Structure and Characteristics of the Solar Transition Region

The solar transition region,in which above the photosphere the temperature increases rapidly and the density drops dramatically,is a rather inhomogeneous and dynamic layer between the chromosphere and corona.The transition region is believed to play an important role in the origin of the solar wind and in coronal heating.The most prominent feature in the transition region is the network structure,which is the upward extension of the supergranular boundary above the photosphere and manifests itself as bright lanes on the radiance images of FUV/EUV lines.It is believed that the network fluxes consist of two components:the magnetic funnel,either open into the corona and guiding the solar wind or being simply the leg of a large coronal loop, and low-lying flux ropes in the rest of the network.Most of the FUV/EUV emission lines formed in the middle transition region show a predominant red shift,which can be caused by the return of spicule materials,siphon flows driven by asymmetric heating through loops,nano-flares and the resulting downward propagating waves,or downward flows after magnetic reconnection between open field lines and closed loops at the upper transition region in the network.While for lines formed in the upper transition region and lower corona,a net blue shift is found,which is believed to be associated with the nascent solar wind outflow.Lots of transient phenomena are found in the transition region,including blinkers,explosive events,micro/nano flares,coronal bright points, and spicules.Blinkers are transient brightenings with a life time of 3-110 minutes and caused by a process which can lead to the increase of density or filling factor of the transition region. Explosive events correspond to outflows(at the order of 100 km/s) in magnetic reconnection. Micro/nano flares are believed to play a very important role in heating the corona.Coronal bright points are local heating phenomena occurring in the transition region and lower corona, the related powering could be due to magnetic cancelation,separator reconnection,or current sheet induced by photospheric horizontal motions.Spicules are rapidly evolving jet-like structures extending from the chromosphere upward into the corona and,according to the new observational evidence of Hinode and TRACE,may be driven by both wave and magnetic reconnection in the chromosphere.