MAGNETIC FIELD CONFIGURATION OF ACTIVE REGION NOAA 6555 AT THE TIME OF A LONG-DURATIONFLARE ON 23 MARCH 1991 An Exception to Standard Flare Reconnection Model DEBI PRASAD CHOUDHARY and G. ALLEN GARY ES82, Space Science Laboratory, George C. Marshall Space Flight Center/NASA, Huntsville, AL 35812, U.S.A. (Received 16 October 1998; accepted 17 May 1999) Abstract. The high-resolution Hα images observed during the decay phase of a long-duration flare on 23 March 1991 are used to study the three-dimensional magnetic field configuration of the active region NOAA 6555. Whereas all the large flares in NOAA 6555 occurred at the location of high magnetic shear and flux emergence, this long-duration flare was observed in the region of low mag- netic shear at the photosphere. The Hα loops activity started soon after the maximum phase of the flare. There were a few long loop at the initial phase of the activity. Some of these were sheared in the chromosphere at an angle of about 45 ◦ to the east-west axis. Gradually, an increasing number of shorter loops, oriented along the east-west axis, started appearing. The chromospheric Dopplergrams show blue shifts at the end points of the loops. By using different magnetic field models, we have extrapolated the photospheric magnetograms to chromospheric heights. The magnetic field lines computed by using the potential field model correspond to most of the observed Hα loops. The height of the Hα loops were derived by comparing them with the computed field lines. From the temporal evolution of the Hα loop activity, we derive the negative rate of appearance of Hα features as a function of height. It is found that the field lines oriented along one of the neutral lines were sheared and low lying. The higher field lines were mostly potential. The paper also outlines a possible scenario for describing the post-flare stage of the observed long-duration flare. 1. Introduction The reconnection of magnetic fields in the corona is the basic ingredient of the solar flare mechanism (Bruzek, 1964). At the present time, it is possible to infer various complex characteristics of the three dimensional structure of the active region magnetic field by using numerical models (Antiochos, Dahlburg, and Klimchuk, 1994). This gives an opportunity to examine several aspects of the reconnection flare models by using the chromospheric loops, photospheric vector magnetograms and numerical models. According to the basic reconnection flare model (RFM), hot loops with temper- atures of ∼ 3 × 10 7 K form soon after the flare onset. These loops are essentially magnetic flux tubes filled with hot plasma. The regions joined by these loops are brightened due to heating by thermal conduction, shocks, and/or non-thermal parti- cles generated in the reconnection process. When the plasma cools to ∼ 10 4 K, the Solar Physics 188: 345–364, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.