Internetwork Areas Research Articles

Sex differences in functional brain networks involved in interoception: An fMRI study.

Interoception can be described as the ability to perceive inner body sensations and it is different between biological sex. However, no previous research correlated this ability with brain functional connectivity (FC) between males and females. In this study, we used resting-state functional magnetic resonance imaging to investigate FC of networks involved in interoception among males and females in a sample of healthy volunteers matched for age. In total, 67 participants (34 females, mean age 44.2; 33 males, mean age 37.2) underwent a functional MRI session and completed the Self-Awareness Questionnaire (SAQ) that tests the interoceptive awareness. To assess the effect of sex on scores obtained on the SAQ we performed a multivariate analysis of variance. A whole-brain seed-to-seed FC analysis was conducted to investigate the correlation between SAQ score and FC, and then to test differences in FC between males and females with SAQ score as a covariate. MANOVA revealed a significant difference in SAQ scores between males and females with higher values for the second ones. Also, significant correlations among interoception scores and FC in Salience network and fronto-temporo-parietal brain areas have been detected, with a sharp prevalence for the female. These results support the idea of a female advantage in the attention toward interoceptive sensations, suggesting common inter-network areas that concur to create the sense of self.

A Model of the Solar Chromosphere: Structure and Internal Circulation

Abstract A model of the solar chromosphere that consists of two fundamentally different regions, a lower region and an upper region, is proposed. The lower region is covered mostly by weak locally closed magnetic field and small network areas of extremely strong, locally open field. The field in the upper region is relatively uniform and locally open, connecting to the corona. The chromosphere is heated by strong collisional damping of Alfvén waves, which are driven by turbulent motions below the photosphere. The heating rate depends on the field strength, wave power from the photosphere, and altitude in the chromosphere. The waves in the internetwork area are mostly damped in the lower region, supporting radiation in the lower chromosphere. The waves in the network area, carrying more Poynting flux, are only weakly damped in the lower region. They propagate into the upper region. As the thermal pressure decreases with height, the network field expands to form the magnetic canopy where the damping of the waves from the network area supports radiation in the whole upper region. Because of the vertical stratification and horizontally nonuniform distribution of the magnetic field and heating, one circulation cell is formed in each of the upper and lower regions. The two circulation cells distort the magnetic field and reinforce the funnel-canopy-shaped magnetic geometry. The model is based on classical processes and is semi-quantitative. The estimates are constrained according to observational knowledge. No anomalous process is invoked or needed. Overall, the heating mechanism is able to damp 50% of the total wave energy.

MAGNETIC UPFLOW EVENTS IN THE QUIET-SUN PHOTOSPHERE. I. OBSERVATIONS

Rapid magnetic upflows in the quiet-Sun photosphere were recently uncovered from both SUNRISE/IMaX and Hinode/SOT observations. Here, we study magnetic upflow events (MUEs) from high-quality, high- (spatial, temporal, and spectral) resolution, and full Stokes observations in four photospheric magnetically sensitive Fe I lines centered at 525.021, 617.334, 630.151, and 630.250 nm acquired with the Swedish Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line Stokes V signals from those in far blue wing whose signal-to-noise ratio (S/N) >= 7. We find a larger number of MUEs at any given time (0.02 per square arcsec), larger by one to two orders of magnitude, than previously reported. The MUEs appear to fall into four classes presenting different shapes of Stokes V profiles with (I) asymmetric double lobes, (II) single lobes, (III) double-humped (two same-polarity lobes), and (IV) three lobes (an extra blue-shifted bump in addition to double-lobes), of which less than half are single-lobed. We also find that MUEs are almost equally distributed in network and internetwork areas and they appear in the interior or at the edge of granules in both regions. Distributions of physical properties, except that of horizontal velocity, of the MUEs (namely, Stokes V signal, size, line-of-sight velocity, and lifetime) are almost identical for the different spectral lines in our data. A bisector analysis of our spectrally resolved observations shows that these events host modest upflows and do not show direct indication of the presence of supersonic upflows reported earlier. Our findings reveal that numbers, types (classes), and properties determined for MUEs can strongly depend on the detection techniques used and the properties of the employed data, namely, S/Ns, resolutions, and wavelengths.

