arXiv subjects

P. Heinzel

Publications and source records attributed to P. Heinzel.

At least 37 records · Page 2Linked to original sources

arXiv2020

Quiet-Sun hydrogen Lyman-alpha line profile derived from SOHO/SUMER solar-disk observations

The solar radiation in the Lyman-alpha spectral line of hydrogen plays a significant role in the illumination of chromospheric and coronal structures, such as prominences, spicules, chromospheric fibrils, cores of coronal mass ejections, and solar wind. Moreover, it is important for the investigation of the heliosphere, Earth's ionosphere, and the atmospheres of planets, moons, and comets. We derive a reference quiet-Sun Lyman-alpha spectral profile that is representative of the Lyman-alpha radiation from the solar disk during a minimum of solar activity. This profile can serve as an incident radiation boundary condition for the radiative transfer modelling of chromospheric and coronal structures. Because the solar radiation in the Lyman lines is not constant over time but varies significantly with the solar cycle, we provide a method for the adaptation of the incident radiation Lyman line profiles (Lyman-alpha and higher lines) to a specific date. Moreover, we analyse how the change in the incident radiation influences the synthetic spectra produced by the radiative transfer modelling. To take into account the Lyman-alpha variation with the solar cycle, we used the LISIRD composite Lyman-alpha$ index. To estimate the influence of the change in the incident radiation in the Lyman lines on the results of radiative transfer models, we used a 2D prominence fine structure model. The analysis of the influence of the change in the incident radiation shows that the synthetic spectra are strongly affected by the modification of the incident radiation boundary condition. The hydrogen H alpha line can also be considerably affected, despite the fact that the H alpha radiation from the solar disk does not vary with the solar cycle.

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arXiv2020

Observational study of chromospheric heating by acoustic waves

Aims. To investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere, observations in strong chromospheric lines are analyzed by comparing the deposited acoustic-energy flux with the total integrated radiative losses. Methods. Quiet-Sun and weak-plage regions were observed in the Ca II 854.2 nm and H-alpha lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope (GST) on 2019 October 3 and in the H-alpha and H-beta lines with the echelle spectrograph attached to the Vacuum Tower Telescope (VTT) on 2018 December 11 and 2019 June 6. The deposited acoustic energy flux at frequencies up to 20 mHz was derived from Doppler velocities observed in line centers and wings. Radiative losses were computed by means of a set of scaled non-LTE 1D hydrostatic semi-empirical models obtained by fitting synthetic to observed line profiles. Results. In the middle chromosphere (h = 1000-1400 km), the radiative losses can be fully balanced by the deposited acoustic energy flux in a quiet-Sun region. In the upper chromosphere (h > 1400 km), the deposited acoustic flux is small compared to the radiative losses in quiet as well as in plage regions. The crucial parameter determining the amount of deposited acoustic flux is the gas density at a given height. Conclusions. The acoustic energy flux is efficiently deposited in the middle chromosphere, where the density of gas is sufficiently high. About 90% of the available acoustic energy flux in the quiet-Sun region is deposited in these layers, and thus it is a major contributor to the radiative losses of the middle chromosphere. In the upper chromosphere, the deposited acoustic flux is too low, so that other heating mechanisms have to act to balance the radiative cooling.

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arXiv2020

Spectral Characteristics and Formation Height of Off-Limb Flare Ribbons

Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off-limb in the H$β$ and Ca II 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2-class solar flare SOL2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in H$β$ line wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the H$β$ wing and the nominal photospheric limb is about 300 - 500 km. The Ca II 8542 Å line wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, towards the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb H$β$ ribbons and their significant elevation above the photosphere. The ribbons in the Ca II 8542 Å line wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the H$β$ line wing intensities can be an useful tracer of flare energy deposition in the lower solar atmosphere

