the solar system. On comparing the calculated values of H²/He⁴ and He³/(He³+He⁴) as a function of energy with the observed data of several investigators, it is found that agreement between the calculated values and most of the observed data is obtained on the basis of: (a) source spectrum in the form of Fermi Spectrum, (b) distribution of path-lengths as in the gaussian form with a mean value of 4 g cm^–2 of hydrogen or as in exponential form with leakage path length of 4 g cm^–2.     

A comparison of the energy spectra of cosmic ray helium and heavy nuclei

Recent observations of the spectra of cosmic ray helium, M, LH and VH nuclei in the energy range from 200 MeV/nuc to>22 GeV/nuc are reported. The differential spectra of all of these groups of nuclei are found to have a maximum at 300–400 MeV/nuc at sunspot minimum. The average exponents on the integral rigidity spectra in the range 5 to 50 GV are 1.54±0.03 for He nuclei, 1.50±0.04 for M nuclei, 1.47±0.06 for LH nuclei and 1.40±0.08 for VH nuclei. The spectra of these groups of nuclei are compared and it is found that the average He/M, He/LH and He/VH ratios are 16±1, 70±3 and 200±15 respectively. These values are reasonably constant from the highest down to the lowest energies measured although some evidence for a dip is present in the 500–1000 MeV/nuc range for both the He/LH and He/VH ratios. Solar modulation effects on these ratios are discussed and it is concluded that the ratios measured at earth are representative of those existing in interstellar space only if energy loss processes in interplanetary space are unimportant. The influence of interstellar propagation on the spectra and charge ratios at low energies arising from ionization energy loss and nuclear spallation during matter traversal is examined. It is found that propagation models that contain a large number of relatively short path lengths significantly modify the expected effects of ionization energy loss at low energies. Specifically it is suggested that the presently measured charge ratios are consistent with the passage of the average cosmic radiation through enough matter to reproduce the abundances of the so-called fragmentation nuclei, Li, Be, B and He³. Two component models are not required to explain our data. Rather we feel that a better representation of the situation results from considering a continuous spatial distribution of sources which, along with the actual interstellar propagation conditions, leads to a particular distribution of matter path lengths. It is pointed out that large differences exist in the approaches used in the literature to calculate the effects of matter traversal in interstellar space at low energies. These differences play an important role in the interpretation of the experimental results. Significant modifications of the charge ratios at low energies can also be obtained by requiring that some of the matter traversal occur in the cosmic ray sources themselves during the cosmic ray acceleration process. This may be sufficient to produce charge ratios that are essentially flat at low energies even in the presence of interstellar ionization loss.     

The comparative spectra of cosmic-ray protons and helium nuclei

We have re-examined and extended the measurements of the primary cosmic ray proton and helium nuclei intensities in the range from a few MeV nuc^–1 to 100 GeV nuc^–1 using a considerable body of recently published data. The differential spectra obtained from this data are determined as a function of both energy and rigidity. The exponents of the energy spectra of both protons and helium nuclei are found to be different at the same energy/nucleon and to increase with increasing energy between 1 and 100 GeV nuc^–1 reaching a value=–2.70 at higher energies and in addition, theP/He ratio changes from a value 5 at 1 GeV nuc^–1 and below to a value 30 at 100 GeV nuc^–1. On a rigidity representation the spectral exponent for each species is nearly identical and remains virtually constant above several GV at a value of –2.70, and in addition, theP/He ratio is also a constant 7 above 3 GeV. The changingP/He ratio and spectral exponent on an energy representation occur at energies well above those at which interplanetary modulation effects or interstellar ionization energy loss effects can significantly affect the spectra. In effect by comparing energy spectra and rigidity spectra in the intermediate energy range above the point where solar modulation effects and interstellar energy loss effects are important, but in the range where there are significant differences between energy and rigidity spectra, we deduce that the cosmic ray source spectra are effectively rigidity spectra. This fact has important implications regarding the mechanism of acceleration of this radiation and also with regard to the form of the assumed galactic spectrum at low energies. The relationship between the proton and helium spectra derived here and the heavier nuclei spectral differences recently reported in the literature is also examined.If rigidity spectra are adopted for protons and helium nuclei, then the source abundance ratio of these two components is determined to be 7:1. Some cosmological implications of this ratio are discussed.     

