Main Objectives:
We wish to explore the height dependence of the mangnetic field vector in the deep photosphere of umbral dots, and to search for any chromospheric response to the umbral dot phenomenon.
For 2018, if there is no sunpot available, a different quiet sun program will run. The science objective for the quiet Sun disk center observations is to study the height variation of the B_perp/B_parallel ratio. Numerical simulations (see Fig.3 in Schuessler & Voegler 2008 and Fig.1 in Steiner et al. 2008) predict that this ratio increases towards higher layers, therefore one would expect it to be smaller in GREGOR's near-IR spectral lines than in Hinode's visible lines. This was attempted several times in the past, however all attempts were unsucessful because GREGOR's AO could not lock on the quiet Sun. The situation has however improved in the past years (new M2-M3 mirrors, new AO set-up, etc), so with some luck we could make it work.
Scientific Justification:
In order to better understand their convective origin and the role that umbral dots play in the subsurface structure of the sunspot magnetic field and the overall energy balance of umbrae, it is important to have a firm observational description of the magnetic field strength and geometry within and surrounding them. Hinode has been used to explore the structure of umbral dots (Sobotka and Jurcak 2010), with the conclusion that little reduction in the field strength and no change in inclination is seen in measurements using the 630 nm lines of Fe I. This observational result in the 630 nm lines from Hinode was foreseen by Degenhardt and Lites (1993), who constructed a theoretical model of a field-free inclusion (and umbral "flux tube") within a surrounding strong vertical field. Degenhardt and Lites synthesized not only the 630 nm Fe I lines, but also the 1565 nm Fe I lines. Unlike the visible lines, the synthesized near-infrared 1565 nm lines showed considerable reduction in field strength because they form at somewhat lower layers than their visible counterparts. In view of these theoretical results, we propose to make simultaneous observations of umbral dots in the Fe I lines at 630 nm and 1565 nm, the latter being observed from the ground with the Grating Infrared Spectrograph (GRIS) at the new 1.5 m GREGOR telescope at Tenerife. The aim is to explore the observed variation of the umbral dot magnetic field vector when observed at the heights where these two diagnostic lines form. It should be noted that field diagnostics are very robust because of the large magnetic splitting present in umbrae, and the GREGOR/GRIS combination has proven observational capability for ~0.4 arcsecond resolution at 1.56 microns.
Little is known about any chromospheric manifestation of the umbral dot
phenomenon. At chromospheric heights umbrae usually exhibit oscillations with a period around 3 min ("umbral flashes"), but there is little exploration of any specific association of chromospheric dynamics with umbral dots in the photosphere. There is a motivation for such observations from theory: a study by Choudhuri (1986) indicated that the magnetic field above a field-free inclusion (umbral dot) acts as a valve that can be forced open by the piling up of the upflowing plasma inside the low field region. He predicted the formation of plasma jets above the dot's apex whenever the pressure of the field-free plasma inside the umbral dot exceeds a certain threshold. Recognizing that there has been little exploration of chromospheric dynamics in association with umbral dots, we propose to use the opportunity of the coordinated high-resolution Hinode and GREGOR/GRIS observations to encompass IRIS spectroscopy as well. |
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