Physics
Scientific paper
Mar 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011phdt........11r&link_type=abstract
PhD Thesis. Università degli Studi di Catania
Physics
Solar Physics, Chromosphere, Flares, Simulations, Ly-Alpha, H-Alpha, Observations, Radiative Transfer
Scientific paper
This thesis is divided into two main parts: a multiwavelength observational study of solar flares, focusing mainly in the chromosphere in Ly-α and Hα, and an application of a radiative transfer code and a radiative hydrodynamic code, to compare the results obtained by observations with the simulated ones. The Ly-α emission is a very interesting line because it is a natural tracer of the solar activity in the chromosphere. The Transition Region And Coronal Explorer satellite observed a small number of flares in the Ly-α passband, but apart from this, these events have not often been observed in this strong chromospheric line. Because TRACE has a broad Ly-α channel, in order to estimate the "pure" Lyα emission, we had to apply an empirical correction. We found that there is a reasonable coverage in TRACE 1216 A and the TRACE 1600 A for two different flares: on 8 September 1999 and on 28 February 1999. Studying them we estimated, for the first time, the pure Ly-α flare signature, being on the order of 10^25 erg/s at the flare peak. The study of the first flare gave us the possibility to calculate the electron energy budget using the X-ray data from Yohkoh/HXT in the context of the collisional thick target model, finding that the Ly-α power is less than 10% of the power inferred by the electrons. The morphology and evolution of the second flare were described in different wavelengths by using imaging data acquired by TRACE and by BBSO in white light and in Hα. We studied the magnetic topology using the magnetic field provided by SOHO/MDI, extrapolating the photospheric magnetic field lines, assuming a potential field. We found different morphologies in the magnetic configuration before and after the flare, confirming the occurrence of a reconnection process. The Hα line is the most important line in the chromosphere. We studied the Hα emission of a flare which occurred on 3 July 2002 using some spectroscopical observations from the Ondrejov Observatory. Analyzing the available data in other wavelengths, we made a morphological study of the active region from three hours before the flare to seven hours after it. The results obtained by observations, both in the form of integrated intensity as a function of time, and detailed line profiles, motivated the second part of the thesis. In this, we used a radiative transfer code (Gouttebroze et al. 1978) applying different atmospheric models as input parameters in order to compute the hydrogen spectral lines and study how they are affected by the temperature and microturbulent stratification. In particular, the intensity of the Ly-α and Hα lines is related to the temperature stratification of the atmospheric model, the position of the transition region being a key factor. The variation of the microturbulent velocity does not significantly affect the resulting intensities, but we observed that an increase of the microturbulent velocity broadens the line profiles. The RADYN Radiative HydroDynamic code (Allred et al. 2005) was applied to solar flares, modelling a flare loop from its footpoints in the photosphere to its apex in the corona by adding non-thermal heating at the lower atmosphere and soft X-ray irradiation. The majority of this work was to deal with investigating the dynamical response of the solar chromosphere to energy injected in the form of non-thermal electrons during solar flares. We studied the flare energy transport and radiation production in the chromosphere as well as the Hα and Ly-α emission. The Ly-α intensity is affected by the flux of the initial beam of electrons injected at the top of the loop, while the Hα intensity appears to be less affected by the flare model. Comparing the observational results in Lyα and Hα with the computed ones from the radiative code and the RADYN code, we found that the RADYN code fits better the Hα intensities to the observations than the Lyα intensities, concluding that the code gives a better description of processes in the lower chromosphere than those in the upper layers.
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