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arXiv2020

Stellar coronal mass ejections II. Constraints from spectroscopic observations

Detections of stellar coronal mass ejections (CMEs) are still rare. Observations of strong Balmer line asymmetries during flare events have been interpreted as being caused by CMEs. Here, we aim to estimate the maximum possible Balmer line fluxes expected from CMEs to infer their detectability in spectroscopic observations. Moreover, we use these results together with a model of intrinsic CME rates to infer the potentially observable CME rates for stars of different spectral types under various observing conditions, as well as the minimum required observing time to detect stellar CMEs in Balmer lines. We find that generally CME detection is favoured for mid- to late-type M dwarfs, as they require the lowest signal-to-noise ratio for CME detection, and the fraction of observable-to-intrinsic CMEs is largest. They may require, however, longer observing times than stars of earlier spectral types at the same activity level, as their predicted intrinsic CME rates are lower. CME detections are generally favoured for stars close to the saturation regime, because they are expected to have the highest intrinsic rates; the predicted minimum observing time to detect CMEs on just moderately active stars is already >100 h. By comparison with spectroscopic data sets including detections as well as non-detections of CMEs, we find that our modelled maximum observable CME rates are generally consistent with these observations on adopting parameters within the ranges determined by observations of solar and stellar prominences.

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arXiv2019

2D non-LTE modelling of a filament observed in the H_alpha line with the DST/IBIS spectropolarimeter

We study a fragment of a large quiescent filament observed on May 29, 2017 by the Interferometric BIdimensional Spectropolarimeter (IBIS) mounted at the Dunn Solar Telescope. We focus on its quiescent stage prior to its eruption. We analyse the spectral observations obtained in the H$α$ line to derive the thermodynamic properties of the plasma of the observed fragment of the filament. We used a 2D filament model employing radiative transfer computations under conditions that depart from the local thermodynamic equilibrium. We employed a forward modelling technique in which we used the 2D model to producesynthetic H_alpha line profiles that we compared with the observations. We then found the set of model input parameters, which produces synthetic spectra with the best agreement with observations. Our analysis shows that one part of the observed fragment of the filament is cooler, denser, and more dynamic than its other part that is hotter, less dense, and more quiescent. The derived temperatures in the first part range from 6,000 K to 10,000$ K and in the latter part from 11,000 K to 14,000 K. The gas pressure is 0.2-0.4 dyn/cm}^{2} in the first part and around 0.15 dyn/cm}^{2} in the latter part. The more dynamic nature of the first part is characterised by the line-of-sight velocities with absolute values of 6-7 km/s and microturbulent velocities of 8-9 km/s. On the other hand, the latter part exhibits line-of-sight velocities with absolute values 0-2.5 km/s and microturbulent velocities of 4-6 km/s.

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arXiv2018

High-Density Off-Limb Flare Loops Observed by SDO

The density distribution of flare loops and the mechanisms of their emission in the continuum are still open questions. On September 10, 2017 a prominent loop system appeared during the gradual phase of an X8.2 flare (SOL2017-09-10), visible in all passbands of SDO/AIA and in the white-light continuum of SDO/HMI. We investigate its electron density by taking into account all radiation processes in the flare loops, i.e. the Thomson continuum, hydrogen Paschen and Brackett recombination continua, as well as free-free continuum emission. We derive a quadratic function of the electron density for a given temperature and effective loop thickness. By absolutely calibrating SDO/HMI intensities, we convert the measured intensities into electron density at each pixel in the loops. For a grid of plausible temperatures between cool (6000 K) and hot (10^6 K) structures, the electron density is computed for representative effective thicknesses between 200 and 20 000 km. We obtain a relatively high maximum electron density, about 10^13 cm^-3. At such high electron densities, the Thomson continuum is negligible and therefore one would not expect a significant polarization degree in dense loops. We conclude that the Paschen and Brackett recombination continua are dominant in cool flare loops, while the free-free continuum emission is dominant for warmer and hot loops.

