Context :

Aims : In this paper we confirm the presence of a globally-ordered , kG-strength magnetic field in the photosphere of the young O star \theta ^ { 1 } Orionis C , and examine the properties of its optical line profile variations .

Methods : A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the rotational cycle of \theta ^ { 1 } Orionis C. Using the Least-Squares Deconvolution ( LSD ) multiline technique , we have succeeded in detecting variable Stokes V Zeeman signatures associated with the LSD mean line profile .
These signatures have been modeled to determine the magnetic field geometry .
We have furthermore examined the profile variations of lines formed in both the wind and photosphere using dynamic spectra .

Results : Based on spectrum synthesis fitting of the LSD profiles , we determine that the polar strength of the magnetic dipole component is 1150 \la B _ { d } \la 1800 G and that the magnetic obliquity is 27 \degr \la \beta \la 68 \degr , assuming i = 45 \pm 20 \degr .
The best-fit values for i = 45 \degr are B _ { d } = 1300 \pm 150 ( 1 \sigma ) G and \beta = 50 \degr \pm 6 \degr ( 1 \sigma ) .
Our data confirm the previous detection of a magnetic field in this star , and furthermore demonstrate the sinusoidal variability of the longitudinal field and accurately determine the phases and intensities of the magnetic extrema .
The analysis of “ photospheric ” and “ wind ” line profile variations supports previous reports of the optical spectroscopic characteristics , and provides evidence for infall of material within the magnetic equatorial plane .

Conclusions :