CEA projection
CEA means Cylindrical Equal Area projection.
For HMI SHARP work, CEA means the active-region patch has already been remapped from the curved solar disk onto a local heliographic grid. That is helpful because most extrapolation codes want a rectangular box, not a patch on a sphere.
The usual JSOC series is:
hmi.sharp_cea_720sand the usual magnetic-field segments are:
Br,Bp,Bt,Br_err,Bp_err,Bt_errWhy it matters for NF2
CEA is the easiest HMI format for NF2 active-region work because the vector field is already supplied as remapped components:
Br: radial / local normal component.Bp: heliographic phi component.Bt: heliographic theta component.
NF2’s FITS loader then maps these into the local boundary convention:
Bx = Bp
By = -Bt
Bz = BrThe minus sign on Bt is part of the coordinate convention. Treat it as a real sign choice, not a cosmetic detail.
What CEA buys us
The Sun is spherical, but for a small active region near disk centre we can treat the patch as a local plane. CEA makes that approximation cleaner by preserving area better than a naive image-plane cutout.
That matters for quantities such as magnetic flux, current density, and magnetic energy. If the projected area is wrong, the boundary integrals are quietly wrong too.
Practical interpretation
For the AR 11158 workflow:
horizontal coordinates: CEA active-region grid
vertical coordinate: height above the patch
bottom boundary: SHARP vector magnetogramSo for a first NF2 reproduction, prefer hmi.sharp_cea_720s over a generic full-disk magnetogram.