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arXiv2018

Statistical analysis of UV spectra of a quiescent prominence observed by IRIS

The paper analyzes the structure and dynamics of a quiescent prominence that occurred on October 22, 2013. We aim to determine the physical characteristics of the observed prominence using MgII k and h, CII (1334 and 1336 A), and SiIV (1394 A) lines observed by IRIS. We employed the 1D non-LTE modeling of MgII lines assuming static isothermal-isobaric slabs. We selected a large grid of models with realistic input parameters and computed synthetic MgII lines. The method of Scargle periodograms was used to detect possible prominence oscillations. We analyzed 2160 points of the observed prominence in five different sections along the slit averaged over ten pixels due to low signal to noise ratio in the CII and SiIV lines. We computed the integrated intensity for all studied lines, while the central intensity and reversal ratio was determined only for both MgII and CII 1334 lines. We plotted several correlations: time evolution of the integrated intensities and central intensities, scatter plots between all combinations of line integrated intensities, and reversal ratio as a function of integrated intensity. We also compared MgII observations with the models. Results show that more than two-thirds of MgII profiles and about one-half of CII 1334 profiles are reversed. Profiles of SiIV are generally unreversed. The MgII and CII lines are optically thick, while the SiIV line is optically thin. The studied prominence shows no global oscillations in the MgII and CII lines. Therefore, the observed time variations are caused by random motions of fine structures with velocities up to 10 km/s. The observed average ratio of MgII k to MgII h line intensities can be used to determine the prominence's characteristic temperature. Certain disagreements between observed and synthetic line intensities of MgII lines point to the necessity of using more complex 2D multi-thread modeling in the future.

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arXiv2018

Visibility of Prominences using the HeI D3 Line Filter on PROBA-3/ASPIICS Coronagraph

We determine an optimal width and shape of the narrow-band filter centered around the He\,{\sc i} D$_{3}$ line for prominence and coronal mass ejection (CME) observations with the ASPIICS ({\it Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun}) coronagraph onboard the PROBA-3 ({\it Project for On-board Autonomy}) satellite, to be launched in 2020. We analyze He\,{\sc i} D$_{3}$ line intensities for three representative non-LTE prominence models at temperatures 8, 30 and 100~kK computed by the radiative transfer code and the prominence visible-light (VL) emission due to Thomson scattering on the prominence electrons. We compute various useful relations at prominence line-of-sight (LOS) velocities of 0, 100, and 300~km~s$^{-1}$ for 20~Å~wide flat filter and three Gaussian filters with full width at half maximum (FWHM) equal to 5, 10, and 20~Å~to show the relative brightness contribution of the He\,{\sc i} D$_{3}$ line and the prominence VL to the visibility in a given narrow-band filter. We also discuss possible signal contamination by Na\,{\sc i} D$_{1}$ and D$_{2}$ lines which otherwise may be useful to detect comets. Results mainly show: i) an optimal narrow-band filter should be flat or somewhere between flat and Gaussian with FWHM of 20~Å~in order to detect fast moving prominence structures, ii) the maximum emission in the He\,{\sc i} D$_3$ line is at 30~kK and the minimal at 100~kK, and iii) the ratio of emission in the He\,{\sc i} D$_3$ line to the VL emission can provide a useful diagnostic for the temperature of prominence structures. This ratio is up to 10 for hot prominence structures, up to 100 for cool structures and up to 1000 for warm structures.

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arXiv2018

Hot prominence detected in the core of a Coronal Mass Ejection: III. Plasma filling factor from UVCS Lyman-$α$ and Lyman-$β$ observations

This work deals with the study of an erupting prominence embedded in the core of a CME and focuses on the derivation of the prominence plasma filling factor. We explore two methods to measure the prominence plasma filling factor that are based on the combination of visible-light and ultraviolet spectroscopic observations. Theoretical relationships for resonant scattering and collisional excitation are used to evaluate the intensity of the H I Lyman-α and Lyman-β lines, in two prominence points where simultaneous and cospatial LASCO-C2 and UVCS data were available. Thermodynamic and geometrical parameters assumed for the calculation are provided by both observations and the results of a detailed 1D non-LTE radiative-transfer model of the prominence, developed in our previous work (Heinzel 2016). The filling factor is derived from the comparison between the calculated and the measured intensities of the two lines. The results are then checked against the non-LTE model in order to verify the reliability of the methods. The resulting filling factors are consistent with the model in both the prominence points when the separation of the radiative and collisional components of the total intensity, required to estimate the filling factor, is performed using both the line intensities. An exploration of the parameter space shows that the results are weakly sensitive to the plasma velocity, but they depends more strongly on the assumed kinetic temperatures. The combination of visible-light and ultraviolet Lyman-α and Lyman-β data can be used to approximately estimate the geometrical filling factor in erupting prominences, but the proposed techniques are reliable only for emission that is optically thin in the lines considered, condition that is not in general representative of prominence plasma.

