On the removal of the sign ambiguity in the photospheric transverse magnetic field

We present a method for removing the ambiguity in the transverse component of the photospheric magnetic field measured by vector magnetographs. The method is based upon the use of the divergence-free condition satisfied by the magnetic field in the sense of least squares. The method requires the measurement of the longitudinal component of the magnetic field at two depth levels in the photosphere. The method is is shown to be efficient when compared to other existing methods on some particular analytical force-free magnetic fields.