H. Wöhl

Magnetic loop emergence within a granule

Aims. We investigate the temporal evolution of magnetic flux emerging within a granule in the quiet-Sun internetwork at disk center. Methods. We combined IR spectropolarimetry of high angular resolution performed in two Fe i lines at 1565 nm with specklereconstructed G-band imaging. We determined the magnetic field parameters by a LTE inversion of the full Stokes vector using the SIR code, and followed their evolution in time. To interpret the observations, we created a geometrical model of a rising loop in 3D. The relevant parameters of the loop were matched to the observations where possible. We then synthesized spectra from the 3D model for a comparison to the observations. Results. We found signatures of magnetic flux emergence within a growing granule. In the early phases, a horizontal magnetic field with a distinct linear polarization signal dominated the emerging flux. Later on, two patches of opposite circular polarization signal appeared symmetrically on either side of the linear polarization patch, indicating a small loop-like structure. The mean magnetic flux density of this loop was roughly 450 G, with a total magnetic flux of around 3 × 10 17 Mx. During the ∼12 min episode of loop occurrence, the spatial extent of the loop increased from about 1 to 2 arcsec. The middle part of the appearing feature was blueshifted during its occurrence, supporting the scenario of an emerging loop. There is also clear evidence for the interaction of one loop footpoint with a preexisting magnetic structure of opposite polarity. The temporal evolution of the observed spectra is reproduced to first order by the spectra derived from the geometrical model. During the phase of clearest visibility of the loop in the observations, the observed and synthetic spectra match quantitatively. Conclusions. The observed event can be explained as a case of flux emergence in the shape of a small-scale loop. The fast disappearance of the loop at the end could possibly be due to magnetic reconnection.

Height correction in the measurement of solar differential rotation determined by coronal bright points

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse solar differential rotation by tracing coronal bright points for the period June 4, 1998 to May 22, 1999. A method for the simultaneous determination of the true solar synodic rotation velocity and the height of the tracers is applied to data sets analysed with interactive and automatic methods. The calculated height of coronal bright points is on average 8000-12000 km above the photosphere. Corrected rotation velocities are transformed into sidereal ones and compared with results from the literature, obtained with various methods and tracers. The differential rotation profile determined by coronal bright points with the interactive method corresponds roughly to the profile obtained by correlating photospheric magnetic fields and the profile obtained from the automatic method corresponds roughly to the rotation of sunspot groups. This result is interpreted in terms of the differences obtained in the latitudinal distribution of coronal bright points using the two methods.

Differential Rotation of Stable Recurrent Sunspot Groups

Stable recurrent sunspot groups from the Greenwich data set which were identified in at least two subsequent solar rotations were traced. The solar rotation was determined by the period method from the time difference of the two central meridian passages of each of the 327 identified groups. Sidereal rotation periods were calculated from the synodic ones by a seasonal-dependent procedure taking into account the details of the Earths motion around the Sun. Growing recurrent sunspot groups rotate on the average faster than decaying recurrent sunspot groups, while sunspot groups of all types taken together rotate faster than both growing and decaying recurrent sunspot groups. A north–south rotational asymmetry and a cycle-dependence of rotational velocity of recurrent sunspot groups were analyzed. Positive rotation velocity deviations are larger, but less numerous than the negative ones. Signatures of torsional oscillations were not found analyzing the rotation velocity residual of recurrent sunspot groups as a function of the distance from the average latitude of activity.

Solar differential rotation determined by tracing coronal bright points in SOHO-EIT images II. Results for 1998/99 obtained with interactive and automatic methods

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse solar dierential rotation by tracing coronal bright points. The results obtained with the interactive and the automatic method for the time period June 4, 1998 to May 22, 1999 are presented and compared. A possible north-south rotational asymmetry and dierences in the rotation velocity curves for various subtypes of tracers are investigated.

