Full-disk observations of the Sun to investigate flow structures in the chromosphere
Final Report Abstract
The chromosphere is a thin dynamic layer in the solar atmosphere, located between the surface and the corona. It is not visible to the naked eye, except for a few seconds during a total eclipse. The spectacular and energetic processes occurring here can be visualized using narrow-band optical filters. For better insights into the as yet poorly understood physical mechanisms causing these chromospheric events, continuous high-quality observations are required. Some of these phenomena are very short-lived and highly unpredictable. The Chromospheric Telescope ChroTel observing the full disk of the Sun, allows nearsimultaneous observations in the three major chromospheric lines observable from the ground, Ca II K (393 nm), Hα (656 nm) and He I (1083 nm). This is achieved by recording narrow-band images at different wavelengths, employing a short cadence of about a minute. In the He channel filtergrams at seven wavelength positions across the line are taken to derive Doppler maps of the chromosphere over the full disk. The combination of this imaging capability with coordinated spectroscopic observations using the nearby high-resolution spectrographic facilities at VTT and Gregor is unique and allows a reliable calibration of the Doppler shifts in the He channel. ChroTel is a refractor with an effective aperture of 100mm and a focal length of 2250mm. All channels use narrow-band Lyot filtes, in the He channel the filter is tunable. The Sun is imaged onto a CCD with 2048x2048 pixels, with a pixel scale of about 1 arcsec. and a spatial resolution of down to just above 2 arcsec. Its mechanical design is a robotically operated domeless fully encapsulated turret system. The domeless construction was one major reason for the use of a turret. All drives, optics, and light paths are located inside the protective housing but accessible for maintenance. A motor-driven shutter that will open only during observations protects the entrance window. The possibility for remote operations minimizes the required on-site-maintenance During this project ChroTel was finalized to become fully operational and first science data were taken and analyzed. The main science result within this project was related to a flow in the penumbra of sunspot. The observed flows match theoretical predictions of chromospheric and coronal siphon flows, with accelerating upflowing plasma at one footpoint with low field strength and decelerating downflowing plasma at the other end. A tube shock at the inner end is probable, but the evidence is not conclusive. Accompanying extreme UV imaging data suggest that the structure forms because of a reorganization of the magnetic field after a reconnection event, and that another later reconnection event is disrupting the flow structure. This emphasizes the role of reconnection in the formation of penubmral (siphon) flows.
Publications
- (2008): SPIE 7014, 701413. ChroTel – a robotic telescope to observe the chromosphere of the Sun
Kentischer T.J., Bethge C., Elmore D., et al.
- (2008): SPIE 7019, 70192T. The control and data concept for the robotic solar telescope ChroTel
Halbgewachs C., Bethge C., Caligari P., et al.
- (2011): Astron. Astrophys. 534, A105. The Chromospheric Telescope
Bethge C., Peter H., Kentischer T.J., et al.
- (2012): Astron. Astrophys. 537, A130. Siphon flow in a cool magnetic loop
Bethge C., Beck C.B., Peter H., Lagg A.
