114 research outputs found
Solar Force-free Magnetic Fields
The structure and dynamics of the solar corona is dominated by the magnetic
field. In most areas in the corona magnetic forces are so dominant that all
non-magnetic forces like plasma pressure gradient and gravity can be neglected
in the lowest order. This model assumption is called the force-free field
assumption, as the Lorentz force vanishes. This can be obtained by either
vanishing electric currents (leading to potential fields) or the currents are
co-aligned with the magnetic field lines. First we discuss a mathematically
simpler approach that the magnetic field and currents are proportional with one
global constant, the so-called linear force-free field approximation. In the
generic case, however, the relation between magnetic fields and electric
currents is nonlinear and analytic solutions have been only found for special
cases, like 1D or 2D configurations. For constructing realistic nonlinear
force-free coronal magnetic field models in 3D, sophisticated numerical
computations are required and boundary conditions must be obtained from
measurements of the magnetic field vector in the solar photosphere. This
approach is currently of large interests, as accurate measurements of the
photospheric field become available from ground-based (for example SOLIS) and
space-born (for example Hinode and SDO) instruments. If we can obtain accurate
force-free coronal magnetic field models we can calculate the free magnetic
energy in the corona, a quantity which is important for the prediction of
flares and coronal mass ejections. Knowledge of the 3D structure of magnetic
field lines also help us to interpret other coronal observations, e.g.,
EUV-images of the radiating coronal plasma.Comment: 49 pages, 11 figures, Living Reviews in Solar Physics, accepte
On the extrapolation of magneto-hydro-static equilibria on the sun
Modeling the interface region between solar photosphere and corona is
challenging, because the relative importance of magnetic and plasma forces
change by several orders of magnitude. While the solar corona can be modeled by
the force-free assumption, we need to take care about plasma forces (pressure
gradient and gravity) in photosphere and chromosphere, here within the
magneto-hydro-static (MHS) model. We solve the MHS equations with the help of
an optimization principle and use vector magnetogram as boundary condition.
Positive pressure and density are ensured by replacing them with two new basic
variables. The Lorentz force during optimization is used to update the plasma
pressure on the bottom boundary, which makes the new extrapolation works even
without pressure measurement on the photosphere. Our code is tested by using a
linear MHS model as reference. From the detailed analyses, we find that the
newly developed MHS extrapolation recovers the reference model at high
accuracy. The MHS extrapolation is, however, numerically more expensive than
the nonlinear force-free field (NLFFF) extrapolation and consequently one
should limit their application to regions where plasma forces become important,
e.g. in a layer of about 2 Mm above the photosphere.Comment: accepted for publication in Ap
Thin current sheets caused by plasma flow gradients in space and astrophysical plasma
Strong gradients in plasma flows play a major role in space and astrophysical
plasmas. A typical situation is that a static plasma equilibrium is surrounded
by a plasma flow, which can lead to strong plasma flow gradients at the
separatrices between field lines with different magnetic topologies, e.g.,
planetary magnetospheres, helmet streamers in the solar corona, or at the
boundary between the heliosphere and interstellar medium. Within this work we
make a first step to understand the influence of these flows towards the
occurrence of current sheets in a stationary state situation. We concentrate
here on incompressible plasma flows and 2D equilibria, which allow us to find
analytic solutions of the stationary magnetohydrodynamics equations (SMHD).
First we solve the magnetohydrostatic (MHS) equations with the help of a
Grad-Shafranov equation and then we transform these static equilibria into a
stationary state with plasma flow. We are in particular interested to study
SMHD-equilibria with strong plasma flow gradients perpendicular to
separatrices. We find that induced thin current sheets occur naturally in such
situations. The strength of the induced currents depend on the Alfv\'en Mach
number and its gradient, and on the magnetic field.Comment: 10 pages, 6 figures, published in Annales Geophysica
Nonlinear force-free coronal magnetic stereoscopy
Getting insights into the 3D structure of the solar coronal magnetic field
have been done in the past by two completely different approaches: (1.)
