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.