Blue Wing Research Articles

New explanation of opposite position angles in red and blue wings of spectral lines

ABSTRACTWe consider the emission of resonance line radiation from rotating circular accretion disc with the progressive increasing height (the inclined ring). Our theory can also be applied to the rotating spot-like sources of resonance radiation. We assume that the atmosphere of inclined ring is homogeneous. In this case, the every part of ring emits the radiation with the wave electric field oscillations perpendicular to plane $(\boldsymbol{nN^{\prime }})$, where ${\boldsymbol{n}}$ is the direction to a telescope and $\boldsymbol{N^{\prime }}$ is the normal to considered local surface of a ring. Geometrical consideration shows that the radiation polarization angle χ (frequently denoted as position angle or PA) depends on the inclination angle α of the ring relative to the central plane of accretion disc, the inclination angle θ of the central plane of accretion disc with the normal $\boldsymbol{N}$, and on azimuthal angle φ of radiating part on the ring. The right and left parts of a ring according to the symmetry plane $(\boldsymbol{nN})$ give rise to opposite polarization angles. For rotating accretion disc, due to Doppler’s effect, this means that polarization angles have opposite signs in the red and blue wings of emerging spectral line radiation. Such behaviour is observed in many objects.

Light guidance in photonic structures of Morpho butterfly wing scales

In this paper, we investigate the impact of the periodicity along three orthogonal axes of the blue wing scales of Morpho butterflies, whereas most of the previous studies modeled the scale structure only considering one or two photonic crystal dimensions. Moreover, the previous optical studies of these scales focused on the light reflected by the wing, whereas we investigate the light propagation along the lamellae, this direction corresponding to the third dimension of the photonic crystal structure. Simulation results obtained using the finite-element method are compared with measurements and/or literature. These calculations, performed for different scale models and orientations, show that a significant part of the non-reflected light, essentially red and infrared, is guided by the lamellae towards the base of the scales where it can be more easily absorbed and the heat more quickly transferred to the hemolymph. This new phenomenon could contribute to the thermal balance of the insect and further illustrates the multifunctionality of the wings of Lepidoptera.

Global 3D hydrodynamic modeling of absorption in Lyα and He 10830 A lines at transits of GJ3470b

Abstract Warm Neptune GJ3470b has been recently observed in 23S-23P transition of metastable helium, yielding absorption of about 1 per cent in Doppler velocity range of [-40; 10] km/s. Along with previous detection of absorption in Lyα with depth of 20–40 per cent in the blue and red wings of the line, it offers a complex target for simulation and testing of the current models. Obtained results suggest that absorption in both these lines comes from interaction of expanding upper planetary atmosphere with stellar plasma wind, allowing to constrain the stellar plasma parameters and the helium abundance in planet atmosphere.

Intensity and Polarization of Radiation Emerging from Rotating Accretion Disc with Increasing Height

We consider the radiation emission in continuum and spectral lines from rotating accretion disc with the progressive increasing height. It is known that for the plane accretion disc with homogeneous atmosphere the wave electric field E is perpendicular to the plane between line of sight n and the normal to the disc N. For the expanding accretion disc the wave electric field from every inclined cone-like part with given azimuthal angle φ has direction perpendicular to the plane between n and local normal N′ to the inclined surface. This behaviour is the consequence of homogeneity of inclined atmosphere and has purely geometrical origin. The geometrical consideration shows that the position angles of polarized radiation in the right and left parts relative to plane (nN) of the inclined accretion disc have opposite values. Therefore, for inclined accretion disc the integral continuum radiation has the usual polarization angle perpendicular to the plane (nN), but smaller degree of polarization and less alongated along the normal N than that for the plane accretion disc. For spectral line, due to the Doppler effect, the polarization (position) angles have opposite signs for the red and blue wings. Such behaviour is frequently observed in Hα-radiation.

Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017

In this paper we present the interpretation of the observations of the flare from 6 September 2017 reported in Paper I. These include gamma-ray (GR), hard X-ray (HXR), soft X-rays, Lyαline, extreme ultraviolet (EUV), Hα, and white light (WL) emission, which were recorded during the two flaring events 1 (FE1) and 2 (FE2) that occurred at 11:55:37 UT (FE1) and 12:06:40 UT (FE2). Paper I also reported the first detection of the sunquake with first and second bounces of seismic waves combined with four other sunquakes in different locations supported with the observations of HXR, GR, EUV, Hα, and WL emission with strongly varying spatial resolution and temporal coverage. In the current paper, we propose some likely scenarios for heating of flaring atmospheres in the footpoints with sunquakes which were supported with EUV and Hαemission. We used a range of parameters derived from the HXR, EUV, and Hαline observations to generate hydrodynamic models, which can account for the blueshifts derived from the EUV emission and the redshifts observed with the EUV Imaging Spectrometer in the He II line and by the CRisp Imaging Spectro-Polarimeter in the Swedish Solar Telescope in Hαline emission. The parameters of hydrodynamic shocks produced by different beams in flaring atmospheres were used as the initial conditions for another type of hydrodynamic models that were developed for acoustic wave propagation in the solar interior. These models simulate the sets of acoustic waves produced in the interior by the hydrodynamic shocks from atmospheres above deposited in different footpoints of magnetic loops. The Hαline profiles with large redshifts in three kernels (two in FE1 and one in FE2) were interpreted with the full non-local thermodynamic equilibrium radiative simulations in all optically thick transitions (Lyman lines and continuum Hα, Hβ, and Pα) applied for flaring atmospheres with fast downward motions while considering thermal and non-thermal excitation and ionisation of hydrogen atoms by energetic power-law electron beams. The observed Hαline profiles in three kernels were fit with the simulate blue wing emission of the Hαline profiles shifted significantly (by 4–6 Å) towards the line red wings, because of strong downward motions with velocities about 300 km s−1by the shocks generated in flaring atmospheres by powerful beams. The flaring atmosphere associated with the largest sunquake (seismic source 2 in FE1) is found consistent with being induced by a strong hydrodynamic shock produced by a mixed beam deposited at an angle of −30° from the local vertical. We explain the occurrence of a second bounce in the largest sunquake by a stronger momentum delivered by the shock generated in the flaring atmosphere by a mixed beam and deeper depths of the interior where this shock was deposited. Indeed, the shock with mixed beam parameters is found deposited deeply into the interior beneath the flaring atmosphere under the angle to the local vertical that would allow the acoustic waves generated in the direction closer to the surface to conserve enough energy for the second bounces from the interior layers and from the photosphere. The wave characteristics of seismic sources 1 and 3 (in FE1) were consistent with those produced by the shocks generated by similar mixed beams deposited at the angles −(0 − 10)° (seismic source 1) and +30° (seismic source 3) to the local vertical. The differences of seismic signatures produced in the flares of 6 September 2011 and 2017 are also discussed.

A review. blue Wing Disease -chicken infectious anaemia molecular study

Blue wing is a diseased condition of discoloured skin of wing caused mainly by Chicken anaemia virus and or reovirus. This review designed to illustrate the causative agent/s and molecular structure of the disease, epidemiology and common characters referring to pubmed as a references search tool. The disease is major cause of anaemia in chicken and the virus possess the ability to induce apoptosis in numerous chicken derived cells and tumor cell lines.

Chandra High Energy Transmission Gratings Spectra of V3890 Sgr

Abstract The recurrent nova (RN) V3890 Sgr was observed during the seventh day after the onset of its most recent outburst, with the Chandra ACIS-S camera and High Energy Transmission Gratings. A rich emission line spectrum was detected, due to transitions of Fe-L and K-shell ions ranging from neon to iron. The measured absorbed flux is ≈10−10 erg cm−2 s−1 in the 1.4–15 Å range (0.77–8.86 keV). The line profiles are asymmetric, blueshifted, and skewed toward the blue side, as if the ejecta moving toward us are less absorbed than the receding ejecta. The full width at half-maximum of most emission lines is 1000–1200 km s−1, with some extended blue wings. The spectrum is thermal and consistent with a plasma in collisional ionization equilibrium with column density 1.3 × 1022 cm−2 and at least two components at temperatures of about 1 and 4 keV, possibly a forward and a reverse shock, or regions with differently mixed ejecta and a red giant wind. The spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we cannot distinguish whether the shocks occurred at a distance of a few au from the red giant, or near the giant’s photosphere, in a high-density medium containing only a low mass. The ratios of the flux in lines of aluminum, magnesium, and neon relative to the flux in lines of silicon and iron probably indicate a carbon–oxygen white dwarf.

