Observational Evidence for Rising Penumbral Flux Tubes?

On 13 May 2000 parts of a penumbra were observed in an active region NOAA 8990 with the La Palma Stokes Polarimeter attached to the Swedish Vacuum Solar Telescope. The stratification over the solar atmosphere of different physical parameters is retrieved from these data by using the Stokes inversion based on response functions. The results confirm the previous findings of the penumbral structure. In general, the magnetic field becomes weaker and more horizontal with increasing distance from the umbra and the line-of-sight velocities are increasing towards the outer boundary of the penumbra. The results also suggest the existence of the unresolved fine structure of the penumbra. The stratifications of the temperature and of the magnetic field strength indicate the presence of rising flux tubes, which were predicted theoretically by Schlichenmaier, Jahn and Schmidt (1998, Astron. Astrophys. 337, 897).     

Large-scale horizontal flows in the solar photosphere IV. On the vertical structure of large-scale horizontal flows

In the recent papers, we introduced a method utilised to measure the flow field. The method is based on the tracking of supergranular structures. We did not precisely know, whether its results represent the flow field in the photosphere or in some subphotospheric layers. In this paper, in combination with helioseismic data, we are able to estimate the depths in the solar convection envelope, where the detected large-scale flow field is well represented by the surface measurements. We got a clear answer to question what kind of structures we track in full-disc Dopplergrams. It seems that in the quiet Sun regions the supergranular structures are tracked, while in the regions with the magnetic field the structures of the magnetic field are dominant. This observation seems obvious, because the nature of Doppler structures is different in the magnetic regions and in the quiet Sun. We show that the large-scale flow detected by our method represents the motion of plasma in layers down to ～10 Mm. The supergranules may therefore be treated as the objects carried by the underlying large-scale velocity field.     

Infrared photometry of a sunspot near the disk center

A time series of broadband images of a sunspot near the disk center was obtained simultaneously in two wavelength bands at 0.56 μm and 1.55 μm at the German VTT on June 17, 1998. We computed intensity difference images of the best frame pairs which reveal information about the facular distribution in the present field of view. Faculae are found around pores, in the quiet granulation and as well around the sunspot penumbra.     

Narrowband dm-spikes in the 2 GHz frequency range and MHD cascading waves in reconnection outflows

Two examples of clouds of narrowband dm-spikes, observed by the Ondejov radiospectrograph in the 1–2 GHz frequency range, are analyzed. After transformation of the frequency scales to distances in the solar atmosphere, the power spectra analysis of size scales reveals a spectral index of –5/3, resembling that of Kolmogorov spectra of turbulent cascades. The narrowband dm-spikes are interpreted as radio emission from electrons accelerated in MHD cascading waves, probably generated in plasma outflows from magnetic field reconnection.     

Book reviews

Exact solutions are found for the static gravitational fields for a matter free space in a self-creation theory of gravitation proposed by G. A. Barber.     

Dynamics of Magnetic Bright Points in an Active Region

A high-quality 80-minute time series of a part of a sunspots moat (18 ″ × 23 ″) in the G-band (4308.64 Å) has been analysed to measure the horizontal velocities of Magnetic Bright Points (MBPs). The observations were carried out in June 2004 at the new 1-meter Swedish Solar Telescope in La Palma. Spatial resolution was estimated to be 0.17 ″ or 125 km on the Sun, and images were taken in a frame selection mode in a 20-seconds interval. Individual feature tracking of MBPs with manual selection and automated tracking has been performed. The intensity of MBPs increases with size. The mean value in a MBP-velocity histogram was found to be 1.11 km s ^?1 and it shows good accordance with an abnormal granulation-velocity histogram. MBP velocity histograms as presented here can be taken as an input for coronal heating models in an active region. However, MBPs move slower in an active region than in the network (presumably because of the higher active region magnetic flux) and hence, a process that includes dissipation of MHD waves through fast MBP motions (>2 km s ^?1) may not alone explain the observed properties of the corona.     

DC-induced acoustic emission in saturated sand models of sedimentary rock

The results of laboratory experimental studies of the DC-induced acoustic emission (AE) in saturated sand models of sedimentary rock are presented. It is shown that direct current acting on a geological medium generates acoustic vibrations in it. The experimental methodology is based on recording acoustic pulses generated by the rock model (sample) when the electric field is applied. The amplitude-frequency parameters of the AE signals depend on petrophysical properties of the solid phase, on the salinity of porous medium, and the electric field (current intensity and voltage). The above research is the first step towards the full identification of the characteristics of the AE signals generated by rocks in a DC electric field.     

Power-law spectra of 1–2 GHz narrowband dm-spikes

Twelve examples of clouds of narrowband dm-spikes, observed by the Ondejov radiospectrograph in the 1–2 GHz frequency range, are analyzed. After transforming of the frequency scales to heights in the solar atmosphere, the indices of the power-law power spectra are determined. The derived power-law indices are scattered in a broad range of values (–0.80––2.85). In some cases they considerably deviate from the previously found value of –5/3. A change of the power-law index above logk2.5 was also found in some cases. In the two longest events the time evolution of their power spectra as well as their indices were studied. While in most parts of the radio spectra the spectral index remains constant, in one part its absolute value increases with the spike intensity increase. Finally, the results, especially the broad range of power-law indices, are briefly discussed.     

Spectroscopic observations and models of umbral light bridges

Profiles of the Nai D lines in two moderately bright umbral light bridges are analyzed. A simple method of elimination of stray light is presented. The corrected continuum intensities ( 588 nm) of the light bridges are 0.51 and 0.43. For each light bridge, using optimization technique, a semi-empirical model is derived. The models exhibit a steep temperature rise towards the subphotospheric layers. Generally, the temperature of the light bridges under investigation is lower than the temperature of the penumbra as well as the temperature of bright umbral dots.     

Infrared photometry of solar active regions

Simultaneous time series of broad-band images of two active regions close to the disk center were acquired at the maximum (0.80Μm) and minimum (1.55Μm) continuum opacities. Dark faculae are detected in images obtained as weighted intensity differences between both wave-length bands. The elements of quiet regions can be clearly distinguished from those of faculae and pores in scatter plots of brightness temperatures. There is a smooth transition between faculae and pores in the scatter plots. These facts are interpreted in terms of the balance between the inhibition of convective energy transport and the lateral radiative heating.