## Seed_Magnetic_Fields: A Module for Seeding a Matter Distribution with Magnetic Fields

Date: 2015-10-12 12:00:00 -0600 (Mon, 12 Oct 2015) 

### Abstract

The Seed_Magnetic_Fields thorn seeds magnetic fields into an initial hydrodynamic configuration. Currently seeding into TOV stars is supported, according to the poloidal magnetic field prescription:

$\begin{array}{rcll}{A}_{x}& =& -y\ast {A}_{b}\ast pow\left(MAX\left(P-{P}_{c}ut,0.0\right),{n}_{s}\right)& \text{(1)}\text{}\text{}\\ {A}_{y}& =& x\ast {A}_{b}\ast pow\left(MAX\left(P-{P}_{c}ut,0.0\right),{n}_{s}\right)& \text{(2)}\text{}\text{}\\ {A}_{z}& =& 0& \text{(3)}\text{}\text{}\\ Phi& =& 0& \text{(4)}\text{}\text{}\end{array}$

as specified in Appendix B of the IllinoisGRMHD code announcement paper:
http://arxiv.org/pdf/1501.07276.pdf

Note that we must be careful if ${A}_{i}$’s are staggered. In this case, the pressure must be interpolated to the staggered point, and the values of $x$ and $y$ must also be shifted.

Both staggered and unstaggered vector potential fields are currently supported in this thorn.

### 1 Parameters

 a_b Scope: restricted REAL Description: Magnetic field strength parameter. Range Default: 1e-3 *:* Any real

 afield_type Scope: restricted KEYWORD Description: A-field prescription Range Default: Pressure_prescription see [1] below A_phi propto (P - P_cut)n^_s see [1] below A_phi propto rho

[1]

Pressure\_prescription


[1]

Density\_prescription


 enable_illinoisgrmhd_staggered_a_fields Scope: restricted BOOLEAN Description: Define A fields on an IllinoisGRMHD staggered grid Default: yes

 enable_varpi_squared_multiplication Scope: restricted BOOLEAN Description: Multiply A_phi by varpi2^? Default: yes

 n_s Scope: restricted REAL Description: Magnetic field strength pressure exponent. Range Default: 1.0 *:* Any real

 p_cut Scope: restricted REAL Description: Cutoff pressure, below which vector potential is set to zero. Typically set to 4% of the maximum initial pressure. Range Default: 1e-5 0:* Positive

 rho_cut Scope: restricted REAL Description: Cutoff density, below which vector potential is set to zero. Typically set to 20% of the maximum initial density. Range Default: 0.2 0:* Positive

### 2 Interfaces

#### General

Implements:

seed_magnetic_fields

Inherits:

grid

hydrobase

### 3 Schedule

This section lists all the variables which are assigned storage by thorn WVUThorns/Seed_Magnetic_Fields. 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.

#### Storage

 Always: HydroBase::rho[1] HydroBase::press[1] HydroBase::eps[1] HydroBase::vel[1] HydroBase::Bvec[1] HydroBase::Avec[1] HydroBase::Aphi[1]

#### Scheduled Functions

CCTK_INITIAL

seed_magnetic_fields

set up seed magnetic field configuration.

 After: hydrobase_initial Before: illinoisgrmhd_id_converter Language: c Type: function