摘要：Context. A flux emergence in the active region AR 111850 was observed on
September 24, 2013 with the Interface Region Imaging Spectrograph (IRIS). Many bright
points are associated with the new emerging flux and show enhancement brightening in the
UV spectra.
Aims. The aim of this work is to compute the altitude formation of the
compact bright points (CBs) observed in Mg II lines in the context of searching Ellerman
bombs (EBs).
Methods. IRIS provided two large dense rasters of spectra in Mg II h and
k lines, Mg II triplet, C II and Si IV lines covering all the active region and slit jaws
in the two bandpasses (1400 Å and 2796 Å) starting at 11:44 UT and 15:39 UT, and lasting
20 min each. Synthetic profiles of Mg II and Hα lines are computed with non-local thermodynamic
equlibrium (NLTE) radiative transfer treatment in 1D solar atmosphere model including a
hotspot region defined by three parameters: temperature, altitude, and width.
Results. Within the two IRIS rasters, 74 CBs are detected in the far
wings of the Mg II lines (at +/−1 Å and 3.5 Å). Around 10% of CBs have a signature in Si IV and CII.
NLTE models with a hotspot located in the low atmosphere were found to fit a sample of Mg
II profiles in CBs. The Hα profiles computed with these Mg II CB models are
consistent with typical EB profiles observed from ground based telescopes e.g. THEMIS. A
2D NLTE modelling of fibrils (canopy) demonstrates that the Mg II line centres can be
significantly affected but not the peaks and the wings of Mg II lines.
Conclusions. We conclude that the bright points observed in Mg II lines
can be formed in an extended domain of altitudes in the photosphere and/or the
chromosphere (400 to 750 km). Our results are consistent with the theory of heating by
Joule dissipation in the atmosphere produced by magnetic field reconnection during flux
emergence.
