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.     

Observation of temporal and spatial variations in the Fe/O charge composition of the solar particle event of 4 July, 1974

Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R _E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles.     

Abundances of carbon,oxygen, and neon in the solar wind during the period from August 1978 to June 1982

From four years of data provided by the Ion Composition Instrument on ISEE-3, we have derived flux ratios of minor elements in the solar wind and found He/O = 75 ± 20 and Ne/O = 0.17 ± 0.02. These results are compared with recent solar energetic particle composition data and photospheric values, and they are discussed in the light of theoretical models of ionization and acceleration of heavy ions in the solar chromosphere and corona.     

CRRES Measurements of Energetic Helium During the 1990–1991 Solar Maximum

During the 1990–1991 solar maximum, the CRRES satellite measured helium from 38 to 110 MeV n^–1, with isotopic resolution, during both solar quiet periods and a number of large solar flares, the largest of which were seen during March and June 1991. Helium differential energy spectra and isotopic ratios are analyzed and indicate that (1) the series of large solar energetic particle (SEP) events of 2–22 June display characteristics consistent with CME-driven interplanetary shock acceleration; (2) the SEP events of 23–28 March exhibit signatures of both CME-driven shock acceleration and impulsive SEP acceleration; (3) below about 60 MeV n^–1, the helium flux measured by CRRES is dominated by solar helium even during periods of least solar activity; (4) the solar helium below 60 MeV n^–1 is enriched in ³He, with a mean ³He/⁴He ratio of about 0.18 throughout most of the CRRES mission `quiet' periods; and (5) an association of this solar component with small CMEs occurring during the periods selected as solar `quiet' times.     

Charge dependence of the solar modulation of multiply charged cosmic ray nuclei

The intensity and energy spectra of multiply charged cosmic ray nuclei, in the energy interval 250–1500 MeV/n, were studied at three different levels of solar activity, viz. in 1963, 1964 and 1967. The same detectors, nuclear emulsion stacks flown from Fort Churchill, Canada, were used to determine simultaneouslty the energy spectra of helium, C, N, O as well as H (Z=10–28) nuclei. An analysis of the measured spectra indicates that these can be interpreted in terms of: (a) the source spectrum as a Fermi spectrum with a spectral index of 2.65; (b) the interstellar propagation as in a Gaussian distribution of path lengths with a mean path length of 4 g cm^–2 and (c) the interplanetary propagation as given by the numerical solution of the Fokker-Planck equation incorporating diffusion, convection and adiabatic deceleration. On comparing the measured ratios of He to H-nuclei (mean Z14) with the theoretically calculated values for the three levels of solar activity, it is found that within experimental uncertainties, the solar modulation is essentially the same for nuclei of same mass to charge ratio and is not dependent on the charge of the nuclei.On leave from Tata Institute of Fundamental Research, Bombay.     

Energy and nuclear charge dependence of abundance enhancements of solar cosmic ray heavy ions in three large solar events

The differential flux and energy spectra of solar cosmic ray heavy ions of He, C, O, Ne, Mg, Si, and Fe were determined in the energy interval E = 3–30 MeV amu^-1 for two large solar events of January 24, 1971 and September 1, 1971 in rocket flights made from Ft. Churchill. From these data the relative abundances and the abundance enhancement factors, ξ, relative to photospheric abundances were obtained for these elements. Similar results were obtained for a third event on August 4, 1972 from the available published data. Characteristic features of ξ vs nuclear charge dependences were deduced for five energy intervals. The energy dependence of ξ for He shows a moderate change by a factor of about 3, whereas for Fe, ξ shows a very dramatic decrease by a factor of 10–20 with increasing energy. It is inferred that these abundance enhancements of solar cosmic ray heavy ions at low energies seem to be related to their ionization states (Z ^*) and hence studies of Z ^* can give information on the important parameters such as temperature and density in the accelerating region in the Sun.     

