Plasma Beta Plot

The plasma-b regime (shaded) above an active region is derived from a single pressure function and an upper and lower limit on the coronal magnetic field. The photosphere-chromosphere-corona pressure curve is combination of a semi-empirical fit through various chromospheric non-LTE readiative transfer calculations for the chromosphere and to an isothermal corona T=1.4Mk. The magnetic field is defined as a weak dipolar field and the upper envelope of a set of coronal loops. A large dipolar field is use to approximately simulate the effect of the outflow of the solar wind.

The plasma b ratio (= 8p p/B²) as a function along the coronal magnetic field lines varies from b > 1 in the photosphere at the base of the field lines, to b << 1 in the mid-corona, to b > 1 in the upper corona. The solar magnetic field extrapolations do not or cannot take into account the full range of b. Most models essentially assume b<<1, since the full boundary conditions do not exist in the b>1 regions.