The Reddening and [O ii] Emissions of the Quasar Mg ii Absorption-line Systems

Abstract Using quasar Mg ii narrow absorption lines (NALs) with velocity offset ( , where c is the speed of light) , this paper investigates the emissions and reddening associated with Mg ii NALs by constructing composite spectra. Dust extinctions of all the inflow ( ), environment (−750 ≤ r < 600 ), outflow ( ), and strong intervening-like (2000 ≤ r < 6000 ) Mg ii NALs can be described by the SMC extinction curve, which suggest that all four types of Mg ii NALs have similar dust properties. The colors of quasars hosting intervening-like Mg ii NALs with Å and intervening Mg ii NALs ( ) are similar to those of control quasars (without Mg ii NALs with ), which suggests that these two types of Mg ii NALs are mainly formed within media unconnected with background quasars. The other three types of Mg ii NALs have much more obvious reddening to background quasars, and the stronger absorptions or the absorptions detected in radio detected quasars produce larger reddening than the weaker absorptions or the absorptions in radio undetected quasars. In addition, the dust-to-gas ratios within inflow Mg ii NALs are possibly lower than those within environment ones. We find that flux ratios / of quasars hosting inflow, outflow, intervening-like, and intervening Mg ii NALs are similar to those of control quasars. For quasars hosting environment Mg ii NALs, (1) the flux ratio / is much higher than that of control quasars, which suggests that there is a high star formation rate within the host galaxies of environment Mg ii NALs; (2) the flux ratio / is positively correlated with absorption strengths; and (3) radio detected quasars have a slightly higher flux ratio / when compared to radio undetected quasars, which suggests that the quasar feedback enhances the star formation rate within host galaxies of environment absorbers. For quasars hosting outflow Mg ii NALs, we find that emission lines display excesses at blue wings with respect to the line profiles of control quasars, and the excesses are positively correlated with absorption strengths.

Structural color in Junonia butterflies evolves by tuning scale lamina thickness.

In diverse organisms, nanostructures that coherently scatter light create structural color, but how such structures are built remains mysterious. We investigate the evolution and genetic regulation of butterfly scale laminae, which are simple photonic nanostructures. In a lineage of buckeye butterflies artificially selected for blue wing color, we found that thickened laminae caused a color shift from brown to blue. Deletion of the optix patterning gene also altered color via lamina thickening, revealing shared regulation of pigments and lamina thickness. Finally, we show how lamina thickness variation contributes to the color diversity that distinguishes sexes and species throughout the genus Junonia. Thus, quantitatively tuning one dimension of scale architecture facilitates both the microevolution and macroevolution of a broad spectrum of hues. Because the lamina is an intrinsic component of typical butterfly scales, our findings suggest that tuning lamina thickness is an available mechanism to create structural color across the Lepidoptera.

H–He collision-induced satellite in the Lyman α profile of DBA white dwarf stars

ABSTRACT The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centred around 1160 Å in the blue wing of the Lyman α line. It is extremely apparent in WD 1425+540 recently observed with Hubble Space Telescope(HST) Cosmic Origins Spectrograph (COS). With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman α seen in this and other white dwarf spectra. Our previous work predicting this absorption feature was limited by molecular potentials that were not adequate to follow the atomic interactions with spectroscopic precision to the asymptotic limit of large separation. A new set of potential energy curves and electronic dipole transition moments for the lowest electronic states of the H–He system were developed to account accurately for the behaviour of the atomic interactions at all distances, from the chemical regime within 1 Å out to where the radiating H atoms are not significantly perturbed by their neighbours. We use a general unified theory of collision-broadened atomic spectral lines to describe a rigorous treatment of hydrogen Lyman α with these potentials and present a new study of its broadening by radiative collisions of hydrogen and neutral helium. These results enable ab initio modelling of radiative transport in DBA white dwarf atmospheres.

Deformation of the emission line profile of optically dense spherical plasma caused by rotation of substance

The transfer of resonance radiation in optically dense spherical plasma, rotating around the axis of symmetry, is studied numerically. Frequency shifts in the red and blue wings of the spectrum, frequency broadening of the emission line profile and the absorption line profile, caused by the rotational broadening effect (RBE), are investigated for calcium plasma at the resonant transition with the wavelength of 397 nm. The RBE of absorption and emission lines is explained by broadening of the elementary ion absorption profiles which demonstrate the Doppler dependence on frequency. Location of the frequency of absorption maximum in broadband radiation transmitted through the plasma, as well as location of the frequencies of intensity maximum of the unreabsorbed emission line and intensity minimum of the reabsorbed emission line, unambiguously determine ray velocity of ions in the plasma. The RBE effect of the emission line leads to an increase in probability of photon escape from the medium. This circumstance is indirect evidence for the fact that Holstein’s effective deexcitation time for rotating plasma should decrease. Full width at half maximum (FWHM) of the emission spectrum for rotating plasma increases linearly with the growth of optical thickness of the medium. Measurement of the FWHM value can be a diagnostic method for determining concentration of ions in the plasma.

