reza rezaei | Islamic Azad University Of Mashhad (original) (raw)

Papers by reza rezaei

Astronomy & Astrophysics, 2006

We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolar... more We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolarimetry of three sunspots ($\theta$ = 27, 50, 75 deg) and their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron lines.} {To calibrate the Doppler shifts, we compare an absolute velocity calibration using the telluric O_2O_2O_2-line at 630.20 nm and a relative velocity calibration using the Doppler shift of Stokes V profiles in the umbra under the assumption that the umbra is at rest. Both methods yield the same result within the calibration uncertainties (~150 m/s). We study the radial dependence of Stokes V profiles in the directions of disk center and limb side. Maps of Stokes V profile shifts, polarity, amplitude asymmetry, field strength and magnetic field azimuth provide strong evidence for the presence of a magnetic canopy and for the existence of a radial outflow in the canopy. Our findings indicate that the Evershed flow does not cease abruptly at the white-light spot boundary, but that at least a part of the penumbral Evershed flow continues into the magnetic canopy.

Astronomy & Astrophysics, 2009

Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromosph... more Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE). Aims: We derived the mechanical wave energy as seen in line-core velocities on disc centre to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model, the Harvard-Smithsonian reference atmosphere (HSRA). Methods: We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we used the increase of internal energy (∝ R ρ Δ T) necessary to lift a RE atmosphere to the HSRA temperature stratification. Results: We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z ~ 500 km is insufficient to maintain on a long-term average the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z ~ 1000 km) can, however, be traced back to the velocity variations of the lowermost forming spectral line considered (z ~ 250 km). Conclusions: The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. The wave energy is found to be insufficient to maintain the temperature stratification of the semi-empirical HSRA model above 500 km. We will in a following paper of this series investigate the energy contained in the intensity variations to see if the semi-empirical model is appropriate for the spectra.

Astronomy & Astrophysics, 2008

We analyze a 1-hour time series of Ca II H intensity spectra and polarimetric spectra around 630 ... more We analyze a 1-hour time series of Ca II H intensity spectra and polarimetric spectra around 630 nm to derive the signature of the chromospheric heating and to investigate its relation to magnetic fields. We derived several characteristic quantities of Ca II H to define the chromospheric atmosphere properties. We study the power of the Fourier transform at different wavelengths and their phase relations. We perform local thermodynamic equilibrium inversions of the data to obtain the magnetic field, once including the Ca spectra. We find that the emission in the Ca II H line core at locations without detectable photospheric polarization signal is due to waves that propagate from low forming continuum layers in the line wing up to the line core. The phase differences of intensity oscillations at different wavelengths indicate standing waves for v < 2 mHz and propagating waves for higher frequencies. The waves steepen into shocks in the chromosphere. On average, shocks are both preceded and followed by intensity reductions. In field-free regions, the profiles show emission about half of the time. The correlation between wavelengths and the decorrelation time is significantly higher in the presence of magnetic fields than for field-free areas. The average Ca II H profile in the presence of magnetic fields contains emission features symmetric to the line core and an asymmetric contribution, where mainly the blue H2V emission peak is increased. We find that acoustic waves steepening into shocks are responsible for the emission in the Ca II H line core for locations without photospheric magnetic fields. We suggest using wavelengths in the line wing of Ca II H, where LTE still applies, to compare theoretical heating models with observations.

We study chromospheric emission to understand the temperature stratification in the solar chromos... more We study chromospheric emission to understand the temperature stratification in the solar chromosphere. We observed the intensity profile of the CaIIH line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 10^5 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative transfer code. A fraction of about 25% of the observed CaIIH line profiles do not show a measurable emission peak in H_{2v} and H_{2r} wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile. The observed reversal-free profiles indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere, but also with a very cool chromosphere as found in some dynamic simulations.

