  We present a novel technique that uses the autocorrelation of the spectrum of
a star to measure the line broadening caused by the modulus of its average
surface magnetic field. The advantage of the autocorrelation comes from the
fact that it can detect very small spectral line broadening effects because it
averages over many spectral lines and therefore gives an average with a very
high signal to noise ratio. We validate the technique with the spectra of known
magnetic stars and obtain autocorrelation curves that are in full agreement
with published magnetic curves obtained with Zeeman splitting. The
autocorrelation also gives less noisy curves so that it can be used to obtain
very accurate curves. We degrade the resolution of spectra of these magnetic
stars to lower spectral resolutions where the Zeeman splitting is undetectable.
At these resolutions, the autocorrelation still gives good quality curves,
thereby showing that it can be used to measure magnetic fields in spectra where
the Zeeman splitting is significantly smaller than the width of the spectral
line. This would therefore allow observing magnetic fields in very faint Ap
stars with low-resolution spectrographs, thereby greatly increasing the number
of known magnetic stars. It also demonstrates that the autocorrelation can
measure magnetic fields in rapidly rotating stars as well as weak magnetic
fields that give a Zeeman splitting smaller than the intrinsic width of the
spectral lines. Finally, it shows that the autocorrelation can be used to find
unknown magnetic stars in low resolution spectroscopic surveys.
