arXiv subjects

R. Rezaei

Publications and source records attributed to R. Rezaei.

42 records · Page 3Linked to original sources

arXiv2007

Freeware solutions for spectropolarimetric data reduction

Most of the solar physicists use very expensive software for data reduction and visualization. We present hereafter a reliable freeware solution based on the Python language. This is made possible by the association of the latter with a small set of additional libraries developed in the scientific community. It provides then a very powerful and economical alternative to other interactive data languages. Although it can also be used for any kind of post-processing of data, we demonstrate the capabities of such a set of freeware tools using THeMIS observations of the second solar spectrum.

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arXiv2007

Temporal evolution of magnetic elements

We study the structure and evolution of the magnetic field of the quiet Sun by investigating weak spectro-polarimetric signals. To this end, we observed a quiet region close to the disk center with the German VTT in Tenerife, July 07, 2006. We recorded 38 scans of the same area. Each scan was eight arcsec wide and observed within about 100 seconds. We used POLIS to simultaneously observe Stokes profiles of the neutral iron lines at 630.15 and 630.25 nm, the Stokes-I profile of the Ca II H line at 396.8 nm, and a continuum speckle channel at 500 nm. We witness two examples of magnetic flux cancellation of small-scale opposite-polarity patches, followed by an enhanced chromospheric emission. In each case, the two opposite-polarity patches gradually became smaller and, within a few minutes, the smaller one completely disappeared. The larger patch also diminished significantly. We provide evidence for a cancellation scenario in the photosphere which leaves minor traces at the chromospheric level.

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arXiv2007

Hinode observations reveal boundary layers of magnetic elements in the solar photosphere

We study the structure of the magnetic elements in network-cell interiors. A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board the Hinode space mission, which yielded the best spatial resolution ever achieved in polarimetric data of the Fe I 630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation. 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 some eight years ago on the basis of numerical simulations. Here, we observationally confirm its existence for the first time. We gather convincing evidence that this pattern of Stokes-V area asymmetry is caused by the funnel-shaped boundary of magnetic elements that separates the flux concentration from the weak-field environment. We also conclude that this kind of magnetic element of the internetwork is accompanied by electric current sheets.

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arXiv2007

Opposite magnetic polarity of two photospheric lines in single spectrum of the quiet Sun

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$σ$ noise level of the data is about 1.8 $\times 10^{-3} I_{c}$. 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.

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arXiv2007

Relation between photospheric magnetic field and chromospheric emission

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.

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arXiv2007

Photospheric magnetic field and chromospheric emission

We present a statistical analysis of network and internetwork properties in the photosphere and the chromosphere. For the first time we simultaneously observed (a) the four Stokes parameters of the photospheric iron line pair at 630.2 nm and (b) the intensity profile of the Ca II H line at 396.8 nm. The vector magnetic field was inferred from the inversion of the iron lines. We aim at an understanding of the coupling between photospheric magnetic field and chromospheric emission.

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