Pavel Kotrc

Hinode, TRACE, SOHO, and Ground-based Observations of a Quiescent Prominence

A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by the Hinode SOT, in X-rays by the Hinode XRT, and in the 195 A channel by TRACE. Moreover, ground-based observatories (GBOs) provided calibrated Hα intensities. Simultaneous extreme-UV (EUV) data were also taken by the Hinode EIS and SOHO SUMER and CDS instruments. Here we have selected the SOT Hα image taken at 13:19 UT, which nicely shows the prominence fine structure. We compare this image with cotemporaneous ones taken by the XRT and TRACE and show the intensity variations along several cuts parallel to the solar limb. EIS spectra were obtained about half an hour later. Dark prominence structure clearly seen in the TRACE and EIS 195 A images is due to the prominence absorption in H I, He I, and He II resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in the XRT images is entirely due to X-ray emissivity blocking. We use TRACE, EIS, and XRT data to estimate the amount of absorption and blocking. The Hα integrated intensities independently provide us with an estimate of the Hα opacity, which is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. However, spatial averaging of the Hα and EUV data have quite different natures, which must be taken into account when evaluating the true opacities. We demonstrate this important effect here for the first time. Finally, based on this multiwavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence.

SOHO/SUMER observations and analysis of the hydrogen Lyman spectrum in solar prominences

The complete hydrogen Lyman spectrum in several prominences has been observed with the UV spectrometer SUMER on-board the SOHO, during the Joint Observing Programme 107, together with other space and ground-based observatories. Based on these observations, we are able to demonstrate, for the first time, that there exists a large variety of intensities and shapes of Lyman lines in different prominences and in various parts thereof. Therefore, no “canonical” Lyman spectrum can be considered for modelling purposes. However, we have identified at least two representative properties of the observed spectra: in one case (May 28, 1999 prominence) we detected high integrated intensities and no reversals in lines higher than Lα. Another prominence (June 2, 1999) exhibited quite similar integrated intensities, but all lines have rather strongly reversed profiles. This behaviour cannot be explained in terms of standard isothermal-isobaric models and we thus consider more general models which are in pressure equilibrium with the magnetic field and which have significant prominencecorona transition region (PCTR) temperature gradients. This type of model, recently suggested by Anzer & Heinzel (1999), is capable of explaining strong emission profiles without reversal. Based on extended non-LTE computations, we suggest that quite different Lyman spectra mentioned above may correspond to two types of PCTRs, one seen along the magnetic-field lines (unreversed profiles) and the other one seen across the field lines (reversed profiles). Finally, we again confirm the importance of partial-redistribution (PRD) scattering processes for Lyman lines in prominences. However, our analysis of new SUMER data also points to a critical role of the PCTR in radiative transport in these lines.

On the occurrence of blue asymmetry in chromospheric flare spectra

We present observations of optical spectra of a flare in which blue line asymmetry was seen for more than 4 min close to the flare onset. The maximum blue asymmetry coincided with the maximum of a hard X-ray and microwave burst. We discuss possible interpretations of the blue asymmetry and conclude that the most plausible one is electron-beam heating with return current. Although this process predicts downflows in the lower transition region and upper chromosphere, its ultimate effect on the line profiles can be blue asymmetry: the upper layers moving away from us absorb the radiation of the red peak thus lowering its intensity in comparison to the blue one.

KAPPA: A PACKAGE FOR SYNTHESIS OF OPTICALLY THIN SPECTRA FOR THE NON-MAXWELLIAN κ-DISTRIBUTIONS BASED ON THE CHIANTI DATABASE

The non-Maxwellian �-distributions have been detected in the solar transition region and flares. These distributions are characterized by a high-energy tail and a near-Maxwellian core and are known to have significant impact on the resulting optically thin spectra arising from collisionally dominated astrophysical plasmas. We developed the KAPPA package a for synthesis of such line and continuum spectra. The package is based on the freely available CHIANTI database and software, and can be used in a similar manner. Ionization and recombination rates together with the ionization equilibria are

IMAGING AND SPECTROSCOPIC OBSERVATIONS OF A TRANSIENT CORONAL LOOP: EVIDENCE FOR THE NON-MAXWELLIAN κ-DISTRIBUTIONS

We report on the SDO/AIA and Hinode/EIS observations of a transient coronal loop. The loop brightens up in the same location after the disappearance of an arcade formed during a B8.9-class microflare three hours earlier. EIS captures this loop during its brightening phase as observed in most of the AIA filters. We use the AIA data to study the evolution of the loop, as well as to perform the DEM diagnostics as a function of �. Fe XI–Fe XIII lines observed by EIS are used to perform the diagnostics of electron density and subsequently the diagnostics of �. Using ratios involving the Fe XI 257.772u selfblend, we diagnose �. 2, i.e., an extremely non-Maxwellian distribution. Using the predicted Fe line intensities derived from the DEMs as a function of �, we show that, with decreasing �, all combinations of ratios of line intensities converge to the observed values, confirming the diagnosed �. 2. These results represent the first positive diagnostics of �-distributions in the solar corona despite the limitations imposed by calibration uncertainties.

