Small-scale explosive phenomena such as jets, spicules and nanoflares are frequently observed around an active region. They are suggested as energy release in the lower atmosphere by magnetic reconnection by analogy to the standard flare model, and thought to be highly related to chromospheric/coronal heating. To solve the heating mechanism, which is one of the most important problems in solar physics, it is crucial to identify the magnetic structure during the energy release.
We observed AR12259 on 2015 January 16th and 17th by Facility InfraRed Spectropolarimeter (FIRS) at National Solar Observatory of New Mexico. This observation was co-operated with Hinode and IRIS (HOP274), and we obtained spectropolarimetric data of 10814-10852 A with FIRS and around 6302 A with Hinode/SP. In the infrared window, there are plenty of lower photospheric to chromospheric lines including Si 10827 A and He 10830 A. By Milne-Eddington inversion and HAZEL codes, we derive three-dimensional magnetic field from these lower atmospheric lines around the sunspot. During the observation, we also obtained chromospheric images of Ca II K, H-alpha, Ca II 8542 with ground-based observation, and Ca II H with Hinode/SOT, and images and spectra of transition region with IRIS. In this presentation, we introduce our observational results and discuss on the relationship between three-dimensional connectivity of the magnetic field from the photosphere to the chromosphere and chromospheric brightening in the active region.