On the modulation and energy spectrum of highly charged cosmic ray nuclei

The intensity and energy spectrum of cosmic ray VH-nuclei (20Z30) has been measured in a stack of nuclear emulsions exposed over Fort Churchill in 1968. The integral intensity above 300 MeV/nucleon was 1.04±0.04 nuclei m^–2 sr^–1 s^–1 and three differential intensities were measured below 750 MeV/nucleon. Because of the current controversy regarding the true intensities of helium nuclei at this phase of the solar cycle we have also measured these nuclei, obtaining results intermediate between those quoted by other workers. Comparison of these results on the VH-and helium nuclei with those obtained in previous observations made at times of low solar modulation leads to the conclusion that there is no significant charge dependence in the modulation process. This conclusion is in conflict with an earlier analysis but depends on results of improved statistical weight and greater reliability for the VH nuclei and on our measurement of the helium nuclei in the same detector.Supported by the Office of Naval Research under Contract No. N00014-67-A-0113-0021.     

Energy Spectra of low-Flux Protons in the Inner Heliosphere under Quiet Solar Conditions

The energy spectra of protons at energies in the range of about 1–100 MeV are investigated during time periods of low solar activity using data sets from near Earth spacecraft. These populations pose a tough experimental and theoretical problem that remains unsolved up to now. We attempt to provide a consistent definition of low-flux quiet-time periods relevant to low solar activity as well as quasi-stationary periods useful at higher levels of solar activity. Using statistical methods, the possible instrumental contribution to the lowest observed proton fluxes for various detectors is estimated. We suggest and prove that there exists a low-flux population of charged particles in the energy range of about 1–10 MeV, which is present in the inner heliosphere even during the quietest conditions at lowest solar activity. The dynamics of the variations of proton spectra over the solar cycle is investigated. A series of low-flux periods is examined in detail and energy spectra of protons are approximated in the form of J(E)=AE ^–+CE. By determining the best fitting parameters to the energy spectra correlations are made among them as well as with monthly sunspot numbers characterizing solar activity. It has been demonstrated that the value of the energy minimum of proton spectrum E _min that `divides' the two populations – `solar/heliospheric' and `galactic' – is shifted towards higher values with increasing solar activity. Protons have been argued to be predominantly of solar origin up to several MeV near the solar cycle minimum and up to 20–30 MeV at maximum. The slope of the lower spectrum branch (parameter ) slightly decreases with increasing solar activity. The minimum fluxes observed during the last 3 minima of solar activity are compared; the lowest fluxes were those during the 1985–1987 period.     

Relativistic electrons from the Sun observed by IMP-4

Data are presented from the IMP-4 satellite of 0.3–12 MeV electrons from the Sun between May 24, 1967 and May 2, 1969. Correlations with contemporary proton intensity increases at energies above 1 MeV are studied. Classical solar flare events such as those frequently observed from 30°W–60°W in solar longitude are not discussed. Categories of unusual events are defined and examples of each type are given. Discussion of these events centers around the emission and propagation of energetic particles from the point of origin on the Sun to the Earth. The results of this study are the following: (1) The differential electron energy spectrum (0.3–12 keV) from solar flares appears to be a constant of the flare process, with the spectral index = (-)3.0 ± 0.2. (2) Particle emission from solar flares contains a prompt component, which is injected into the interplanetary medium beyond the Sun and which is responsible for the diffusion characteristics of solar particle events, and a delayed component which is effectively contained in the lower solar atmosphere where it diffuses typically ± 100° in longitude and gradually escapes into interplanetary space. The delayed component gives rise to the corotating features commonly observed after the impulsive and diffusive onset from the prompt component. This is not the same as the two component model discussed by Lin (1970a) in which 40 keV electrons are often observed as a separate phenomenon and frequently precede higher energy particles observed at 1 AU. (3) Storage of electrons > 300 keV and protons > 1 MeV is essential to explain emission and propagation characteristics of solar particle events. In some rare cases the storage mechanism appears to be very efficient, culminating in a catastrophic decay of the trapping region. (4) The events with low proton/electron ratios all occur at least three weeks after the previous relativistic electron producing flare.     

Enhancement of solar heavy nuclei at high energies in the 4 July 1974 event

Relative abundances of energetic nuclei in the 4 July 1974 solar event are presented. The results show a marked enhancement of abundances that systematically increase with nuclear charge numbers in the range of the observation, 6 Z 26 for energies above 15 MeV nucl.^–1 While such enhancements are commonly seen below 10 MeV nucl^–1, most observations at higher energies are found to be consistent with solar system abundances. The energy spectrum of oxygen is observed to be significantly steeper than most other solar events studied in this energy region. It is proposed that these observations are characteristic of particle populations at energies 1 MeV nucl^–1, and that the anomalous features observed here may be the result of the high energy extension of such a population that is commonly masked by other processes or populations that might occur in larger solar events.     

