  The aim of this paper is to look at the magnetic helicity structure of an
emerging active region and show that both emergence and flaring signatures are
consistent with a same sign for magnetic helicity. We present a
multi-wavelength analysis of an M1.6 flare occurring in the active region NOAA
10365 on 27 May, 2003, in which a large new bipole emerges in a decaying active
region. The diverging flow pattern and the "tongue" shape of the magnetic field
in the photosphere with elongated polarities are highly suggestive of the
emergence of a twisted flux tube. The orientation of these tongues indicates
the emergence of a flux tube with a right hand twist, i.e. positive magnetic
helicity. The flare signatures in the chromosphere are ribbons observed in
H-alpha by the MSDP spectrograph in the Meudon solar tower and in 1600 A by
TRACE. These ribbons have a `J' shape and are shifted along the inversion line.
The pattern of these ribbons suggests that the flare was triggered by magnetic
reconnection at coronal heights below a twisted flux tube of positive helicity,
corresponding to that of the observed emergence. It is the first time that such
a consistency between the signatures of the emerging flux through the
photosphere and flare ribbons is clearly identified in observations. Another
type of ribbons observed during the flare at the periphery of the active region
by the MSDP and SOHO/EIT are related to the existence of a null point, which is
found high in the corona in a potential field extrapolation. We discuss the
interpretation of these secondary brightenings in terms of the "breakout" model
and in terms of plasma compression/heating within large-scale separatrices.
