User:Tohline/2DStructure/ToroidalCoordinates

Using Toroidal Coordinates to Determine the Gravitational Potential

The detailed derivations and associated scratch-work that support the summary discussion of this chapter can be found under the Appendix/Ramblings category of this H_Book.

Preamble

As I have studied the structure and analyzed the stability of (both self-gravitating and non-self-gravitating) toroidal configurations over the years, I have often wondered whether it might be useful to examine such systems mathematically using a toroidal — or at least a toroidal-like — coordinate system. Is it possible, for example, to build an equilibrium torus for which the density distribution is one-dimensional as viewed from a well-chosen toroidal-like system of coordinates?

I should begin by clarifying my terminology. In volume II (p. 666) of their treatise on Methods of Theoretical Physics, Morse & Feshbach (1953; hereafter MF53) define an orthogonal toroidal coordinate system in which the Laplacian is separable.¹ (See details, below.) It is only this system that I will refer to as the toroidal coordinate system; all other functions that trace out toroidal surfaces but that don't conform precisely to Morse & Feshbach's coordinate system will be referred to as toroidal-like.

I became particularly interested in this idea while working with Howard Cohl (when he was an LSU graduate student). Howie's dissertation research uncovered a Compact Cylindrical Greens Function technique for evaluating Newtonian potentials of rotationally flattened (especially axisymmetric) configurations.^2,3 The technique involves a multipole expansion in terms of half-integer-degree Legendre functions of the <math>2^\mathrm{nd}</math> kind — see NIST digital library discussion — where, if I recall correctly, the argument of this special function (or its inverse) seemed to resemble the radial coordinate of Morse & Feshbach's orthogonal toroidal coordinate system — see more on this, below.

Statement of the Problem

Cohl & Tohline (1999; hereafter CT99) derive an expression for the Newtonian gravitational potential in terms of a Compact Cylindrical Green's Function expansion. They show, for example, that when expressed in terms of cylindrical coordinates, the potential at any meridional location, <math>\varpi = a</math> and <math>~Z = Z_0</math>, due to an axisymmetric mass distribution, <math>~\rho(\varpi, Z)</math>, is

<math> \Phi(a,Z_0) = - \frac{2G}{a^{1/2}} q_0 , </math>

where,

<math> q_0 = \int\int \varpi^{1/2} Q_{-1/2}(\Chi) \rho(\varpi, Z) d\varpi dZ, </math>

and the dimensionless argument (the modulus) of the special function, <math>~Q_{-1/2}</math>, is,

<math> \Chi \equiv \frac{a^2 + \varpi^2 + (Z_0 - Z)^2}{2a \varpi} . </math>

Next, following the lead of CT99, we note that according to the Abramowitz & Stegun (1965),

<math>Q_{-1/2}(\Chi) = \mu K(\mu) \, ,</math>

where, the function <math>~K(\mu)</math> is the complete elliptical integral of the first kind and, for our particular problem,

<math>~\mu^2</math>	<math>~\equiv</math>	<math>~2(1+\Chi)^{-1}</math>
	<math>~=</math>	<math>~ 2\biggl[ 1+\frac{a^2 + \varpi^2 + (Z_0 - Z)^2}{2a \varpi}\biggr]^{-1} </math>
	<math>~=</math>	<math>~ \biggl[\frac{4a\varpi}{(a + \varpi)^2 + (Z_0 - Z)^2} \biggr] \, . </math>

Hence, we can write,

<math> q_0 = \int\int \varpi^{1/2} \mu K(\mu) \rho(\varpi, Z) d\varpi dZ \, . </math>

References

1. Morse, P.M. & Feshmach, H. 1953, Methods of Theoretical Physics — Volumes I and II
2. Cohl, H.S. & Tohline, J.E. 1999, ApJ, 527, 86-101
3. Cohl, H.S., Rau, A.R.P., Tohline, J.E., Browne, D.A., Cazes, J.E. & Barnes, E.I. 2001, Phys. Rev. A, 64, 052509

 

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