Pulsations in solar hard X-ray bursts

We have analyzed the hard X-ray emission from 28 large solar events, searching for pulsations in intensity profiles. Periodicity occurred in 26 events, usually soon after the onset, with periods in the range 10–100 s. Pulsations occurring at common frequencies in different energy bands are observed to be closely in phase. Periodic behavior in hard X-ray emission is related to that at microwave and decametric wavelength. We discuss our observations briefly in terms of two models: that of McClean et al. (1971), applied to X-ray emission, and that of Brown and Hoyng (1975). As periodicity is normal in extended hard X-ray bursts and occurs through a broad energy band, it is probably directly related to a principal flare acceleration mechanism. Our observations constrain possible mechanisms of flare acceleration and physical properties of the acceleration region.This work began when the author was at the Institute for Plasma Research, Stanford University.     

Heavy solar cosmic rays in the January 25, 1971 solar flare

A detailed study of the charge composition of heavy solar cosmic rays measured in the January 25, 1971 solar flare including differential fluxes for the even charged nuclei from carbon through argon is presented. The measurements are obtained for varying energy intervals for each nuclear species in the energy range from 10 to 35 MeV nucleon^?1. In addition, abundances relative to oxygen are computed for all the above nuclei in the single energy interval from 15 to 25 MeV nucleon^?1. This interval contains measurements for all of the species and as a result requires no spectral extrapolations. An upper limit for the abundance of calcium nuclei is also presented. These measurements, when combined with other experimental results, enable the energy dependence of abundance measurements as a function of nuclear charge to be discussed. It is seen that at energies above about 10 MeV nucleon^?1, the variations of abundance ratios are limited to about a factor of 3 from flare to flare, in spite of large variations in other characteristics of these solar events.     

Balloon observations of solar protons on September 29–30, 1968, over Iceland

On September 29, 1968 a proton event has been recorded during three balloon flights performed at Reykjavik, Iceland (64.2 N, 21.7 W) with GM telescopes and scintillation detector. Solar X-rays have been recorded at 1620 UT when a flare of Importance 2B occurred at N 16, W 52. A comparison between X-rays and microwave emissions is made; the time of the maximum of X-ray intensity is taken as the time of the acceleration and ejection of the particles. The beginning of the proton event is at 1650 UT, and particles were observed for almost 24 h. The spectrum of solar protons E>120 MeV is given for several periods between 7 and 20 h after the flare using three independent methods. The solar particle source spectrum is found as: 321-01 (particles/MeV ster), which implies that (1.2±0.1) × 10³¹ protons (E>120 MeV)/ster have been ejected by the Sun.The time behaviour of the event fits well with Krimigis' model for solar particles diffusion in the interplanetary space. Comparison with other events shows that the radial dependence of the diffusion coefficient is the same (1) on September 28, 1961, July 7, 1966 and September 29, 1968. The diffusion mean free path at 1 AU is 0.11 AU for 1966, period of low solar activity, and decreases with solar activity (0.08 AU for 1961 and 1968). The fit of the time behaviour of the event with Burlaga's ADB model is also discussed.     

Observation of solar particle fluxes over extended solar longitudes

Detailed particle observations from various Pioneer Spacecrafts located at different heliolongitudes during the complex solar flare events of March 30–April 10, 1969 have been utilised to investigate the energy dependence of azimuthal gradients of cosmic ray particles and its effect on the decay of the flare intensity. For an observer located to the east of the centroid of the population, the azimuthal corotation term and the convection term will be additive, resulting in a short decay time constant. An observer located to the west of the centroid of the population will experience a much longer decay time constant, the corotation term partially or completely compensating the loss of particles due to convection. At very low energies, the azimuthal corotation term may even be more than the convection term, thus resulting in a rise in intensity instead of decay during late in the event. Using the relationship showing the dependence of the spectral exponent of the cosmic ray flux late in a flare event on the azimuth from the centroid of the population given by McCracken et al., the energy dependence of the decay time constant and the cross-over energy at which the azimuthal gradient term equals the convection term are investigated. The experimental observations are shown to be generally consistent with the theoretical picture, confirming the importance of convection and the azimuthal gradient in determining the decay profile of flare events.On leave from Physical Research Laboratory, Ahmedabad, India.Now at CSIRO, G.P.O. Box 136, North Ryde, N.S.W., Australia.     

