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tokens: of the sigmoid (Figure 6 ). Unfortunately, restricted by the projection effect, we cannot determine the real height of the erupting structure and its evolution with time, but can only derive its projection distance from the source region as shown in Figure 6 (b) and the velocity component in the plane of sky in Figure 6 (a). Assuming that the eruption is along the radial direction, the linear velocity is corrected to be around 60 km s −1 . Figure 5. Time-slice plots of HMI line-of-sight magnetograms showing the emergence phase (a) and cancellation phase (c) of the bipolar region. The directions used for making the time-slice plots are indicated by the blue lines in the two magnetograms at 19:35 UT on 2017 January 28 (b) and 06:36 UT on 2017 January 30 (d). The slopes of the oblique lines represent the moving velocities of the positive and negative polarities. Figure 6. (a) Time-slice plot of AIA 211 Å images showing the eruption process. (b) The slice is taken along the direction to the northwest (the line in blue) as indicated in the AIA 211 Å image at 14:05 UT on 2017 January 30. The slope of the oblique line in panel (a) represents the moving velocity of the erupting structure along the plane of sky. 2.3. Thermal Property of Mini-sigmoid The observed flux of passband i can be written as , where the R i ( T ) is the temperature response function and DEM( T ) is the plasma DEM in the corona. Knowing the observed flux F i and response function R i ( T ), we resolve the DEMs of each pixel in the mini-sigmoid source region following the method in Cheng et al. ( 2012 ), using the “xrt_dem_iterative2.pro” routine in SSW package. The emission measure maps at the four temperatures are shown in Figure 7 . One can see that the temperature of the sigmoidal structure is around 2.0 × 10 6 K (Figures 7 (b4) and (c4)). It is interesting that, before the appearance of the sigmoid, two J-shaped structures that constitute an apparent sigmoid have a higher temperature of 2.5 × 10 6 K (Figure 7 (d3)). Such a difference in temperature between the double J-shaped loops and the S-shaped structure is in agreement with the expectation that the sigmoidal field lines, are formed by the reconnection between two sheared arcades near the PIL. With the J-shaped arcades being closer to the reconnection site, they tend to be heated up to a higher temperature. Shortly afterwards, these J-shaped arcades transform into the sigmoidal structures, which then cool down probably due to the expansion. This is basically consistent with the observations by Tripathi et al. ( 2009 ). Figure 7. Temporal evolution of the EM distributions of the sigmoid source region at the four temperatures intervals ranging from 80 to 250 MK. 3. Nonlinear Force-free Field Structures of Mini-sigmoid 3.1. Flux-rope Insertion Method Through the flux-rope insertion method (van Ballegooijen 2004 ; Bobra et al. 2008 ; Su et al. 2009 ), we further reconstruct the nonlinear force-free field structures of