ON THE NATURE OF BRIGHT RIMS OF FILAMENTS

A filament with a very bright rim was observed in the Hα line on May 11, 1989, using the Meudon spectroheliograph. Absolute calibration of the spectroheliogram allows us to express the intensities in particular sites of the filament, in its bright rim and inside the surrounding chromosphere. From a large number of photometric scans, we obtained a histogram of the intensity excess of the bright rim relative to the quiet chromosphere. The mean value of this excess amounts to about 4%. We present a theoretical explanation of bright rims, based on the nature of Hα radiative diffusion in the filaments. Computed NLTE model of the filament leads to a rim intensity excess which is in good agreement with our observations.

Total mass of six quiescent prominences estimated from their multi-spectral observations

Context. Total masses of six solar prominences were estimated using prominence multi-spectral observations (in EUV, X-rays, Hα, and Ca ii H). The observations were made during the observing campaign from April through June 2011. Aims. The aim of the work was to apply a complex method for the prominence mass estimations that can be used later for other prominences observed during the observing campaign. Methods. Our method is based on the fact that intensity of the EUV solar corona at wavelengths below 912A is reduced by the absorption in resonance continua of hydrogen and helium (photoionisation) and at the same time also by a deficit of the coronal emissivity in volume occupied by the cool prominence plasma. Both mechanisms contribute to intensity decrease simultaneously. The observations in X-rays allow us to separate these mechanisms from each other. Coronal emission behind a prominence is not estimated by any temporal or spatial interpolation, but by using a new method based on comparing the ratio of the optical thickness at 193A and 211A determined from the observations with the theoretical ratio. Results. Values of the total mass estimated for six prominences are between 2.9 × 10 11 and 1.7 × 10 12 kg. The column density of hydrogen is of the order of 10 18 −10 19 cm −2 . Our results agree with results of other authors. Conclusions. The method is now ready to be used for all 30 prominences observed during the campaign. Then in the near future it will be possible to obtain a statistics of the total mass of quiescent solar prominences.

Reconstruction of the HSFA telescopes

At present, two large horizontal solar telescopes with spectrographs, located at the Ondřejov Observatory, are undergoing an important reconstruction. The original designation of these two identical instruments will mostly be preserved. The telescope/spectrograph HSFA1 will continue to be used for the measurement of solar magnetic and velocity fields, while HSFA2 is in the process of rebuilding to a multichannel spectrograph equipped with CCD cameras. The reconstruction of the electronic control systems is the most important item. The up-to-date electronic equipment will enable a remote control of all functions of the instruments, will offer a large amount of automated procedures and should resist to disturbances caused by atmospheric electricity. The whole telescope/spectrograph control system is designed to reduce and simplify the observer’s work as much as possible.

Suppression of Hydrogen Emission in an X-Class White-Light Solar Flare

We present unique NUV observations of a well-observed X-class flare from NOAA 12087 obtained at the Ondřejov Observatory. The flare shows a strong white-light continuum but no detectable emission in the higher Balmer and Lyman lines. Reuven Ramaty High-Energy Solar Spectroscopic Imager and Fermi observations indicate an extremely hard X-ray spectrum and γ-ray emission. We use the RADYN radiative hydrodynamic code to perform two types of simulations: one where an energy of 3 × 1011 erg cm−2 s−1 is deposited by an electron beam with a spectral index of ≈3, and a second where the same energy is applied directly to the photosphere. The combination of observations and simulations allows us to conclude that the white-light emission and the suppression or complete lack of hydrogen emission lines is best explained by a model where the dominant energy deposition layer is located in the lower layers of the solar atmosphere, rather than the chromosphere.

Solar spectral observations at the Ondřejov Observatory with Moscow State University cooperation

On the occasion of the 175-year anniversary of the Moscow State University Sternberg Astronomical Institute we would like to outline the history and some results obtained by the Institute in cooperation with the Ondřejov Astronomical Institute of the Czech Republic Academy of Sciences. The development and current state of this cooperation and the mutual solar observation results are presented. At the end of the 20th century, by a special international contract between the two institutes, one of the five Carl Zeiss new solar instruments, namely the ‘Horizontal Sonnen Forschungs Anlage’ (HSFA) was installed at the High-altitude Sternberg Institute Observatory near Alma-Ata. In the 1980s four other instruments were mounted in Czechoslovakia (two of them in Ondřejov and the other two in Stara Lesna and Hurbanovo). Both Ondřejov instruments (HSFA 1 and 2) were modernized over the period in the 2000–2004, mainly to improve their electronic control systems. A new spectral grating, corresponding optics and detect...