Spicule emission profiles observed in \ion{He}{i} 10830 \AA

Off-the-limb observations with high spatial and spectral resolution will help us understand the physical properties of spicules in the solar chromosphere Spectropolarimetric observations of spicules in the \ion{He}{i} 10830 \AA\ multiplet were obtained with the Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope at the Observatorio del Teide (Tenerife, Spain). The analysis shows the variation of the off-limb emission profiles as a function of the distance to the visible solar limb. The ratio between the intensities of the blue and the red components of this triplet $({\cal R}=I_{\rm blue}/I_{\rm red})$ is an observational signature of the optical thickness along the light path, which is related to the intensity of the coronal irradiation. We present observations of the intensity profiles of spicules above a quiet Sun region. The observable ${\cal R}$ as a function of the distance to the visible limb is also given. We have compared our observational results to the intensity ratio obtained from detailed radiative transfer calculations in semi-empirical models of the solar atmosphere assuming spherical geometry. The agreement is purely qualitative. We argue that future models of the solar chromosphere and transition region should account for the observational constraints presented here.Comment: letter accepted in A&A. 5 figure

Estimation of solar prominence magnetic fields based on the reconstructed 3D trajectories of prominence knots

We present an estimation of the lower limits of local magnetic fields in quiescent, activated, and active (surges) promineces, based on reconstructed 3-dimensional (3D) trajectories of individual prominence knots. The 3D trajectories, velocities, tangential and centripetal accelerations of the knots were reconstructed using observational data collected with a single ground-based telescope equipped with a Multi-channel Subtractive Double Pass imaging spectrograph. Lower limits of magnetic fields channeling observed plasma flows were estimated under assumption of the equipartition principle. Assuming approximate electron densities of the plasma n_e = 5*10^{11} cm^{-3} in surges and n_e = 5*10^{10} cm^{-3} in quiescent/activated prominences, we found that the magnetic fields channeling two observed surges range from 16 to 40 Gauss, while in quiescent and activated prominences they were less than 10 Gauss. Our results are consistent with previous detections of weak local magnetic fields in the solar prominences.Comment: 14 pages, 12 figures, 1 tabl

On the properties of faculae at the poles of the Sun

Context.Faculae at the poles of the Sun, or polar faculae (PFe), take part in the solar magnetic cycle. Their occurrence maximum is shifted by 5–6 years with respect to the sunspot cycle. PFe are stable phenomena, with lifetimes of several hours to days, and harbour magnetic fields of kilo-Gauss strength. Yet their role for the global magnetic field at the solar poles is unknown. Aims.To contribute to the knowledge of the physical properties of PFe and to the understanding of their role in the global magnetism of the Sun. Methods.PFe were observed on 21–24 August 2005 with the Vacuum Tower Telescope at the Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer was used with a Stokes V polarimeter to scan the Fe 

Fast events and waves in an active region of the Sun observed in H

Context. We study the chromosphere of an active region of the Sun in the Hα line. Aims. The development of new instrumentation and new methods of data analysis allows to scrutinize the dynamics of the solar chromosphere with high spatial, spectral, and temporal resolution. The observations we present shed light on some magneto-dynamic processes occurring above an active region in the chromosphere. Methods. We took a time series of 55 min in Hα from AR 10875 at \vartheta\approx36\degr. We used the “Göttingen” Fabry-Perot spectrometer at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, to obtain two-dimensional spectrograms in Hα. Adaptive optics and image reconstruction yielded a spatial resolution better than 0\farcs5 throughout the time sequence. From the wealth of structures, we selected areas of interest to study further, in detail, some ongoing processes. Results. A small straight surge developed aside of a pore with upward phase speed of 100 km s-1 and line-of-sight (LOS) velocity of 15 km s-1. The surge retreated rapidly with LOS velocity of 45 km s-1 at its mouth. It underwent a rebound and fell back again. Two sympathetic mini-flares were observed that lasted only approximately 40 s, but showed strong Hα emission. We found magnetoacoustic waves in long fibrils as mainly short wave trains, short packets or pulses, i.e., solitary waves consisting of small (1´´-2´´) blobs. They start at either end of the fibrils and travel with phase speeds of 12-14 km s-1, i.e., close to the tube speed and approximately the sound velocity for sufficiently large magnetic field strengths. Some waves speed up to reach velocities of the order of 30 km s-1. This is much lower than the expected Alfvén velocity of ≥200 km s-1 for reasonable magnetic field strengths and mass densities. We suggest that slow waves are not purely longitudinal, but possess gas velocities perpendicular to the direction of propagation of few km s-1. Also, fast waves travel along sinuous lines suggesting entangled magnetic fields. They spread out along the direction of propagation in the course of their evolution and often vanish. We discuss the implications

Astronomy Astrophysics On the properties of faculae at the poles of the Sun

Context. Faculae at the poles of the Sun, or polar faculae (PFe), take part in the solar magnetic cycle. Their occurrence maximum is shifted by 5–6 years with respect to the sunspot cycle. PFe are stable phenomena, with lifetimes of several hours to days, and harbour magnetic fields of kilo-Gauss strength. Yet their role for the global magnetic field at the solar poles is unknown. Aims. To contribute to the knowledge of the physical properties of PFe and to the understanding of their role in the global magnetism of the Sun. Methods. PFe were observed on 21–24 August 2005 with the Vacuum Tower Telescope at the Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer was used with a Stokes V polarimeter to scan the Fe i 6173 Å line (Landé factor g = 2.5) and the Hα line in two-dimensional fields of view (FOVs). A large observational coverage of the polar caps was obtained. The data were analysed with speckle methods. Magnetic field strengths were determined with the weak field approximation, with the approximation of the strong field regime, and with the centre of gravity (COG) method. Velocities were measured with the COG method and from the zero-crossing of the Stokes V profiles. Results. PFe show a decrease of the continuum and broadband intensity contrast towards the disc centre and no decrease of contrast towards the limb, similar to as faculae in active regions near the equator. Extrapolating from the observed FOVs to the total areas of PF occurrence around the solar poles, we find 4 120 PFe in the northern polar cap and, asymmetrically to this number, 1 250 PFe near th