- When ther is no magnetic field then pressure gradient and
gravitational force balance gives

Thus, we can neglect gravity and the exponential decrease of the pressure with height if . - Compare the Lorentz force with the pressure gradient force so
that

Hence, we may neglect the pressure gradient term if

and we may alternatively neglect the Lorentz force if

so that

and the magnetic field is called ``force-free''.

- Coronal active regions where the magnetic field is closed.

This gives kg m and pascals. Therefore, the plasma beta is

Thus, is small in the corona. - Coronal holes have a weaker magnetic field strength and a
lower temperature. Typically we take

Thus, in a coronal hole the plasma beta has a typical value of

and is even smaller than the active region value.