Current sheet formation in quasi-separatrix layers and hyperbolic flux tubes
Guillaume Aulanier Submitted: 2005-07-22 07:49
In 3D magnetic field configurations, quasi-separatrix
layers (QSLs) are defined as volumes in which field lines
locally display strong gradients of connectivity. Considering
QSLs as the preferential locations for current sheet development
and magnetic reconnection in general, and as a natural model
for solar flares and coronal heating in particular, have been
strongly debated issues over the last decade.
In this paper, we perform zero-eta resistive MHD simulations
of the development of electric currents in smooth magnetic
configurations, which are strictly speaking bipolar though they
are formed by four flux concentrations, and whose potential fields
contain QSLs. The configurations are driven by smooth and large-scale
sub-Alfvénic footpoint motions.
Extended electric currents naturally form in the configurations,
which evolve through a sequence of quasi non-linear force-free
Narrow current layers also develop. They spontaneously form at small
scales, all around the QSLs, whatever the footpoint motions are.
For long enough motions, the strongest currents develop where the
QSLs are the thinnest, namely at the Hyperbolic Flux Tube (HFT) which
generalizes the concept of separator. These currents progressively take
the shape of an elongated sheet, whose formation is associated with
a gradual steepening of the magnetic field gradients over tens of
Alfvén times, due to the different motions applied to the field
lines which pass on each side of the HFT.
Our model then self-consistently accounts for the long-duration energy
storage prior to a flare, followed by a switch-on of reconnection when
the currents reach the dissipative scale at the HFT.
In configurations whose potential fields contain broader QSLs, when
the magnetic field gradients reach the dissipative scale, the currents
at the HFT %quasi-separator
reach higher magnitudes. This implies that major
solar flares, that are not related with an early large-scale ideal instabilities,
must occur in regions whose corresponding potential fields have broader
Our results lead us to conjecture that physically, current layers
must always form on the scale of the QSLs. This implies that electric
currents around QSLs may be gradually amplified in time only if the
QSLs are broader than the dissipative length-scale.
We also discuss the potential role of QSLs in coronal heating in
bipolar configurations made of a continuous distribution of flux
Authors: G. Aulanier, E. Pariat and P. Demoulin
Publication Status: A&A (accepted)
Last Modified: 2005-07-22 07:49