[Users] New Code Announcement: GiRaFFE

[Zach Etienne]

Dear ET folks,

We are pleased to announce the public release of *GiRaFFE*, an open-source
General Relativistic Force-Free Electrodynamics (GRFFE) code for dynamical
spacetimes.

*-= Code Release Paper =-*
Our code release paper--including a full description of the GiRaFFE code, a
discussion of potential applications of GiRaFFE, and code validation test
results--has been uploaded to the arXiv: https://arxiv.org/abs/1704.00599 .

*-= Executive Summary =-*
GiRaFFE adopts the strategy pioneered by McKinney [1] and modified by
Paschalidis and Shapiro [2] to convert a general relativistic
magnetohydrodynamic (GRMHD) code into a GRFFE code. In short, GiRaFFE
exists as an extensive modification to IllinoisGRMHD [3]. Both GiRaFFE and
IllinoisGRMHD leverage the Einstein Toolkit’s highly-scalable
infrastructure [4,5] to make possible large-scale simulations of magnetized
plasmas in strong, dynamical spacetimes on adaptive-mesh refinement (AMR)
grids.

As you are aware, IllinoisGRMHD is an open-source rewrite of the Illinois
Numerical Relativity Group's GRMHD code. GiRaFFE, on the other hand, is not
based on any existing GRFFE code and is designed only to solve the
equations of GRFFE in the context of fixed or dynamically-generated
spacetimes. GiRaFFE has passed a large suite of GRFFE code validation tests
passed by other state-of-the-art GRFFE codes, leading us to conclude that
GiRaFFE is now ready for large-scale, state-of-the-art GRFFE simulations in
dynamical spacetimes.

*-= New Thorns for the Einstein Toolkit =-*
All GiRaFFE thorns are listed below. Next to the name of each thorn, we
include the number of lines of code to highlight the fact that *GiRaFFE is
quite compact and designed to be welcoming to new users*. In addition, the
comment-to-line-of-code ratio is very nearly 50%, and the code release
paper provides a clean, broad-brush overview of the formalism and
algorithms adopted. To become acquainted with any given thorn, first take a
look at its README file.

** GiRaFFE (~1,600 lines of code)*: Solves the GRFFE equations in the
formalism of Paschalidis & Shapiro [2] using the same reconstruction (PPM)
and staggered vector potential methods as IllinoisGRMHD.

** GiRaFFEfood** (~1,300 lines of code)*: Code validation initial data and
basic diagnostics. Contains

   - Five 1D tests in flat spacetime (fast wave, Alfvén wave,
   degenerate Alfvén wave, "three-wave", FFE breakdown),
   - One 3D test in flat spacetime (aligned rotator), and
   - Three 3D tests in curved spacetimes ("Exact Wald" solution,
   "Magnetospheric Wald" solution, and split monopole).

** ShiftedKerrSchild (~230 lines of code)*: Sets up all 3+1 quantities for
a Kerr-Schild spacetime with an arbitrary radial shift, to mitigate effects
of extreme spacetime curvature near r_{KS}=0. See Appendix of code release
paper for full discussion.

** VolumeIntegrals_slab_Gfood (~310 lines of code)*: Performs volume
integrals over arbitrary rectangular prisms as specified in parameter files.

** GiRaFFE_to_HydroBase* & *ID_converter_GiRaFFE (~230 lines of code)*:
Acts as a translation layer to convert 3-velocities in HydroBase's Valencia
formalism to/from the 3-velocity found in the induction equation (v^i =
u^i/u^0; native to GiRaFFE).

*-= License =-*
To maximize compatibility with existing thorns in ET, all of the
aforementioned GiRaFFE thorns are either licensed GPLv2+ or 2-clause BSD
(FreeBSD). See LICENSE file in each.

*-= Download GiRaFFE! =-*
*The collection of thorns comprising GiRaFFE may be downloaded here:*
*https://bitbucket.org/zach_etienne/wvuthorns
<https://bitbucket.org/zach_etienne/wvuthorns>*

Sincerely,

The GiRaFFE team

-Zach Etienne
-Mew-Bing Wan
-Maria Babiuc
-Sean McWilliams
-Ashok Choudhary

References:
[1] J.C. McKinney. Mon. Not. R. Astron. Soc., 367:1797, 2006.
[2] V. Paschalidis and S. L. Shapiro. Phys. Rev. D, 88:104031, 2013.
[3] Z. B. Etienne, V. Paschalidis, R. Haas, P. M¨osta, and S. L. Shapiro.
Class. Quantum Grav., 32(17):175009, 2015.
[4] https://einsteintoolkit.org/
[5] https://carpetcode.org/
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