Astronomy & Astrophysics, 2007

Aims. We study the structure of magnetic elements of network-cell interiors. Methods. A quiet Sun... more Aims. We study the structure of magnetic elements of network-cell interiors. Methods. A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board of the Hinode space mission, which yielded the best ever achieved spatial resolution in polarimetric data of the Fe  630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation. Results. We find several examples of magnetic elements, either roundish (tube) or elongated (sheet), which show a central area of negative Stokes-V area asymmetry framed or surrounded by a peripheral area with larger positive asymmetry. This pattern was predicted on the basis of numerical simulations some eight years ago. Here, we observationally confirm its existence for the first time.

Astronomy & Astrophysics, 2007

We study the structure of the photospheric magnetic field of the quiet Sun by investigating weak ... more We study the structure of the photospheric magnetic field of the quiet Sun by investigating weak spectro-polarimetric signals. We took a sequence of Stokes spectra of the Fe I 630.15 nm and 630.25 nm lines in a region of quiet Sun near the disk center, using the POLIS spectro-polarimeter at the German VTT on Tenerife. The line cores of these two lines form at different heights in the atmosphere. The 3$\sigma$ noise level of the data is about 1.8 times10−3Ic\times 10^{-3} I_{c}times10−3Ic. We present co-temporal and co-spatial Stokes-$V$ profiles of the Fe I 630 nm line pair, where the two lines show opposite polarities in a single spectrum. We compute synthetic line profiles and reproduce these spectra with a two-component model atmosphere: a non-magnetic component and a magnetic component. The magnetic component consists of two magnetic layers with opposite polarity: the upper one moves upwards while the lower one moves downward. In-between, there is a region of enhanced temperature. The Stokes-$V$ line pair of opposite polarity in a single spectrum can be understood as a magnetic reconnection event in the solar photosphere. We demonstrate that such a scenario is realistic, but the solution may not be unique.

Astronomy & Astrophysics, 2009

Recent observations with the HINODE satellite have found abundant horizontal magnetic fields in t... more Recent observations with the HINODE satellite have found abundant horizontal magnetic fields in the internetwork quiet Sun. We compare the results on the horizontal fields with ground-based observations. We obtained 30 sec-integrated data of quiet Sun on disc centre during a period of very good seeing. The data have a rms noise in polarization of around 2 10^-4 of the continuum intensity. The low noise level allowed for an inversion of the spectra. We compare the inversion results with proxies for the determination of magnetic flux. We confirm the presence of the horizontal fields in the quiet Sun internetwork, with voids of some granules extent of nearly zero linear polarization signal. Voids in the circular polarization signal are only of granular scale. More than 60 % of the surface show polarization signals above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (gamma > 45 deg) is smaller by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are seen to suffer from a strong influence of the thermodynamic state of the atmosphere. We suggest that the difference of the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We caution that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to estimated by an explicit modeling with the observed spectra as boundary conditions, or has to be taken from MHD simulations.

Astronomy & Astrophysics, 2007

We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm a... more We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm and the intensity profile of the chromospheric Ca II H line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53 deg. We perform a statistical analysis of network and inter-network properties.The H-index is the integrated emission in a 0.1 nm band around the Ca core. We separate a non-magnetically, H_non, and a magnetically, H_mag, heated component from a non-heated component, H_co in the H-index. The average network and inter-network H-indices are equal to 12 and 10 pm, respectively. The emission in the network is correlated with the magnetic flux density, approaching a value of H 10 pm for vanishing flux. The inter-network magnetic field is dominated by weak field strengths with values down to 200 G and its mean absolute flux density is 11 Mx cm$^{-2}$. We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network, has non-magnetic origin (H_mag = 2pm, H_non = 3pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (H_co = 6pm) is about half of the observed H-index in the inter-network. The H-index in the inter-network is not correlated to any property of the photospheric magnetic field, suggesting that magnetic flux concentrations have a negligible role in the chromospheric heating in this region. The height range of the thermal coupling between the photosphere and low/mid chromosphere increases in presence of magnetic field. In addition, we demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a significant over-estimation of the magnetic field strength.

