  Aims. Convective collapse, a theoretically predicted process that intensifies
existing weak magnetic fields in the solar atmosphere, was first directly
observed in a single event by Nagata et al. (2008) using the high resolution
Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space
telescope, we performed a statistical analysis of convective collapse events.
  Methods. Our data sets consist of high resolution time series of polarimetric
spectral scans of two iron lines formed in the lower photosphere and filter
images in Mg I b2 and Ca II H, spectral lines that are formed in the high
photosphere and the lower chromosphere, respectively. We were thus able to
study the implication of convective collapse events on the high photospheric
and the chromospheric layers. The physical parameters from the full Stokes
profiles were obtained with the MERLIN Milne-Eddington inversion code.
  Results. We found that in all cases, the event was accompanied by a continuum
bright point and nearly always by a brightening in the Ca II H images. The
magnesium dopplergram exhibits a strong downflow in about three quarters of the
events that took place within the field of view of the magnesium dopplergram.
For each of the 49 events we determined the duration, maximum photospheric
downflow, field strength increase and size. We found event durations of about
10 minutes, magnetic element radii of about 0.43 arcsec and 0.35 arcsec, before
and after the event, respectively, and field strengths of up to 1.65 kG.

Solar and Stellar Astrophysics