Chromospheric Models Research Articles

On the Peak Times of Thermal and Nontheral Emissions in Solar Flares

We selected a sample of 859 flares observed in hard X-rays (HXRs) by BATSE and in soft X-rays (SXRs) by GOES to study the timing of the SXR and HXR emission. Each sample event presents a simple light curve; i.e., the HXR has only one short time peak and the SXR seems to be a response to the pulse HXR emission. The statistical study shows that the peak time differences between the SXR and HXR emissions conform to a decay exponential function. This distribution, being similar to that for the flaring loop size discovered previously, suggests that the flaring loop size is the reason for the peak time difference between the SXR and HXR emissions. The longer the flaring loop is, the longer the time for chromospheric evaporation reaching the loop top is, i.e., the later the SXR peaks. Our result here provides a support for the traditional chromospheric evaporation model. The fact that the SXR can increase for some time after the end of HXR emission does not seem to be in contradiction with the Neupert effect, if one considers that the electron-driven evaporation needs some time to reach the loop top. Further hydrodynamic modeling is obviously necessary to explain the observations.

The dynamics of the solar chromosphere: comparison of model predictions with millimeter-interferometer observations

We analyze the millimeter intensity spectrum expected from the dynamic model of Carlsson & Stein together with the interferometric observations of the quiet Sun obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array. The observational data products (Fourier and wavelet spectra, brightness histograms) are compared with the corresponding products obtained for the Carlsson & Stein (CS) models. We estimate how the limited spatial resolution of the observations influences the comparison with the predictions of chromospheric dynamic models and discuss the limitations of a one-dimensional non-magnetic modeling approach. In addition, we test the effect of the integration time of the BIMA observations on the dynamic signatures. The dependence of the observed brightness variations on spatial resolution is studied by employing artificial image degradation and approximating the obtained dependence by power laws. We are able to establish a correspondence between the CS model predictions and the observational data under assumptions on the horizontal coherence length of the oscillations. The reconstructed brightness rms values indicate that, assuming the coherence length of oscillating elements to be of order of 1, the oscillation power in the observations recorded with 10 resolution agrees within a factor of 2 with the power predicted by the CS model. We argue that millimeter continuum observations promise to be an important diagnostic of chromospheric structure and dynamics. Based on the analysis carried out in this work, the appropriate wavelengths to look for dynamic signatures are in the range 0.8-5.0 mm. Further millimeter interferometric observations with longer sequences and higher spatial resolution are highly desirable along with the development of realistic three-dimensional radiation magnetohydrodynamic simulations.

Dynamic models of the sun from the convection zone to the chromosphere

AbstractThe chromosphere in internetwork regions of the quiet Sun was regarded as a static and homogeneous layer for a long time. Thanks to advances in observations and numerical modelling, the wave nature of these atmospheric regions received increasing attention during the last decade. Recent three-dimensional radiation magnetohydrodynamic simulations with CO5BOLD feature the chromosphere of internetwork regions as a dynamic and intermittent phenomenon. It is a direct product of interacting waves that form a mesh-like pattern of hot shock fronts and cool post-shock regions. The waves are excited self-consistently at the top of the convection zone. In the middle chromosphere above an average height of 1000 km, plasma beta gets larger than one and magnetic fields become more important. The model chromosphere exhibits a magnetic field that is much more homogeneous than in the layers below and evolves much faster. That includes fast propagating (MHD) waves. Further improvements of the simulations like time-dependent hydrogen ionisation are currently in progress. This class of models is capable of explaining apparently contradicting diagnostics such as carbon monoxide and UV emission at the same time.

Modelling chromospheric line profiles in NGC 2808: evidence of mass loss from RGB stars

In this study we test the possibility that the asymmetry in the profiles of the H-alpha and Ca II K lines in red giant stars is due to the presence of an active chromosphere rather than to mass loss. To this end, we compare line profiles computed using relevant model chromospheres to profiles of the H-alpha and Ca II K lines observed in five red giant stars of the globular cluster NGC 2808. The spectra were taken with FLAMES during Science Verification, using the UVES mode at high resolution (R=43,000) for the H-alpha line, and GIRAFFE in MEDUSA mode (R=20,000) for the Ca II K line. We find that the observed profiles are better described if a negative (outward) velocity field is included in the model chromospheres. This leads to mass loss rates of a few 10**(-9) solar masses per year, very close to the requirements of the stellar evolution theory.

