Appendix/Ramblings/ConcentricEllipsoidalCoordinates

Concentric Ellipsoidal (T6) Coordinates

Background

Building on our general introduction to Direction Cosines in the context of orthogonal curvilinear coordinate systems, and on our previous development of T3 (concentric oblate-spheroidal) and T5 (concentric elliptic) coordinate systems, here we explore the creation of a concentric ellipsoidal (T6) coordinate system. This is motivated by our desire to construct a fully analytically prescribable model of a nonuniform-density ellipsoidal configuration that is an analog to Riemann S-Type ellipsoids.

Orthogonal Coordinates

Primary (radial-like) Coordinate

We start by defining a "radial" coordinate whose values identify various concentric ellipsoidal shells,

${\lambda }_1$

$\equiv$

$(x^2+q^2{y}^2+p^2{z}^2{)}^{1/2}.$

When ${\lambda }_1=a$, we obtain the standard definition of an ellipsoidal surface, it being understood that, $q^2=a^2/b^2$ and $p^2=a^2/c^2$. (We will assume that $a>b>c$, that is, $p^2>q^2>1$.)

A vector, $\hat{\mathbf{n}}$, that is normal to the ${\lambda }_1$ = constant surface is given by the gradient of the function,

$F(x,y,z)$

$\equiv$

$(x^2+q^2{y}^2+p^2{z}^2{)}^{1/2}-{\lambda }_1.$

In Cartesian coordinates, this means,

$\hat{\mathbf{n}}(x,y,z)$	$=$	$\hat{ı}\left(\frac{\partial F}{\partial x}\right)+\hat{ȷ}\left(\frac{\partial F}{\partial y}\right)+\hat{k}\left(\frac{\partial F}{\partial z}\right)$
	$=$	$\hat{ı}[x(x^2+q^2{y}^2+p^2{z}^2{)}^{-1/2}]+\hat{ȷ}[q^{2}y(x^2+q^2{y}^2+p^2{z}^2{)}^{-1/2}]+\hat{k}[p^{2}z(x^2+q^2{y}^2+p^2{z}^2{)}^{-1/2}]$
	$=$	$\hat{ı}\left(\frac{x}{{\lambda }_1}\right)+\hat{ȷ}\left(\frac{q^{2}y}{{\lambda }_1}\right)+\hat{k}\left(\frac{p^{2}z}{{\lambda }_1}\right),$

where it is understood that this expression is only to be evaluated at points, $(x,y,z)$, that lie on the selected ${\lambda }_1$ surface — that is, at points for which the function, $F(x,y,z)=0$. The length of this normal vector is given by the expression,

$[\hat{\mathbf{n}}\cdot \hat{\mathbf{n}}{]}^{1/2}$	$=$	$\left[\right(\frac{\partial F}{\partial x}{)}^2+(\frac{\partial F}{\partial y}{)}^2+(\frac{\partial F}{\partial z}{)}^2{]}^{1/2}$
	$=$	$\left[\right(\frac{x}{{\lambda }_1}{)}^2+(\frac{q^{2}y}{{\lambda }_1}{)}^2+(\frac{p^{2}z}{{\lambda }_1}{)}^2{]}^{1/2}$
	$=$	$\frac{1}{{\lambda }_1{\ell }_{3D}}$

where,

${\ell }_{3D}$

$\equiv$

$[x^2+q^4{y}^2+p^4{z}^2{]}^{-1/2}.$

It is therefore clear that the properly normalized normal unit vector that should be associated with any ${\lambda }_1$ = constant ellipsoidal surface is,

${\hat{e}}_1$

$\equiv$

$\frac{\hat{\mathbf{n}}}{[\hat{\mathbf{n}}\cdot \hat{\mathbf{n}}{]}^{1/2}}=\hat{ı}(x{\ell }_{3D})+\hat{ȷ}(q^{2}y{\ell }_{3D})+\hat{k}(p^{2}z{\ell }_{3D}).$

From our accompanying discussion of direction cosines, it is clear, as well, that the scale factor associated with the ${\lambda }_1$ coordinate is,

$h_1^2$

$=$

${\lambda }_1^2{\ell }_{3D}^2.$

We can also fill in the top line of our direction-cosines table, namely,

Direction Cosines for T6 Coordinates ${\gamma }_{ni}=h_n\left(\frac{\partial {\lambda }_n}{\partial x_i}\right)$
$n$	$i=x,y,z$
$1$	$x{\ell }_{3D}$	$q^{2}y{\ell }_{3D}$	$p^{2}z{\ell }_{3D}$
$2$	---	---	---
$3$	---	---	---

Other Coordinate Pair in the Tangent Plane