mi014.zuccarello01
Posted:  18-May-96
Updated:
Events specified: N/A


Antiparallel currents in close loops

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