ap.analysis of pdes – Classification of homogeneous distributions

On page 92 of these notes, there is a discussion on how to find the fundamental solution to the D’Alembertian operator. It is firstly proposed that $c_n(t^2 – |x|^2 )^{-(n-1)/2}$ may be a good candidate. Then the author says that we can use the homogeneous family of distributions $j_a$ to get a homogeneous distribution that agrees with the above function in $t>|x|$.

However, I am unable to find the definition of $j_a.$

Could anyone give me a reference/clue on what result we are using here?