Astronomy & Astrophysics, 2006

We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolar... more We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolarimetry of three sunspots ($\theta$ = 27, 50, 75 deg) and their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron lines.} {To calibrate the Doppler shifts, we compare an absolute velocity calibration using the telluric O_2O_2O_2-line at 630.20 nm and a relative velocity calibration using the Doppler shift of Stokes V profiles in the umbra under the assumption that the umbra is at rest. Both methods yield the same result within the calibration uncertainties (~150 m/s). We study the radial dependence of Stokes V profiles in the directions of disk center and limb side. Maps of Stokes V profile shifts, polarity, amplitude asymmetry, field strength and magnetic field azimuth provide strong evidence for the presence of a magnetic canopy and for the existence of a radial outflow in the canopy. Our findings indicate that the Evershed flow does not cease abruptly at the white-light spot boundary, but that at least a part of the penumbral Evershed flow continues into the magnetic canopy.

Astronomy & Astrophysics, 2009

Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromosph... more Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE). Aims: We derived the mechanical wave energy as seen in line-core velocities on disc centre to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model, the Harvard-Smithsonian reference atmosphere (HSRA). Methods: We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we used the increase of internal energy (∝ R ρ Δ T) necessary to lift a RE atmosphere to the HSRA temperature stratification. Results: We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z ~ 500 km is insufficient to maintain on a long-term average the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z ~ 1000 km) can, however, be traced back to the velocity variations of the lowermost forming spectral line considered (z ~ 250 km). Conclusions: The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. The wave energy is found to be insufficient to maintain the temperature stratification of the semi-empirical HSRA model above 500 km. We will in a following paper of this series investigate the energy contained in the intensity variations to see if the semi-empirical model is appropriate for the spectra.

Astronomy & Astrophysics, 2008

We analyze a 1-hour time series of Ca II H intensity spectra and polarimetric spectra around 630 ... more We analyze a 1-hour time series of Ca II H intensity spectra and polarimetric spectra around 630 nm to derive the signature of the chromospheric heating and to investigate its relation to magnetic fields. We derived several characteristic quantities of Ca II H to define the chromospheric atmosphere properties. We study the power of the Fourier transform at different wavelengths and their phase relations. We perform local thermodynamic equilibrium inversions of the data to obtain the magnetic field, once including the Ca spectra. We find that the emission in the Ca II H line core at locations without detectable photospheric polarization signal is due to waves that propagate from low forming continuum layers in the line wing up to the line core. The phase differences of intensity oscillations at different wavelengths indicate standing waves for v < 2 mHz and propagating waves for higher frequencies. The waves steepen into shocks in the chromosphere. On average, shocks are both preceded and followed by intensity reductions. In field-free regions, the profiles show emission about half of the time. The correlation between wavelengths and the decorrelation time is significantly higher in the presence of magnetic fields than for field-free areas. The average Ca II H profile in the presence of magnetic fields contains emission features symmetric to the line core and an asymmetric contribution, where mainly the blue H2V emission peak is increased. We find that acoustic waves steepening into shocks are responsible for the emission in the Ca II H line core for locations without photospheric magnetic fields. We suggest using wavelengths in the line wing of Ca II H, where LTE still applies, to compare theoretical heating models with observations.

We study chromospheric emission to understand the temperature stratification in the solar chromos... more We study chromospheric emission to understand the temperature stratification in the solar chromosphere. We observed the intensity profile of the CaIIH line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 10^5 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative transfer code. A fraction of about 25% of the observed CaIIH line profiles do not show a measurable emission peak in H_{2v} and H_{2r} wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile. The observed reversal-free profiles indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere, but also with a very cool chromosphere as found in some dynamic simulations.

