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The time profile of the neutron capture line of 2.223 MeV for the flare of 16 December 1988 (its third peak) is analyzed. The enhancements of plasma density in the deep photospheric layers of the Sun under the flare region (an effect of density enhancement) have been deduced by the SINP calculation code. These enhancements are studied based on several modified models of the lower chromosphere, photosphere, and convection zone. Additionally, the case has been examined when the neutron source is placed at the top of the photosphere. The energy spectrum of accelerated particles (protons) is shown to evolve with time. Assuming a stochastic mechanism of acceleration and a Bessel function for the spectrum, we find the spectral index αT to increase from 0.005 to 0.1 during the decay phase of the burst; i.e., the proton spectrum becomes harder. Density enhancements found in the flare of 16 December 1988 are consistent with our previous results on the density enhancement effects deduced for two other flares, 6 November 1997 and 22 March 1991. It is suggested that density enhancements in the deep layers of the photosphere may be rather common features of powerful solar flares. B.M. Kuzhevskij is deseased. E.V. Troitskaia ( ) · B.M. Kuzhevskij

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