Influence of NLTE calculations on the hydrogen lines in chromospheric models

We present extensive NLTE calculations for a semi-empirical solar 1D chromosphere model based on the VAL C model. We report on a significant influence of nitrogen, oxygen, and sulfur on the emergent hydrogen emission with respect to LTE vs. NLTE calculations for these elements. Moreover, we present a model spectrum with 20 light and iron group elements computed in NLTE. We compare this to an observed solar spectrum and to a photospheric model spectrum. We find that the agreement for this model with the data is less good than the original VAL C model especially in the UV. This may imply the need of changes in the chromospheric temperature structure or may point to general problems of static 1D chromospheric models.

Identification of the broad solar emission features near 117 nm

Wilhelm et al. have recently called attention to the unidentified broad emission features near 117 nm in the solar spectrum. They discuss the observed properties of these features in detail but do not identify the source of this emission. We show that the broad autoionizing transitions of neutral sulfur are responsible for these emission features. Autoionizing lines of occur throughout the spectrum between Lyman alpha and the Lyman limit. Sulfur is a normal contributor to stellar spectra. We use non-LTE chromospheric model calculations with line data from the Kurucz 2004 line list to simulate the solar spectrum in the range 116 to 118 nm. We compare the results with SUMER disk-center observations from Curdt et al. and limb observations from Wilhelm et al. Our calculations generally agree with the SUMER observations of the broad autoionizing emission features, the narrow emission lines, and the continuum in this wavelength region, and agree with basic characteristics of the center-to-limb observations. In addition to modeling the average spectrum, we show that a change of ±200 K in the temperature distribution causes the intensity to change by a factor of 4. This exceeds the observed intensity variations 1) with time in quiet regions at these wavelengths, and 2) with position from cell centers to bright network. These results do not seem compatible with current dynamical models that have temporal variations of 1000 K or more in the low chromosphere.

Chromospheric models of solar analogues with different activity levels

We computed chromospheric models of the Sun as a star and of nine solar analogues. The atmospheric models were constructed to obtain the best possible match with the Ca II K and Hβ lines, including the asymmetry of the lines due to macroscopic velocity fields. The stars were chosen with 0.62 < B − V < 0.68 (the solar B − V = 0.65) and have a wide variety of magnetic activity levels, which allows us to study the differences in atmospheric structures induced by activity. For the less active stars we found that the changes with activity are in the region of the temperature minimum, while the most active stars show changes all along their atmospheric structures, mainly in the upper chromosphere. Regarding the macroscopic velocity fields, we can distinguish between the two groups. The most active group has a velocity field in the temperature-minimum region, and the other group in the chromospheric plateau. We also computed the net radiative losses for each model, and found that they depend linearly on the usual index of chromospheric activity, S CaII.

On the Validity of Acoustically Heated Chromosphere Models

Theoretical models of solar and stellar chromospheres heated by acoustic waves have so far been constructed by using time-dependent, one-dimensional, radiation-hydrodynamic numerical codes that are based on the approximation of plane-parallel geometry. The approach seems to be justified by the fact that the chromospheres of most stars extend over very narrow height ranges compared to the stellar radius. It is demonstrated that this commonly used assumption may lead to unrealistic shock mergings, to the artificial formation of unusually strong shocks and the artificial destruction of high-frequency acoustic wave power. Comparing one-dimensional calculations with observations may lead to severe misjudgment about the nature of chromospheric heating.

PHOENIX model chromospheres of mid- to late-type M dwarfs

We present semi-empirical model chromospheres computed with the atmosphere code PHOENIX. The models are designed to fit the observed spectra of five mid- to late-type M dwarfs. Next to hydrogen lines from the Balmer series we used various metal lines, e.g. from Fe , for the comparison between data and models. Our computations show that an NLTE treatment of C, N, O impacts on the hydrogen line formation, while NLTE treatment of less abundant metals such as nickel influences the lines of the considered species itself. For our coolest models we investigated also the influence of dust on the chromospheres and found that dust increases the emission line flux. Moreover we present an (electronically published) emission line list for the spectral range of 3100 to 3900 and 4700 to 6800 A for a set of 21 M dwarfs and brown dwarfs. The line list includes the detection of the Na  D lines in emission for a L3 dwarf.