Spectroscopy and polarimetry of the gravitationally lensed quasar SDSS J1004+4112 with the 6m SAO RAS telescope

Context. We present new spectroscopic and polarimetric observations of the gravitational lens SDSS J1004+4112 taken with the 6 m telescope of the Special Astrophysical Observatory (Russia). Aims. In order to explain the variability that is observed only in the blue wing of the C IV emission line, corresponding to image A, we analyze the spectroscopy and polarimetry of the four images of the lensed system. Methods. Spectra of the four images were taken in 2007, 2008, and 2018, and polarization was measured in the period 2014–2017. Additionally, we modeled the microlensing effect in the polarized light, assuming that the source of polarization is the equatorial scattering in the inner part of the torus. Results. We find that a blue enhancement in the C IV line wings affects component A in all three epochs. We also find that the UV continuum of component D was amplified in the period 2007–2008, and that the red wings of CIII] and C IV appear brighter in D than in the other three components. We report significant changes in the polarization parameters of image D, which can be explained by microlensing. Our simulations of microlensing of an equatorial scattering region in the dusty torus can qualitatively explain the observed changes in the polarization degree and angle of image D. We do not detect significant variability in the polarization parameters of the other images (A, B, and C), although the averaged values of the polarization degree and angle are different for the different images. Conclusions. Microlensing of a broad line region model including a compact outflowing component can qualitatively explain the C IV blue wing enhancement (and variation) in component A. However, to confirmed this hypothesis, we need additional spectroscopic observation in future.

Chicken Anaemia Virus in Egyptian Broiler; Clinical Signs and Molecular Characterization

A total of 18 flocks , 45 day old broiler from Ismailia, Egypt were tested for chicken anemia virus (CAV) using serological evidence and molecular characterization. Tested birds had history of diarrhea, blue wing, depression, and deaths. Necropsy findings were pale liver, severe atrophy of bursa Fabricius and thymus, pale discoloration of the bone marrow and subcutaneous hemorrhages. The viral DNA of the VP1 gene was detected and sequenced. Nucleic acid alignment showed 100% similarities with many CAV viruses. Antibodies against CAV were detected in 73.9% of the tested samples using commercial ELISA kit. Hemoglobin (Hb) revealed low percentage (4.3% to 8.02%), and packed cell volume (PCV) were measured and showed anemic level (22.5 to 34.2). The data strengthen the concept that CAV may have a negative economic impact on the chicken industry in Ismailia.

Transition Region Loops in the Very Late Phase of Flux Emergence in IRIS Sit-and-stare Observations

Abstract Loops are one of the fundamental structures that trace the geometry of the magnetic field in the solar atmosphere. Their evolution and dynamics provide a crucial proxy for studying how the magnetized structures are formed and heated in the solar atmosphere. Here, we report on spectroscopic observations of a set of transition-region loops taken by the Interface Region Imaging Spectrograph (IRIS) at Si iv 1394 Å in the sit-and-stare mode. The loops are corresponding to the flux emergence at its very late phase when the emerged magnetic features in the photosphere have fully developed. We find the transition-region loops are still expanding and moving upward with a velocity of a few kilometers per second (≲10 km s−1) at this stage. The expansion of the loops leads to interactions between the loops themselves and with the ambient field, which can drive magnetic reconnection evidenced by multiple intense brightenings, including transition-region explosive events and IRIS bombs in the footpoint region associated with the moving polarity. A set of quasi-periodic brightenings with a period of about 130 s is found at the loop apex, from which the Si iv 1394 Å profiles are significantly non-Gaussian with enhancements at both blue and red wings at Doppler velocities of about 50 km s−1. We suggest that the transition-region loops in the very late phase of flux emergence can be powered by heating events generated by the interactions between the expanding loops and the ambient fields and also by (quasi-)periodic processes, such as oscillation-modulated braiding reconnection.

