Observational Studies of the Magnetic Field
in Solar Active Regions
by Wang Tongjiang
Supervisor Professor Ai Guoxiang
Beijing Astronomical Observatory,Chinese Academy of Sciences
Viva Date: March, 1998
- Abstract [ps-file]
The author focuses his much interest on the photospheric magnetic
field in active regions, because only in these regions observations can
provide both the longitudinal and transverse components of the magnetic field,
which allow us to approach the problems how free magnetic energy is built up
and where energy is released in configurations, through analyses of the
features such as magnetic non-potentiality, current helicity and magnetic
topology. In this thesis, the author at first introduces the observational
calibration for the Multi-Channel Solar Telescope (MCST) of Huairou Solar
Observing Station (HSOS) and his software used for HSOS data reduction,
then presents his studies during 1994-1997. The obtained results mainly
include as follows:
1. The author tested two observational methods in calibrating the longitudinal
field measurements. One is the line profile method, and the other is the
Doppler velocity method. The both methods showed that the calibrated results
working at FeI5324.19A and H_beta were in good agreement with those
in previous theoretical calibration.
2. The author designed a software called DCP for reductions of the HSOS vector
magnetograms. DCP includes the functions such as calibration, removal of
180-ambiguity of the transverse field measurement, calculation of the vertical
electric current and elimination of projection effect, etc.
3. The author calculated the imbalance of current helicity over the whole AR's
area for 40 active regions and obtained: (i) In 90% of cases there existed
significant excess of current helicity of some sign. So one can suggest that
the build up of large-scale currents in an active region due to small-scale
fluctuations is rather typical phenomenon. (ii) In 82.5% of cases the excess of
current helicity in the northern (southern) hemisphere was negative (positive).
4. The author applied the Poincare index method to diagnose the 3-D singular
structures in an observed transverse field. He found that in some cases the
deduced 2-D saddle points and magnetic lanes correspond to the separatrices
on the photosphere. When comparing with the bright kernels in H_beta and
the emitting loops in SXR in some flaring events, it showed that the energy
release contributing to the coronal heating appeared to be closely associated
with these structures.
5. The author analysed the evolution of structures of magnetic fields and
electric currents. Combining the emissions in H_beta and SXR, he deduced
the spatial magnetic configuration of a 1N/M1.1 flare and its relationship
with the photospheric current distribution, which suggested that the flare
may be due to the interaction of two emerging current loops.
- Content
Chapter 1. Extensive Introduction
1.1. Solar Activity and Magnetic Fields
1.2. Measurement of Solar Magnetic Fields
1.3. Subjects of Vector Magnetic Measurements
Chapter 2. Observational Calibration
2.1. Introduction
2.1.1. Solar Multi-Channel Telescope
2.1.2. Theoretical Interpretation of Polarization
2.1.3. Weak Field Approximation
2.2. Calibration Methods
2.2.1. Calibration Principle
2.2.2. Line Profile Slope Method
2.2.3. Doppler Method by Solar Rotation
2.3. Measurements and Results
2.3.1. Tests with Solar Magnetic Field Telescope
2.3.2. Estimates from the Line Profile
2.3.3. Some Results on Nine-Channel Solar Telescope
2.4. Discussions and Conclusions
Chapter 3. Software DCP for HSOS Data Reduction
3.1. Introduction
3.2. Removal of the 180$^{o}$ Ambiguity
3.3. Electric Current Calculation
3.4. Elimination of Projection Effect
3.5. Summary
Chapter 4. Electric Current Helicity in Active Regions
4.1. Introduction
4.2. Observational Data
4.3. Method
4.4. Results
4.5. Conclusions
Chapter 5. Magnetic Topology of Active Regions
5.1. Introduction
5.1.1. Magnetic Reconnection in Solar Flares
5.1.2. Determination of the Topology of Magnetic Configurations
5.2. Saddle Point and Separator
5.3. Observations
5.3.1. Active Region NOAA 7321
5.3.2. Fourier Filtering Method
5.4. Singularity in the Observed Vector Field
5.4.1. 2D Topology of the Observed Transvers
5.4.2. Topological Comparison with Linear Force-free Fields
5.4.3. 3D Field Line Patterns of the Modelli ng Field
5.4.4. Flare and High Energy Emissions
5.5. Summary
Chapter 6. Non-potential Features of the Magnetic Field
6.1. Introduction
6.1.1. Magnetic Shear
6.1.2. Electric Current System
6.1.3. Spatial Field Configuration
6.2. Observations
6.2.1. Active Region NOAA 7321
6.2.2. Evolution of Magnetic Configuration
6.2.3. Evolution of the Vertical Current System
6.3. Field Configuration of a 1N/M1.1 Flare
6.3.1. Spatial Correlation between the Flare and Magnetic Loops
6.3.2. Vertical Current System and Flares
6.3.3. Scenario of the 1N/M1.1 Flare
6.3.4. Energy Budget
6.4 Conclusions
References