A Steady-state Supersonic Downflow in the Transition Region above a Sunspot Umbra

Abstract

We investigate a small-scale ($\approx$ 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 ($N_{\mathrm{e}}=10^{10.6\pm 0.25}{\mathrm{cm}}^{-3}$) is only a factor 2 smaller than the one inferred from the main components ($N_{\mathrm{e}}=10^{10.95\pm 0.20}{\mathrm{cm}}^{-3}$). Consequently, this implies a substantial mass flux ($\approx 5\times 10^{-7}$ 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.

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Cited by in corpus (16)

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• Long-Period Intensity Pulsations in Coronal Loops Explained by Thermal Non-Equilibrium Cycles
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• A Hot Downflowing Model Atmosphere For Umbral Flashes And The Physical Properties Of Their Dark Fibrils
• A Si IV/O IV electron density diagnostic for the analysis of IRIS solar spectra
• Coronal Condensation as the Source of Transition Region Supersonic Downflows above a Sunspot
• Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes
• Umbral chromospheric fine structure and umbral flashes modelled as one: the corrugated umbra
• Statistical Investigation of Supersonic Downflows in the Transition Region above Sunspots
• Magnetic filed dynamics and varying plasma emission in large coronal loops
• Evolution Of Super-Sonic Downflows In A Sunspot
• ALMA Discovery of Solar Umbral Brightness Enhancement at λ=3 mm
• Magnetized supersonic downflows in the chromosphere. A statistical study using the He I 10830 Å lines
• Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales
• Sustained heating of the chromosphere and transition region over a sunspot light bridge
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)