The O9IV star HD 57682 , discovered to be magnetic within the context of the MiMeS survey in 2009 , is one of only eight convincingly detected magnetic O-type stars .
Among this select group , it stands out due to its sharp-lined photospheric spectrum .
Since its discovery , the MiMeS Collaboration has continued to obtain spectroscopic and magnetic observations in order to refine our knowledge of its magnetic field strength and geometry , rotational period , and spectral properties and variability .
In this paper we report new ESPaDOnS spectropolarimetric observations of HD 57682 , which are combined with previously published ESPaDOnS data and archival H \alpha spectroscopy .
This dataset is used to determine the rotational period ( 63.5708 \pm 0.0057 d ) , refine the longitudinal magnetic field variation and magnetic geometry ( dipole surface field strength of 880 \pm 50 G and magnetic obliquity of 79 \pm 4 \degr as measured from the magnetic longitudinal field variations , assuming an inclination of 60 \degr ) , and examine the phase variation of various lines .
In particular , we demonstrate that the H \alpha equivalent width undergoes a double-wave variation during a single rotation of the star , consistent with the derived magnetic geometry .
We group the variable lines into two classes : those that , like H \alpha , exhibit non-sinusoidal variability , often with multiple maxima during the rotation cycle , and those that vary essentially sinusoidally .
Based on our modelling of the H \alpha emission , we show that the variability is consistent with emission being generated from an optically thick , flattened distribution of magnetically-confined plasma that is roughly distributed about the magnetic equator .
Finally , we discuss our findings in the magnetospheric framework proposed in our earlier study .