Rapid Changes in the Longitudinal Magnetic Field Associated with the July gamma -ray Flare Vasyl Yurchyshyn, Haimin Wang, Valentyna Abramenko, Thomas J. Spirock and Säm Krucker Big Bear Solar Observatory, Big Bear City, CA Space Sciences Lab, University of California, Berkeley, CA 94720

Introduction Earlier studies report on flare related variations in the magnetic field: –local changes associated with the major polarity inversion line ( Severny 1964; Zvereva&Severny 1970; Moore et al. 1984; Wang&Tang 1993; Wang et al. 1994; Kosovichev&Zharkova 1999, 2001; Spirock et al. 2002; Wang et al ) –changes of magnetic shear ( Chen et al. 1994, Wang et al. 1994, Hagyard et al ) –global changes, when both the photospheric and the coronal field of an active region are involved in a flare ( van Driel-Gesztelyi et al. 1997, Aschwanden et al. 1999; Abramenko et al ). 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.

Goal of the Study 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 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.

The 2002 July 23 Gamma -ray Flare BBSO H  at 00:27 UT Contours are RHESSI keV (red) and keV (blue) BBSO H  at 00:27 UT BBSO DMG at 00:37 UT 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 ).

Magnetic Flux From SOHO/MDI The flux time profiles are plotted for the following (positive) and the leading (negative) polarity by the thin solid lines. 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

Magnetograms From BBSO/DMG 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?

Magnetic Flux Changes From Simulated Data The simulated magnetogram contains an “S”- shaped NL and, projected on the east limb, it resembles the observed magnetogram. A pre- (a= arcsec -1 ) and a post-flare (a= 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.

i) the magnetic flux and ii) the inclination of the magnetic field change rapidly during a major flare? What Can We Learn What Can We Learn From the Fact That

Magnetic Flux Changes and Flare Models reconnection models – rapid formation of an unstable flux rope by reconnection between sheared arcades flux rope models – eruption of a pre-existing (in equilibrium) flux rope and a gradual (~ 1-2 hours) reconnection process Figures courtesy of Amari et al. 2003, ApJ, 585, 1073 erupted flux rope reconnection reconnection sheared arcades

Summary 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 2x10 20 Mx (6%), while the following polarity decreased by 1x10 20 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. 1.a location, which was most probably associated with new flux emergence and it showed an increase in the magnetic field and a new penumbral area 2.a location, which coincided with foot points of a growing post-flare loop system and it showed a shift of the neutral line in longitudinal magnetograms. 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.