tokens: 12 . Panel (a) shows that current sheet disruption takes place earlier with a lower growth rate in Run H5, where the initial noise level is three orders of magnitude higher than that of Run S5. Because current sheet disruption marks the onset of fast reconnection, the onset also occurs earlier in Run H5. However, as can be seen from panel (b), the reconnection rates after nonlinear saturation are essentially the same for the two runs. Therefore, we can conclude that reconnection rate after saturation is nearly unaffected by the noise level and the condition at the onset. This conclusion is also in line with the earlier study by Huang Bhattacharjee ( 2010 ), where the dependence of the averaged reconnection rate on the external forcing amplitude is found to be fairly weak. Figure 12. Comparison of time histories of (a) the linear growth rate and (b) the reconnection rate for Run S5 (with an initial noise level ) and Run H5 (). The initial noise level affects the disruption time and the linear growth rate at disruption, but the reconnection rate after nonlinear saturation remains approximately the same. From an observational point of view, the period between current sheet disruption and nonlinear saturation is especially interesting. During this period, the outflow profile changes from a nearly monotonic Sweet–Parker-like profile to a highly fluctuating profile, as shown in Figure 13 (a). Accordingly, the histogram of the outflow profile changes from double-peak-shaped to triangle-shaped, as shown in Figure 13 (b). A triangle-shaped histogram is a natural consequence of the outflow profile being highly fluctuating. To understand this, we consider an outflow profile of the form , where and the first term mimics the Sweet–Parker-like outflow profile, and the second term mimics short-wavelength fluctuations. If the fluctuation is much smaller than the background, i.e., , the histogram is double-peak-shaped. Conversely, if the fluctuation is on the order of the background, i.e., , which is the case in the saturated phase, then the histogram is triangle-shaped. The transition from a double-peak-shaped histogram to triangle-shaped histogram can lead to observable effects through spectroscopy of emission spectral line profiles, because the observed spectral line profile is a convolution of the Doppler-shifted line profile along the line-of-sight direction. For reconnection events with outflows along the line-of-sight direction, the histogram of the outflow profile can be viewed as a proxy for the shape of the line profile. A triangle-shaped line profile has been reported in IRIS (Interface Region Imaging Spectrograph (De Pontieu et al. 2014 )) observations of the Si iv line during transition region explosive events (Innes et al. 2015 ). With the high-cadence mode of the IRIS spectrograph, it is possible to observe transitions from a double-peak-shaped line profile to a triangle-shaped line profile in transition region explosive events (L.-J. Guo
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