McDonald, Lee; (1999) The energetics of solar flares and bright points. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis examines the supply and transfer of energy in solar flares and bright points in the solar atmosphere. The thesis starts with a description of the solar atmosphere and the theoretical physics that describe the solar magnetic fields and plasmas. The physics of solar flares and X-ray bright points are then discussed, highlighting difficulties in our understanding of these events. The atomic physics that underlies the solar ultra-violet and soft X-ray emission is also introduced. The satellites and instruments that provided data for the thesis are introduced, with a section on each major instrument (Yohkoh SXT, HXT, BCS, SOHO CDS, EIT, MDI, TRACE and the NSO Spectromagnetograph) describing their capabilities and limitations. Common analysis techniques for some of the instruments are also described. Most solar flare observations show that an increase in the hard X-ray flux during the impulsive phase is usually accompanied by an increase in the flare's soft X-ray emission. The thesis considers a sample of flares that do not exhibit this property, and instead have large hard X-ray flux and small GOES class. It is found that the amount of chromospheric evaporation (and soft X-ray emission) a flare exhibits depends on where in the solar atmosphere the nonthermal electrons deposit their energy, which strongly depends on the spectral index of the energy of the nonthermal electron beam population. Sources of energy other than nonthermal electrons are considered, and the speed of the evaporating plasma is predicted. X-ray bright points (XBPs) are believed to be powered by magnetic reconnection in the corona, and therefore direct links between coronal soft X-ray and photospheric magnetic parameters are expected. The luminosity and temperature of a sample of XBPs are found to be directly proportional to the magnetic flux contained in the photospheric fragments, consistent with the magnetic reconnection model. Other soft X-ray parameters are found to be independent of the magnetic fields in the fragments. Many examples of magnetic dipoles existing in the photosphere without any corresponding XBP emission are observed. In most of these cases the separation of the magnetic fragments is larger than the interaction distance predicted by Longcope (1998), and the fragments should be unconnected. The morphologies of a sample of bright points observed by TRACE were also studied. These indicated that energy is released into the bright points on the magnetic field lines nearest the reconnection sites. These do not necessarily reflect the shapes of the simple loops joining the pairs of magnetic fragments. An examination of the energetics of a single XBP using SOHO and Yohkoh is also described. The XBP is observed to be connected to a nearby active region by a magnetic flux tube. The observations indicate that energy is flowing along this loop, providing a substantial amount of power to the XBP. The most likely mechanism for energy transfer across this loop is thought to be either Alfven or fast-mode magnetoacoustic waves.

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