We investigate a small-scale (≈ 1.5 Mm along the slit), supersonic
downflow of about 90 km s-1 in the transition region above the
light-bridged sunspot umbra in AR 11836. The observations were obtained with
the Interface Region Spectrograph (IRIS) on 2013 September 2, from 16:40 to
17:59 UT. The downflow shows up as red-shifted "satellite" lines of the Si IV
and O IV transition region lines and is remarkably steady over the observing
period of nearly 80 min. The downflow is not visible in the chromospheric
lines, which only show an intensity enhancement at the location of the
downflow. The density inferred from the line ratio of the red-shifted
satellites of the O IV lines (Ne = 1010.6±0.25
cm-3) is only a factor 2 smaller than the one inferred from the
main components (Ne = 1010.95±0.20 cm-3).
Consequently, this implies a substantial mass flux (≈ 5 x 10-7
g cm-2 s-1), which would evacuate the overlying corona on time scales
of the order of 10 s. We interpret these findings as evidence of a stationary
termination shock of a supersonic siphon flow in a cool loop rooted in the
central umbra of the spot.
Authors: Thomas Straus and Bernhard Fleck and Vincenzo Andretta
Projects: IRIS
Publication Status: Accepted for A&A
Last Modified: 2015-07-23 10:11