Migration of Ca II H bright points in the internetwork

The migration of magnetic bright point-like features (MBP) in the lower solar atmosphere reflects the dispersal of magnetic flux as well as the horizontal flows of the atmospheric layer they are embedded in. We analyse trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca II H MBPs (mean lifetime 461 +- 9 s) to obtain their diffusivity behaviour. We use seeing-free high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca II H 3968 {\AA} passband of the Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory. Small MBPs in the internetwork are automatically tracked. The trajectory of each MBP is then calculated and described by a diffusion index ({\gamma}) and a diffusion coefficient (D). We further explore the distribution of the diffusion indices with the help of a Monte Carlo simulation. We find {\gamma} = 1.69 +- 0.08 and D = 257 +- 32 km^2/s averaged over all MBPs. Trajectories of most MBPs are classified as super-diffusive, i.e., {\gamma} > 1, with the determined {\gamma} being to our knowledge the largest obtained so far. A direct correlation between D and time-scale ({\tau}) determined from trajectories of all MBPs is also obtained. We discuss a simple scenario to explain the diffusivity of the observed, relatively short-lived MBPs while they migrate within a small area in a supergranule (i.e., an internetwork area). We show that the scatter in the {\gamma} values obtained for individual MBPs is due to their limited lifetimes. The super-diffusive MBPs can be well-described as random walkers (due to granular evolution and intergranular turbu- lence) superposed on a large systematic (background) velocity, caused by granular, mesogranular and supergranular flows.

A new perspective on quiet Sun magnetism

The Hinode mission has provided us with a new, quantitative view of the magnetism of the quiet Sun. It has revealed that the quiet internetwork areas are blanketed by horizontal fields that appear at first sight to have more flux than the vertical fields resolved on the same 0.3″ size scale. These measurements point to the possibility that the horizontal fields might be the primary source of the “hidden turbulent flux” of the quiet Sun anticipated from Hanle effect depolarization. In this paper, evidence is presented suggesting that the “seething” horizontal fields observed by Harvey in 2007 and the horizontal fields revealed by Hinode are the same phenomenon. Because the seething fields appear to be of uniform fluctuation over the whole disk, the phenomenon is most likely not associated with the dynamo source of solar activity. Thus, the small-scale “hidden turbulent flux” lends support to the notion of a local solar dynamo acting on granular sizes and time scales.

Dynamics of small-scale magnetic fields on the Sun: observations and numerical simulations

Context. Small-scale magnetic fields play an important role in the structure and the dynamics of the solar atmosphere. Aims. This study aims at revealing the evolution of magnetic fields, together with granular convection in the quiet Sun by means of observations with high spatial, spectral, and temporal resolution and of numerical MHD simulations. Methods. Time sequences from quiet Sun disc centre were obtained with the upgraded “Gottingen” Fabry-Perot spectropolarimeter at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the $\ion{Fe}{i}$ 6173 A line. The data were reconstructed with speckle methods. For comparison with the observations, numerical simulations of granular magnetoconvection were carried out with the MURaM code. The intensities and Stokes vectors emerging from the simulation box were degraded in wavelength, spatial co-ordinates, and noise to the quality of the observations. Results. The noise in the observed magnetograms from the centre-of-gravity method is $\sigma_B\approx2$ G, yielding a polarimetric sensitivity of $3 \times 10^{15}$ Mx, at a cadence of 23 s with 0$\farcs$33 spatial resolution in a field of view of ${\sim}33\arcsec\times 25\arcsec$. Many of the observed V profiles in network and internetwork (IN) areas exhibit strong asymmetries that indicate strong magnetoconvection. The temporal evolutions of IN structures and of a bright point (BP), as seen in broadband and line-minimum images, in Dopplergrams, and in magnetograms, are presented. The magnetic field structure in the numerical MHD simulations is even more complex than seen in the observations. Correspondingly, the emergent Stokes profiles are often very abnormal. The degradation yields a reduction of the intrinsic field strength to the “observed” one by a factor 4–5. The spectral resolution of the spectrometer is adequate, yet the limitation in spatial resolution and by noise filtering swamps the details seen in the non-degraded simulations. A BP was not found in the simulations, presumably because BP's are not so common and the simulated box is quiet small: only 6 Mm wide in both horizontal directions. Conclusions. The combination of high-resolution observations with numerical simulation is a highly valuable means for studying small-scale magnetic fields on the Sun. Two-dimensional, low-noise data with spectral resolution as good as achieved here and with spatial resolution of 0$\farcs$1 and better are needed to better understand this important part of the solar magnetism.

