Our institute is pleased to announce the public defence of L. Dumitru's thesis.

Study of the Helicity in Solar Active Regions

on

October 15, 2020 at 11:00

The Romanian announcement (pdf version) is available here
The Romanian announcement (text version) is available here

The PhD Thesis may be consulted at AIRA's Library
The abstract of this thesis is available in Romanian at this link

L. Dumitru's CV

Committee members CVs:
Prof. Habil. Dr. Gelu Pașa CV (Publications)
Dr. Dan Alin Nedelcu
Dr. Cristiana Dumitrache
Prof. Dr. Alexandru Marcu
Conf. Dr. Cristiana Blaga

Here is a short summary:
Studying the Sun and its activity represents the main objective of the heliophysics research. The interest in these studies is connected with the consequences that the solar activity may have on the Earth. This study’s main objective is the description of the magnetic field lines in solar active regions that triggered major flares and the study of the helicity evolution, and the field line torsion, respectively, during these phenomena.
The first chapter is an introductory one where general data about the Sun is described, along with its structure and cyclic activity. Babcock’s theory is presented, as well as the role that the magnetic field plays in the active region formation.
Chapter 2 presents some mathematical aspects of the helicity concept that is common in fluid dynamics, tied to Herman von Helmholtz and William Thomson (lord Kelvin) researches. The chapter ends with a display of the connection between the helicity and the number of twists that two adjacent curves have (curves that are situated at a small distance).
Chapter 3 shows the introductory notions related to magnetic helicity, as well as the computation method. Results from other researchers are presented in order to justify the hemispherical rule of the helicity signs, as well as the helicity characteristics. The Romanian mathematician Gheorghe Călugăreanu’s contribution to the helicity study is also presented. Computation methods for the free force magnetic field parameter, known as the torsion parameter, are also presented, as well as the correlation with this parameter. A description of the "Local Correlation Tracking" (LCT) method and the role that velocities play in the computation of helicity for active regions. The last section of this chapter presents the correlation of the helicity sign with the torsion sign.
Chapter 4 contains a short review of the literature results. Given that the solar magnetic field can be measured only in the solar photosphere and chromosphere, obtaining values for the solar coronal magnetic field can only be done by MHD modelling on the photospheric observational data, using 3D extrapolation algorithms. Thus, coronal magnetic field "Nonlinear force-free field" (NLFFF) extrapolation methods are presented. Then, the two methods that are used in this study. For analysing the data Lee’s code (2002) and Valori’s code (2016) are implemented in IDL. For each one of them, there are results presented. Lee’s method was applied for active regions NOAA11035, NOAA10930, as well as for two active regions situated approximately symmetrical from the solar equator (NOAA11073 and NOAA11076). Valori’s code was used to analyse the active region NOAA12673 and the obtained results are presented. The results obtained following the application of the two extrapolation methods NLFFF of the coronal magnetic field, for the active region NOAA12673 were compared. At the end of the chapter conclusions obtained following the two methods of extrapolation are presented.
Chapter 5 presents an extensive analysis of the torsion parameter, as well as the magnetic flux, obtained using Valori’s code, for a number of 62 solar active regions that triggered major solar flares. The respective regions have been selected such as they were not close to the solar disk in the moment of the analysed solar flare. There were numerical simulations run for each active region, on a two hour interval encompassing the flares, and based on processing 620 magnetograms graphics for the evolution of the torsion parameter and the magnetic flux were done, for each active region. By analysing these graphs five cases of evolution were determined for the torsion parameter, as well as for the magnetic flux.
Chapter 6 summarises all the general conclusion of these studies, as well as an insight into possible future research.