SPATIAL AND TEMPORAL DISTRIBUTIONS OF TRANSIENT HORIZONTAL MAGNETIC FIELDS WITH DEEP EXPOSURE

We obtained a long-exposure vector magnetogram of the quiet Sun photosphere at the disk center with a wide field of view of 51'' x 82''. The observation was performed at Fe I 525.0 nm with the shutterless mode of the Narrow Band Filter Imager of the Solar Optical Telescope (SOT) on board the Hinode satellite. We summed the linear polarization (LP) maps taken with a time cadence of 60 s for 2 hr to obtain a map with as long of an exposure as possible. The polarization sensitivity would be more than 4.6 (21.2 in exposure time) times the standard observation with the SOT Spectropolarimeter. The LP map shows a cellular structure with a typical scale of 5''-10''. We find that the enhanced LP signals essentially consist of the isolated sporadic transient horizontal magnetic fields (THMFs) with a lifetime of 1-10 minutes and are not contributed by long-duration weak horizontal magnetic fields. The cellular structure coincides in position with the negative divergence of the horizontal flow field, i.e., mesogranular boundaries with downflows. Azimuth distribution appears to be random for the scale size of the mesogranules. Some pixels have two separate appearances of THMFs, and the measured time intervals are consistent with the random appearance. THMFs tend to appear at the mesogranular boundaries, but appear randomly in time. We discuss the origin of THMFs based on these observations.