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tokens: to study the variation of LOS plasma velocity during the formation of the penumbra, we also analyzed the SHARPs Dopplergrams, which are taken in the Fe i 617.3 nm with a resolution of 1′′. The velocities inferred by the Dopplergrams were calibrated by assuming that the plasma flow in the sunspot umbrae is at rest. We set the umbra region at rest in order to avoid the effect of plasma convection blueshift in the quiet Sun region (Dravins et al. 1981 ). We also used the images of the He ii 30.4 nm line taken by the SDO /Atmospheric Imaging Assembly (AIA) to investigate the upper atmospheric layers above the developing sunspot. The He ii 30.4 nm line is one of the strongest lines in the AIA observed in the solar spectrum, providing information on the upper chromosphere or lower transition region (Andretta et al. 2012 ; Lemen et al. 2012 ). The NST/TiO and SDO /HMI observations were coaligned by using the first image of TiO taken by the NST at 16:38 UT on 2016 August 11 as a reference and the continuum intensity images at 16:36 UT taken by SDO /HMI. We used the IDL Solarsoft mapping routines to solve the problem of the differential rotation. In addition, we traced the photospheric horizontal motions of the plasma flow by using the Fourier local correlation tracking (FLCT) method in the TiO images (Welsch et al. 2004 ; Fisher Welsch 2008 ). 3. Results Figure 1 shows the surroundings of the interesting sunspot in the continuum intensity images and LOS magnetograms taken by SDO /HMI. The black boxes in Figure 1 outline the region of the interesting sunspot. The interesting sunspot was a developing sunspot. The developing sunspot had two umbrae with some preexisting penumbral filaments. It was formed as a result of the coalescence of sunspots “S1” and “S2” (see Figure 1 (a1)). The newly formed penumbra sector was located at the northwest side of the developing sunspot, as shown by the red boxes in Figures 1 (a2) and (b2). The developing sunspot changed its initial shape in the process of new penumbra sector formation, as shown in Figures 1 (a1)–(a3). With the growth of the developing sunspot, sunspots S1 and S2 gradually combined. The developing sunspot was characterized by negative polarity. Some thin filaments around the developing sunspot stretched to the region that was characterized by positive polarity (see Figures 1 (a1), (b1), (a2), and (b2)). Figure 1. Evolution of active region NOAA 12574. (a1–a2): continuum intensity images observed by SDO /HMI. (b1–b2): corresponding LOS magnetograms. The white contours indicate the boundary of the umbra as seen in the continuum intensity images. The developing sunspot is marked by the black boxes. The two sunspots in the black box of Figure 1 (a1) are marked “S1” and “S2”. The black boxes also indicate the field of view of Figures 2 – 5 , 7 , and 8 . Each red box in Figures 1 (a2) and (b2) indicates the position of the newly formed penumbra sector. Figure 2 shows the evolution of the developing