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 as 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,

$$p \equiv n\,T,$$ (1.34)

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

$$\beta_s = \frac{2\,\mu_0\,p_s}{B^{2}}.$$ (1.35)

The total $\beta$ is written

$$\beta = \sum_s \beta_s.$$ (1.36)