[ET Trac] #2754: Inconsistent definition of B in evolution thorns

[Samuel Cupp]

#2754: Inconsistent definition of B in evolution thorns

 Reporter: Samuel Cupp
   Status: new
Milestone: 
  Version: 
     Type: bug
 Priority: minor
Component: EinsteinToolkit thorn

GRHydro and IllinoisGRMHD use different definitions for several quantities. This is mostly fine, as IllinoisGRMHD converts `vel` to get its desired velocity variable. However, the magnetic variable `B` also differs between these two thorns by a factor of `sqrt(4pi)`. This means that GRHydro couldn't use, for example, initial data from the Seed\_Magnetic\_Fields thorns without rescaling them. Without an expectation for what `B` is, diagnostic thorns that need this variable will produce incorrect results if used with anything except for the evolution thorn they were designed for. Presumably, all the WVUThorns are using IlliniosGRMHD’s `B`, and GRHydro\_InitData uses GRHydro’s `B`. Anything that isn't explicitly tied to an evolution thorn would be ambiguous. Is there a consensus on what `B` in HydroBase is expected to be?

Especially with the transition to CarpetX, having a single definition for B would likely help keep thorns mutually compatible.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2754/inconsistent-definition-of-b-in-evolution
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