Studies of the chemical composition of cosmic rays withZ=3–30 at high and low energies

We have measured the chemical composition of cosmic rays withZ2 over an energy range from 100 MeV/nuc to >2 GeV/nuc using 2 new large area counter telescopes. One of these instruments was a 4 element dE/dx×E× Range telescope, the other a 4 element dE/dx×Cerenkov× ×Range telescope. Two balloon flights with these telescopes at Ft. Churchill in the summer of 1970 provided a total of nearly 1000 Fe nuclei with a charge resolution ranging from 0.10 charge unit at Carbon to 0.25 charge unit at Fe. A detailed charge spectrum is obtained at both high and low energies. Some important differences exist between the present results and those obtained earlier, due in part to the improved statistical accuracy and in part to the improved background rejection of the present data. In particular, the abundance of Cr and Mn are each found to be 0.10×Fe in contrast to the earlier ratio of 0.30 found by some workers for each of these nuclei. The abundance of these two nuclei, as well as others in the 15–25 range, shows no strong dependence on energy. We have extrapolated our composition data to the cosmic ray sources using a variety of interstellar path length distributions. The abundances ofall secondary nuclei withZ between 3–25 are consistent only with propagation models which have vacuum path length distributions which do not differ greatly from exponential. The source abundances of nuclei withZ=15, 17, 18, 19, 21, 22, 23, 24, and 25 are found to be <0.02×Fe. For the remaining nuclei, Na, Al, S, and Ca are found to have source abundances of 0.07, 0.11, 0.18 and 0.13 of Fe respectively. The source abundance of C and O relative to Fe is also much different than some earlier compilations. A comparison of solar and cosmic ray abundances reveals certain selective differences, rather than a systematic overabundance of heavy nuclei in cosmic rays, as has been suggested in the past. These differences are discussed in terms of a common nucleosynthesis origin of the two species of particles.Research sponsored by the National Aeronautics and Space Administration under Grant No. NGR-30-002-052.     

Measurements of cosmic-ray Li,Be and B nuclei in the energy range 100 MeV/nuc to >22 BeV/nuc

We report on new measurements of the spectra of Li, Be and B nuclei in the primary cosmic radiation in the energy range 100 MeV/nuc to >22 BeV/nuc. The differential spectrum of these light nuclei is found to have a maximum at 400 MeV/nuc in 1966. The L/M ratio is found to be equal to 0.25±0.01, constant over the entire energy range of the measurement. Atmospheric and solar modulation effects on the L nuclei and the L/M ratio are discussed. It is concluded that this ratio is representative of conditions in interstellar space. Using the most recently available fragmentation parameters gives a material path length of 3.6 g/cm² of hydrogen for the particles producing the L nuclei. The absence of any variation of the L/M ratio with energy places severe constraints on models for the propagation of cosmic rays. Models in which the material path length is a strong function of energy — or that exhibit an exponential path-length distribution for a fixed energy are incompatible with these results. An examination of the abundance ratios of the individual L nuclei separately reveals major discrepancies with the predictions of interstellar diffusion theory based on presently accepted fragmentation parameters. The constancy of the measured Li/M and B/M ratios with energy is not in accord with the large energy dependence of these ratios expected from the energy dependence of the fragmentation cross-sections. The low Li/M ratio and high B/M ratio to be expected if these nuclei are created at a much lower energy than we observe are also not found. This presents difficulties for theories which suggest that the passage through matter has occurred at low energies subsequently followed by considerable acceleration.The Be/M ratio in cosmic rays is anomalous in that it is 40% larger than expected on the basis of the fragmentation cross-sections. Evidence presented here on the isotopic composition of Be nuclei suggests that this discrepancy is due to an enhanced abundance of Be⁹ or Be¹⁰ in cosmic rays. This discrepancy complicates the determination of a cosmic-ray age using the decay of Be¹⁰ into B.Nevertheless the Be/B ratio is observed to remain constant at 0.42±0.03 over the energy range from 100 MeV/nuc to over 10 BeV/nuc. Unless the fragmentation parameters into the various isotopes of Be and B are such that e.g. (Be/B)<0.05 as a result of this decay, then the age of cosmic rays is either >3×10⁸ years or <10⁶ years. The further observation that the mass to charge ratio of all Be nuclei of energy 1 BeV/nuc is =2.05±0.1 suggests that Be¹⁰ is present at these energies. This supports the idea of a short lifetime.     