Solar flares and solar wind helium enrichments: July 1965–July 1967

It has previously been suggested that the very high relative abundances of helium occasionally observed in the solar wind mark the plasma accelerated by major solar flares. To confirm this hypothesis, we have studied the 43 spectra with He/H 15% that were observed among 10300 spectra collected by Vela 3 between July 1965–July 1967. The 43 spectra were distributed among 16 distinct periods of helium enhancement, 12 of which (containing 75% of the spectra) were associated with solar flares. Six new flare-enhancement events are discussed in this paper. It is concluded that the association of helium enhancements with major flares is real, non-random and very strong.With this study, there are 12 cases of reliable associations between helium enhancements (He/H 15%) and flares reported in the literature. The general characteristics of these events are discussed. It is found that the flares are typically large and bright (2B or 3B), often they produce cosmic ray protons, and they are widely distributed in solar longitude. The average transit velocity of the pistons (i.e., flare accelerated driver gas) is in excellent agreement with earlier observations of flare shock velocities. The degree to which the pistons have been slowed in transit is in good agreement with theory. The average percentage of helium in the enhanced regions is 15%, but this number should not be considered more than an extremely rough estimate because of very arbitrary decisions that had to be made as to when we would consider an enhancement had ended. The number of positively charged particles in the enhanced region is estimated to be of the order of 4 × 10³⁹.A qualitative discussion of some of the possibilities for the source of helium enhanced plasma is presented. It is suggested that the helium enriched plasma may be the piston producing the shock causing the Type II radio emission. The size of the Type II emission region and the number of particles in the helium enhancement permit an estimate to be made of the density of the corona at the origin of the piston. From this it is estimated further that the piston must come from below about 0.5 R , in agreement with the 0.2–0.3 R often given for the initial height of the Type II emission source. Recent theoretical discussions have indicated that the corona as a whole can be expected to show helium enrichments at these levels.It is pointed out that observations of solar wind helium enhancement can be expected to be a useful tool in studying the distribution and relative abundance of helium in different layers of the solar corona, as well as mechanisms for the acceleration of plasma by solar flares.     

Proton activity of the Sun in current solar cycle 24

We present a study of seven large solar proton events in the current solar cycle 24(from 2009 January up to the current date). They were recorded by the GOES spacecraft with the highest proton fluxes being over 200 pfu for energies 10 Me V. In situ particle measurements show that:(1) The profiles of the proton fluxes are highly dependent on the locations of their solar sources, namely flares or coronal mass ejections(CMEs), which confirms the "heliolongitude rules" associated with solar energetic particle fluxes;(2) The solar particle release(SPR) times fall in the decay phase of the flare emission, and are in accordance with the times when the CMEs travel to an average height of 7.9 solar radii; and(3) The time differences between the SPR and the flare peak are also dependent on the locations of the solar active regions. The results tend to support the scenario of proton acceleration by the CME-driven shock,even though there exists a possibility of particle acceleration at the flare site, with subsequent perpendicular diffusion of accelerated particles in the interplanetary magnetic field. We derive the integral time-of-maximum spectra of solar protons in two forms: a single power-law distribution and a power law roll-over with an exponential tail. It is found that the unique ground level enhancement that occurred in the event on 2012 May 17 displays the hardest spectrum and the largest roll-over energy which may explain why this event could extend to relativistic energies.     

Chemical abundance analysis of the Be star HD 171054

In this work the results of a spectroscopic study of the southern field narrow-line Be star HD 171054 are presented. High dispersion and signal-to-noise ratio spectra allowed the estimation of the fundamental photospheric parameters such as the projected rotational velocity, effective temperature and superficial gravity from non-LTE stellar atmosphere models. From these parameters and microturbulence, the abundances of He, C, N, O, Mg, Al and Si for this object are estimated. Results show that C is depleted whereas N is overabundant compared with the sun and OB stars in the solar vicinity. Oxygen and helium are close to the solar value. Magnesium is down by 0.43 dex and aluminium and silicon are overabundant.     

