From an article, they use Matern Covariance function : $$C_v (d)=\frac{1}{2^{\nu-1}\Gamma(\nu)}(\frac{d}{\phi})^\nu\kappa_\nu(\frac{d}{\phi})$$ with $\phi, \nu>0$ and $\kappa_\nu$ a modified Bessel function of order $\nu$. I have looked this up on google and found Bessel function has two kinds. But there is not anything about subscript $\nu$ of the Bessel function. Also, I googled 'Mathern Covariance function' on wiki. It says, $\rho$ and $\nu$ are just non-negative parameters of the covariance. I want to know what the meanings are.
In short, I want to know the parameters of Matern Covariance function. How do they work, what do they mean? Thanks in advance.