GREGOR acquires multidimensional data cubes

At GREGOR, the largest European solar telescope, another essential instrument is now operational: the slit scanner. Up to now, the slit of the spectropolarimeter GRIS (GRegor Infrared Spectrograph) could only stare at one position on the solar surface. Therefore, only spectra in one spatial dimension could be taken.

In a first scan with the new scanning unit, a sunspot was scanned in the wavelength regime of a neutral iron line at 1564.8 nm. The five subpanels from left to right: (P1) The intensity spectrum of the iron line at 1564.8 nm along the slit is displayed. (P2) The scanned intensity map, assembled from all slit positions. The black vertical line marks the positon of the slit for the spectrum in (P1). The spectrum clearly shows the splitting of the spectral line due to the magnetic field present in the spot. In the umbra of the sunspot the splitting is largest, i.e., there the magnetic field is strongest. The panels (P3), (P4), and (P5) display maps of the lineraly polarised light in two directions, and the circularly polarised light. From the three different polarization states, not only the strength, but also the inclination of the magnetic field vector can be determined for all points on the map. From the Dopplershift of the line, the small-scale velocity field in the spot can be inferred.

With the new scanning unit the slit can be moved precisely across the solar image, such that maps in two spatial dimensions can be acquired. For all points in a two dimensional map, GRIS can now acquire spectra of intensity, 2 orientations of linearly polarised light, and circularly polarised light.

All four physical quantities are a function of wavelength and two spatial dimensions. Such a multidimensional data set is sometimes called 'data cube'. Hence, GRIS acquires a set of 4 data cubes.

The spectropolarimetric data cube has a spatial resolution of some 0.4 arcsec, unprecedented at a wavelength of 1564.8 nm. Since the used line forms in the very deep photosphere, where the crucial magnetohydrodynamic processes presumably take place, these data are ideally suited to understand the nature of the sunspot fine structure.

 (Text and Image: Rolf Schlichenmaier and Dirk Soltau)