Research example:
Constant cross section of
coronal loops
The corona of the Sun is dominated by emission from loop-like
structures. When observed in X-ray or extreme ultraviolet emission,
these million K hot coronal loops show a more or less constant cross
section.
Aims: In this study we show how the interplay of heating, radiative
cooling, and heat conduction in an expanding magnetic structure can
explain the observed constant cross section.
We employ a three-dimensional magnetohydrodynamics (3D MHD)
model of the corona. The heating of the coronal plasma is the result
of braiding of the magnetic field lines through footpoint motions and
subsequent dissipation of the induced currents. From the model we
synthesize the coronal emission, which is directly comparable to
observations from, e.g., the Atmospheric Imaging Assembly on the
Solar Dynamics Observatory (AIA/SDO).
We find that the synthesized observation of a coronal loop
seen in the 3D data cube does match actually observed loops in count
rate and that the cross section is roughly constant, as observed. The
magnetic field in the loop is expanding and the plasma density is
concentrated in this expanding loop; however, the temperature is not
constant perpendicular to the plasma loop. The higher temperature in
the upper outer parts of the loop is so high that this part of the
loop is outside the contribution function of the respective emission
line(s). In effect, the upper part of the plasma loop is not bright
and thus the loop actually seen in coronal emission appears to have a
constant width.
From this we can conclude that the underlying
field-line-braiding heating mechanism provides the proper spatial and
temporal distribution of the energy input into the corona - at least
on the observable scales.
click on image to view movie
View of the modeled corona above an active region as would be seen by
AIA/SDO in the 171 Å channel dominated by emission from below about
106 K. The rectangle indicates the boundaries of the computational
domain (50x50x30 Mm) at the coronal base. The most prominent loop
connects the periphery of the two magnetic concentrations at the
surface (not shown here) and has a roughly semicircular shape with
about 30 Mm length. In the vicinity of the active region, hazy
emission can be seen that is associated with the diffuse background
corona in the quiet Sun. The white dashed line indicates the position
of a loop shown in the figure below. The temporal evolution
over 50 min is shown in a movie available when clicking on the image.
View of the synthesized coronal loop in AIA 171 Å seen in the top figure when
horizontally integrating through the box along a line-of-sight
perpendicular to the loop plane. The AIA 171 Å point spread function
(PSF) was applied and shows the image with the same pixel size as AIA
(0.6 arcsec corresponding to 435 km on the Sun). As in observations, a
background subtraction was applied to enhance the contrast. Here the
constant cross section of the loop is obvious. When analysingmeasuring the
width of this loop, the constant cross section is confirmed.
More inforation can be found in the following publication:
Constant cross section of loops in the solar corona
Peter H., Bingert S. (2012) A&A 548, A1
Link to ADS
Structure of solar coronal loops: from miniature to large-scale
Peter H., Bingert S., Klimchuk J.A., et al. (2013) A&A 556, A104
Link to ADS
