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Brian Welsch   Submitted: 2007-09-27 13:34

Several studies correlated observations of impulsive solar activity -- flares and coronal mass ejections (CMEs) -- with the amount of magnetic flux near strong-field polarity inversion lines (PILs) in active regions' photospheric magnetic fields, as measured in line-of-sight (LOS) magnetograms. Practically, this empirical correlation holds promise as a space weather forecasting tool. Scientifically, however, the mechanisms that generate strong gradients in photospheric magnetic fields remain unknown. Hypotheses include: the (1) emergence of highly twisted or kinked flux ropes, which possess strong, opposite-polarity fields in close proximity; (2) emergence of new flux in close proximity to old flux; and (3) flux cancellation driven by photospheric flows acting fields that have already emerged. If such concentrations of flux near strong gradients are formed by emergence, then increases in unsigned flux near strong gradients should be correlated with increases in total unsigned magnetic flux -- a signature of emergence. Here, we analyze time series of MDI line-of-sight (LOS) magnetograms from several dozen active regions, and conclude that increases in unsigned flux near strong gradients tend to occur during emergence, though strong gradients can arise without flux emergence. We acknowledge support from NSF-ATM 04-51438.

Authors: B. T. Welsch and Y. Li
Projects: SoHO-MDI

Publication Status: in press (Proc. of 2007 Sac Pk Workshop)
Last Modified: 2007-09-28 05:01
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Subject will be restored when possible  

Brian Welsch   Submitted: 2007-09-27 13:30

Estimates of velocities from time series of photospheric and/or chromospheric vector magnetograms can be used to determine fluxes of magnetic energy (the Poynting flux) and helicity across the magnetogram layer, and to provide time-dependent boundary conditions for data-driven simulations of the solar atmosphere above this layer. Velocity components perpendicular to the magnetic field are necessary both to compute these transport rates and to derive model boundary conditions. Here, we discuss some possible approaches to estimating perpendicular flows from magnetograms. Since Doppler shifts contain contributions from flows parallel to the magnetic field, perpendicular velocities are not generally recoverable from Doppler shifts alone. The induction equation's vertical component relates evolution in Bz to the perpendicular flow field, but has a finite null space, meaning some ``null'' flows, e.g., motions along contours of normal field, do not affect Bz. Consequently, additional information is required to accurately specify the perpendicular flow field. Tracking methods, which analyze partial_t B_z in a neighborhood, have a long heritage, but other approaches have recently been developed. In a recent paper, several such techniques were tested using synthetic magnetograms from MHD simulations. Here, we use the same test data to characterize: 1) the ability of the induction equation's normal component, by itself, to estimate flows; and 2) a tracking method's ability to recover flow components that are perpendicular to mathbf{B} and parallel to contours of Bz. This work has been supported by NASA Heliophysics Theory grant NNG05G144G.

Authors: B. T. Welsch and G. H. Fisher
Projects: None

Publication Status: in press (Proc. of 2007 Sac Pk Workshop)
Last Modified: 2007-09-28 05:01
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Magnetic Flux Cancellation and Coronal Magnetic Energy  

Brian Welsch   Submitted: 2005-06-14 17:29

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 magnetic energy in the coronal magnetic field that can power solar flares and CMEs. After defining cancellation mathematically, I derive equations that quantify the evolution of free magnetic energy in response to arbitrary plasma flows on the boundary, including flows consistent with cancellation. While cancellation can reduce the magnetic energy in both the actual coronal field and the potential field matching the same normal field boundary condition, cancellation can, in the process, increase the difference between the two, i.e., cancellation can increase the free magnetic energy in the corona. By making simple assumptions based upon typical observed field configurations in filament channels, I show that cancellation tends to add free energy to these fields. Finally, I discuss the implications of this fact, as well as wider applications of the free energy flux formalism developed here. I also briefly address related issues, including the relationship of cancellation to Taylor's hypothesis.

Authors: B. T. Welsch
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2005-10-05 15:38
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ILCT: Recovering Photospheric Velocities from Magnetograms by Combining the Induction Equation with Local Correlation Tracking  

Brian Welsch   Submitted: 2004-02-03 12:54

We introduce a new method for deriving the velocity field in magnetized regions of the Sun's photosphere, with the goal of using time sequences of magnetograms to drive boundary conditions for MHD simulations of the solar corona. Our technique enforces consistency between the normal component of the induction equation and results obtained from local correlation tracking (LCT). When used with photospheric vector magnetograms, our method determines all three components of the photospheric velocity normal to the magnetic field. Our method explicitly employs the assumption advanced by Demoulin & Berger (2003), that results determined by LCT applied to magnetograms involve a combination of all components of the velocity and magnetic fields. We present results from our method applied to vector magnetograms of NOAA AR 8210 on 1 May 1998.

Authors: Welsch, B.T.; Fisher, G.H.; Abbett, W.P.; Regnier, S.
Projects:

Publication Status: ApJ (accepted)
Last Modified: 2007-06-24 17:25
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Magnetic Flux Cancellation and Coronal Magnetic Energy
ILCT: Recovering Photospheric Velocities from Magnetograms by Combining the Induction Equation with Local Correlation Tracking

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