  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. 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.