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arXiv2017

On the nature of off-limb flare continuum sources detected by SDO/HMI

The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory has provided unique observations of off-limb flare emission. White-light (WL) continuum enhancements were detected in the "continuum" channel of the Fe 6173 A line during the impulsive phase of the observed flares. In this paper we aim to determine which radiation mechanism is responsible for such an enhancement being seen above the limb, at chromospheric heights around or below 1000 km. Using a simple analytical approach, we compare two candidate mechanisms, the hydrogen recombination continuum (Paschen) and the Thomson continuum due to scattering of disk radiation on flare electrons. Both mechanisms depend on the electron density, which is typically enhanced during the impulsive phase of a flare as the result of collisional ionization (both thermal and also non-thermal due to electron beams). We conclude that for electron densities higher than $10^{12}$ cm$^{-3}$, the Paschen recombination continuum significantly dominates the Thomson scattering continuum and there is some contribution from the hydrogen free-free emission. This is further supported by detailed radiation-hydrodynamical (RHD) simulations of the flare chromosphere heated by the electron beams. We use the RHD code FLARIX to compute the temporal evolution of the flare heating in a semi-circular loop. The synthesized continuum structure above the limb resembles the off-limb flare structures detected by HMI, namely their height above the limb, as well as the radiation intensity. These results are consistent with recent findings related to hydrogen Balmer continuum enhancements, which were clearly detected in disk flares by the IRIS near-ultraviolet spectrometer.

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arXiv2016

Chromospheric heating by acoustic waves compared to radiative cooling

Acoustic and magnetoacoustic waves are among the possible candidate mechanisms that heat the upper layers of solar atmosphere. A weak chromospheric plage near a large solar pore NOAA 11005 was observed on October 15, 2008 in the lines Fe I 617.3 nm and Ca II 853.2 nm with the Interferometric Bidimemsional Spectrometer (IBIS) attached to the Dunn Solar Telescope. Analyzing the Ca II observations with spatial and temporal resolutions of 0.4" and 52 s, the energy deposited by acoustic waves is compared with that released by radiative losses. The deposited acoustic flux is estimated from power spectra of Doppler oscillations measured in the Ca II line core. The radiative losses are calculated using a grid of seven 1D hydrostatic semi-empirical model atmospheres. The comparison shows that the spatial correlation of maps of radiative losses and acoustic flux is 72 %. In quiet chromosphere, the contribution of acoustic energy flux to radiative losses is small, only of about 15 %. In active areas with photospheric magnetic field strength between 300 G and 1300 G and inclination of 20-60 degrees, the contribution increases from 23 % (chromospheric network) to 54 % (a plage). However, these values have to be considered as lower limits and it might be possible that the acoustic energy flux is the main contributor to the heating of bright chromospheric network and plages.

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arXiv2016

Numerical RHD simulations of flaring chromosphere with Flarix

Flarix is a radiation-hydrodynamical (RHD) code for modeling of the response of the chromosphere to a beam bombardment during solar flares. It solves the set of hydrodynamic conservation equations coupled with non-LTE equations of radiative transfer. The simulations are driven by high energy electron beams. We present results of the Flarix simulations of a flaring loop relevant to the problem of continuum radiation during flares. In particular we focus on properties of the hydrogen Balmer continuum which was recently detected by IRIS.

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arXiv2016

New Observations of Balmer Continuum Flux in Solar Flares, Instrument Description and First Results

Increase in the Balmer continuum radiation during solar flares was predicted by various authors but never firmly confirmed observationally using ground-based slit spectrographs. Here we describe a new post-focal instrument - Image Selector - enabling to measure the Balmer continuum flux from the whole flare area, in analogy of successful detections of flaring dMe stars. The system was developed and put into operation at the horizontal solar telescope HSFA-2 of the Ondřejov Observatory. We measure the total flux by a fast spectrometer from a limited but well defined region on the solar disk. Using a system of diaphragms, the disturbing contribution of a bright solar disk can be eliminated as much as possible. Light curves of the measured flux in the spectral range 350 - 440 nm are processed, together with the Hα images of the flaring area delimited by the appropriate diaphragm. The spectral flux data are flat-fielded, calibrated and processed to be compared with model predictions. Our analysis of the data proves that the described device is sufficiently sensitive to detect variations in the Balmer continuum during solar flares. Assuming that the Balmer-continuum kernels have at least a similar size as those visible in Hα, we find the flux increase in the Balmer continuum to reach 230 % - 550 % of the quiet continuum during the observed X-class flare. We also found temporal changes in the Balmer continuum flux starting well before the onset of the flare in Hα.

