45 research outputs found
Effects of partitioning and extrapolation on the connectivity of potential magnetic fields
Coronal magnetic field may be characterized by how its field lines
interconnect regions of opposing photospheric flux -- its connectivity.
Connectivity can be quantified as the net flux connecting pairs of opposing
regions, once such regions are identified. One existing algorithm will
partition a typical active region into a number of unipolar regions ranging
from a few dozen to a few hundred, depending on algorithmic parameters. This
work explores how the properties of the partitions depend on some algorithmic
parameters, and how connectivity depends on the coarseness of partitioning for
one particular active region magnetogram. We find the number of connections
among them scales with the number of regions even as the number of possible
connections scales with its square.
There are several methods of generating a coronal field, even a potential
field. The field may be computed inside conducting boundaries or over an
infinite half-space. For computation of connectivity, the unipolar regions may
be replaced by point sources or the exact magnetogram may be used as a lower
boundary condition. Our investigation shows that the connectivities from these
various fields differ only slightly -- no more than 15%. The greatest
difference is between fields within conducting walls and those in the
half-space. Their connectivities grow more different as finer partitioning
creates more source regions. This also gives a quantitative means of
establishing how far away conducting boundaries must be placed in order not to
significantly affect the extrapolation. For identical outer boundaries, the use
of point sources instead of the exact magnetogram makes a smaller difference in
connectivity: typically 6% independent of the number of source regions
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
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine
[This corrects the article DOI: 10.1186/s13054-016-1208-6.]
Plasma beta limits for magnetic annihilation models
Magnetic annihilation occurs when two oppositely directed magnetic fields are brought together by a plasma flow. Several exact nonlinear solutions exist which typically depend on the ratios of plasma pressure to magnetic pressure (the plasma beta), inflow speed to global Alfvén speed (the Alfvén Mach number) and of the advective to diffusive terms of the induction equation (the Lundquist number). Ensuring that the plasma pressure is everywhere positive restricts the freedom of choice of these parameters, however. Restrictions on the plasma beta are derived for the cases of two- and three-dimensional annihilation and two-dimensional reconnective annihilation. At the inflow speeds typically required for fast reconnection in diffuse astrophysical plasmas the minimum plasma beta is several orders of magnitude larger than the observed values of unity or less. In other words, at the observed plasma beta the models are only valid for extremely small annihilation rates.</p