Ian Hinder

01 Jun 2010


The Multipole thorn performs spherical harmonic mode decomposition of Cactus grid functions on coordinate spheres.

1 Introduction

This thorn allows the user to compute the coefficients of the spherical harmonic expansion of a field stored in a Cactus grid function on coordinate spheres of given radii. A set of radii for these spheres, as well as the number of angular points to use, can be specified. Complex fields can be used, but they must be stored in pairs of real Cactus grid functions (the CCTK_COMPLEX type is not supported).

2 Physical System

The angular dependence of a field \(u(t, r, \theta , \varphi )\) can be expanded in spin-weight \(s\) spherical harmonics [1]:

\begin {eqnarray} u(t, r, \theta , \varphi ) = \sum _{l=0}^\infty \sum _{m=-l}^l C^{lm}(t,r) {}_s Y_{lm}(\theta ,\varphi ) \end {eqnarray}

where the coefficients \(C^{lm}(t,r)\) are given by

\begin {eqnarray} C^{lm}(t, r) = \int {}_s Y_{lm}^* u(t, r, \theta , \varphi ) r^2 d \Omega \label {eqn:clmint} \end {eqnarray}

At any given time, \(t\), this thorn can compute \(C^{lm}(t,r)\) for a number of grid functions on several coordinate spheres with radii \(r_i\). The coordinate system of the Cactus grids must be Cartesian and the coordinates \(r\), \(\theta \), \(\varphi \) are related to \(x\), \(y\) and \(z\) by the usual transformation between Cartesian and spherical polar coordinates (\(\theta \) is the polar angle and \(\varphi \) is the azimuthal angle).

The spin-weighted spherical harmonics are computed using Eq. 3.1 in Ref. [1].

3 Numerical Implementation

The coordinate sphere on which the multipolar decomposition is performed is represented internally as a 2-dimensional grid evenly spaced in \(\theta \) and \(\varphi \) with coordinates

\begin {eqnarray} \theta _k &=& \frac {\pi k}{n_\theta } \quad k = 0, 1, ..., n_\theta \\ \varphi _k &=& \frac {2\pi k}{n_\varphi } \quad k = 0, 1, ..., n_\varphi , \end {eqnarray}

so \(n_\theta \) and \(n_\varphi \) count the number of cells (not the number of points). The gridfunction to be decomposed, \(u\), is first interpolated from the 3D Cactus grid onto this 2D grid at a given radius, \(r_i\), and the \(C^{lm}\) are computed by evaluating the integral in Eq. ?? for different values of \(l\) and \(m\). The interpolation is performed using the Cactus interpolation interface, so any Cactus interpolator can be used. The numerical method used for interpolation should be specified in the documentation for the thorn which provides it. One such thorn is AEILocalInterp. The integration is performed using either the midpoint rule, yielding a result which is second order accurate in the angular spacings \(\Delta \theta = \pi /n_\theta \) and \(\Delta \varphi = 2\pi /n_\theta \), or Simpson’s rule, which is fourth order accurate.

4 Using This Thorn

4.1 Obtaining This Thorn

This thorn is available as part of the Einstein Toolkit via:

svn checkout Multipole

or by using the Einstein Toolkit GetComponents script and thornguide.

4.2 Basic Usage

Suppose that you have a real grid function, \(u\), for which you want to compute the spherical harmonic coefficients \(C^{lm}\). Start by including Multipole and an interpolator (for example AEILocalInterp) in the ActiveThorns line of your parameter file (for interpolation thorns other than AEILocalInterp, you will need to modify the interpolator and interpolator_options accoording to the documentation of the interpolation thorn). Next decide the number and radii of the coordinate spheres on which you want to decompose. Set the number of spheres with the nradii parameter, and the radii themselves with the radius[i] parameters (the indices \(i\) are zero-based). For example,

ActiveThorns = ".... AEILocalInterp Multipole"

Multipole::nradii       = 3
Multipole::radius[0]    = 10
Multipole::radius[1]    = 20
Multipole::radius[2]    = 30
Multipole::variables    = "MyThorn::u"

The default parameters will compute all \(l = 2\) modes assuming a spin-weight \(s = 0\) on every iteration of your simulation. The coefficients \(C^{lm}\) will be output in the files with names of the form mp_<var>_l<lmode>_m<mmode>_r<rad>.asc, for example


For the filename, the radius is converted to a string with two decimal places, which should be sufficient. These are ASCII files where each line has columns

\(t\), \(\mathrm {Re} \, C^{lm}\), \(\mathrm {Im} \, C^{lm}\).

4.3 Special Behaviour

Often it will be necessary to go beyond the basic usage described in the previous section.

