3.1 Statistics of detachment rate
The ‘detachment of arc per hour’ is defined as the ratio between total number of detachment events and total hours of data. Figure 3 shows the yearly (2006-2020) rate of detachment for red, red+green arcs, and STEVE. The yearly averaged variation of solar wind speed (Vx), Ap index, and F10.7 is superposed to study the association of arc detachment with the solar and geomagnetic activity. In general, the detachment rate of red+green arcs exhibits dependence on geomagnetic activity with higher detachment rate when Ap and solar wind speed were high and no occurrence for low Ap and solar wind speed years (2009-2011 and 2020). However, the detachment rate of both red and red+green arcs was low during 2016-2017 when the Ap index was the highest. Comparing the two solar minimum, the detachment rate of red arc was highest during 2018-2019 when both the solar wind and Ap index was at the higher levels as compared to 2008-2009. In comparison to red+green arcs, the detachment rate of red arcs was higher even for the lowest Ap years (2009-2010). These results suggest that the detachment rate of red and red+green arc exhibits dependence on both the geomagnetic activity (Ap and solar wind) and solar flux. The detachment of STEVE was maximum during the solar minimum year 2019, and then for 2010 and 2018. The high detachment rate of STEVE coincided with the high detachment rate of red arcs.
Figure 4 shows the detachment rate of red, red+green arcs, and STEVE as a function of MLT. The MLT distribution of detachment rate was calculated for each arc-type separately by dividing the total events observed in a particular 2-hour time slot to the total number of arc events. In general, the detachment rate of all arcs exhibits asymmetric distribution with highest rate in the premidnight sector. For STEVE, detachment rate is highest at 22-24 MLT, and then at 20-22 MLT, before midnight. The detachment rate of red and red+green arcs maximized around midnight, that is at 22-24 and 00-02 MLT.