Opposite magnetic polarity of two photospheric lines in single spectrum of the quiet Sun (original) (raw)
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Solar atmospheric oscillations and the chromospheric magnetic topology
Astronomy and Astrophysics, 2007
Aims. We investigate the oscillatory properties of the quiet solar chromosphere in relation to the underlying photosphere, with particular regard to the effects of the magnetic topology. Methods. For the first time we perform a Fourier analysis on a sequence of line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0 nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained from full spectroscopic data acquired at high spatial resolution with the Interferometric BIdimensional Spectrometer (IBIS). The field of view encompasses a full supergranular cell, allowing us to discriminate between areas with different magnetic characteristics. Results. We show that waves with frequencies above the acoustic cut-off propagate from the photosphere to upper layers only in restricted areas of the quiet Sun. A large fraction of the quiet chromosphere is in fact occupied by "magnetic shadows", surrounding network regions, that we identify as originating from fibril-like structures observed in the core intensity of the Ca II line. We show that a large fraction of the chromospheric acoustic power at frequencies below the acoustic cut-off, residing in the proximity of the magnetic network elements, directly propagates from the underlying photosphere. This supports recent results arguing that network magnetic elements can channel low-frequency photospheric oscillations into the chromosphere, thus providing a way to input mechanical energy in the upper layers. Conclusions.
Spectropolarimetric diagnostics of unresolved magnetic fields in the quiet solar photosphere
Proceedings of the International Astronomical Union, 2012
A few years before the Hinode space telescope was launched, an investigation based on the Hanle effect in atomic and molecular lines indicated that the bulk of the quiet solar photosphere is significantly magnetized, due to the ubiquitous presence of an unresolved magnetic field with an average strength 〈B〉, ≈ 130 G. It was pointed out also that this “hidden” field must be much stronger in the intergranular regions of solar surface convection than in the granular regions, and it was suggested that this unresolved magnetic field could perhaps provide the clue for understanding how the outer solar atmosphere is energized. In fact, the ensuing magnetic energy density is so significant that the energy flux estimated using the typical value of 1 km/s for the convective velocity (thinking in rising magnetic loops) or the Alfvén speed (thinking in Alfvén waves generated by magnetic reconnection) turns out to be substantially larger than that required to balance the chromospheric energy los...
Measurement of the evolution of the magnetic field of the quiet photosphere during a solar cycle
arXiv: Solar and Stellar Astrophysics, 2017
The solar photosphere is filled by a magnetic field which is tangled on scales much smaller than the resolution capability of solar telescopes. This hidden magnetic field can be investigated via the Hanle effect. In 2007 we started a synoptic program to explore if the magnetic flux of the quiet photosphere varies with the solar cycle. For this purpose we applied a differential Hanle effect technique based on observations of scattering polarization in C$_2$ molecular lines around 514.0 nm, taken generally every month. Our results now span almost one complete solar cycle.
We used improved data processing on simultaneous multi-wavelength observations at high spatial resolution with THEMIS, in order to identify thermodynamic structures and show that both magnetic and velocity gradients appear in every solar structure (quiet region as well as faculae). The asymmetries of polarized spectra were analyzed accurately for different line formation heights. We present observations that can be useful to investigate the validity of inversion techniques, to detect the presence of canopies and magnetic oscillations, and to estimate accurately line formation heights. We also present evidence of the coupling between the local thermodynamic parameters and the magnetic field, using a selection of both magnetically sensitive and insensitive lines.
Analysis of photospheric magnetic fields in AR 12546: a case study
Astrophysics and Space Science
We investigate high-resolution observations with the spectropolarimeter (SP) aboard the Hinode satellite of the Solar Optical Telescope (SOT) of a positive polarity sunspot of an active region (AR) (NOAA 12546). We present a case study for the properties of the thermal, magnetic field, and plasma flows as a function of the optical depth from the inversion of the observed Stokes profiles, covering a wide field of view area. Particular attention is paid to the examination of the net circular polarization (NCP) and area asymmetry of spectral lines in sunspots. We detect a large red-shifted velocity of 10 km sec−1 localized with the presence of a strong magnetic field corresponding to the NCP best fit of the inverted profiles. In addition, the comparison between the observed and calculated NCPs or Stokes V area asymmetries of spectral lines fitted well for most pixels in the field of view region, with a significant indication of a single-component inversion. We study the vertical gradie...