4.3.1 Higher modes

By default, Multipole computes only \(l = 2\) modes. You can choose whether to extract a single mode, or all modes from \(l = \) l_min to \(l = \) l_mode, with \(|m| \le \) m_mode. This is controlled by the mode_type parameter, which can be set to "all_modes" or "specific_mode". The parameters l_min and l_mode specify the lowest and highest modes to compute. The parameter m_mode specifies up to which value of \(m\) to compute. When using mode_type = "specific_mode", the mode \(l\) and \(m\) are given by l_mode and m_mode respectively.

4.3.2 Variable options

Several variable-specific options can be listed as tags in the variables parameter:

Multipole::variables = "<imp>::<var>{<tagname> = <tagvalue> ... } ..."

Valid tags are:


A string giving the fully qualified variable name for the imaginary part of a complex field, assuming that <imp>::<var> is the real part.


A string giving an alias to use for the decomposed variable in the output filename, for use in the case of complex variable when otherwise the name of the real part would be used, which might be confusing.


The integer spin-weight of the spherical harmonic decomposition to use.

Strings should be enclosed in single quotes within the list of tags.

4.3.3 Complex variables

In order to decompose a complex quantity, Multipole currently requires that the field is stored in two separate CCTK_REAL grid functions, one for the real and one for the imaginary part. Suppose the complex function \(u\) is stored in two gridfunctions u_re and u_im. In order to correctly decompose \(u\), specify the real variable in the variables parameter, and use the tag cmplx to specify the name of its imaginary companion:

Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’}"

4.3.4 Renaming variables

In some cases, you might want the name of the variable in the output filename to be different to the name of the grid function. This can be done by setting the name tag of the variable:

Multipole::variables = "MyThorn::u{name = ’myfunction’}"

For example, in the case of a complex variable where the output file contains the name of the real part, you can rename it as follows:

Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’ name = ’u’}"

4.3.5 Spin weight

Depending on the nature of the field to be decomposed, a spin-weight other than 0 might be required in the spherical harmonic basis. Use the tag sw for this

Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’ name = ’u’ sw = -2}"

4.3.6 Interpolator options

The interpolator to be used can be specified in the interpolator_name parameter, and a string containing interpolator parameters can be specified in the interpolator_pars parameter. See the interpolator (for example AEIThorns/AEILocalInterp) documentation for details of interpolators available and their options.

4.3.7 Output

When used with mesh-refinement, it is common to require mode decomposition less frequently than every iteration. The parameter out_every can be used to control this. 1D and 2D output of the coordinate spheres can be enabled using out_1d_every and out_2d_every.

4.4 Interaction With Other Thorns

This thorn obtains grid function data via the standard Cactus interpolator interface. To use this, one needs the parallel driver (for example PUGHInterp or CarpetInterp) as well as the low-level interpolator (e.g. AEILocalInterp).

4.5 Examples

To use this thorn with WeylScal4 to compute modes of the complex \(\Psi _4\) variable, one could use the following example:

ActiveThorns = ".... WeylScal4 CarpetInterp AEILocalInterp Multipole"

Multipole::nradii    = 3
Multipole::radius[0] = 10
Multipole::radius[1] = 20
Multipole::radius[2] = 30
Multipole::variables = "WeylScal4::Psi4r{sw=-2 cmplx=’WeylScal4::Psi4i’}"
Multipole::l_mode    = 4
Multipole::m_mode    = 4

5 History

This thorn was developed in the Penn State Numerical Relativity group and contributed to the Einstein Toolkit.

5.1 Acknowledgements

This thorn was written by Ian Hinder and Andrew Knapp, with contributions from Eloisa Bentivegna and Shaun Wood.


[1]   J. N. Goldberg, A. J. MacFarlane, E. T. Newman, F. Rohrlich, and E. C. G. Sudarshan. Spin s spherical harmonics and edth. J. Math. Phys., 8:2155, 1967.

6 Parameters

Scope: restricted  STRING

Description: What is the coord system?

Range   Default: cart3d
Any smart string will do

Scope: restricted  BOOLEAN

Description: whether to set a spherical harmonic in the ’harmonic’ grid functions

  Default: no

Scope: restricted  INT

Description: How many iterations to preallocate in extensible HDF5 datasets

Range   Default: 200
Any integer

Scope: restricted  KEYWORD

Description: How to do surface integrals

Range   Default: midpoint
Midpoint rule (2nd order)
Trapezoidal rule (2nd order)
Simpson’s rule (4th order) [requires even ntheta and nphi]
Driscoll & Healy (exponentially convergent) [requires even ntheta]

Scope: restricted  STRING

Description: Which interpolator should I use

Range   Default: Hermite polynomial interpolation
Any nonempty string

Scope: restricted  STRING

Description: Parameters for the interpolator

Range  Default: order=3 boundary_off_centering_tolerance={0.0 0.0 0.0 0.0 0.0 0.0} boundary_extrapolation_tolerance={0.0 0.0 0.0 0.0 0.0 0.0}
”Any string that Util_TableSetFromStr ing() will take”

