35 research outputs found
Is null-point reconnection important for solar flux emergence?
The role of null-point reconnection in a 3D numerical MHD model of solar
emerging flux is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to interact and
reconnect with a horizontal overlying magnetic field in the atmosphere. Null
points appear as the reconnection begins and persist throughout the rest of the
emergence, where they can be found mostly in the model photosphere and
transition region, forming two loose clusters on either side of the emerging
flux tube. Up to 26 nulls are present at any one time, and tracking in time
shows that there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality of the nulls
in terms of their methods of creation and destruction, their balance of signs,
their long lifetimes, and their geometrical stability. We then show that due to
the low parallel electric fields associated with the nulls, null-point
reconnection is not the main type of magnetic reconnection involved in the
interaction of the newly emerged flux with the overlying field. However, the
large number of nulls implies that the topological structure of the magnetic
field must be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc
Effects of fieldline topology on energy propagation in the corona
We study the effect of photospheric footpoint motions on magnetic field
structures containing magnetic nulls. The footpoint motions are prescribed on
the photospheric boundary as a velocity field which entangles the magnetic
field. We investigate the propagation of the injected energy, the conversion of
energy, emergence of current layers and other consequences of the non-trivial
magnetic field topology in this situation. These boundary motions lead
initially to an increase in magnetic and kinetic energy. Following this, the
energy input from the photosphere is partially dissipated and partially
transported out of the domain through the Poynting flux. The presence of
separatrix layers and magnetic null-points fundamentally alters the propagation
behavior of disturbances from the photosphere into the corona. Depending on the
field line topology close to the photosphere, the energy is either trapped or
free to propagate into the corona.Comment: 14 pages, 15 figure
Why are flare ribbons associated with the spines of magnetic null points generically elongated?
Coronal magnetic null points exist in abundance as demonstrated by
extrapolations of the coronal field, and have been inferred to be important for
a broad range of energetic events. These null points and their associated
separatrix and spine field lines represent discontinuities of the field line
mapping, making them preferential locations for reconnection. This field line
mapping also exhibits strong gradients adjacent to the separatrix (fan) and
spine field lines, that can be analysed using the `squashing factor', . In
this paper we make a detailed analysis of the distribution of in the
presence of magnetic nulls. While is formally infinite on both the spine
and fan of the null, the decay of away from these structures is shown in
general to depend strongly on the null-point structure. For the generic case of
a non-radially-symmetric null, decays most slowly away from the spine/fan
in the direction in which increases most slowly. In particular,
this demonstrates that the extended, elliptical high- halo around the spine
footpoints observed by Masson et al. (Astrophys. J., 700, 559, 2009) is a
generic feature. This extension of the halos around the spine/fan
footpoints is important for diagnosing the regions of the photosphere that are
magnetically connected to any current layer that forms at the null. In light of
this, we discuss how our results can be used to interpret the geometry of
observed flare ribbons in `circular ribbon flares', in which typically a
coronal null is implicated. We conclude that both the physics in the vicinity
of the null and how this is related to the extension of away from the
spine/fan can be used in tandem to understand observational signatures of
reconnection at coronal null points.Comment: Pre-print version of article accepted for publication in Solar
Physic
The theory of magnetic connectivity in the solar corona
International audienc