This is partly for my understanding and, hopefully, doesn’t imply that there are (too many) errors in what I’m doing.

I’ve been downloading SHARP magnetograms with the JSOC client and have performed calculations with these. I’ve then replotted the maps and plotted the SHARP regions in Carrington longitude and latitude, using code such as:

coord = SkyCoord(rcx*u.deg, rcy*u.deg, 695700000*u.m, obstime=obsTime, observer=‘self’, frame=‘heliographic_carrington’)

where rcx and rcy are the (lon,lat) from Map.reference_coordinate.

I have two questions.

(1) I get negative values for some Carrington longitudes. Is this correct and, if, so, how is the alignment calculated? In other words, why is it not from 0 to 360 degrees? Apologies if this is obvious but I can’t find a reference that explains the procedure through an example.

(2) Without going into detail, I’m currently working on some photospheric signatures and relating them to CMEs. I have been checking the locations of my signatures with someone else’s code, which estimates CME locations. I’m getting lots of matches (which means that others are generating negative Carringtion longitudes too) but for the cases that don’t match, there is a difference of approximately 360 degrees in the longitudes. For someone with experience of these coordinates, could a phase shift of 360 degrees be possible? I have three instances of this situation. It could be a fluke but I thought I’d ask.

Thank you in advance to anyone with information.

Best wishes,

David