30 research outputs found
How to use magnetic field information for coronal loop identification?
The structure of the solar corona is dominated by the magnetic field because
the magnetic pressure is about four orders of magnitude higher than the plasma
pressure. Due to the high conductivity the emitting coronal plasma (visible
e.g. in SOHO/EIT) outlines the magnetic field lines. The gradient of the
emitting plasma structures is significantly lower parallel to the magnetic
field lines than in the perpendicular direction. Consequently information
regarding the coronal magnetic field can be used for the interpretation of
coronal plasma structures. We extrapolate the coronal magnetic field from
photospheric magnetic field measurements into the corona. The extrapolation
method depends on assumptions regarding coronal currents, e.g. potential fields
(current free) or force-free fields (current parallel to magnetic field). As a
next step we project the reconstructed 3D magnetic field lines on an EIT-image
and compare with the emitting plasma structures. Coronal loops are identified
as closed magnetic field lines with a high emissivity in EIT and a small
gradient of the emissivity along the magnetic field.Comment: 14 pages, 3 figure
3D Coronal Density Reconstruction and Retrieving the Magnetic Field Structure during Solar Minimum
Measurement of the coronal magnetic field is a crucial ingredient in
understanding the nature of solar coronal phenomena at all scales. We employed
STEREO/COR1 data obtained during a deep minimum of solar activity in February
2008 (Carrington rotation CR 2066) to retrieve and analyze the
three-dimensional (3D) coronal electron density in the range of heights from
1.5 to 4 Rsun using a tomography method. With this, we qualitatively deduced
structures of the coronal magnetic field. The 3D electron density analysis is
complemented by the 3D STEREO/EUVI emissivity in the 195 A band obtained by
tomography for the same CR. A global 3D MHD model of the solar corona was used
to relate the reconstructed 3D density and emissivity to open/closed magnetic
field structures. We show that the density maximum locations can serve as an
indicator of current sheet position, while the locations of the density
gradient maximum can be a reliable indicator of coronal hole boundaries. We
find that the magnetic field configuration during CR 2066 has a tendency to
become radially open at heliocentric distances greater than 2.5 Rsun. We also
find that the potential field model with a fixed source surface (PFSS) is
inconsistent with the boundaries between the regions with open and closed
magnetic field structures. This indicates that the assumption of the potential
nature of the coronal global magnetic field is not satisfied even during the
deep solar minimum. Results of our 3D density reconstruction will help to
constrain solar coronal field models and test the accuracy of the magnetic
field approximations for coronal modeling.Comment: Published in "Solar Physics
Tomographic 3D-Modeling of the Solar Corona with FASR
The "Frequency-Agile Solar Radiotelescope" (FASR) litteraly opens up a new
dimension in addition to the 3D Euclidian geometry: the frequency dimension.
The 3D geometry is degenerated to 2D in all images from astronomical
telescopes, but the additional frequency dimension allows us to retrieve the
missing third dimension by means of physical modeling. We call this type of 3D
reconstruction "Frequency Tomography". In this study we simulate a realistic 3D
model of an active region, composed of 500 coronal loops with the 3D geometry
[x(s),y(s),z(s)] constrained by magnetic field extrapolations and the physical
parameters of the density n_e(s) and temperature T_e(s) given by hydrostatic
solutions. We simulate a series of 20 radio images in a frequency range of
f=0.1-10 GHz, anticipating the capabilities of FASR, and investigate what
physical information can be retrieved from such a dataset. We discuss also
forward-modeling of the chromospheric and Quiet Sun density and temperature
structure, another primary goal of future FASR science.Comment: 10 Figure