Near-IR internetwork spectro-polarimetry at different heliocentric angles

Aims. The analysis of near infrared spectropolarimetric data at the internetwork in different regions on the solar surface could offer constraints that reject current modeling of these quiet areas. Methods. We present spectro-polarimetric observations of very quiet regions for different values of the heliocentric angle for the Fe i lines at 1.56 µm, from disc centre to positions close to the limb. The spatial resolution of the data is 0.7−1 �� . We analyse direct observable properties of the Stokes profiles as the amplitude of circular and linear polarisation, as well as the total degree of polarisation. The area and amplitude asymmetries are also studied. Results. We do not find any significant variation in the properties of the polarimetric signals with the heliocentric angle. This means that the magnetism of the solar internetwork remains the same regardless of the position on the solar disc. This observational fact discards the possibility of modeling the internetwork as a network-like scenario. The magnetic elements of internetwork areas seem to be isotropically distributed when observed at our spatial resolution.

On the fine structure of the quiet solar Ca II K atmosphere

Aims. We investigate the morphological, dynamical, and evolutionary properties of the internetwork and network fine structure of the quiet sun at disk centre. Methods. The analysis is based on a ∼6h time sequence of narrow-band filtergrams centred on the inner-wing Call K 2v reversal at 393.3 nm. To examine the temporal evolution of network and internetwork areas separately we employ a double-Gaussian decomposition of the mean intensity distribution. An autocorrelation analysis is performed to determine the respective characteristic time scales. In order to analyse statistical properties of the fine structure we apply image segmentation techniques. Results. The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. The average evolutionary time scale of the internetwork in our observations is 52 s. Internetwork grains show a tendency to appear on a mesh-like pattern with a mean cell size of ∼4-5 arcsec. Based on this size and the spatial organisation of the mesh we speculate that this pattern is related to the existence of photospheric downdrafts as predicted by convection simulations. The image segmentation shows that typical sizes of both network and internetwork grains are in the order of 1.6 arcsec.

Characterization of Magnetic Flux in the Quiet Sun

Observations of the quiet Sun observed with the Advanced Stokes Polarimeter are used to explore the distribution of apparent flux density (Bapp) and size scales of internetwork regions. For a typical disk-center quiet-Sun observation with excellent seeing (at a quantifiable angular resolution of 1''), average |Bapp| of 18.6 and 10.7 Mx cm-2 are found for the entire quiet region map and the internetwork region (2 σ noise < |Bapp| < 40 Mx cm-2), respectively. The weak internetwork flux appears to consist of two components: the spatially concentrated "granular" internetwork fields and a more diffuse, weaker component that has a characteristic size scale of a few arcseconds. Most of the internetwork area is occupied by measurable fields: 69% of the area has apparent flux density greater than 4.5 Mx cm-2 (3 σ), or 84% with flux density greater than 3.0 Mx cm-2 (2 σ). If the results of Hanle depolarization measurements are accepted, the internetwork flux detected here must be close (within a factor of 2-3) to being spatially resolved and must be intrinsically weak (10-30 G). Examination of Stokes polarimetry in quiet regions away from disk center indicates that the central cores of network flux, as indicated by high-polarization signals, do not have a high degree of mixed polarity. In contrast, regions immediately surrounding the network elements have stronger linear polarization than would be expected from locally vertical fields, indicating a high degree of mixed polarity. This mixed polarity may be the result of the continual "sweeping" of mixed polarity internetwork flux toward the network boundaries. When a quiet region has a significant imbalance of flux of opposite polarities, the internetwork imbalance is of the same sign and typically that of the network. The smaller imbalance in the internetwork suggests that a local dynamo produces most of the internetwork flux.

