Date of Award

Spring 2001

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

James Ryan

Abstract

The X-ray and gamma-ray emission from solar flares provides important information about high-energy particles in solar flares. Energetic protons and ions interact with the solar atmosphere, giving rise to nuclear line emission at MeV energies and higher energy photons from the decay of neutral and charged pions. Electrons interact with the solar atmosphere producing a bremsstrahlung continuum. The solar flare spectrum is generally a superposition of these spectra with nuclear line emission dominating from ∼1--8 MeV and the bremsstrahlung at lower and higher energies. The main goal of this thesis has been to explain a small part of a gamma-ray flare observed by COMPTEL in June 1991.

A difficult interval to explain in the 11 June 1991 solar flare is the Intermediate (Rank 1997) or Interphase (Murphy and Share 1999; Dunphy et al. 1999) immediately following the peak of the impulsive phase. All three analyses of this flare using COMPTEL, OSSE and EGRET data yielded a hard proton spectrum with a power law index around 2 using the 2.2 to 4.44 MeV fluence ratio. This hard of a spectrum would indicate the presence of a high-energy component above eight MeV and emission due to spallation products. However, none of the three instruments observed such a component. We discuss the standard techniques used in solar flare spectral deconvolution and introduce a new technique we use with the COMPTEL observations.

This work presented the explanation that the proton spectrum is soft during this interval of the 11 June 1991 solar flare based on this new analysis of the COMPTEL observations. This means that the region of 2.223/4--7 MeV fluence space is largely unexplored for soft proton spectra. The use of this ratio must be reexamined for proton spectra with indices greater than 5 or 6. We then applied a model we developed for the transport of neutrons created from a soft proton spectrum to determine the photospheric 3He abundance during this flare. We calculated a 3He/H ratio of 8.7e-05 with a 1 a range of 1.96e-04 to 1.75e-05 for this flare using this new model. This is larger than all previous values reported.

In addition, we presented an additional flare observation from COMPTEL. In response to a BACODINE cosmic gamma-ray burst alert, COMPTEL on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTEL's sensitivity, clearly shows a nuclear line excess above the continuum. Using new spectroscopy techniques we were able to resolve individual lines. This allowed us to make a basic comparison of this event with the GRL (gamma ray line) flare distribution from SMM and also compare this flare with a well-observed large GRL flare seen by OSSE. We showed this flare is normal, i.e., it is a natural extension of the SMM distribution of flares. The analysis of this flare means there is no evidence for a lower flare size for proton acceleration. Protons even in small flares contain a large part of the accelerated particle energy.

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