On the temporal variation of the solar continuous brightness fluctuations

From a sequence of white-light photographs of solar granulation at the centre of the disk, obtained by Spectro-Stratoscope on May 17, 1975, two-dimensional spatial power spectra of photospheric intensity fluctuations were deduced. These show periodicities of 1000 s, 250–450 s (5-min oscillation), and shorter ones in the range 30–120 s. The reality of the shorter periods, however, seems to be questionable.The weighted mean wavenumber of the spatial power spectra and rms of the intensity fluctuation (I _rms) are also computed, showing the same periodicities as the power.Mitteilungen aus dem Kiepenheuer-Institut Nr. 188.On leave of absence from the Institute of Geophysics, University of Tehran.     

The gamma ray spectrometer for the Solar Maximum Mission

The Solar Maximum Mission Gamma Ray Experiment (SMM GRE) utilizes an actively shielded, multicrystal scintillation spectrometer to measure the flux of solar gamma rays. The instrument provides a 476-channel pulse height spectrum (with energy resolution of 7% at 662 keV) every 16.38 s over the energy range 0.3–9 MeV. Higher time resolution (2 s) is available in three windows between 3.5 and 6.5 MeV to study prompt gamma ray line emission at 4.4 and 6.1 MeV. Gamma ray spectral analysis can be extended to 15 MeV on command. Photons in the energy band from 300–350 keV are recorded with a time resolution of 64 ms. A high energy configuration also gives the spectrum of photons in the energy range from 10–100 MeV and the flux of neutrons 20 MeV. Both have a time resolution of 2 s. Auxiliary X-ray detectors will provide spectra with 1-sec time resolution over the energy range of 10–140 keV. The instrument is designed to measure the intensity, energy, and Doppler shift of narrow gamma ray lines as well as the intensity of extremely broadened lines and the photon continuum. The main objective is to use this time and spectral information from both nuclear gamma ray lines and the photon continuum in a direct study of the dynamics of the solar flare/particle acceleration phenomena.     

Adiabatic Deceleration Effects on the Formation of Heavy Ion Charge Spectra in Interplanetary Space

We investigate the effects of interplanetary propagation on the energy dependence of the mean ionic charge of ~0.1–1 MeV/n iron observed during impulsive solar particle events at 1 AU. A Monte-Carlo approach is applied to solve the transport equation which takes into account spatial diffusion as well as convection and adiabatic deceleration. We find that interplanetary propagation results in a shift of charge spectra observed at 1 AU towards lower energies due to adiabatic deceleration. Taking the above effect into account, we compare predictions of our model of charge-consistent stochastic acceleration with recent ACE observations. A detailed analysis of two particle events shows that our model can give a consistent explanation of the observed iron charge and energy spectra, and allows one to put constraints on the temperature, density, and the acceleration and escape time scales in the acceleration region.     

Relativistic electrons in solar particle events

We present here the results of the first systematic study of electrons of energies greater than 10 MeV associated with solar flares. We have made direct measurements of the frequency with which these particles are found in solar flare related events, the spectra of the particles, and the evolution of the spectra during these events. In addition the nature of the propagation of these electrons is studied and the degree of anisotropy in their diffusion is measured for the first time.The observations were made aboard the spacecraft OGO-5 from 1968, March through 1969, August. It is found that electrons in the energy range 12–45 MeV are normally present in major solar particle events. The time-intensity profiles of the fluxes indicate diffusive propagation; and the time-to-maximum intensity is found to vary with solar longitude in a way which can only be the result of anisotropic propagation with the perpendicular diffusion coefficient comparable in magnitude to, but smaller than, the parallel diffusion coefficient. Spectra during the observed events fit the power law AE ^– with 2.5 3.8. The time evolution of the spectrum throughout the course of the 4 most intense events shows that the spectrum steepens rapidly during the initial phase but retains a constant slope through the decay phase.Events which behave in a manner which is not described by the normal diffusion picture are also discussed. These examples show phenomena other than direct flare injection followed by anisotropic diffusion.This work was supported in part by the National Aeronautics and Space Administration under Contract NAS 5-9096 and Grant NGL 14-001-005.Submitted to the Department of Physics, University of Chicago, Chicago, Illinois in partial fulfillment of the requirements for the Ph. D. degree.NASA Trainee 1967–1969.     