Decrease in imprisonment of resonance radiation due to the broadening of the absorption line contour caused by the rotation of the plasma sphere

Imprisonment of resonance radiation in the spherical barium plasma rotating around its symmetry axis is numerically studied. Photoexcitation of barium ions at resonance transition with the wavelength λ0= 455.4 nm by laser radiation is modeled. A numerical algorithm allows one to calculate the emission spectrum and the Holstein's escape factor of resonant radiation using the afterglow intensity of rotating plasma. The full width at half maximum of the emission line is considerably wider than the one for the static medium, and the central frequency of emission is shifted to either red or blue wing of the spectrum. Location of the frequency of intensity maximum of the emission line unambiguously determines the projection of velocity of ions on the ray direction in the plasma. For rotational plasma, the imprisonment of resonant quanta decreases due to the broadening of the ion absorption line. The appearance of two qualitatively novel effects that distinguish rotating plasma from static one is predicted: 1) more facilitated diffusion of resonant photons to the medium surface and more facilitated escape of them out of it (effect strengthened escape); 2) presence of a velocity gradient along the sphere radius leads to “spreading” of the line over a wider frequency range, impeding free propagation of continual photons (effect of frequency band narrowing).

Global 3D Hydrodynamic Modeling of In-transit Lyα Absorption of GJ 436b

Abstract Using a global 3D, fully self-consistent, multifluid hydrodynamic model, we simulate the escaping upper atmosphere of the warm Neptune GJ 436b, driven by the stellar X-ray and ultraviolet (XUV) radiation impact and gravitational forces and interacting with the stellar wind. Under the typical parameters of XUV flux and stellar wind plasma expected for GJ 436, we calculate in-transit absorption in Lyα and find that it is produced mostly by energetic neutral atoms outside of the planetary Roche lobe, due to the resonant thermal line broadening. At the same time, the influence of radiation pressure has been shown to be insignificant. The modeled absorption is in good agreement with the observations and reveals such features as strong asymmetry between blue and red wings of the absorbed Lyα line profile, deep transit depth in the high-velocity blue part of the line reaching more than 70%, and the timing of early ingress. On the other hand, the model produces significantly deeper and longer egress than in observations, indicating that there might be other processes and factors, still not accounted for, that affect the interaction between the planetary escaping material and the stellar wind. At the same time, it is possible that the observational data, collected in different measurement campaigns, are affected by strong variations of the stellar wind parameters between the visits, and therefore they cannot be reproduced altogether with the single set of model parameters.

Properties of radio-loud quasars in the Sloan Digital Sky Survey

We present a study of a sample of 223 radio-loud quasars (up to redshift < 0.3) in order to investigate their spectral properties. Twenty-six of these radio-loud quasars are identified as flat-spectrum radio quasars (FSRQs), and 54 are identified as steep-spectrum radio quasars (SSRQs) based on their radio spectral index. We study the [O III] line properties of these quasars to investigate the origin and properties of blue wings (shift of the profile toward lower wavelengths) and blue outliers (shift of the whole spectroscopic feature). Most of the quasars show blue wings with velocities of up to 420 km s−1. We find that about 17% of the quasars show outliers whose velocities span from 419 to −315 km s−1. Finally, we revisit the MBH − σ relation of our sample using the [S II]λ 6716,6731 and [O III] line widths as surrogates for stellar velocity dispersions, σ, to investigate their location on the MBH − σ relation for quiescent galaxies. Because [S II] is strongly blended with Hα, we were able to estimate σ[S II] for only 123 quasars. We find that the radio-loud quasars do not show a relationship between MBH and σ[S II]/[O III] up to a redshift of 0.3, although they cluster around the local relation. We find an overall offset of 0.12 ± 0.05 dex of our sample of radio-loud quasars from the MBH − σ relation of quiescent galaxies. Quasars in our highest redshift bin (z = 0.25 − 0.3) show a deviation of ∼0.33 ± 0.06 dex from the local relation. Implications of the results are discussed.

