Plasma beta

The fundamental measure of a magnetic field's effect on a plasma is the magnetization parameter $\delta$. The fundamental measure of the inverse effect is called $\beta$, and is defined to be the ratio of the thermal energy density $n\,T$ to the magnetic energy density $B^2/2\,\mu_0$. It is conventional to identify the plasma energy density with the pressure,

$$p \equiv n\,T,$$ (33)

as in an ideal gas, and to define a separate $\beta_s$ for each plasma species. Thus,

$$\beta_s = \frac{2\,\mu_0\,p_s}{B^2}.$$ (34)

The total $\beta$ is written

$$\beta = \sum_s \beta_s.$$ (35)