  Using high-resolution transition region (TR) observations taken by the
Interface Region Imaging Spectrograph (IRIS) mission, Tian et al. (2014b)
revealed numerous short-lived subarcsecond bright dots (BDs) above sunspots
(mostly located in the penumbrae), which indicate yet unexplained small-scale
energy releases. Moreover, whether these subarcsecond TR brightenings have any
signature in the lower atmosphere and how they are formed are still not fully
resolved. This paper presents a multi-wavelength study of the TR penumbral BDs
using a coordinated observation of a near disk-center sunspot with IRIS and the
1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory. NST provides
high-resolution chromospheric and photospheric observations with narrow-band
H-alpha imaging spectroscopy and broad-band TiO images, respectively,
complementary to IRIS TR observations. A total of 2692 TR penumbral BDs are
identified from a 37-minute time series of IRIS 1400 A slitjaw images. Their
locations tend to be associated more with downflowing and darker fibrils in the
chromosphere, and weakly associated with bright penumbral features in the
photosphere. However, temporal evolution analyses of the BDs show that there is
no consistent and convincing brightening response in the chromosphere. These
results are compatible with a formation mechanism of the TR penumbral BDs by
falling plasma from coronal heights along more vertical and dense magnetic
loops. The BDs may also be produced by small-scale impulsive magnetic
reconnection taking place sufficiently high in the atmosphere that has no
energy release in the chromosphere.
