Rapid Changes in the Longitudinal Magnetic Field Associated with the July 23 2002 gamma -ray Flare. Vasyl Yurchyshyn, Haimin Wang, Valentyna Abramenko, Thomas J. Spirock and S ä m Krucker Big Bear Solar Observatory, Big Bear City, CA 92314
Vasyl Yurchyshyn, Haimin Wang, Valentyna Abramenko,
Thomas J. Spirock and Säm Krucker
Big Bear Solar Observatory, Big Bear City, CA 92314
Space Sciences Lab, University of California, Berkeley, CA 94720
Earlier studies report on flare related variations in the magnetic field:
Wang et al. (2002) studied six X-class flares and found impulsive and permanent changes in the magnetic flux during the flares, which were not balanced:
the leading flux always increased while the following tended to decrease.
Spirock, Yurchyshyn and Wang (2002) and Wang et al. (2002) explained the unbalanced flux increase by several possible mechanisms:
i) emergence of a very inclined flux tube,
ii) change of the orientation of the magnetic field and
iii) expansion of the preceding sunspot as a result of the relaxation of the magnetic field after a flare.
In the present paper we will show that the first two mechanisms (new flux
emergence and the orientation and/or inclination) may indeed be
responsible for the observed changes.
We will analyze Halpha, magnetograph and X-ray data for the 2002 July 23 gamma-ray flare, which occurred in Active Region NOAA 0039. It was a long duration event that peaked around 0028UT. We will focus here on the evolution of the magnetic field associated with the flare.
To understand the changes that the magnetic field underwent during the early phase of the flare, we reconstruct basic signatures of a 3D coronal magnetic field by using a linear force-free field extrapolation model.
BBSO H at 00:27 UT
BBSO DMG at 00:37 UT
Contours are RHESSI 12-20 keV (red) and 100-150 keV (blue)
HXR emission, which dominated above 30 keV, was related to the photospheric foot points, while at lower energy range a gradual coronal HXR source had been found (Krucker, Hurford and Lin 2003).
BBSO H at 00:27 UT
The flux time profiles are plotted for
the following (positive) and the leading
(negative) polarity by the thin solid
Immediately after the flare (at 0047UT,
the right vertical dashed line) the total
following (positive) flux decreased by
about 14%, while the total leading flux
increased by approximately 6%.
These changes were permanent and
the flux did not return back to the pre-
flare level after the flare ended.
Bold lines are RHESSI flux in the 100 to 150 keV energy range
BBSO: the leading (S) flux up by about 5%, while the following (N) flux down by about 13% (The corresponding MDI flux changes are 6% and 14%).
Circled area: the northern HXR source at the flare onset; the peak intensity increased from 800Gs to -1100Gs; S flux up by 30%; the penumbral bridge became wider (compared the red contours); the transverse field has changed orientation. New flux emergence?
Boxed area: the northern HXR source and the foot points of the PFL system in the late phase of the flare; S flux up threefold; the transverse field has changed orientation . Change in the inclination?
The simulated magnetogram contains an “S”-shaped NL and, projected on the east limb, it resembles the observed magnetogram.
A pre- (a=-0.025 arcsec-1) and a post-flare (a=-0.001 arcsec-1) magnetic field was simulated by matching general curvature of the calculated field lines to the observed Ha fibrils and post-flare loops.
The magnetograms were projected at the east solar limb (l=-65°) and the line-of-sight components were determined.
The leading (S) flux increased by 24%, while the following (N) flux decreased by about 59%. Note, that the observed data gives 6% and 14%, respectively.
The flux variations in the simulated line-of-sight magnetograms are due to changes in the inclination and/or orientation of the magnetic field.
What Can We Learn with From the Fact That
i) the magnetic flux
ii) the inclination of the magnetic field
change rapidly during a major flare?
flux rope models – eruption of a pre-existing (in equilibrium) flux rope and a gradual (~ 1-2 hours) reconnection process
erupted flux rope
reconnection models – rapid formation of an unstable flux rope by reconnection between sheared arcades
erupted flux rope
Figures courtesy of Amari et al. 2003, ApJ, 585, 1073
We presented study on rapid changes in the magnetic field associated with the July 23, 2002 gamma-ray flare.
MDI and BBSO data showed that immediately after the flare the leading polarity of the magnetic field had increased by 2x1020 Mx (6%), while the following polarity decreased by 1x1020 Mx (20%). The observed changes were permanent and seem not to be caused by variations in the profile of the spectral lines that were used to measure magnetic fields.
We distinguish two separate locations, which show the most dramatic changes in the magnetic field.
Linear force-free field simulations showed that the re-orientation of the magnetic field was capable of producing the observed changes in the total magnetic flux.