Force-Freeness of Solar Magnetic Fields in the Photosphere
Yong-Jae Moon Submitted: 2002-04-05 17:47
It is widely believed that solar magnetic fields are force-free
in the solar corona, but not in the solar photosphere at all.
In order to examine the force-freeness of active region magnetic fields
at the photospheric level, we have calculated the integrated magnetic forces
for 12 vector magnetograms of three
flare-productive active regions. The magnetic field vectors are
simultaneous Stokes profiles of the Fe I doublet 6301.5 and 6302.5
by the Haleakala Stokes Polarimeter of Mees Solar Observatory,
with a non-linear least square method adopted for field calibration.
The resulting vertical Lorentz force normalized to the total magnetic pressure force
|F_z/F_p| ranges from 0.06 to 0.36 with a median value of
0.13, which is smaller than the values (sim 0.4)
obtained by Metcalf et al.
who applied a weak field derivative method to the Stokes profiles of Na I 5896.
Our results indicate that the photospheric
magnetic fields are not so far from force-free as conventionally regarded.
As a good example of a linear force-free field, AR 5747 is
examined. By applying three different methods (a most probable value method,
a least square fitting method, and comparison with linear force-free
solutions), we have derived
relatively consistent linear force-free coefficients for AR 5747.
It is found that the scaled downward Lorentz force
(|F_z/F_p|) in the solar photosphere
decreases with increasing α .
Our results also show that the
force-freeness of photospheric
magnetic fields depends not only on the character of the active region,
but also on
its evolutionary status.
Authors: Y.-J. Moon, G. S. Choe, H. S. Yun, Y. D. Park, D. L. Mickey
Publication Status: ApJ, 2002, 568, 422-431
Last Modified: 2002-04-05 17:55