HPR

Helioprojective Radial - A 3-D orthonormal(left-handed) coordinate system that is primarily intended to specify withtwo dimensions a point on the solar disk. The Z axis points from theobserver to the center of the solar disk. The Y axis lies in the planedefined by the solar spin vector and the Z axis, positive northward. The Xaxis is perpendicular to the Y and Z axes, positive toward solar west. Givenas the distance between the observer and the center of the solar disk, thestandard representation for this system of an (x,y) point on the solar diskis via the point's latitude angle theta {= arctan [SQRT(x2 + y2)]/d]} orequivalent declination parameter delta (= theta - 90 deg), and its phaseangle psi as measured counter- clockwise from the +Y axis [psi = arctan(-y/x)].