Radial gradients and anisotropies due to galactic cosmic rays

Numerical calculations have been made of the radial gradients and the anisotropyvector atr=1 AU due to galactic cosmic-ray protons and helium nuclei. The model used assumes transport by convection and anisotropic diffusion, and includes the energy losses due to adiabatic deceleration. The present calculations are for the 1964–65 solar minimum. An important constraint applied ineach case was that the model reproduces the electron modulation known from deductions of the galactic spectrum and observations of the near-Earth spectrum; and also reproduces the near-Earth proton and helium nuclei spectra. The diffusion coefficients have been based upon those deduced from magnetic-field power spectra.The principal aim has been to provide estimates of radial gradients and anisotropies, particularly at kinetic energiesT100 MeV/nucleon, by the complete solution of realistic models. Typical values for protons, obtained with a galactic differential number density (total energy)^–2.5, atT50 MeV are: radial gradient, 25%/AU; radial anisotropy, –0.2%; azimuthal anisotropy, 0.2%. These values change markedly when the galactic spectrum is cut-off or greatly enhanced atT<150 MeV, but the intensity spectrum near Earth remains substantially unchanged.It has been shown that it is possible to obtain negative radial gradients and positive radial anisotropies atT50 MeV for galactic particles and thus to mimic solar sources. The radial gradient for 1964–65 reported by Anderson (1968) and by Krimigis and Venkatesan (1969) are shown to be consistent with the diffusion coefficient deduced from the magnetic-field power spectrum; those reported by O'Gallagher are higher than expected and that for 20T30 MeV protons appears to be inconsistent. More precise data on conditions throughout the solar cavity are required if more definitive gradients and anisotropies are to be determined.     

71 GHz (4.2 mm) solar radio bursts in the period July 1967 to December 1969

The results of 2 1/2 years (July 1967 – December 1969) monitoring of solar radio bursts at 71 GHz ( = 4.2 mm) at the Radio and Space Research Station, Slough are presented. During this period only seven events were positively identified as 71 GHz bursts. One of these events (6 July, 1968) is among the largest solar bursts ever recorded anywhere in the microwave-millimetre wave band (47000 × 10^–22Wm^–2Hz^–1), and the associated magnetic field may possibly have exceeded 7200 G. Another event (27 March, 1969) has demonstrated that bursts at 71 GHz can be both intense (4700 × 10^–22Wm^–2Hz^–1) and complex. On other occasions, the absence of any detectable event at 71 GHz helps to define the high frequency spectrum of the burst, this being an important factor in determining the initial energy distribution of the electrons ejected by the associated flare. On one such occasion (21 March, 1969) the derived energy distribution index is 8, in contrast with the more usual values of 2–4.1969–1970 NCR-OAR Senior Post-Doctoral Research Associate at Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass., U.S.A.     

Solar microwave bursts as indicators of the occurrence of solar proton emission

A study has been made of the relation of 19 GHz( = 1.58 cm) solar radio bursts to solar proton emission, with particular reference to the usefulness of relatively long duration bursts with intensities exceeding 50% of the quiet Sun flux (or exceeding 350 × 10^–22 W m^–2 Hz^–1) as indicators of the occurrence of proton events during the four years from 1966–69. 76 to 88% of such bursts are directly associated with solar protons and 60 to 85% of the moderate to large proton events in the four year period could have been predicted from these bursts. The complete microwave spectra of the proton events have also been studied, and have been used to extend the results obtained at 19 GHz to other frequencies, particularly in the 5–20 GHz band. The widely used frequency of 2.8 GHz is not the optimum frequency for this purpose since proton events have a minimum of emission in this region. Most of the radio energy of proton events is at frequencies above 10 GHz. The radio spectra of proton events tend to peak at higher frequencies than most non-proton events, the overall range being 5 to 70 GHz, with a median of 10–12 GHz and a mean of 17 GHz.On leave from the Radio and Space Research Station, Slough, England, as 1969–1970 National Research Council-National Academy of Sciences Senior Post-Doctoral Research Associate at AFCRL.     