Dynamics of the solar chromosphere

We analyze oscillations in the solar atmosphere using image sequences from the Transition Region and Coronal Explorer (TRACE) in three ultraviolet passbands which sample the upper solar photosphere and low chromosphere. We exploit the absence of atmospheric seeing in TRACE data to furnish comprehensive Fourier diagnostics (amplitude maps, phase-dierence spectra, spatio-temporal decomposition) for quiet-Sun network and internetwork areas with excellent sampling statistics. Comparison displays from the ground-based Ca ii Hs pec- trometry that was numerically reproduced by Carlsson & Stein are added to link our results to the acoustic shock dynamics in this simulation. The TRACE image sequences conrm the dichotomy in oscillatory behaviour between network and internetwork and show upward propagation above the cuto frequency, the onset of acoustic shock formation in the upper photosphere, phase-dierence contrast between pseudo-mode ridges and the inter- ridge background, enhanced three-minute modulation aureoles around network patches, a persistent low-intensity background pattern largely made up of internal gravity waves, ubiquitous magnetic flashers, and low-lying mag- netic canopies with much low-frequency modulation. The spatio-temporal occurrence pattern of internetwork grains is found to be dominated by acoustic and gravity wave interference. We nd no sign of the high-frequency sound waves that have been proposed to heat the quiet chromosphere, but such measurement is hampered by non-simultaneous imaging in dierent passbands. We also nd no signature of particular low-frequency fluxtube waves that have been proposed to heat the network. However, internal gravity waves may play a role in their excitation.

Dynamics of the Solar Chromosphere. II. CaiiH2Vand K2VGrains versus Internetwork Fields

We use the Advanced Stokes Polarimeter at the NSO/Sacramento Peak Vacuum Tower Telescope to search for spatio-temporal correlations between enhanced magnetic fields in the quiet solar internetwork photosphere and the occurrence of Ca II H2V grains in the overlying chromosphere. We address the question of whether the shocks that produce the latter are caused by magnetism-related processes, or whether they are of purely hydrodynamic nature. The observations presented here are the first in which sensitive Stokes polarimetry is combined synchronously with high-resolution Ca II H spectrometry. We pay particular attention to the nature and significance of weak polarization signals from the internetwork domain, obtaining a robust estimate of our magnetographic noise level at an apparent flux density of only 3 Mx cm-2. For the quiet Sun internetwork area analyzed here, we find no direct correlation between the presence of magnetic features with apparent flux density above this limit and the occurrence of H2V brightenings. This result contradicts the one-to-one correspondence claimed by Sivaraman & Livingston. We also find no correspondence between H2V grains and the horizontal-field internetwork features discovered by Lites et al.

Ultraviolet Jets and Bright Points in the Solar Chromosphere. II. Statistical Correlations

We use HRTSVI rocket observations of the solar chromosphere to search for relationships between high-Dopplershift ii jets ˇˇ observed in the C I lines near j 156 nm and internetwork ii bright points ˇˇ observed in the j 160 nm continuum, in sequel to the analysis by Cook et al. which failed to —nd a direct connection between these phenomena. We now use the same data to establish statistical corre- lations between C I Dopplershift and 160 nm brightness modulation in internetwork areas. These mean relations emerge only after extensive spatial averaging and have small amplitude, but are de—nitely sig- ni—cant. They show that both C I Dopplershift and 160 nm brightness participate in oscillatory behavior with 3 minute periodicity and mesoscale (8 Mm wavelength) as well as small-scale (1.4 Mm wavelength) spatial patterning. We —nd spatial and temporal phase relations between Dopplershift and brightness that con—rm that jets and bright points should not be interpreted as isolated entities. Rather, they are chromospheric manifestations, with much pattern interference, of the oscillatory acoustic shock dynamics in the internetwork which also cause Ca II grains. Additional small-scale modulation is present K 2V which we attribute to waves with f-mode character. Subject headings: Sun: chromosphereSun: oscillationsSun: UV radiation

Dynamics of the solar chromosphere. I - Long-period network oscillations

We analyze differences in solar oscillations between the chromospheric network and internetwork regions from a 1 hr sequence of spectrograms of a quiet region near disk center. The spectrograms contain Ca II H, Ca I 422.7 nm, and various Fe I blends in the Ca II H wing. They permit vertical tracing of oscillations throughout the photosphere and into the low chromosphere. We find that the rms amplitude of Ca II H line center Doppler fluctuations is about 1.5 km/s for both network and internetwork, but that the character of the oscillations differs markedly in these two regions. Within internetwork areas the chromospheric velocity power spectrum is dominated by oscillations with frequencies at and above the acoustic cutoff frequency. They are well correlated with the oscillations in the underlying photosphere, but they are much reduced in the network. In contrast, the network Ca II H line center velocity and intensity power spectra are dominated by low-frequency oscillations with periods of 5-20 min. Their signature is much clearer in our Ca II H line center measurements than in previously used diagnostics which are contaminated by signals from deeper layers. We find that these long-period oscillations are not correlated with underlying photospheric disturbances, and we discuss their nature.