--- Circular arc approx.---
radius : 115 (pix), center angle : 121.334 (deg)
arc length : 243.532 (pix),
             146.119 (arcsec),     ; *0.60(arcsec/pix) aia_index.cdelt1 after aia_prep
             0.000708407 (rad),    ; /3600*!dtor    
          or 107678 (km)           ; aia_index.dsun_obs * 0.001
time lag : 00:31:12 = 1872 (sec)
speed of expansion : 57.5202 (km/s)

--- Speed of Chromospheric motion ---
sound speed : ~10 km/s,
chromospheric jets  : 5 - 20 (km/s)  by Nishizuka et al. (2011) ApJ 731, 43,
20 - 40 (km/s) in mottles (Tsiropoula & Tziotziou 2004), 
flows up to (40 km/s) related to an active region filament (Chae et al. 2000),
large-scale dynamical events like solar flares, sprays, and surges : > 100 km/s    according to Foukal's text book (2004)

Trying a little sophisticated mothod

--- Approx. by 2 circular arcs. and make a stack plot ---
[Arc-1] radius : 150 (pix), angle range :  90 to 145 (deg)
[Arc-2] radius : 150 (pix), angle range : 170 to 270 (deg)

sampling scale of the arcs : 1 (pix/deg)
slope of the fastest part of the stack plot : 
      87(=133-46) pix in hrizontal scale of the stack plot.
           ---> 87 deg.       ;  on 150 pix radius arc.
           ---> 228 pix       ;  2*!pi*150*(87/360) on the AIA image scale.
           ---> 137 arcsec    ;  288 * 0.60 (arcsec/pix, AIA pix-res)
           ---> 100958 km     ;  137 /3600*!dtor * aia_index.dsun_obs * 0.001 

      (111-132) pix in vertical direction  
           ---> time difference of the No.111 (16:36:44) and No.132 (16:49:22) images
           ---> 758 sec.

speed of expansion : 133 (km/s)