Solar Gamma-ray Lines Research Articles

The Calculation of Solar Gamma-Rays by TALYS

Solar gamma-ray lines, produced from nuclear reactions of accelerated particles interacting with the solar atmospheric medium, are the most direct diagnosis for the acceleration and transportation of energetic electrons and ions in solar flares. Much information about composition, spectrum, and angular distribution of the accelerated ions, as well as the elemental abundances of the ambient solar atmosphere can be derived from solar gamma-ray line spectra. A new gamma-ray calculation program has been developed by using an efficient nuclear code − TALYS. The theory of gamma-ray production in solar flares is treated in detail. The characteristics of gamma-ray spectrum are also presented.

New Results from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) Mission

The primary scientific objective of RHESSI Small Explorer mission is to investigate the physics of particle acceleration and energy release in solar flares, through imaging and spectroscopy of X-ray/gamma-ray continuum and gamma-ray lines emitted by accelerated electrons and ions, respectively. Here we briefly describe the mission and summarize the new solar observations, including the first hard X-ray imaging spectroscopy, the first high resolution spectroscopy of solar gamma-ray lines, and the first imaging of solar gamma-ray lines and continuum.

The RHESSI Spectrometer

RHESSI observes solar photons over three orders of magnitude in energy (3 keV to 17 MeV) with a single instrument: a set of nine cryogenically cooled coaxial germanium detectors. With their extremely high energy resolution, RHESSI can resolve the line shape of every known solar gamma-ray line except the neutron capture line at 2.223 MeV. High resolution also allows clean separation of thermal and non-thermal hard X-rays and the accurate measurement of even extremely steep power-law spectra. Detector segmentation, fast signal processing, and two sets of movable attenuators allow RHESSI to make high-quality spectra and images of flares across seven orders of magnitude in intensity. Here we describe the configuration and operation of the RHESSI spectrometer, show early results on in-flight performance, and discuss the principles of spectroscopic data analysis used by the RHESSI software.

Atmospheric gamma rays from solar energetic particles and cosmic rays penetrating the magnetosphere

We detail observations of gamma rays produced by interactions of cosmic rays and solar‐energetic particles on the Earth's atmosphere. The Solar Maximum Mission (SMM) spectrometer accumulated the quiescent atmospheric spectrum over its full 9‐year lifetime and our analysis revealed 20 resolved line features. We compare this spectrum with one collected on October 20, 1989, when SMM observed gamma rays produced by shock‐accelerated protons that impacted the atmosphere in the polar region following an intense solar flare and coronal mass ejection. Observed nuclear‐line intensities increased by over a factor of 50 during this event. Because this event was on the horizon and subtended a limited solid angle as viewed from SMM, the local increase was several times higher. We compare the gamma ray line energies, widths, and intensities from the solar event and quiescent atmospheric spectra and discuss their identifications. From this comparison we confirm direct observations that the solar proton spectrum is considerably softer than the spectrum of cosmic rays. Extension of an existing code for calculating solar gamma ray lines will provide information on the spectra of the protons reaching the Earth's atmosphere and thus on their transport through the magnetosphere. Broad spectral features found in the spectra are likely to be caused by a multitude of unresolved lines excited by neutron capture in the instrument, spacecraft, and Earth's atmosphere.

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is ≲ 0.8 ph cm-2 in the flares, and the upper limit flux is 1.8 × 10-4 ph s-1 cm-2 in the absence of flares. These limits imply that ≲ 6 × 1030 (2σ) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is ≲ 4 × 1026 ergs, but this limit can be more than 1030 ergs if the spectrum extends down to ∽1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 1030 ergs if the spectrum goes down to ∽ 7 keV. Quiet-Sun observations indicate that ≲ 1026erg s-1 are deposited by energetic protons >1 MeV, well below the1027 –1028 erg s-1 required for coronal heating, while <3 × 1027 erg s-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least ∽2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for ≳ 104 s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.

On the correlation between the solar gamma–ray line emission, radio bursts and proton fluxes in the interplanetary space

AbstractIt is shown that the correlation takes place between the 4–7 MeV gamma–ray line flare fluence F4–7 and the intensity of the &gt; 10 MeV proton flux in the interplanetary space as well as between F4–7 and the peak flux density of microwave bursts. Besides, the energy spectral index of protons displays the definite dependence from parameters of the radio burst frequency spectrum. These testify that: a) there is a close physical association between the acceleration of electrons and protons in flares; b) protons, giving gamma–ray lines, and ones, registered in the interplanetary space, belong to the same population.

Solar Gamma-Ray Lines and Interplanetary Solar Protons

Solar gamma-ray lines and protons were simultaneously observed for six flares on 1 April, 4 April, 27 April, 13 May 1981, 1 Feb., and 6 June, 1982 by Hinotori and Himawari satellites. The flare list is presented in Table I and the time histories of gamma-rays and protons are shown in Figs. 1 to 6. The relation-ship between the gamma-ray line fluences and peak proton fluxes for these flares is shown in Fig. 7. It does not reveal an apparent correlation between them. The present results imply that the protons producing gamma-ray lines in the flare region and protons observed near the Earth do not always belong to the same population, and favor the downward streaming model for the gamma-ray line production.

Ratios of the Fluence of 2.22 MeV Gamma-Ray Line to the Fluence of 4.44 MeV Gamma-Ray Line in Solar Flares

The Hinotori satellite observed eight solar gamma-ray line flares between April, 1981 and June, 1982. Ratios of the fluence of 2.22 MeV gamma-ray line restulting from neutron capture on proton to the fluence of 4.44 MeV gamma-ray line resulting from the nuclear deexcitation of C are shown as a function of the heliolongitude of flare. The ratios are roughly constant for disk flares, but decreased for limb flares. The present result suggests that the particle acceleration produces energy spectra that do not vary much from flare to flare. The limb darkening of the ratios can be interpreted in terms of the attenuation of the 2.22 MeV gamma-ray line by Compton scattering in the photosphere.

Solar gamma-ray lines as probes of accelerated particle directionalities in flares

Anisotropies of charged particles accelerated in solar flares can be studied by observing Doppler shifts of selected gamma-ray lines. The spectral shape of the 6.1-MeV line of O-16 is calculated. If the accelerated particles are isotropic, the line remains centered at an emitting-nucleus rest-frame energy of 6129.4 keV, and its width (FWHM) is about 100 keV. However, for particle anisotropies that may be produced in solar flares, the line is shifted to lower energies by about 30 to 40 keV.

Solar Gamma Ray Lines observed during the Solar Activity of August 2 to August 11, 1972

WE report preliminary observations from OSO-7 on the emission of gamma ray lines associated with the solar flares on August 4 and 7, 1972. This is the first observation of such solar gamma rays.

High resolution search for solar gamma-ray lines

A balloon-borne directional spectrometer for gamma rays from 30 kev to 6.3 Mev is described, based on a high-resolution Ge(Li) detector. The sun was observed from 4.7 g cm−2 on May 5, 1968. Seven lines present in the background are attributed to neutron capture reactions in the detector. No evidence of monochromatic solar gamma-ray emission was detected. An upper limit of 6.9×10−3 cm−2 sec−1 was found for 511 kev radiation from the quiet sun. Upper limits 4.5×10−3 cm−2 sec−1 and 3.2×10−2 cm−2 sec−1 were found for the flux of 2.223-Mev gamma rays at the earth for the quiet sun and for a period of subflare activity, respectively.