Observations and Models of the Dynamical Evolution of Solar Flares

Details Observations and Models of the Dynamical Evolution of Solar Flares Observations and Models of the Dynamical Evolution of Solar Flares

Nov 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phdt.........6g&link_type=abstract

Ph.D dissertation, 2006 ETH Zurich (Switzerland)

Statistics

Computation

Sun: Flares, Sun: X-Rays, Acceleration Of Particles

Scientific paper

Solar flares and associated Coronal Mass Ejections (CMEs) are the biggest explosions in the solar system, converting huge amounts of magnetic energy into kinetic energy of accelerated particles and heat. The key questions at the core of flare physics research are: how is the energy stored in the solar corona before the flare? What triggers the sudden release of that energy? How are the particles accelerated and heated during the flare? Notwithstanding the strong theoretical and observational progress of the last few decades, this questions still remain open.
Hard X-ray observations of the Sun, such as provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), are the best tools to probe the population of flare-accelerated particles, because X-rays are the direct signature of energetic electrons. In this thesis, novel RHESSI hard X-ray observations of solar flares are compared with quantitative predictions from modern theoretical models of stochastic acceleration of electrons. The focus lies on the spectral evolution, which has been discovered in the early days of hard X-ray observations, but, with a few exceptions, neglected by theorists.
The work presented here starts with RHESSI observations of the spectral evolution of the non-thermal component in the hard X-ray spectrum of solar flares. A representative sample of 24 M class impulsive flares is analyzed. They show rapid changes in the spectral hardness during distinct emission spikes. The maximum hardness is reached at peak time, thus the spectral behavior can be classified as soft-hard-soft. A quantitative relation between the normalization of the power-law component and its spectral index is found, holding for single emission spikes, as well as for the whole dataset comprising all events.
The analysis is then expanded, transforming the data from photon space to electron space and comparing the results with predictions from simple available electron acceleration models featuring soft-hard-soft behavior. This simple approach yields plausible best-fit model parameters for about 77% of the 141 events consisting of rise and decay phases of individual hard X-ray spikes. This success suggests that stochastic acceleration is a viable mechanism to explain the observed spectral evolution.
Therefore, a recent stochastic acceleration model, the transit-time damping acceleration scenario, was chosen for further investigation. A mechanism that accounts for particle trapping in the accelerator was added in order to account for changes in the spectral hardness. The model predictions for the spectral evolution were compared with spectral observations of emph{looptop} hard X-ray sources, delivering a snapshot of the particles still residing in the accelerator. A novel parameter was used for the comparison, the emph{pivot
point} (that is, a common crossing point of the accelerated particle spectra at different times). The model computations show the presence of a pivot point at an energy of 10 keV. This value can be brought in agreement with the observed value of 20 keV by enhanced trapping through an electric potential.
Lastly, some puzzling observations of the motion of hard X-ray sources during an impulsive M class flare are reported. The double sources, interpreted as footpoints of magnetic loops, show continuous motion along an arcade of magnetic loops, contradicting the predictions of the translation invariant 2.5D reconnection models, where motion perpendicular to the arcade is expected. Therefore, the development of more realistic 3D models is needed to account for such behavior.

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