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arXiv2016

Solar science with the Atacama Large Millimeter/submillimeter Array - A new view of our Sun

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere - a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases.

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arXiv2015

Mg II Lines Observed during the X-class Flare on 29 March 2014 by the Interface Region Imaging Spectrograph

Mg II lines represent one of the strongest emissions from the chromospheric plasma during solar flares. In this article, we studied the Mg II lines observed during the X1 flare on March 29 2014 (SOL2014-03-29T17:48) by IRIS. IRIS detected large intensity enhancements of the Mg II h and k lines, subordinate triplet lines, and several other metallic lines at the flare footpoints during this flare. We have used the advantage of the slit-scanning mode (rastering) of IRIS and performed, for the first time, a detailed analysis of spatial and temporal variations of the spectra. Moreover, we were also able to identify positions of strongest HXR emissions using RHESSI observations and to correlate them with the spatial and temporal evolution of Mg II spectra. The light curves of the Mg II lines increase and peak contemporarily with the HXR emissions but decay more gradually. There are large red asymmetries in the Mg II h and k lines after the flare peak. We see two spatially well separated groups of Mg II line profiles, non-reversed and reversed. In some cases, the Mg II footpoints with reversed profiles are correlated with HXR sources. We show the spatial and temporal behavior of several other line parameters (line metrics) and briefly discuss them. Finally, we have synthesized the Mg II k line using our non-LTE code with the MALI technique. Two kinds of models are considered, the flare model F2 of Machado et al. (1980) and the models of Ricchiazzi and Canfield (1983). Model F2 reproduces the peak intensity of the unreversed Mg II k profile at flare maximum but does not account for high wing intensities. On the other hand, the RC models show the sensitivity of Mg II line intensities to various electron-beam parameters. Our simulations also show that the microturbulence produces a broader line core, while the intense line wings are caused by an enhanced line source function.

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arXiv2015

SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network

The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at coordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA.

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arXiv2013

Dynamics of the solar atmosphere above a pore with a light bridge

Context: Solar pores are small sunspots lacking a penumbra that have a prevailing vertical magnetic field component. They can include light bridges at places with locally reduced magnetic field. Like sunspots, they exhibit a wide range of oscillatory phenomena. Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to obtain characteristics of a chromospheric filamentary structure around the pore, to analyse oscillations and waves in and around the pore, and to understand the structure and brightness of the light bridge. Methods: Spectral imaging observations in the line Ca II 854.2 nm and complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter, were used to measure photospheric and chromospheric velocity fields, oscillations, waves, the magnetic field in the photosphere, and acoustic energy flux and radiative losses in the chromosphere. Results: The chromospheric filamentary structure around the pore has all important characteristics of a superpenumbra: it shows an inverse Evershed effect and running waves, and has a similar morphology and oscillation character. The granular structure of the light bridge in the upper photosphere can be explained by radiative heating. Acoustic waves leaking up from the photosphere along the inclined magnetic field in the light bridge transfer enough energy flux to balance the total radiative losses of the light-bridge chromosphere. Conclusions: The presence of a penumbra is not a necessary condition for the formation of a superpenumbra. The light bridge is heated by radiation in the photosphere and by acoustic waves in the chromosphere.

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arXiv2013

Atmosphere above a large solar pore

A large solar pore with a granular light bridge was observed on October 15, 2008 with the IBIS spectrometer at the Dunn Solar Telescope and a 69-min long time series of spectral scans in the lines Ca II 854.2 nm and Fe I 617.3 nm was obtained. The intensity and Doppler signals in the Ca II line were separated. This line samples the middle chromosphere in the core and the middle photosphere in the wings. Although no indication of a penumbra is seen in the photosphere, an extended filamentary structure, both in intensity and Doppler signals, is observed in the Ca II line core. An analysis of morphological and dynamical properties of the structure shows a close similarity to a superpenumbra of a sunspot with developed penumbra. A special attention is paid to the light bridge, which is the brightest feature in the pore seen in the Ca II line centre and shows an enhanced power of chromospheric oscillations at 3-5 mHz. Although the acoustic power flux in the light bridge is five times higher than in the "quiet" chromosphere, it cannot explain the observed brightness.

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