34th Solar Physics Division Meeting, June 2003
Session 15 Flares and Microflares I
Oral, Wednesday, June 18, 2003, 1:30-3:30pm, Auditorium

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[15.07] Signature of Avalanche in Solar Flares as Measured by Photospheric Magnetic Fields

V. I. Abramenko, V. B. Yurchyshyn, H. Wang, T. J. Spirock, P. R. Goode (Big Bear Solar Obs.)

Turbulent/fractal parameters of the longitudinal magnetic field, Bz, for four powerful solar flares were analyzed utilizing the correlation length, l, of the magnetic energy dissipation field and the scaling exponent, b, which characterizes the measure of intermittency of the Bz structure. We select a set of four two-ribbon flares, which were followed by coronal mass ejections, for the study of magnetic structure. During the course of each flare, we found a peak in b which was followed by a peak in l in all of the cases studied in this paper. These two peaks were separated by the time interval tl during which a rapid growth of the soft X-ray and Halpha flux occurred. The peak in b was preceded by a time period tb during which b increased gradually. For all of the flares tb was longer than the time interval tl. The maximum of l occurred nearly simultaneously, within an accuracy of about 2-5 minutes, with the maximum of the hard X-ray emission. For the four flares considered in this paper, we concluded that the more impulsive and/or more powerful a flare is, the shorter the b growth time, tb, and the l growth time, tl, are. In the framework of the theory of non-linear dissipative processes, these results may be interpreted as follows. Before a solar flare occurs there is a significant increase in the number of magnetic field discontinuities (b increasing), which is followed by an avalanche (increase of the correlation length) of magnetic energy dissipation events. The avalanche event occupies the entire active region from the corona to the photosphere. Our study indicates that the more abrupt is the avalanche, the stronger and/or more impulsive a flare is. The time profiles of an avalanche is either Gaussian, which satisfies the logistic avalanche model, or exponential with an abrupt drop, which satisfies the exponential avalanche model. The driving time, tb, was longer than the avalanching time, tl, for all of the events. This qualitatively agrees with the requirements of the self-organized criticality theory.

This work was supported by NSF-ATM 0076602, 0205157, 9903515 and NASA NAG5-12782 grants.

The author(s) of this abstract have provided an email address for comments about the abstract: avi@bbso.njit.edu

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