Helicity of Active Regions in the Photosphere and Corona

ar080.canfield05
Posted:  18-Feb-95
Updated: 02-Aug-95, 7-May-96, 27-Jan-97
Events specified: N/A

R. Canfield, A. Pevtsov, T. Metcalf, L. Acton; other contributors are welcome

ar068.harvey_j01 Soft X-Ray Coronal Structures and Magnetic and Chromospheric Patterns qs068.sakurai06 Helicity of filaments and of overlying coronal arcades ar076.matsumoto02 Formation of a Kinky Alignment of Solar Active Regions

We propose to use Mees vector magnetograms and Yohkoh SXT images to study the relationship between the average magnetic helicity density of active regions in the photosphere and vortical structure of active regions in the corona.

Using photospheric vector magnetograms from Mees Solar Observatory, we have shown that active regions have an average helicity that depends on latitude in an interesting way, and that the sign of the helicity is opposite in opposite hemispheres (Pevtsov, Canfield, & Metcalf, ApJ Letters, in press), but opposite to what a magnetohydrodynamic dynamo in the convection zone would produce. At the AAS meeting in January 1995, Harvey reported on his study of active region structures, including the vortical structure of the corona, and noted a systematic twist of the coronal loops, and that the twist has a opposite sense in opposite hemispheres.

We plan to explore the role of reconnection in the corona by comparing
our photospheric results on the average magnetic helicity of over 150
active regions, observed by us at Mees Solar Observatory, to the twist
shown by the corona in SXT observations.  We will use the routine SXT

Update 27-Jan-97

I am pleased to tell you that this project can be closed. I expect that the ApJ will soon accept our manuscript entitled "On the Sub-Photospheric Origin of Coronal Electric Currents", by Alexei A. Pevtsov, Richard C. Canfield & Alexander N. McClymont.

The abstract is atached.

R. C. Canfield

Abstract

Using photospheric vector magnetograms from the Haleakala Stokes Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray Telescope, we infer values of the force-free field parameter alpha at both photospheric and coronal levels within 140 active regions. We determine the value of alpha for a linear force-free field that best fits each magnetogram in a least-squares sense. We average values from all available magnetograms to obtain a single mean photospheric alpha value for each active region. From the SXT images we estimate alpha in the corona by determining (pi/L) sin gamma for individual loops, where gamma is the observed shear angle of X-ray loops of length L. We then average these values to obtain a single coronal alpha value, , for each active region.

In active regions whose photospheric alpha map is predominantly of one sign, we find that the values of and are well correlated. Only for active regions in which both signs of alpha are well represented, for which our analysis method breaks down, are the values of and poorly correlated. The former correlation implies that coronal electric currents typically extend down to at least the photosphere. However, other studies imply subphotospheric origin of the currents, and even current systems, that are observed in the photosphere. We therefore conclude that the currents responsible for sinuous coronal structures are of subphotospheric origin.

Update 7-May-96

We have just submitted the following paper for publication in ApJ. The abstract follows:

Helicity of Solar Magnetic Fields in Photosphere and Corona

Alexei A. Pevtsov, Richard C. Canfield, Alexander N. McClymont Institute for Astronomy, University of Hawaii Loren W. Acton Dept. of Physics, Montana State University

Using photospheric vector magnetograms and coronal X-ray images of 140 active regions, we determine and compare values of magnetic helicity density in the photosphere and corona. In the photosphere we use the observed magnetic field vectors to compute maps of the local force-free field parameter \a, which, in some regions, varies significantly in both amplitude and sign, as well as the best single value of \aph, which characterizes the active region as a whole. In the corona we determine $\alpha_c = (\pi/L)~ sin~\gamma,$ where $\gamma$ is the observed shear angle of observed X-ray loops of length L. The amplitude and sign of \ac\ also vary significantly from loop to loop, in some regions. Because neither the photospheric vector magnetograms nor the X-ray images have enough spatial resolution, we cannot directly compare photospheric and coronal \a\ values for individual loops. Hence, we study active-region averages.

In active regions whose photospheric helicity is predominantly of one sign, we find that the average values of \aph\ and \ac\ are well corellated. They are not well corellated in active regions in which both signs of \a\ are well represented. Hence, we conclude that the shear of coronal loops reflects the presence of coronal electric currents that are of sub-photospheric origin.

Update 02-Aug-95

Canfield, Pevtsov, and Acton presented a paper at the Spring AGU meeting in Baltimore in May/June.

Using Haleakala Stokes Polarimeter vector magnetograms from Mees Solar Observatory and Soft X-ray Telescope (SXT) emages from Yohkoh, we compared the helicity density of photospheric magnetic fields and coronal loops of 96 solar active regions. To minimize projection artifacts, we studied regions that are close to the central meridian.

We measured the helicity density of photospheric magnetic fields using the force-free field parameter alpha. For each magnetogram of our dataset we computed sets of linear force-free fields of varying alpha values and establish the best value (for the region as a whole) by minimizing the difference between the computed and observed horizontal magnetic fields.

We estimated the helicity density of the coronal fields using the morphology of observed X-ray loops. Analysis of a model linear force-free dipole showed that lines of force form loops having evident twist. In projection onto a horizontal plane, the twisted loops look like hooked structures connecting the poles of the dipole. The curvature of these structures is determined by the sign and value of alpha. Using the model we established the relationship between the alpha parameter and the average tilt of the line of force to the axis of the bipole in projection upon the horizontal plane. We used this tilt as a parameter to characterize the helicity density of the observed coronal loops.

Using the Yohkoh movie, we have identified 50 hook-like coronal structures in connection with the 96 active regions in our vector magnetogram dataset; in the remaining 46 cases, no clear evidence of helicity is present. We approximated the form of each hook-like structure by a third-degree polynomial, and estimated the sign and value of alpha using the average tilt as a measure. We find that in 78% of these regions the sign of the helicity of the coronal loops is the same as that of the photospheric magnetic fields. In 22% it is opposite. Qualitative analysis of the Yohkoh SXT movie shows that typically the coronal loops maintain the same sign of helicity for at least several days.