Definition and significance of average temperatures in time-dependent solar chromosphere models

We assess different types of average temperatures in time-dependent solar chromosphere models. They include the conventional definition of mean and median temperature, and a formal definition related to the model-dependent hydrogen ionization degree, referred to as ionization temperature. It is found that the latter is always higher than the mean and median temperatures, except in the photosphere, and that the mean temperatures are always higher than the median temperatures, especially in models with frequency spectra. The most dramatic differences are attained in the topmost portion of one of our models with the ionization temperatures up to a factor 150 higher than the mean and median temperatures. The differences between the mean, median, and ionization temperatures are a direct consequence of nonlinearities (spikyness) of the. temperatures in the models mostly due to strong shocks. The main results hold for both acoustic and magnetic models despite significant differences in the initial wave energy fluxes, densities, and geometrical settings.

Detection of red line asymmetries in LHS 2034

We report very pronounced line asymmetries during a long duration flare on the dM6 star LHS 2034 (AZ Cnc). While all lines of the Balmer series and all strong He  lines show these asymmetries, the metal lines do not. This can be explained with the help of PHOENIX model chromospheres considering the formation depth of the lines involved. Moreover, the asymmetries persist over about one hour changing shape and amplitude. Fitting the asymmetries with an additional broad Gaussian component leads us to the scenario of a series of downward propagating condensations that decelerate due to the higher density of the lower chromosphere. In addition, similar but weaker line asymmetries were found in LHS 2397a.

A Simple Procedure for Optimizing the Height Resolution in Spectral Line Inversions

This paper presents a method to determine how much detail of the height stratification of atmospheric parameters can be retrieved from the inversion of spectral lines (and their polarization profiles, when available) via χ2-minimization algorithms. An a priori analysis of the response functions and their correlations permits an evaluation of the optimal number of inversion nodes and their locations. A number of problems of practical interest are studied by applying this method to different photospheric and chromospheric models of quiet Sun and sunspots. As a result of this analysis I obtain the optimal discretization for retrieving the temperature and magnetic field stratifications in such atmospheres. I show that the Fe I lines at 6302 A provide a height resolution for temperature inversions close to Δτ500 0.5 in the middle photosphere. In comparison, the chromospheric Hα line allows for a resolution of Δτ500 1.5 through the chromosphere. This kind of analysis may also be useful in other contexts in which χ2-minimization algorithms are employed.

Chromospheric structure and activity in solar analogues

We computed chromospheric models of the Sun as a star and of nine solar analogues. The stars have a wide variety of magnetic activity levels, which allows us to study the differences in atmospheric structures induced by activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Acoustic Heating Models for the Basal Flux Star Ceti Including Time-dependent Ionization: Results for Ca ii and Mg ii Emission

We present new calculations of chromospheric heating for τ Ceti (G8 V), a star exhibiting an extremely low level of chromospheric activity, thus also referred to as a basal flux star or a flat activity star. Our simulations consider energy deposition by acoustic shocks and also take into account time-dependent (i.e., noninstantaneous) ionization processes of hydrogen, magnesium, and calcium, allowing us to attain a new generation of chromospheric heating models previously obtained for the Sun. We consider both monochromatic waves and acoustic frequency spectra. The latter are calculated using new models of acoustic energy generation based on an extended Kolmogorov spectrum with a modified Gaussian frequency factor. Our models show that the theoretically deduced emergent Ca II and Mg II emission very much agree with observations, adding to the argument that the chromospheres of basal flux stars are predominantly heated by acoustic shocks.

