Small-scale magnetic fields of the Sun
Final Report Abstract
We investigated in this research project structures and dynamics of the Sun at small scales, down to 100 km. Especially solar magnetic fields influence very much the behaviour of the gas in the solar atmosphere. Their study yields knowledge about the physical processes on the Sun, about their impact on the ambient heliosphere and solar wind. We used the Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife to obtain the data for our studies. The VTT is equipped with an “Adaptive Optics System” to improve the sharpness of the images. Image reconstructions, leading to image sharpness close to the VTT's resolution limit, were carried out with so called speckle methods, which require short exposure times of _10 ms. In the course of our work, we have substantially improved a post-focus instrument which was developed by our group and which allows to observe spectroscopically two-dimensional areas of size 35 000km×35 000km on the Sun. It is based on Fabry-Perot etalons which are tuned rapidly along wavelength. The spectrometer can scan a Fraunhofer line of the solar spectrum within few seconds. At the same time, magnetic fields on the Sun can be measured with good accuracy by means of polarisers which can be switched rapidly. We present results and new questions about the physics of the solar atmosphere: 1. Our observations confirm the flux tube model for the fine structuring of the penumbra which surrounds the dark area - the umbra - of sunspots. The model says that magnetic tubes filled with hot gas rise from deep down. An explanation for the variation of brightness in the structures could be found. Depending on the viewing angle onto the outward flow in the tubes - the Evershed flow, the gas shows different line shifts from the Doppler effect. During outflow, the gas cools down and may become dark while still flowing out. 2. We could find magneto-acoustic waves along lengthy fibrils in high layers of the solar atmosphere. The fibrils connect sunspots and/or bright faculae of different magnetic polarity. One kind of waves travels with sound speed. The other kind, which is dominated by magnetic tension, travels faster, but still much too slowly to be identified as a magnetic Alfvén wave. 3. The high precision of our magnetic field measurements together with the possibility to observe time sequences with fast cadence, allowed to study the dynamics of weak magnetic fields in so-called quiet, non-active regions of the Sun. Also, we could compare such observations with numerical simulations. One sees the interaction of solar gas flows, i.e. convective flows, with magnetic fields, e.g. the enforcement of fields by shrinking of the cross section of a flux tube, the field weakening by expansion, and the emergence of fields in hot, up-flowing gas. 4. By means of spectral lines that split strongly in the presence of magnetic fields, it was shown that the small bright areas near the rotation poles of the Sun, so called polar faculae, harbour strong fields in the range of 1 000-1 500 Gauss (0.10-0.15 Tesla). Their magnetic polarity is the same as that of the global solar magnetic field. 5. Observations with high spatial resolution and fast cadence of 15 s allowed to see “short-period” acoustic waves in the solar atmosphere. They could transport acoustic energy from the “noise” in the photoshperic flows into the high atmosphere, the chromosphere, and heat the gas up in this region.
Publications
- On super-resolution in astronomical imaging. A&A 436, 373 (2005)
Puschmann, K. G., Kneer, F.
- On super-resolution in astronomical imaging. A&A 436, 373 (2005)
Puschmann, K.G., Kneer, F.
- Polarimetry of sunspot penumbrae with high spatial resolution. A&A 434, 317 (2005)
Bello González, N., Okunev, O. V., Domínguez Cerdeña, I., Kneer, F., Puschmann, K. G.
- Polarimetry of sunspot penumbrae with high spatial resolution. A&A 434, 317 (2005)
Bello González, N., Okunev, O.V., Domínguez Cerdeña, I., Kneer, F., Puschmann, K.G.
- Spectropolarimetry of a sunspot at disk centre. A&A 441, 1157 (2005)
Sánchez Cuberes, M., Puschmann, K. G., Wiehr, E.
- The structure of a penumbral connection between solar pores. A&A 442, 1079 (2005)
Hirzberger, J., Stangl, S., Gersin, K., Jurčàk, J., Puschmann, K. G., Sobotka, M.
- Speckle reconstruction of photometric data observed with adaptive optics. A&A 454, 1011 (2006)
Puschmann, K. G., Sailer, M.
- The flux-gap between bright and dark solar magnetic structures. A&A 445, 337 (2006)
Puschmann, K. G., Wiehr, E.
- The new Göttingen Fabry-Pérot spectrometer for two-dimensional observations of the Sun. A&A 451, 1151 (2006)
Puschmann, K. G., Kneer, F., Seelemann, T., Wittmann, A. D.
- The new Göttingen Fabry-Pérot spectrometer for two-dimensional observations of the Sun. A&A 451, 1151 (2006)
Puschmann, K. G., Kneer, F., Seelemann, T., Wittmann, A. D.
- On the properties of faculae at the poles of the Sun. A&A 474, 251 (2007)
Blanco Rodríguez, J., Okunev, O. V., Puschmann, K. G., Kneer, F., Sánchez-Andrade Nuño, B.
- Spicule emission profiles observed in He i 10 830°A. A&A 472, L51 (2007)
Sánchez-Andrade Nuño, B., Centeno, R., Puschmann, K. G., Trujillo Bueno, J., Blanco Rodríguez, J., Kneer, F.
- Fast events and waves in an active region of the Sun observed in H_ with high spatial resolution. A&A 486, 577 (2008)
Sánchez-Andrade Nuño, B., Bello González, N., Blanco Rodríguez, J., Kneer, F., Puschmann, K. G.
- Small-scale magnetic field dynamics on the Sun at high spatial and temporal resolution. A&A 490, L23 (2008)
Bello González, N., Okunev, O., Kneer, F.
- Acoustic waves in the solar atmosphere at high spatial resolution. A&A 508, 941 (2009)
Bello González, N., Flores Soriano, M., Kneer, F., Okunev, O.
- Dynamics of smallscale magnetic fields on the Sun: observations and numerical simulations. A&A 494, 1091 (2009)
Bello González, N., Yelles Chaouche, L., Okunev, O., Kneer, F.