Simulation of heavy ion spectra for the solar event of October 5, 2002

The results of numerical simulation of stochastic acceleration of the heavy ions (³He, ⁴He, ¹⁶O, and ⁵⁶Fe) are presented for the impulsive solar event of October 5, 2002. The energy spectra of the aforementioned particles have peculiarities (depressions) in the low-energy region (≤1 MeV nucleon⁻¹). Coulomb losses in the flare plasma and adiabatic losses during interplanetary propagation are considered in the study as possible causes of such peculiarities.     

Sectorial anisotropy of solar cosmic rays

A hitherto unobserved sectorial pattern of anisotropy that was limited to a narrow and stable region was displayed by the ground level event (GLE) of January 24, 1971. For the entire 11/2 h interval following onset before isotropy set in, the anisotropy was limited to a 10° cone centered about 60° from the spiral magnetic field line. It is also the first solar particle event for which it is possible, by analytical procedures based upon a theoretical propagation model, to distinguish between two rival candidates for the parent flare.This research was sponsored by the National Science Foundation and Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract No. F19628-70-C-0190, but the report does not necessarily reflect endorsement by the sponsor.     

Helium abundance variations in the solar wind

Helium abundance variations in the solar wind have been studied using data obtained with Los Alamos plasma instrumentation on IMP 6, 7, and 8 from 1971 through 1978. For the first time, average flow characteristics have been determined as a function of helium abundance, A(He). Low and average values of A(He) are each preferentially identified with a different characteristic plasma ‘state’ these correspond to what have previously been recognized as the signatures of interplanetary magnetic field polarity reversals and high speed streams, respectively. Helium enhancements at 1 AU also can be identified with a characteristic plasma state, which includes high magnetic field intensity and low proton temperature. This is further evidence that such enhancements are a signal of coronal transient mass ejections. Long-term averages of A(He) at least partially reflect the relative frequency with which coronal streamers, holes, and transients extend their influence into the ecliptic plane at 1 AU. As a result, there is a real and pronounced solar cycle variation of solar wind H(He).     

Upper cutoff of high energy solar protons

By studying the data from the worldwide neutron monitor network the spectra of most of the solar proton events in cycles 19–20 have been determined. These spectra are best represented by a power law with an upper cutoff R _m . This holds over a wide range in energy or rigidity. For the events examined R _m had values between 3 GV and 20 GV. It is shown that there is no correlation between R _m and the amplitude of the events.The equation describing continuous particle acceleration in a confining medium is solved in the non-stationary case. This solution shows the existence of a cutoff in the spectrum, and is compared with the experimental results in connection with the problem of particle acceleration time.Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico.     

A Study of the Abundance of Low-Z Elements in the Sun During its Whole Predicted Life

The study of the chemical composition of stars and galaxies is a key topic for understanding their origin and evolution. In this study, we present the results of the calculation of solar abundances of the isotopes ¹H, ⁴He, ¹²C, ¹⁴N, ¹⁵O, ¹⁶O, ¹⁷O, and ¹⁸O during the four phases of the solar life: hydrogen burning, onset of rapid growth and red giant, helium burning and helium exhaustion. The open source package “NucNet Tools” from the Webnucleo Group in Clemson University, SC, USA was used for this purpose. The results for all isotopes are listed in tables for future use. Abundances found, globally, agree fairly well with those predicted in the literature. Results obtained for the last two phases have no equivalents elsewhere.     

The effect of particle precipitation events on the neutral and ion chemistry of the middle atmosphere—I. Odd nitrogen

A one-dimensional, time-dependent model of the neutral and ion composition of the middle atmosphere is used to study the processes controlling the production and loss of odd nitrogen species during particle ionization events. From consideration of the cross-sections for the relevant ionization and dissociation reactions we conclude that between 1.3 and 1.6 odd nitrogen atoms per ion pair are produced in the middle atmosphere. The value in the thermosphere is larger due to the role of atomic oxygen. The time-dependent mutual destruction of odd nitrogen by the reaction N(⁴S) +NO→ N₂+O must be included and the assumption of a nitric oxide production normalized to the ionization rate is invalid. A simulation of the 1972 August solar proton event is presented. The calculated ozone depletion occurring during the event due to the increase in odd nitrogen agrees well with the measured ozone changes.