Astronomy & Astrophysics, 2007

Aims. We study the structure of magnetic elements of network-cell interiors. Methods. A quiet Sun... more Aims. We study the structure of magnetic elements of network-cell interiors. Methods. A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board of the Hinode space mission, which yielded the best ever achieved spatial resolution in polarimetric data of the Fe  630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation. Results. We find several examples of magnetic elements, either roundish (tube) or elongated (sheet), which show a central area of negative Stokes-V area asymmetry framed or surrounded by a peripheral area with larger positive asymmetry. This pattern was predicted on the basis of numerical simulations some eight years ago. Here, we observationally confirm its existence for the first time.

Astronomy & Astrophysics, 2007

We study the structure of the photospheric magnetic field of the quiet Sun by investigating weak ... more We study the structure of the photospheric magnetic field of the quiet Sun by investigating weak spectro-polarimetric signals. We took a sequence of Stokes spectra of the Fe I 630.15 nm and 630.25 nm lines in a region of quiet Sun near the disk center, using the POLIS spectro-polarimeter at the German VTT on Tenerife. The line cores of these two lines form at different heights in the atmosphere. The 3$\sigma$ noise level of the data is about 1.8 times10−3Ic\times 10^{-3} I_{c}times10−3Ic. We present co-temporal and co-spatial Stokes-$V$ profiles of the Fe I 630 nm line pair, where the two lines show opposite polarities in a single spectrum. We compute synthetic line profiles and reproduce these spectra with a two-component model atmosphere: a non-magnetic component and a magnetic component. The magnetic component consists of two magnetic layers with opposite polarity: the upper one moves upwards while the lower one moves downward. In-between, there is a region of enhanced temperature. The Stokes-$V$ line pair of opposite polarity in a single spectrum can be understood as a magnetic reconnection event in the solar photosphere. We demonstrate that such a scenario is realistic, but the solution may not be unique.

Astronomy & Astrophysics, 2009

Recent observations with the HINODE satellite have found abundant horizontal magnetic fields in t... more Recent observations with the HINODE satellite have found abundant horizontal magnetic fields in the internetwork quiet Sun. We compare the results on the horizontal fields with ground-based observations. We obtained 30 sec-integrated data of quiet Sun on disc centre during a period of very good seeing. The data have a rms noise in polarization of around 2 10^-4 of the continuum intensity. The low noise level allowed for an inversion of the spectra. We compare the inversion results with proxies for the determination of magnetic flux. We confirm the presence of the horizontal fields in the quiet Sun internetwork, with voids of some granules extent of nearly zero linear polarization signal. Voids in the circular polarization signal are only of granular scale. More than 60 % of the surface show polarization signals above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (gamma > 45 deg) is smaller by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are seen to suffer from a strong influence of the thermodynamic state of the atmosphere. We suggest that the difference of the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We caution that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to estimated by an explicit modeling with the observed spectra as boundary conditions, or has to be taken from MHD simulations.

Astronomy & Astrophysics, 2007

We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm a... more We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm and the intensity profile of the chromospheric Ca II H line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53 deg. We perform a statistical analysis of network and inter-network properties.The H-index is the integrated emission in a 0.1 nm band around the Ca core. We separate a non-magnetically, H_non, and a magnetically, H_mag, heated component from a non-heated component, H_co in the H-index. The average network and inter-network H-indices are equal to 12 and 10 pm, respectively. The emission in the network is correlated with the magnetic flux density, approaching a value of H 10 pm for vanishing flux. The inter-network magnetic field is dominated by weak field strengths with values down to 200 G and its mean absolute flux density is 11 Mx cm$^{-2}$. We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network, has non-magnetic origin (H_mag = 2pm, H_non = 3pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (H_co = 6pm) is about half of the observed H-index in the inter-network. The H-index in the inter-network is not correlated to any property of the photospheric magnetic field, suggesting that magnetic flux concentrations have a negligible role in the chromospheric heating in this region. The height range of the thermal coupling between the photosphere and low/mid chromosphere increases in presence of magnetic field. In addition, we demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a significant over-estimation of the magnetic field strength.