  In solar filament formation mechanisms, magnetic reconnection between two
sets of sheared arcades forms helical structures of the filament with numerous
magnetic dips, and cooling and condensation of plasma trapped inside the
helical structures supply mass to the filament. Although each of these
processes, namely, magnetic reconnection and coronal condensation have been
separately reported, observations that show the whole process of filament
formation are rare. In this Letter, we present the formation of a sigmoid via
reconnection between two sets of coronal loops, and the subsequent formation of
a filament through cooling and condensation of plasma inside the newly formed
sigmoid. On 2014 August 27, a set of loops in the active region 12151
reconnected with another set of loops that are located to the east. A longer
twisted sigmoidal structure and a set of shorter lower-lying loops then formed.
The observations coincide well with the tether-cutting model. The newly formed
sigmoid remains stable and does not erupt as a coronal mass ejection. From the
eastern endpoint, signatures of injection of material into the sigmoid (as
brightenings) are detected, which closely outline the features of increasing
emission measure at these locations. This may indicate the chromospheric
evaporation caused by reconnection, supplying heated plasma into the sigmoid.
In the sigmoid, thermal instability occurs, and rapid cooling and condensation
of plasma take place, forming a filament. The condensations then flow
bi-directionally to the filament endpoints. Our results provide a clear
observational evidence of the filament formation via magnetic reconnection and
coronal condensation.
