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We report multi-wavelength ultraviolet observations taken with the IRISsatellite, concerning the emergence phase in the upper chromosphere andtransition region of an emerging flux region (EFR) embedded in the pre-existingfield of active region NOAA 12529. IRIS data are complemented by full-diskobservations of the Solar Dynamics Observatory satellite, relevant to thephotosphere and the corona. The photospheric configuration of the EFR is alsoanalyzed by measurements taken with the spectropolarimeter aboard the Hinodesatellite, when the EFR was fully developed. Recurrent intense brighteningsthat resemble UV bursts, with counterparts in all coronal passbands, areidentified at the edges of the EFR. Jet activity is also observed atchromospheric and coronal levels, near the observed brightenings. The analysisof the IRIS line profiles reveals the heating of dense plasma in the low solaratmosphere and the driving of bi-directional high-velocity flows with speed upto 100 km/s at the same locations. Compared with previous observations andnumerical models, these signatures suggest evidence of several long-lasting,small-scale magnetic reconnection episodes between the emerging bipole and theambient field. This process leads to the cancellation of a pre-existingphotospheric flux concentration and appears to occur higher in the atmospherethan usually found in UV bursts, explaining the observed coronal counterparts.

IRIS observations of magnetic interactions in the solar atmosphere between pre-existing and emerging magnetic fields. I. Overall evolution