Relativistic heavy cosmic rays

During three balloon flights of a 1 m² sr ionizationchamber erenkov counter detector system, we have measured the atmospheric attenuation, flux, and charge composition of cosmic-ray nuclei with 16Z30 and rigidity greater than 4.5 GV.The attenuation mean-free-path in air of VH (20Z30) nuclei is found to be 19.7±1.6 g cm^–2, a value somewhat greater than the best previous measurement. The attenuation mean-free-path of iron is found to be 15.6±2.2 g cm^–2, consistent with predictions of geometric cross-section formulae.We measure an absolute flux of VH nuclei 10 to 20% higher than earlier experiments at similar geomagnetic cutoff and level of solar activity. The relative abundances of evencharged nuclei are found to be in good agreement with results of other recent high-resolution counter experiments.We calculate that our observed cosmic ray chemical composition implies relative abundances at the cosmic-ray source of Ca/Fe=0.12±0.04 and S/Fe=0.14±0.05. The results are consistent with all other elements of charge between 16 and 26 being absent at the source and being produced by cosmic-ray fragmentation in interstellar hydrogen. The results show the ratios A/Fe and S/Fe to be significantly lower in the cosmic-ray source than in the solar system.     

Solar Microwave Bursts from Electron Populations with a 'Broken’ Energy Spectrum

Usually the gyrosynchrotron emission of microwave bursts from electron populations with a power-law (PL) energy distribution has been considered under the assumption that the spectral index of the distribution is constant over a wide range of energies. Meanwhile, there is strong evidence, in particular from hard X-ray and -ray, but also from cm/mm wavelength radio observations, that in many solar flare events the spectrum of the emitting electrons is characterized by a significant hardening at energies above 100–500 keV. We present some examples of calculated microwave burst spectra at cm/mm wavelengths taking into account the above evidence. It is shown that a break in the energy spectrum of the PL electrons can indeed result in a spectral hardening sometimes observed in microwave bursts at frequencies above 10–30 GHz.     

Solar cosmic ray composition above 10 MeV/nucleon and its energy dependence in the 4 August 1972 event

Observations of the proton, helium, (C, N, O) and Fe-group nuclei fluxes made during the large 4 August 1972 solar particle event are presented. The results show a small, but significant variation of the composition of multiply-charged nuclei as a function of energy in the energy region above 10 MeV nucleon^–1. In particular, the He/(C, N, O) abundance ratio varies by a factor 2 between 10 and 50 MeV nucleon^–1 and the Fe-group/(C, N, O) ratio suggests a similar variation. Abundance ratios from the 4 August 1972 event are compared as a function of energy with ratios measured in other solar events to show that several of the earlier results are consistent with an energy variation like that observed in August 1972, while certain other events must have had a substantially different dependence of composition on energy. At energies 50 MeV nucleon^–1, the He/(C, N, O) abundance ratio for August 1972 is consistent with all earlier measurements made above that energy which suggests that variations may vanish at high energies.NASA/NAS Senior Resident Research Associate, on leave from TATA Institute of Fundamental Research, Bombay.     

Upper limit on the steady emission of the 2.223 MeV neutron capture γ-ray line from the Sun

Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) have been searched for evidence of the 2.223 MeV neutron capture line from the Sun, outside the times of -ray-emitting solar flares. Background-corrected spectra accumulated over 3-day intervals between 1980 and 1989 show no evidence of the line. Upper limits are reported separately for periods of high and low solar activity.A conservative 3 upper limit of 5.7 × 10^–5 (cm² s)^–1 is placed on the steady flux in the 2.223 MeV line during inactive periods, which is nearly two orders of magnitude lower than previously published results. After correction for limb darkening of the line emission from off-center positions, this upper limit becomes 7.1 × 10^–5 (cm²s)^–1. Our 3 upper limit on the steady flux in the line during periods of high solar activity is 6.9 × 10^–5 (cm² s)^–1, or 8.6 × 10^–5 (cm² s)^–1 after correction for limb darkening. Our results imply that the quiescent solar corona cannot be heated by ions accelerated above 1 MeV in microflares (or a continuous acceleration process), so long as the ion energy spectrum is similar to that measured in large flares. We also use our results to derive the rate of tritium production at the solar surface; our upper limit of 9 nuclei (cm² s)^–1 is about a factor of 9 below the upper limit from searches for ³H in the solar wind. We place upper limits of the order 10³³ on the number of energetic (> 30 MeV) protons which can be stored in active regions prior to being released in solar flares, which imply that the strongest observed flares cannot be produced by such a mechanism.