A. Kučera

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.

Emerging magnetic flux, flares and filaments — FBS interval 16–23 June 1980

Abstract 17 emerging magnetic flux regions with arch filaments related to new sunspots were identified in Hale Active Region No. 16918 during the 7 day interval from 16–22 June. Most of the new flux regions were clustered around the filament channel between the old opposite polarity fields as were most of the flares. The two largest regions of new magnetic flux and a few of the smaller flux regions developed very near the end points of filaments. This suggests that the emergence of flux in existing active regions might be non-random in position along a filament channel as well as in distance from a filament channel. We have analyzed the positions of 88 flares to date during about half of each day. We find that slightly more than half (50%) of the flares, irrespective of their size, are centered within the new flux regions. About 1/5 (20%) were centered on the border between the new flux and the adjacent older magnetic field. Less than 1/3 occurred outside of the newly emerging flux regions but in many cases were very close to the newly emerging flux. We conclude that at least 2/3 of the flares are intimately related to the emerging flux regions while the remaining 1/3 might be either indirectly related or unrelated to the emerging flux.

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.

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.

Relationships of a growing magnetic flux region to flares

Some sites for solar flares are known to develop where new magnetic flux emerges and becomes abutted against opposite polarity pre-existing magnetic flux (review by Galzauskas/1/). We have identified and analyzed the evolution of such flare sites at the boundaries of a major new and growing magnetic flux region within a complex of active regions, Hale No. 16918. This analysis was done as a part of a continuing study of the circumstances associated with flares in Hale Region 16918, which was designated as an FBS target during the interval 18 – 23 June 1980. We studied the initiation and development of both major and minor flares in Hα images in relation to the identified potential flare sites at the boundaries of the growing flux region and to the general development of the new flux. This study lead to our recognition of a spectrum of possible relationships of growing flux regions to flares as follows: (1) intimate interaction with adjacent old flux — flare sites centered at new/old flux boundary, (2) forced or “intimidated” interaction in which new flux pushes old field having lower flux density towards a neighboring old polarity inversion line where a flare then takes place, (3) “influential” interaction — magnetic lines of force over an old polarity inversion line, typically containing a filament, reconnect to the new emerging flux; a flare occurs with erupting filament when the magnetic field overlying the filament becomes too weak to prevent its eruption, (4) inconsequential interaction — new flux region is too small or has wrong orientation for creating flare conditions, (5) incidental — flare occurs without any significant relationship to new flux regions.

SOHO/CDS observations of waves above the network

We analyze temporal variations in the intensities and the Doppler shifts of He i 584.33 A (chromosphere), O v 629.73 A (transition region), and Mg ix 368.07 A (corona) measured in and above chromospheric network near disk center with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO). There is significant correlation between the He i and O v modulations, with O v intensity leading He i intensity by 27.3 s ± 4.6 s but no significant time shift in the Doppler shift. Cross-correlation between the O v and Mg ix intensities reveals multiple maxima without correlation between their Doppler shifts. Wavelet power analysis gives evidence of intermittent chromospheric and transition-region oscillations with periodicities in the 250−450 s range and of coronal oscillations in the 110−300 s range. Wavelet phase difference analysis shows that the determined time shift between variations of the He i and O v intensities is dominated by waves with about 300 s periodicity. We interpret these results as giving evidence of compressive waves that propagate downward from the transition region to the chromosphere in the particular chromospheric network. We discuss different scenarios regarding origin and source localization of waves, and we speculate on their role in coronal heating above chromospheric network.

The Location of Solar Oscillations in the Photosphere

Applying a correlation analysis to time series of granulation it has been shown that due to the influence of enhanced turbulent motions near the downflow regions in the intergranular lanes the turbulent motions predominate.

Flare-induced changes of the photospheric magnetic field in a δ-spot deduced from ground-based observations

Aims. Changes of the magnetic field and the line-of-sight velocities in the photosphere are being reported for an M-class flare that originated at a δ -spot belonging to active region NOAA 11865. Methods. High-resolution ground-based near-infrared spectropolarimetric observations were acquired simultaneously in two photospheric spectral lines, Fe i 10783 A and Si i 10786 A, with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The observations covered several stages of the M-class flare. Inversions of the full-Stokes vector of both lines were carried out and the results were put into context using (extreme)-ultraviolet filtergrams from the Solar Dynamics Observatory (SDO). Results. The active region showed high flaring activity during the whole observing period. After the M-class flare, the longitudinal magnetic field did not show significant changes along the polarity inversion line (PIL). However, an enhancement of the transverse magnetic field of approximately 550 G was found that bridges the PIL and connects umbrae of opposite polarities in the δ -spot. At the same time, a newly formed system of loops appeared co-spatially in the corona as seen in 171 A filtergrams of the Atmospheric Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the magnetic connection between the umbrae already existed in the upper atmosphere before the M-class flare and became visible only later when it was filled with hot plasma. The photospheric Doppler velocities show a persistent upflow pattern along the PIL without significant changes due to the flare. Conclusions. The increase of the transverse component of the magnetic field after the flare together with the newly formed loop system in the corona support recent predictions of flare models and flare observations.

Influence of the 5-min oscillations on solar photospheric layers. I. Quiet region

Time series of 1D spectrograms are used to study the influence of the 5-min oscillations on intensity and velocity fields of different layers of the quiet solar photosphere. We study the continuum intensity field along with intensity and corresponding velocity patterns of the mid and upper photosphere, obtained from two Fe lines. Oscillations seem to dominate the intensity and velocity fields of the higher atmospheric layers. Our results confirm the fast decay of the granular intensity structure with height. From correlations of temperature structures at three different photospheric levels we conclude that there are rapid changes of the structures in the lower photosphere, which are valid for the duration of the time series, while for the upper levels changes of the stuctures are fainter and show significant periodic character. The velocity pattern, on the other hand, shows a periodic propagation through the photosphere. The tests of the influence of seeing conditions on the data are considered.

Height-Dependent Solar Plage Temperature Distribution

We present a temperature model of solar plage based on the simultaneous observations of UV, visible and IR spectral lines, both in plages and the nearby quiet chromosphere. Our calculations are carried out with the atomic model consisting of the nine H, six Ca II and thirteen He I levels. Using technique of the “equivalent two-level atom” we established temperature distributions with a sharp increase on the height 1200 km. Results are compared with the two-component models of Ayres et al. (1986).