  We study the magnetohydrostatic equilibrium of magnetic flux tubes with
circular cross sections embedded in a magnetic surrounding atmosphere. We solve
the static momentum equation in 2.5D to obtain the thermodynamics that are
consistent with a prescribed velocity and magnetic fields. We show that force
balance is roughly satisfied if the flux tube's magnetic field is aligned with
its axis. Equilibrium is guaranteed if this magnetic field possesses a
transverse component. Several forms of this transverse field are investigated.
The resulting magnetic field configurations are critically reviewed in terms of
the results from spectropolarimetric observations. The thermodynamic structure
that allows the flux tube to be in mechanical equilibrium is also calculated.
We show that the inferred pressure, density and temperature stratification
reproduce intensity features similar to dark core penumbral filaments and
penumbral grains.