Nonlinear force-free field (NLFFF) extrapolations, which use photospheric
vector magnetograms as boundary condition. (2.) Stereoscopy of coronal magnetic
loops observed in EUV coronal images from different vantage points. Both
approaches have their strength and weaknesses. Extrapolation methods are
sensitive to noise and inconsistencies in the boundary data and the accuracy of
stereoscopy is affected by the ability of identifying the same structure in
different images and by the separation angle between the view directions. As a
consequence, for the same observational data, the computed 3D coronal magnetic
field with the two methods do not necessarily coincide. In an earlier work
(Paper I) we extended our NLFFF optimization code by the inclusion of
stereoscopic constrains. The method was successfully tested with synthetic data
and within this work we apply the newly developed code to a combined data-set
from SDO/HMI, SDO/AIA and the two STEREO spacecraft. The extended method
(called S-NLFFF) contains an additional term that monitors and minimizes the
angle between the local magnetic field direction and the orientation of the 3D
coronal loops reconstructed by stereoscopy. We find that prescribing the shape
of the 3D stereoscopically reconstructed loops the S-NLFFF method leads to a
much better agreement between the modeled field and the stereoscopically
reconstructed loops. We also find an appreciable decrease by a factor of two in
the angle between the current and the magnetic field which indicates the
improved quality of the force-free solution obtained by S-NLFFF.Comment: 9 pages, 7 figure
The Magnetic Field in the Solar Atmosphere
This publication provides an overview of magnetic fields in the solar
atmosphere with the focus lying on the corona. The solar magnetic field couples
the solar interior with the visible surface of the Sun and with its atmosphere.
It is also responsible for all solar activity in its numerous manifestations.
Thus, dynamic phenomena such as coronal mass ejections and flares are
magnetically driven. In addition, the field also plays a crucial role in
heating the solar chromosphere and corona as well as in accelerating the solar
wind. Our main emphasis is the magnetic field in the upper solar atmosphere so
that photospheric and chromospheric magnetic structures are mainly discussed
where relevant for higher solar layers. Also, the discussion of the solar
atmosphere and activity is limited to those topics of direct relevance to the
magnetic field. After giving a brief overview about the solar magnetic field in
general and its global structure, we discuss in more detail the magnetic field
in active regions, the quiet Sun and coronal holes.Comment: 109 pages, 30 Figures, to be published in A&AR
Fragmentation of electric currents in the solar corona by plasma flows
We consider a magnetic configuration consisting of an arcade structure and a
detached plasmoid, resulting from a magnetic reconnection process, as is
typically found in connection with solar flares. We study spontaneous current
fragmentation caused by shear and vortex plasma flows. An exact analytical
transformation method was applied to calculate self-consistent solutions of the
nonlinear stationary MHD equations. The assumption of incompressible
field-aligned flows implies that both the Alfven Mach number and the mass
density are constant on field lines. We first calculated nonlinear MHS
equilibria with the help of the Liouville method, emulating the scenario of a
solar eruptive flare configuration with plasmoids and flare arcade. Then a Mach
number profile was constructed that describes the upflow along the open
magnetic field lines and implements a vortex flow inside the plasmoid. This
Mach number profile was used to map the MHS equilibrium to the stationary one.
We find that current fragmentation takes place at different locations within
our configuration. Steep gradients of the Alfven Mach number are required,
implying the strong influence of shear flows on current amplification and
filamentation of the MHS current sheets. Crescent- or ring-like structures
appear along the outer separatrix, butterfly structures between the upper and
lower plasmoids, and strong current peaks close the lower boundary. Impressing
an intrinsic small-scale structure on the upper plasmoid results in strong
fragmentation of the plasmoid. Hence fragmentation of current sheets and
plasmoids is an inherent property of MHD theory. Transformations from MHS into
MHD steady-states deliver fine-structures needed for plasma heating and
acceleration of particles and bulk plasma flows in dissipative events that are
typically connected to magnetic reconnection processes in flares and coronal
mass ejections.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
MHD flows at astropauses and in astrotails
The geometrical shapes and the physical properties of stellar wind --
interstellar medium interaction regions form an important stage for studying
stellar winds and their embedded magnetic fields as well as cosmic ray
modulation. Our goal is to provide a proper representation and classification
of counter-flow configurations and counter-flow interfaces in the frame of
fluid theory. In addition we calculate flows and large-scale electromagnetic
fields based on which the large-scale dynamics and its role as possible
background for particle acceleration, e.g. in the form of anomalous cosmic
rays, can be studied. We find that for the definition of the boundaries, which
are determining the astropause shape, the number and location of magnetic null
points and stagnation points is essential. Multiple separatrices can exist,
forming a highly complex environment for the interstellar and stellar plasma.
