The accuracy of the center-of-gravity method for measuring velocity and magnetic field strength in the solar photosphere

I investigate the accuracy with which the line-of-sight velocity and magnetic field strength in the solar photosphere can be recovered from spatially resolved spectral line profiles with the center-of-gravity ( COG) method. For this purpose, theoretical non-LTE polarized line profiles of a series of Fe I lines were calculated through a two-dimensional slice from a snapshot of a three-dimensional solar magnetoconvection simulation. The calculated profiles were analyzed with the COG method for all positions along the slice, and retrieved values of velocity and field strength were compared with actual values at the heights of formation of the lines. The average formation heights of the employed lines range from 60 to almost 400 km above the average photospheric level. The COG method appears reliable for measuring velocities in the lower half of these formation heights and for measuring field strength over the whole range of heights, for fields up to intermediate strength. Moreover, it is shown that the COG determination is independent of spectral resolution, making it particularly suitable for applications that require high throughput and a correspondingly large spectral bandpass, such as high spatial resolution observations with a large-diameter telescope. Finally, the effect of broad-angle scattering, which includes a schematic representation of image deterioration through seeing, on the retrieved velocity and field strength was investigated.