73 research outputs found
An Improved Virial Estimate of Solar Active Region Energy
The MHD virial theorem may be used to estimate the magnetic energy of active
regions based on vector magnetic fields measured at the photosphere or
chromosphere. However, the virial estimate depends on the measured vector
magnetic field being force-free. Departure from force-freeness leads to an
unknown systematic error in the virial energy estimate, and an origin
dependence of the result. We present a method for estimating the systematic
error by assuming that magnetic forces are confined to a thin layer near the
photosphere. If vector magnetic field measurements are available at two levels
in the low atmosphere (e.g. the photosphere and the chromosphere), the
systematic error may be directly calculated using the observed horizontal and
vertical field gradients, resulting in an energy estimate which is independent
of the choice of origin. If (as is generally the case) measurements are
available at only one level, the systematic error may be approximated using the
observed horizontal field gradients together with a simple linear force-free
model for the vertical field gradients. The resulting `improved' virial energy
estimate is independent of the choice of origin, but depends on the choice of
the model for the vertical field gradients, i.e. the value of the linear
force-free parameter . This procedure is demonstrated for five vector
magnetograms, including a chromospheric magnetogram.Comment: 17 pages, 1 figur
Dynamic topology and flux rope evolution during non-linear tearing of 3D null point current sheets
In this work, the dynamic magnetic field within a tearing-unstable three-dimensional current sheet about a magnetic null point is described in detail. We focus on the evolution of the magnetic null points and flux ropes that are formed during the tearing process. Generally, we find that both magnetic structures are created prolifically within the layer and are non-trivially related. We examine how nulls are created and annihilated during bifurcation processes, and describe how they evolve within the current layer. The type of null bifurcation first observed is associated with the formation of pairs of flux ropes within the current layer. We also find that new nulls form within these flux ropes, both following internal reconnection and as adjacent flux ropes interact. The flux ropes exhibit a complex evolution, driven by a combination of ideal kinking and their interaction with the outflow jets from the main layer. The finite size of the unstable layer also allows us to consider the wider effects of flux rope generation. We find that the unstable current layer acts as a source of torsional magnetohydrodynamic waves and dynamic braiding of magnetic fields. The implications of these results to several areas of heliophysics are discussed
The magnetic field topology associated to two M flares
On 27 October, 2003, two GOES M-class flares occurred in the lapse of three
hours in active region NOAA 10486. The two flares were confined and their
associated brightenings appeared at the same location, displaying a very
similar shape both at the chromospheric and coronal levels. We focus on the
analysis of magnetic field (SOHO/MDI), chromospheric (HASTA, Kanzelhoehe Solar
Observatory, TRACE) and coronal (TRACE) observations. By combining our data
analysis with a model of the coronal magnetic field, we compute the magnetic
field topology associated to the two M flares. We find that both events can be
explained in terms of a localized magnetic reconnection process occurring at a
coronal magnetic null point. This null point is also present at the same
location one day later, on 28 October, 2003. Magnetic energy release at this
null point was proposed as the origin of a localized event that occurred
independently with a large X17 flare on 28 October, 2003, at 11:01 UT. The
three events, those on 27 October and the one on 28 October, are homologous.
Our results show that coronal null points can be stable topological structures
where energy release via magnetic reconnection can happen, as proposed by
classical magnetic reconnection models.Comment: 14 pages, 7 figure
Monitoring the Processes of Interseismic Accumulation and Coseismic Release of Stresses in the Medium Nearby Earthquake Sources Based on the Analysis of Tidal Response of the Earth’s Surface
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