On changes of the rotation velocities of stable, recurrent sunspots and their interpretation with a flux tube model

The angular rotation velocities of stable, recurrent sunspots were investigated using data from the Greenwich Photoheliographic Results 1940 until 1968. We found constant rotation velocities during the passages on the solar disk with errors of about ±4 m s−1. During their lifetime these spots show a decreasing braking of their rotation velocities from 0.8 to 0.3 m s−1 per day. A plausible interpretation is found by assuming the spots to be coupled to a slowly rising subsurface flux tube and a rotation velocity which increases with depth.

The relation between the synodic and sidereal rotation period of the Sun

The relation between the synodic and sidereal rotation period of the Sun for an arbitrary date of observation is derived taking into account details of the Earths motion. The transformation procedure between the synodic (apparent) and sidereal rotation period presented here can be performed without using the annual ephemerides.

A Method to Determine the Solar Synodic Rotation Rate and the Height of Tracers

The dependence of the measured apparent synodic solar rotation rate on the height of the chosen tracer is studied. A significant error occurs if the rotation rate is determined by tracing the apparent position of an object above the photospheric level projected on the solar disc. The centre-to-limb variation of this error can be used to determine simultaneously the height of the object and the true synodic rotation rate. The apparent (projected) heliographic coordinates are presented as a function of the height of the traced object and the coordinates of its ‘footpoint’. The relations obtained provide an explicit expression for the apparent rotation rate as a function of the observed heliographic coordinates of the tracer, enabling an analytic least-squares fit expression to determine simultaneously the real synodic rotation rate and the height of the tracer.

Precise reduction of solar spectra obtained with large CCD arrays

A precise procedure suitable for the reduction of solar spectra taken with large CCD arrays and the retrieval of correct spectral characteristics is presented. Various eects, which one should take into account, are considered and several improvements of the standard reduction are introduced. A special flat-field procedure is suggested for the reduction of spectra registered in dierent flat-field conditions than those when the flat-field matrix was taken. The original flat-field matrix is split into several components to eliminate the influence of the drift of the spectrograph and temporal changes of the flat-field conditions on the reduced spectrum. The importance of every flat-field matrix component is tested and discussed and the noise propagation through data reduction is analyzed. It is documented that the errors of the basic spectral line characteristics, continuum intensity, line centre intensity and full width at the half maxima of the line have variations between 0.5% and 15% and the errors of the line centre Doppler velocity and bisectors fluctuate by up to 200 m s 1 , if derived from imprecise reductions, compared to precise ones.

Meridional Motions of Stable Recurrent Sunspot Groups

Stable recurrent sunspot groups from the Greenwich data set which were identified in at least two subsequent solar rotations were traced and meridional motions were determined from the two central meridian passages. In total, 327 meridional velocities were calculated and the results for the northern and the southern solar hemisphere were compared. A dependence of the solar meridional velocity vectors on the development status, latitude and position respectively to the activity belt of sunspots is investigated. The results indicate that sunspot groups are moving on the average away from the center of activity. This was found for sunspot groups growing and decreasing in area.

Indications of shock waves in the solar photosphere

High resolution observations of solar granulation near the solar limb are used in a search for hydrodynamic shocks caused by an abrupt braking of the fast (probably supersonic) horizontal flow of the granular plasma towards the intergranular lane. Shock signatures in the spectral line of Fe II 6456.38 A of one particular observed shock event are investigated in detail. Evolution, amplitude, and spatial relation of the spectral line characteristics of the shock event are in agreement with predictions from numerical simulations for such shock phenomena in the solar photosphere. The dimensions and amplitudes of the observed shock signatures are comparable to predicted values when seeing and instrumental effects as well as a possible obliqueness of the shock front with respect to the observers line-of-sight are taken into account. The temporal evolution of such an event is observed for the first time. The stable and declining phase of the event were studied for a time period of almost 2 min. A particular relationship was found between the shock event and a nearby G-band bright point located 2 �� from the shock event. It is suggestive that the observed shock is a causal consequence of the magnetic flux concentration, traced by the G-band bright point. Such a type of shock can appear outside the flux concentrations as a consequence of a rapid flux-tube motion.