Astronomy and Astrophysics, 2005
This paper presents and interprets some observations of the limb polarization of Sr 4607 Å obtained with the spectropolarimeter of the French-Italian telescope THEMIS in quiet regions close to the solar North Pole on 2002 December 7-9. The linear polarization was measured for a series of limb distances ranging from 4 to 160 arcsec, corresponding to heights of optical depth unity in the line core ranging from about 330 to 220 km, respectively, above the τ 5000 = 1 level. To increase the polarimetric sensitivity, the data were averaged along the spectrograph slit (one arcmin long) set parallel to the solar limb. Since the data show no rotation of the linear polarization direction with respect to the limb direction, the observed depolarization is ascribed to the Hanle effect of a turbulent weak magnetic field, the zero-field polarization being derived from a model. The interpretation is performed by means of an algorithm which describes the process of line formation in terms of the atomic density matrix formalism, the solar atmosphere being described by an empirical, plane-parallel model. The collisional rates entering the model (inelastic collisions with electrons, elastic depolarizing collisions with neutral hydrogen), have been computed by applying fast semi-classical methods having a typical accuracy of the order of 20% or better (see Derouich 2004), leading to 6% inaccuracy on the magnetic field strength determination. We assume a unimodal distribution for the intensity of the turbulent field. The computed intensity profile has been adjusted to the observed one in both depth and width, by varying both microturbulent and macroturbulent velocities. The best adjustment is obtained for respectively 1.87 km s −1 (micro) and 1.78 km s −1 (macro). The evaluation of the magnetic depolarization leads then to the average value of 46 Gauss for the turbulent magnetic field strength, with a gradient of −0.12 Gauss/km. Our results are in very good agreement with the value of 60 Gauss determined at large µ, in the volume-filling field case, by Trujillo Bueno et al. (2004, Nature, 430, 326), using a 3D magneto-convective simulation. This validates our method.
Spectropolarimetric Investigations of the Deep Photospheric Layers of Solar Magnetic Structures
Solar surface magnetism manifests itself in a variety of structures with sizes often comparable or even below our spatial resolution capabilities. Nevertheless, sub-resolution information about the intrinsic atmospheric structure can be obtained via indirect techniques. We use state-of-the-art spectropolarimetric observations in carefully selected photospheric lines which include C i (5380.3Å) as well as strong lines of Fe i, Ti i covering also the deep layers of the photosphere and obtain ratios of their Stokes V amplitudes. From there we deduce that the temperature within magnetic features is higher at locations of smaller magnetic flux.
Magnetic reconnection signatures in the solar atmosphere: results from multi-wavelength observations
2011
In the solar atmosphere magnetic reconnection is invoked as the main mechanism causing very energetic events (1028 - 1032 erg), like flares and coronal mass ejections, as well as other less energetic phenomena, like microflares, X-ray jets and chromospheric surges. In the last decade, thanks to high spatial resolution, multi-wavelength observations carried out by both ground-based telescopes (THEMIS, SST, VTT, DST) and space-born satellites (SOHO, TRACE, RHESSI, HINODE), it has been possible to study these phenomena and several signatures of the occurrence of magnetic reconnection have been singled out. In this paper, we describe some results obtained from the analysis of multi-wavelength observations carried out in the last years, with special emphasis on those events that were characterized by plasma outflows from the reconnection site. The events here discussed are relevant to some active regions observed on the Sun, characterized by the interaction of different bundles of magnet...
Long-term studies of photospheric magnetic fields on the Sun
Journal of Space Weather and Space Climate, 2020
We briefly review the history of observations of magnetic fields on the Sun, and describe early magnetograps for full disk measurements. Changes in instruments and detectors, the cohort of observers, the knowledge base etc may result in non-uniformity of the long-term synoptic datasets. Still, such data are critical for detecting and understanding the long-term trends in solar activity. We demonstrate the value of historical data using studies of active region tilt (Joy’s law) and the evolution of polar field and its reversal. Using the longest dataset of sunspot field strength measurements from Mount Wilson Observatory (1917-present) supplemented by shorter datasets from Pulkovo (1956–1997) and Crimean (1956-present) observatories we demonstrate that the magnetic properties of sunspots did not change over the last hundred years. We also show that the relationship between the sunspot area and its magnetic flux can be used to extend the studies of magnetic field in sunspots to period...