mi014.zuccarello01 Posted: 18-May-96 Updated: Events specified: N/A
F. Zuccarello
Institute of Astronomy, Citta' Universitaria, Viale A. Doria 6, I-95125 Catania, Italy
It is now widely accepted that the energy suddenly released during a flare had been previously stored in a non-potential configuration of the magnetic field and that, in some circumstances, the entire process is driven by photospheric motions. In order to describe the magnetic configuration, we may use the force-free field approximation, i. e. : rot B = alpha B where alpha is the force-free field parameter. An interesting situation arises when the force free magnetic configuration is characterized by changing sign alpha. In fact, as J = alpha B, it is immediately seen that when alpha changes sign, also the current density changes its sign and therefore the current results to be antiparallel to the magnetic field. Force-free magnetic configurations where alpha changes sign, and consequently currents flowing in the loops in opposite directions have been studied by Levine (1976) and Sakurai (1981). In their works, coronal loops seen in X and EUV were compared with magnetic topologies calculated by theoretical extrapolations of the line of sight components of the magnetic field, in the hypothesis that the force free field approximation could be used. In all the examples shown by Levine (1976), the best fit of observed structures to calculated magnetic fields required currents to be flowing in opposite directions in nearby flux tubes. Moreover, Levine suggested that if two nearby flux tubes are thought of as simple wires, currents flowing in opposite directions would tend to push the loops apart. The result of this process would be loops still rooted at nearby footpoints but forced apart nearer their tops, causing them to bend over and lie very close to the solar surface. Such loops configurations have been often observed by X-ray instruments. Moreover, when in such configurations a flare takes place, Levine observed that the loops which were previously "repelling" themselves, and were therefore very close to the solar surface, tend to approach each other and lie perpendicularly to the solar surface. In this framework, we are very interested to compare Yohkoh X-ray images showing "diverging" coronal loops with photospheric images and magnetograms. The comparison between X-ray images and photospheric ones could indicate what kind of footpoint motion could give rise to coronal "diverging" loops, while the comparison with the magnetograms would allow to confirm whether "diverging" loops are associated with anti-parallel currents. Once this relationship has been confirmed, the next step would be to investigate whether and how much these configurations are flare-productive and to determine the current distribution (and the loops configuration) after the flare.
References
Levine, R.H., 1976, Solar Phys. 46, 159 Sakurai, T., 1981, Solar Phys., 69, 343