Particle Acceleration in the 6 November 1997 Event as viewed from Gamma-Rays and Solar Energetic Particles
M. Yoshimori
      Rikkto University, Toshima-ku, Tokyo, Japan

A gamma-ray spectrometer on Yohkoh observed strong high-energy photon emission from a flare (X9/2B, S13 W64) on 6 November 1997. It showed an impulsive temporal variation with a duration of about 4 min and the photon spectrum extended to 100 MeV. The spectrum exhibited narrow and broad gamma-ray lines superposed on the electron bremsstrahlung continuum. The narrow lines are the C, O, Ne, Mg, Si and Fe nuclear deexcitation lines which are produced by nuclear interactions of accelerated protons and He nuclei with the ambient heavy nuclei, while the broad lines are the Ne+Mg, C and O lines around 1.6, 4 and 6 MeV, respectively, which result from interactions of accelerated heavy ions with ambient protons. The Yohkoh observation suggests that protons and heavy ions were efficiently accelerated to high-energies in a short time scale and streamed down to the chromosphere which nuclear interactions take place. Yohkoh hard X-ray images at 53-93 keV showed typical double sources, indicating that accelerated electrons streamed down to two footpoints of a flaring magnetic loop. A distance between the two hard X-ray sources were almost constant during the rising and maximum phases but increased gradually in the decay phase. The Sakao et al's scenario suggests that the separating footpoints imply a rising of the particle acceleration site (energy release site) in the decay phase. That is, magnetic reconnection and subsequent energy release occur at a X-point located at the tip of a cusp-shaped magnetic field above a closed loop. Further, the temporal variation in the Yohkoh gamma-ray spectrum revealed that a Low-FIP (Fe, Mg, Si) line to High-FIP (C, N, O, Ne) line ratio was enhanced by a factor of three in the decay phase. This proposes the possibility that the magnetic mirror points moved upward to the corona where the abundances of Low-FIP elements are higher. On the other hand, solar energetic particles and strong CME were observed in association with the 6 November event. The time profile of SEPs showed a feature of gradual event with a long duration and the energy spectra of protons and heavy ions were very hard. The charge distribution of Fe ions peaked at 11-14 at 0.1-2.5 MeV/nuc and the mean charge increased with energy. These SEP results are typical of gradual events in which particles are thought to be energized by CME-driven shock. However, the other SEP data indicated a high Fe/O ratio and a tail of the Fe ion charge distribution extending to higher values, characterizing impulsive events. From the gamma-ray and SEP observations the 6 November 1997 event is mixtures of gradual and impulsive events. The gamma-ray observations characterize the impulsive event, while the SEP observations features the gradual event. Both observations suggest that particles were accelerated by two different mechanisms at two different sites: One is that magnetic reconnection occurs above@ the loop top and ions are accelerated by shocks or Alfven waves in a short time scale, streaming down to the chromosphere to produce impulsive gamma-ray emission. The other is that a large amount of heavy ions of solar wind were accelerated by CME-driven shock in the higher corona and propagate in interplanetary space, causing the gradual SEP event.