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tokens: the text. A close examination of Figure 1 discloses that all UVBs evolve with time and most UVBs last for several minutes. In particular, we note that UVB 8, when viewed in Figure 1 (e), appears to be one larger UVB, while when it is viewed in Figure 1 (d) 68 s earlier, it appears to be two closely related but seemingly separate UVBs. UVB 7 appear to last for the shortest time: slightly more than one minute. Figure 1. Panels (a)–(f): time series of IRIS SJI 1400 A logarithmic intensity (DN/s). All panels are normalized between 0.1 and 1000 DN/s, with black being the lowest intensity and white being the highest intensity. The preceding/next three slit positions for each plot are marked with dotted lines. The contours mark the identified UVBs where the intensity is a factor of 24 above the image median. We enumerate the UVBs that have been measured by the slit. Each UVB’s number is mark on top of itself. Table 1 UVBs with Spectrogram (SG) Measurements Notes. If more than one UVBs are observed at a time on the slit, we enumerate them from the top to the bottom along the slit. a We refer to Figure 1 SJI image panels (a)–(f) to denote slit position in this table. If there is no corresponding SJI image at a SG data time, the preceding/next SJI image is referred to. The slit position is subsequently noted as the ±(1–3) slit position of that SJI image. For example, “a+2” means that the corresponding slit position is along the second dotted line to the right of the slit position in Figure 1 (a), and “b−2” means that the corresponding slit position is along the second dotted line to the left of the slit in Figure 1 (b). Note that “a+2” and “b−2” mark the same slit position, likewise “b+1” and “c−3” also mark the same slit position. b Each UVB’s position along Solar-Y is taken from the center (highest intensity) of the UVB as measured by the slit. Figure 2 is an overview of UVB 1's optically thin Si iv 1393.78/1402.77 A lines. Note that the Si iv 1394 A/1402 A ratio can be used to test whether the ion is formed under optically thin or thick conditions. A ratio close to 2, which is the case for the UVBs studied in this paper, is compatible with the lines being optically thin (e.g., Peter et al. 2014 ; Yan et al. 2015 ). However, optically thick lines can still have a ratio of about 2, so one cannot use the line ratio to conclusively state that the lines are optically thin. Panels (a) and (b) show that the UVB has remarkably intensive brightening and broadening of the Si iv lines, being orders of magnitude brighter in spectral radiance and significantly wider in wavelengths than the background Si iv emissions. To form the Si iv line, one needs a temperature of at least ∼80,000 K (Peter et al. 2014 ) or higher. The strong brightening of this line indicate strong heating of the UVB region plasmas to ∼10 5 K. Note that there is a significant absorption line (Ni ii 1393.33 A) on the left wing of the 1393.78 A line, as seen before (e.g., Young et al. 2018 ).