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Session 26 - Solar Flares II.
Oral session, Monday, June 10
Wisconsin Center,
Gamma-ray measurements performed with the COMPTEL instrument on the Compton Gamma Ray Observatory provide strong evidence for an abrupt change in the spectrum of the energetic protons accelerated in the flare of 11 June 1991. The spectrum measured with the COMPTEL burst detectors during the impulsive phase of the flare shows the presence of the 2.223 MeV deuterium-formation line and many other nuclear lines similar to that seen in many flares in the last solar cycle. However, shortly after the maximum of the impulsive phase the spectrum changes to one with a strong 2.223 MeV line and greatly reduced intensities of the other lines detected during the impulsive phase. The ratio of the flux in the 2.223 MeV line to the integrated flux in the 4-7 MeV band is a measure of the shape of the proton spectrum responsible for the emission and, after correcting for the slow behavior of the neutron-capture process, we use this measure to conclude that the spectrum undergoes a significant change in shape. This change occurs over a period of approximately 1 minute or less. We interpret these two spectra to be the effects of two phases of the flare, the second of which has a much harder proton spectrum. Moreover, we believe that the two phases of the flare can be associated with different structures in the solar corona that produce, through transport and acceleration processes, different proton spectra over different time scales.
