The Extrapolation Engine may be selected by choosing
one of the built-in or user
defined IDL wrappers located in the '\\gx_simulator\userlib\Extrapolation
folder. Upon Selection, the IDL routine template is displayed in a text
field where the user may manually edit
the template to specify any optional input
parameters.
To allow integration in the GX_Simulator, any user
defined IDL Wrapper, an Extrapolation Engine must comply with the following IDL
procedure template:
IDL_Wrapper, Bin, nz, dr,Bout
where
-
Bin: input map in a form of a 2D
array (LOS magnetogram) or 3D array (vector magnetogram)
-
nz: requested number of equally
spaced vertical extrapolation layers
-
dr: a vector [dz, dy, dz] indicating
the steps in each direction, measured in arcseconds (not necessarily used by
the extrapolation engine, but needed by the GX_Simulator for proper scaling of
the output datacube.
-
Bout: output variable that must
return a 4D magnetic field datacube, with the fourth dimension indexing the
Cartesian components Bx, By and Bz
NOTE: Although the user defined IDL
Wrapper may use any name for these four manadatory variables, GX_Simulator
automatically rename them according with its internal naming convention before
displaying the template in the corresponding text field. If the IDL Wrapper does
not have at least four variable arguments, an error is issued and the selection
process is stopped.
In addition to these mandatory araguments, the IDL
Wrapper may contain any number of aditional variables of keyword, which may be
manually edited at any time in the input text window, according with the IDL syntax rules.
The keywords
are
alpha1=alpha1
seehafer=seehafer
sub_b00=sub_b00
sub_plan=sub_plan
alpha1 is the input parameter for the currents in force free. The
unit is 1/(pixel size). The pixel distance is the one from the
magnetogram.
The Seehafer is the keyword to double the
magnetogram due to lines near the
border are not closed then it mirror the
magnetogram. It gives better results
accordingly to the paper of Seehafer (1978, ref below). Used by default.
The sub_b00 is because if you see the equation of
the magnetic field in the
paper of Nakagawa and Radu (1972) it is a Fourier
transform but there is
no term of frequency zero (the average of the
magnetic field). So, for
the process to work it is necessary that the piece
of magnetogram you selected
have average equal zero. That is almost true but
you will see that the average
is
sometimes about 10 to 100 Gauss. Thus, you may have to decide what is
better:
- to subtract the average (the program uses the
flag sub_b00 to do it),
- to subtract
a plan (see the program when using the flag
sub_plan) or
- do nothing (default).
References:
Nakagawa and Raadu 1972, Solar Physics, 25, 127
Seehafer
1978, Solar Physics, 58, 215
NOTE:The IDL
Wrapper syntax is not checked for errors after it is edited. GX_Simulator
reports instead any compilation or execution errors after the
extrapolation is launched, and suggests the user the steps that may be taken in
order to fix the error.
An edited syntax of the IDL Wrapper may be reverted to
its original definition by pushing the Reset 
button.
Selecting the Extrapolation Engine
