M. Yoshimori

Gamma-ray observations from Hinotori

Some interesting results on gamma-ray line emission and its time profiles observed by Hinotori are presented. Possible explanations of gamma-ray line and hard X-ray emissions for the impulsive and gradual flares are discussed. Relationship between the gamma-ray line emission and acceleration and escape of the solar particles is also studied.

Solar Neutron Event in Association with a Large Solar Flare on 2000 November 24

Solar neutrons have been detected using the neutron monitor located at Mount Chacaltaya, Bolivia, in association with a large solar flare on 2000 November 24. This is the first detection of solar neutrons by a neutron monitor that has been reported so far in solar cycle 23. The statistical significance of the detection is 5.5 σ. In this flare, the intense emission of hard X-rays and γ-rays has been observed by the Yohkoh Hard X-ray Telescope (HXT) and Gamma Ray Spectrometer (GRS), respectively. The production time of solar neutrons is better correlated with those of hard X-rays and γ-rays than with the production time of soft X-rays. The observations of the solar neutrons on the ground have been limited to solar flares with soft X-ray class greater than X8 in former solar cycles. In this cycle, however, neutrons were detected associated with an X2.3 solar flare on 2000 November 24. This is the first report of the detection of solar neutrons on the ground associated with a solar flare with an X-ray class smaller than X8.

Observations of Solar Flare Photon Spectra from 20 keV to 7 MeV

Solar flare photon energy spectra in the 20 keV to 7 MeV range are derived from the 1981 Apr. 1, Apr. 4, Apr. 27 and May 13 flares. The flares were observed with a hard X-ray and a gamma-ray spectrometers on board the Hinotori satellite. The results reveal that the shape of hard X-ray spectrum for the gamma-ray flares does not much vary from flare to flare, and the spectra harden apparently at energy about 400 keV. Effects of nuclear line emission on the continuum and of higher energy electron bremsstrahlung are considered to explain the spectral hardening.

Yohkoh observation of a gamma-ray flare on November 6, 1997

Abstract Yohkoh observed strong γ-ray emission from a flare ( X9 2B ) at 11:52 UT on November 6, 1997. We found that fluxes of narrow lines from elements with similar FIPs correlated with one another during the flare, while there is a temporal variation of a ratio of low-FIP (Mg, Si and Fe) to high-FIP (C, N, O and Ne) narrow line fluxes. The ratio measured for the decay phase was enhanced by about factor of 3 compared to those for the rise and peak phases. It suggests two possibilities: (1) the low-FIP/high-FIP elemental abundance ratio at the γ-ray emission site was increased in the decay phase or (2) the location of the γ-ray production site could have changed with time, progressing from the chromosphere-photosphere toward the corona.

A Statistical Study of Gamma-Ray Emitting Solar Flares Observed with Yohkoh

Gamma-ray emitting solar flares observed with Yohkoh were analyzed from a statistical viewpoint. The four-band hard X-ray (15--95 keV) photometric data, taken with the Hard X-ray Telescope onboard Yohkoh, were utilized in combination with the spectro-photometric gamma-ray (0.2--30 MeV) data obtained with the Gamma-Ray Spectrometer. The GOES class was also incorporated. Out of 2788 X-ray flares observed from 1991 October to 2001 December, 178 events with strong hard X-ray emission were selected. Among them, 40 flares were further found to show significant gamma-ray emission. A fractal dimension analysis and multi-band color--color plots of the 40 flares suggest that their soft X-ray to MeV gamma-ray spectral energy distributions involve at least four independent parameters. These are: (1) the overall flare size; (2) the relative intensities of the thermal vs. non-thermal signals; (3) the gamma-ray to hard X-ray intensity ratio; and (4) the hard X-ray spectral slope. These results are examined for possible selection effects. Also, the meanings of the third parameter are briefly considered.

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 Proton Energy Spectra Derived from Gamma-Ray Observations

Energy spectra of accelerated protons at flare sites are derived by comparing the observed ratios of the 4.44 MeV gamma-ray line fluence to the 2.22 MeV gamma-ray line fluence with the calculated ratios. The proton spectra derived for the 1981 Apr. 1, 1981 May 13, 1981 Oct. 15 and 1982 June 6 disk flares do not much vary from flare to flare, and are in agreement with average proton spectra observed in interplanetary space within the errors. It implies that both the protons which produce gamma-ray lines at the Sun and those which escape from the Sun are probably accelerated by the same mechanism.

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.

Gamma-ray spectrometer aboard solar flare observation satellite HINOTORI

Abstract A gamma-ray spectrometer aboard the solar flare obsevation satellite HINOTORI launched on 21 Feb. 1981 is described. The gamma-ray spectrometer consists of a phoswich type scintillator of CsI(Tl) crystal surrounded by a plastic scintillator, which can discriminate between gamma-rays and charged particles. The CsI(Tl) scintillator, whose size is 62 cm2 in area and 5.1 cm in height. covers the energy range 0.21–6.67 MeV. The energy resolution is 0.1E 1 2 (MeV) fwhm and the full energy peak efficiency is 0.40 at 0.662 MeV. The primary data output is a full resolution 128 channel pulse height spectrum every 2 s. Since the launch HINOTORI recorded about forty solar gamma-ray flares and four cosmic gamma-ray bursts, and several interesting results were reported. The typical results of the large solar flare on 27 April 1981 and the cosmic gamma-ray burst on 21 July 1981 are presented in this paper. These observed data could provide important clues for advancing our understanding of high enegry phenomena in solar flares and cosmic gamma-ray bursts.

Response of a 7.6 cm × 7.6 cm bismuth germanate spectrometer for solar gamma ray observations

Abstract The response of a 7.6 cm diameter × 7.6 cm long bismuth germanate (BGO) scintillator to gamma rays from 0.511 to 17.23 MeV has been measured. The full energy peak efficiency of the BGO scintillator is 0.90 at 0.511 MeV, 0.40 at 3.08 MeV and 0.33 at 4.95 MeV. The total peak (full energy + single escape + double escape) efficiency is 0.45 at 10.76 MeV and 0.40 at 17.23 MeV. The energy resolution (fwhm) as a function of gamma-ray energy is approximated by 3.89E0.48 (keV) for energies below 5 MeV. This is about twice that of a NaI scintillator of the same size. The present results indicate that the BGO spectrometer has attractive properties for solar gamma-ray observations.