Adaption of example plotting open closed

[Richardjmorton]

Provide a general description of the issue or problem.

Hi,

I have been working with the PFSS model, and the example showing the open closed flux (https://docs.sunpy.org/projects/sunkit-magex/en/stable/generated/gallery/using_sunkit_magex_pfss/open_closed_field_map.html) led to an issue, which took me some time to figure out.

The issue is the open closed flux map is not on the same grid as the gong map. This make direct comparision of the two, e.g., overplotting of contours, complicated. There is an easy fix that can align the two, which I think would make it clearer.

I would suggest changing the example.

The new portion of the code is (without imports):

pfss_in = pfss.Input(gong_map, nrho, rss)
pfss_out = pfss.pfss(pfss_in)

# Here the gong coordinates are used to make the seeds
coords = all_coordinates_from_map(gong_map) 
hgc    = coords.transform_to('heliographic_carrington')    

lon2d = hgc.lon.wrap_at(360*u.deg)    
lat2d = hgc.lat             

r = np.full(ny*nx, 1.01) * u.solRad
seeds = SkyCoord(lon2d.ravel(), lat2d.ravel(), r, frame=pfss_out.coordinate_frame)

tracer = tracing.PerformanceTracer()
field_lines = tracer.trace(seeds, pfss_out)

pols = field_lines.polarities.reshape(ny,nx)

wcs_header = gong_map.wcs.to_header(relax=True)
wcs_header['NAXIS']  = 2
wcs_header['NAXIS1'] = pols.shape[1]
wcs_header['NAXIS2'] = pols.shape[0]
wcs_header['BTYPE']  = 'polarity'   
wcs_header['BUNIT']  = ''    

pols_map = sunpy.map.Map(pols.astype(float), wcs_header)
pols_map.plot_settings['cmap'] =  mcolor.ListedColormap(['tab:red', 'black', 'tab:blue'])

This leads to a polarity map that follows the gong map, enabling direction comparison. Some of this code might be superfluous.

Maybe I have missed the intention of the example. Let me know what you think.

Richard

[STBadman]

Hi @Richardjmorton , thanks for this, I like the suggestion. I think the intention of the example was just to demonstrate that an open closed map could be derived from field line tracing on a grid, but I think it makes things nice and consistent to trace from the coordinates of the gong map, and also to create an open closed sunpy.map.Map from the outcome definitely adds value and more accurately matches the title of the example.

I also see a couple of typos and unclear statements in the current text of the example.

Please can you open a pull request with your changes and we can refine a bit?