10. A long, straight, solid cylinder, oriented with its axis in the z-direction, carries a current whose current density is ( oldsymbol{J} ). The current density, although symmetrical about the cylinder axis, is not constant but varies according to the relationship
[
J=left{egin{array}{cc}
frac{3 I_{0}}{2 pi a^{2}}left[1-left(frac{r}{a}
ight)^{4}
ight] oldsymbol{k} & ext { for } r leq a \
0 & ext { for } r geq a
end{array}
ight.
]
where ( a ) is the radius of the cylinder, ( r ) is the radial distance from the cylinder axis, and ( I_{0} ) is a constant having units of amperes. Find the magnitude of the magnetic field.