Counter-rotation and High-velocity Outflow in the Parsec-scale Molecular Torus of NGC 1068

Abstract We present 1.4 pc resolution observations of 256 GHz nuclear radio continuum and HCN (J = 3 → 2) in the molecular torus of NGC 1068. The integrated radio continuum emission has a flat spectrum consistent with free–free emission and resolves into an X-shaped structure resembling an edge-brightened bicone. HCN is detected in absorption against the continuum, and the absorption spectrum shows a pronounced blue wing that suggests a high-velocity molecular outflow with speeds reaching 450 km s−1. Analysis of the off-nucleus emission line kinematics and morphology reveals two nested, rotating disk components. The inner disk, inside r ∼ 1.2 pc, has kinematics that are consistent with the nearly edge-on, geometrically thin H2O megamaser disk in Keplerian rotation around a central mass of 1.66 × 107 M ⊙. The outer disk, which extends to ∼7 pc radius, counter-rotates relative to the inner disk. The rotation curve of the outer disk is consistent with rotation around the same central mass as the megamaser disk but in the opposite sense. The morphology of the molecular gas is asymmetric around the nuclear continuum source. We speculate that the outer disk formed from more recently introduced molecular gas falling out of the host galaxy or from a captured dwarf satellite galaxy. In NGC 1068, we find direct evidence that the molecular torus consists of counter-rotating and misaligned disks on parsec scales.

The CGM–GRB Study. I. Uncovering the Circumgalactic Medium around GRB Hosts at Redshifts 2–6

Abstract Recent studies have revealed a dynamic interplay between the galaxy ecosystem and circumgalactic medium (CGM). We investigate the CGM at high redshifts (z ≳ 2) by using bright afterglows of gamma-ray bursts (GRBs) as background sources. We compiled a sample of medium-resolution (Δv < 50 km s−1) and high signal-to-noise ratio (typical S/N ∼ 10) spectra from 27 GRB afterglows covering z ∼ 2–6, with six of them at z ≳ 4. We analyzed the high- and low-ionization absorption features within ±400 km s−1 to extract the CGM and interstellar medium (ISM) kinematics. In the stacked spectra, high-ionization absorption profiles (e.g., C iv, Si iv) present significant absorption excess in their blue wings (v < −100 km s−1) relative to the red wings (v > 100 km s−1). The stronger blue wings in high-ionization species are indicative of the presence of ubiquitous warm outflows in the GRB hosts at high redshifts. We used simple toy models to kinematically distinguish the CGM and ISM absorption and estimate the CGM mass and outflow velocity. We find tentative evidence of the evolution of the CGM metal mass by ∼0.5 dex between two redshift bins, each spanning 1 Gyr, z1: 2–2.7 and z2: 2.7–5. By comparing with past studies, we find that over the course of evolution of present-day galaxies with M * > 1010 M ⊙, the ratio of C iv mass in the CGM to the stellar mass remains fairly uniform, with log(M C iv /M *) ∼ −4.5 within ±0.5 dex from z ∼ 4 to z ∼ 0, suggesting CGM–galaxy coevolution.

Height variation of magnetic field and plasma flows in isolated bright points

Aims. The expansion with height of the solar photospheric magnetic field and the plasma flows is investigated for three isolated bright points (BPs). Methods. The BPs were observed simultaneously with three different instruments attached to the 1.5 m GREGOR telescope: (1) filtergrams of Ca II H and blue continuum (4505 Å) with the HiFI, (2) imaging spectroscopy of the Na I D2 line at 5890 Å with the GFPI, and (3) slit spectropolarimetry in the 1 μm spectral range with the GRIS. Spectral-line inversions were carried out for the Si I 10827 Å Stokes profiles. Results. Bright points are identified in the Ca II H and blue continuum filtergrams. Moreover, they are also detected in the blue wing of the Na I D2 and Si I 10827 Å lines, as well as in the Ca I 10839 Å line-core images. We carried out two studies to validate the expansion of the magnetic field with height. On the one hand, we compare the photospheric Stokes V signals of two different spectral lines that are sensitive to different optical depths (Ca I vs. Si I). The area at which the Stokes V signal is significantly large is almost three times larger for the Si I line – sensitive to higher layers – than for the Ca I one. On the other hand, the inferred line-of-sight (LOS) magnetic fields at two optical depths (log τ = −1.0 vs. −2.5) from the Si I line reveal spatially broader fields in the higher layer, up to 51% more extensive in one of the BPs. The dynamics of BPs are tracked along the Na I D2 and Si I lines. The inferred flows from Na I D2 Doppler shifts are rather slow in BPs (≲1 km s−1). However, the Si I line shows intriguing Stokes profiles with important asymmetries. The analysis of these profiles unveils the presence of two components, a fast and a slow one, within the same resolution element. The faster one, with a smaller filling factor of ∼0.3, exhibits LOS velocities of about 6 km s−1. The slower component is slightly blueshifted. Conclusions. The present work provides observational evidence for the expansion of the magnetic field with height. Moreover, fast flows are likely present in BPs but are sometimes hidden because of observational limitations.