Magnetic Fields in Coronal Mass Ejection Source Regions II
Presiding: M K Georgoulis, Applied Physics Laboratory, Johns Hopkins University; Y Liu, Stanford University
SH53B-01 INVITED 13:30h
Evolution of Magnetic Fields Associated with Flares and CMEs
This talk reviews recent progress on the study of relationship between evolution of solar magnetic fields and eruptive events such as flares and Coronal Mass Ejections. There are two different time scales of field evolution: (1) longer term in the scale of days, and (2) rapid changes in the time scale of minutes to one hour. The longer term evolution such as magnetic flux emergence and shear motions are well known to play important roles in the energy buildup and triggering of flares. Recent BBSO and MDI magnetograph observations demonstrate more and more evidences of rapid changes of photospheric magnetic fields associated with the core regions of flares and CMEs. We concentrate on the two aspects of such rapid changes: (1) We observed rapid increases of transverse field strength and magnetic shear in flaring neutral line associated with flare. (2) We found that in more than 40 events, part of the penumbral segments in the outer Δ spot structure decay rapidly after major flares; meanwhile, the neighboring umbral cores and/or inner penumbral regions become darker. Above changes typically take place in about 10 minutes to one hour and are permanent. In this review, I will also compare several well established flare/CME models that can explain different aspects of our observational results.
http://bbso.njit.edu
SH53B-02 INVITED 13:45h
Chromospheric Vector Magnetic Field Measurements of Active Regions
Active regions, as the locations of complex and rapidly evolving magnetic fields, are also the source of many coronal mass ejections. The chromospheric magnetic field, being force-free, should provide direct insights into both the energy storage for solar energetic events, and possibly their trigger mechanism as well. In this talk I will review recent chromospheric vector field observations from the U. Hawai`i/Mees Solar Observatory Imaging Vector Magnetograph, obtained in the Na-I line at 589.6nm. The focus will be on the differences between the photosphere and chromosphere, and chromospheric field measurements relevant to CME production such as free energy storage and evolution prior to an energetic event.
SH53B-03 14:00h
The Source of Magnetic Shear in CME Source Regions
We present a two and three-dimensional numerical magnetohydrodynamic simulations of magnetic flux emergence in three geometries including horizontal layers, arcades and flux ropes. In all cases, the magnetic structures are initially embedded in gravitationally stratified plasma in non-force-free states in which plasma pressure confines the magnetic fields. As the magnetic fields buoyantly rise, they greatly expand in the reduced pressure of the upper atmosphere. In all cases, we find that the legs of ascending bipole loop structures move in opposite horizontal directions drawing the magnetic field parallel to the neutral line. The shearing motions naturally occur as the magnetic field expands in the stratified atmosphere which produces a gradient in the axial field. This gradient results in a horizontal Lorentz force that drives the legs of the bipole loop in opposite directions. The shearing motions transport axial flux and energy from the submerged portion of the field to the expanding portion, which may cause it to violently erupt. This shearing process is very robust and explains the highly sheared state of the magnetic field associated with prominences, flares and coronal mass ejections.
SH53B-04 14:15h
Spatio-temporal dynamics of magnetic fields in the photosphere and flaring productivity of active regions
The role of spatio-temporal dynamics of the photospheric magnetic flux tubes in processes of energy built-up and energy release seems to be a subject of much controversy. To provide an insight into the problem, we propose to study the turbulence state, the multifractality and the fragmentation process in the photospheric magnetic structures and examine their relationship with the flare activity of different active regions (ARs). By calculating magnetic power spectra and structure functions of the line-of-sight magnetograms, as well as distribution functions of the the magnetic flux in magnetic elements, we found that parameters of these functions correlate with the flare productivity of an AR. Namely, flare-productive ARs display non-stationary non-homogeneous turbulent regime, high degree of multifractality and very intensive fragmentation of magnetic elements. On the contrary, for flare-quiet ARs we found a Kolmogorov-type homogeneous stationary turbulent regime, nearly monofractal structure of the field and low fragmentation rate. This study demonstrates that structural and dynamical characteristics of the magnetic field as measured in the photosphere are relevant to the intensity of non-stationary processes in the entire magnetic configuration.
http://www.bbso.njit.edu/~avi/
SH53B-05 14:30h
Distinguishing Between Eruptive and Quiescent Solar Active Regions
We present a method to fully evaluate the energy-helicity formula in solar active regions by using only photospheric vector magnetograms of these active regions. At the moment, the method relies on the linear force-free approximation and provides the total magnetic energy, the magnetic energy of the vacuum (potential) magnetic field, and the non-potential (free) magnetic energy relating to the total magnetic helicity in an active region. The formulation of the technique allows an upgrade to a nonlinear force-free evaluation of the energy-helicity formula, which will be a more realistic approach especially when chromospheric vector magnetograms of solar active regions become available. Even with the linear force-free approximation, however, we find that the magnitudes of the total helicity, as well as the ratios of the free magnetic energy to the total magnetic energy are distinctly higher for eruptive active regions as compared to quiescent active regions. Eruptive active regions produce flares and might trigger CMEs, so the method presents a viable way to discriminate between these two types of active regions even in case a single vector magnetogram of these active regions is available.
SH53B-06 14:45h
Magnetic Flux Cancellation and Coronal Magnetic Energy
I investigate the processes at work in the cancellation of normal magnetic flux in solar magnetograms, and study the relationships between cancellation and the budget of free energy in the coronal magnetic field that can power solar flares and CMEs. I begin by defining cancellation, and, focusing on reconnective cancellation, present the relevant equations describing the field evolution during cancellation. I then analyze these equations, to show that: 1) cancellation tends to reduce the magnetic energy of the hypothetical "open" field state; 2) in a sheared arcade field, steady cancellation adds free energy to the actual coronal field; and 3) horizontal magnetic flux can diffuse at a rate much lower than does the cancelling normal flux, even in the presence of constant resistivity. Finally, I briefly discuss the implications of boundary conditions in numerical simulations of flux cancellation.