[aspect-devel] Earth radius in spherical shell models

[Wolfgang Bangerth]

On 01/14/2018 04:22 PM, Max Rudolph wrote:
> I am wondering why the default outer radius of the Earth in ASPECT is set to 
> 6336 km (based on the default value in Geometry model/Spherical shell)?
> 
> 6,336 km is the equatorial radius of curvature
> 6,378 km is the equatorial radius
> 6,371 km is the radius of a sphere with equal volume
> 
> I know that these differ only at the 0.5% level and for most practical 
> purposes may not make any difference, but was wondering if there is a 
> justification for defaulting to one over the other?

That number dates back to the very early days of ASPECT, and indeed step-32 
has this line:

     const double R0      = 6371000.-2890000.;     /* m          */
     const double R1      = 6371000.-  35000.;     /* m          */

That would suggest that I used the radius of the sphere with equal volume 
minus the average depth of the crust. It seems like a coincidence that it's 
also the radius of curvature at the equator.

I'll document this in the parameter description in a minute.

Best
  Wolfgang

-- 
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Wolfgang Bangerth          email:                 bangerth at colostate.edu
                            www: http://www.math.colostate.edu/~bangerth/