Scope: restricted  INT

Description: The maximum l mode to extract

Range   Default: 2
l >= 0

Scope: restricted  INT

Description: all modes: above which l mode to calculate/ specific mode: which l mode to extract

Range   Default: -1

Scope: restricted  INT

Description: The maximum l mode to extract

Range   Default: -1

Scope: restricted  INT

Description: all modes: Up to which m mode to calculate/ specific mode: which m mode to extract

Range   Default: -100

Scope: restricted  KEYWORD

Description: Which type of mode extraction do we have

Range   Default: deprecated
all modes
Extract all modes up to (l_mode, m_mode).
specific mode
Select one specific (l_mode, m_mode) mode

Scope: restricted  INT

Description: The number of points in the phi direction, minus one. (E.g., if this is set to 100, then 101 points will be chosen.)

Range   Default: 100

Scope: restricted  INT

Description: How many extraction radii?

Range   Default: 1

Scope: restricted  INT

Description: The number of points in the theta direction, minus one. (E.g., if this is set to 50, then 51 points will be chosen.)

Range   Default: 50
Positive please

Scope: restricted  INT

Description: How often to output 1d data

Range   Default: (none)
no output
output every to many iterations

Scope: restricted  STRING

Description: Output directory for Extract’s files, overrides IO::out_dir

Range   Default: (none)
A valid directory name
An empty string to choose the default from IO::out_dir

Scope: restricted  INT

Description: How often to output

Range   Default: 1
no output
output every to many iterations

Scope: restricted  BOOLEAN

Description: Output a simple ASCII file for each mode at each radius

  Default: yes

Scope: restricted  BOOLEAN

Description: Output an HDF5 file for each variable containing one dataset per mode at each radius

  Default: no

Scope: restricted  REAL

Description: The radii for extraction

Range   Default: 0.0
Please keep it in the grid

Scope: restricted  INT

Description: which mode to put into the test variables

Range   Default: 2
Any integer

Scope: restricted  INT

Description: which mode to put into the test variables

Range   Default: 2
Any integer

Scope: restricted  BOOLEAN

Description: whether the test spherical harmonic coefficient is proportional to the radial coordinate

  Default: no

Scope: restricted  INT

Description: which spin weight to put into the test variables

Range   Default: -2
Any integer

Scope: restricted  STRING

Description: What variables to decompose

Range   Default: (none)
A list of variables

Scope: restricted  BOOLEAN

Description: Output detailed information about what is happening

  Default: no

Scope: shared from IO STRING

7 Interfaces






Grid Variables


  Group Names     Variable Names     Details   

harmonics   compact0
harmonic_re   descriptionSpherical harmonics
harmonic_im   dimensions3
  group typeGF
 variable typeREAL

test_integration_convergence_orders   compact0
test_midpoint_convergence_order   descriptionTest integration convergence orders
test_trapezoidal_convergence_order  dimensions0
test_simpson_convergence_order   distributionCONSTANT
  group typeSCALAR
 variable typeREAL

test_integration_results   compact0
test_midpoint_result_low   descriptionTest integration results
test_midpoint_result_high   dimensions0
test_trapezoidal_result_low   distributionCONSTANT
test_trapezoidal_result_high   group typeSCALAR
test_simpson_result_low   timelevels1
test_simpson_result_high  variable typeREAL

test_integration_symmetries   compact0
test_midpoint_pi_symmetry   descriptionTest integration symmetries
test_trapezoidal_pi_symmetry   dimensions0
test_simpson_pi_symmetry   distributionCONSTANT
test_driscollhealy_pi_symmetry   group typeSCALAR
 variable typeREAL

test_orthonormality test_orthonormality   compact0
  descriptionTest orthonormality of spherical harmonics
  group typeARRAY
 variable typeREAL

8 Schedule

This section lists all the variables which are assigned storage by thorn EinsteinAnalysis/Multipole. Storage can either last for the duration of the run (Always means that if this thorn is activated storage will be assigned, Conditional means that if this thorn is activated storage will be assigned for the duration of the run if some condition is met), or can be turned on for the duration of a schedule function.




Scheduled Functions

CCTK_ANALYSIS (conditional)


  calculate multipoles


 After: calc_np
 Options: global
 Type: function

CCTK_INITIAL (conditional)


  populate grid functions with spherical harmonics


 Reads: grid::coordinates(everywhere)
 Type: function
 Writes: multipole::harmonics(interior)

CCTK_INITIAL (conditional)


  loop over modes and integrate them to check orthonormality


 Type: function

CCTK_PARAMCHECK (conditional)


  check multipole parameters


 Options: global
 Type: function

CCTK_PARAMCHECK (conditional)


  test convergence of integration


 Type: function

CCTK_PARAMCHECK (conditional)


  test symmetry of integration


 Type: function