Furthermore, the formation of extended tail structures occur naturally, and
their stretched field and streamlines provide surroundings and mechanisms for
the acceleration of particles by field-aligned electric fields.Comment: 10 pages, 4 Figure
The Magnetic Properties of Heating Events on High-Temperature Active Region Loops
Understanding the relationship between the magnetic field and coronal heating
is one of the central problems of solar physics. However, studies of the
magnetic properties of impulsively heated loops have been rare. We present
results from a study of 34 evolving coronal loops observed in the Fe XVIII line
component of AIA/SDO 94 A filter images from three active regions with
different magnetic conditions. We show that the peak intensity per unit
cross-section of the loops depends on their individual magnetic and geometric
properties. The intensity scales proportionally to the average field strength
along the loop () and inversely with the loop length () for a
combined dependence of . These loop properties are
inferred from magnetic extrapolations of the photospheric HMI/SDO line-of-sight
and vector magnetic field in three approximations: potential and two Non Linear
Force-Free (NLFF) methods. Through hydrodynamic modeling (EBTEL model) we show
that this behavior is compatible with impulsively heated loops with a
volumetric heating rate that scales as .Comment: Astrophysical Journal, in pres
Coronal magnetic field extrapolation using a specific family of analytical 3D MHS equilibria
Funding: LN acknowledges financial support by the University of St Andrews, and TN acknowledges support by the United Kingdom’s Science and Research Council (STFC) via Consolidated Grant ST/W001195/1.With current observational methods it is not possible to determine the magnetic field in the solar corona accurately. Therefore, coronal magnetic field models have to rely on extrapolation methods using photospheric magnetograms as boundary conditions. In recent years, due to the increased resolution of observations and the need to resolve non-force-free lower regions of the solar atmosphere, there have been increased efforts to use magnetohydrostatic (MHS) field models instead of force-free extrapolation methods. Although numerical methods to calculate MHS solutions can deal with non-linear problems and hence provide more accurate models, analytical three-dimensional MHS equilibria can also be used as a numerically relatively “cheap” complementary method. We discuss a family of analytical MHS equilibria that allows for a transition from a non-force-free region to a force-free region. The solution involves hypergeometric functions and while routines for the calculation of these are available, this can affect both the speed and the numerical accuracy of the calculations. Therefore, we look into the asymptotic behaviour of this solution in order to numerically approximate it through exponential functions aiming to improve the numerical efficiency. We present an illustrative example by comparing field line profiles, density and pressure differences between the exact solutions, the asymptotic solution and a hybrid model where the use of the hypergeometric function is restricted to an area around the transitional region between the non-force-free and the force-free domain.Publisher PDFNon peer reviewe
Doppler shift of hot coronal lines in a moss area of an active region
The moss is the area at the footpoint of the hot (3 to 5 MK) loops forming
the core of the active region where emission is believed to result from the
heat flux conducted down to the transition region from the hot loops. Studying
the variation of Doppler shift as a function of line formation temperatures
over the moss area can give clues on the heating mechanism in the hot loops in
the core of the active regions. We investigate the absolute Doppler shift of
lines formed at temperatures between 1 MK and 2 MK in a moss area within active
region NOAA 11243 using a novel technique that allows determining the absolute
Doppler shift of EUV lines by combining observations from the SUMER and EIS
spectrometers. The inner (brighter and denser) part of the moss area shows
roughly constant blue shift (upward motions) of 5 km/s in the temperature range
of 1 MK to 1.6 MK. For hotter lines the blue shift decreases and reaches 1 km/s
for Fe xv 284 {\AA} (~2 MK). The measurements are discussed in relation to
models of the heating of hot loops. The results for the hot coronal lines seem
to support the quasi-steady heating models for non-symmetric hot loops in the
core of active regions.Comment: 11 pages, 15 Figures, Astronomy and Astrophysics (in press
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