3‐D hydrodynamic simulations of the solar chromosphere

AbstractWe present first results of three‐dimensional numerical simulations of the non‐magnetic solar chromosphere, computed with the radiation hydrodynamics code CO5BOLD. Acoustic waves which are excited at the top of the convection zone propagate upwards into the chromosphere where the waves steepen into shocks. The interaction of the waves leads to the formation of complex structures which evolve on short time scales. Consequently, the model chromosphere is highly dynamical, inhomogeneous, and thermally bifurcated.

Detection of the Neupert Effect in the Corona of an RS Canum Venaticorum Binary System byXMM‐Newtonand the Very Large Array

The RS CVn-type binary σ Geminorum was observed during a large, long-duration flare simultaneously with XMM-Newton and the Very Large Array. The light curves show a characteristic time dependence that is compatible with the Neupert effect observed in solar flares: the time derivative of the X-ray light curve resembles the radio light curve. This observation can be interpreted in terms of a standard flare scenario in which accelerated coronal electrons reach the chromosphere, where they heat the cool plasma and induce chromospheric evaporation. Such a scenario can hold only if the amount of energy in the fast electrons is sufficient to explain the X-ray radiative losses. We present a plausibility analysis that supports the chromospheric evaporation model.

Acoustic and magnetic wave heating in stars

Theoretical chromospheric models described in the two previous papers of this series are used to study the relationship between the chromospheric emission and the lling factor. This theoretically determined rela- tionship shows that the chromospheric emission flux in Ca II (H+K) is approximately proportional to the square root of the magnetic lling factor at the stellar surface. To relate the lling factor to stellar rotation rate, we compare the theoretical fluxes with observations of stars with known rotation period. The comparison shows that the Rossby number is probably a more appropriate measure of the rotation influence on activity of main-sequence stars than the rotation period. Our theoretical Mg II (h+k) and Ca II (H+K) emission fluxes are also found to be well correlated, which is in a good agreement with the observational data.

Acoustic and magnetic wave heating in stars

In the first paper of this series we developed a method to construct theoretical, time-dependent and two-component chromosphere models for late-type main sequence stars. The models consist of non-magnetic regions heated by acoustic waves and magnetic flux tube regions heated by magnetic tube waves. By specifying the magnetic filling factor, theoretical models of stellar atmospheres with different chromospheric activity can be calculated. Here, these models are used to simulate the emerging Ca II and Mg II chromospheric emission fluxes and compare them with observations. The comparison shows that the wave heating alone can explain most but not all of the observed range of chromospheric activity. In addition, the results obtained clearly imply that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave (e.g., reconnective) heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres.

Chromospheric models of a solar flare including velocity fields

We study the chromospheric structure of a small flare, before, during and after the rst hard X-ray spike. We construct 5 semiempirical models for dierent times, which will reproduce the proles of the H ,C a II Ka nd Si I 3905 lines during the flare evolution. In order to reproduce the intensity and the main characteristics of the line asymmetry, we introduce the velocity elds in the prole calculations. We nd that the whole chromosphere undergoes a strong upward motion within 1 s of the maximum of the rst hard X-ray spike, when the temperature and the density begin to rapidly increase. This seems the rst chromospheric response to the energy deposition and/or release. Only 6 s after the hard X-ray peak, a strong downflow begins in the low chromosphere and its velocity continues to increase even during the cooling phase.

Acoustic and magnetic wave heating in stars

We describe a method to construct theoretical, time-dependent, two-component and wave heated chromosphere models for late-type dwarfs. The models depend only on four basic stellar parameters: effective temperature, gravity, metallicity and filling factor, which determines the coverage of these stars by surface magnetic fields. They consist of non-magnetic regions heated by acoustic waves and vertically oriented magnetic flux tubes heated by longitudinal tube waves with contributions from transverse tube waves. Acoustic, longitudinal and transverse wave energy spectra and fluxes generated in stellar convection zones are computed and used as input parameters for the theoretical models. The waves are allowed to propagate and heat both components by shock dissipation. We compute the time-dependent energy balance between the dissipated wave energy and the most prominent chromospheric radiative losses as function of height in the stellar atmosphere. For the flux tube covered stars, the emerging radiative fluxes in the Ca II and Mg II lines are computed by using a newly developed multi-ray radiative transfer method.