Magnetic properties of G-band bright points in a sunspot moat

Beck, C.; