Properties and origin of transient horizontal magnetic fields

My talk is a comprehensive summary of the properties of the enigmatic transient horizontal magnetic fields in the photosphere. It is found that the internetwork regions are covered with ubiquitous granular-sized horizontal magnetic fields (e.g., Lites et al. 2008). Using the Solar Optical Telescope on board Hinode, we study the temporal evolution of these horizontal fields. These horizontal fields are highly transient with lifetime ranging from 1 to 10 minutes and the occurrence rate is quite high. Thus, the term of “transient horizontal fields (THMFs)” is used for them. We carry out the full Stokes inversion with the SIRGAUS code (Bellot Rubio 2003) for a THMF, and identify an omega-shaped flux tube with magnetic field strength of 400 G rising through the line forming layer of the Fe I 630.2 nm lines. These horizontal magnetic fields are observed in the quiet Sun, a weak plage region, and the polar regions (Ito et al. 2010). Occurrence rate and magnetic field strength distribution are the same in these different regions in spite of considerable difference in the amount of vertical magnetic flux. We present statistical properties of transient horizontal magnetic fields such as lifetime, size, and the location of their appearance and disappearance with respect to granular structure. I will summarize their magneto-hydrodynamic properties, and discuss their origin as inferred from these comprehensive observations.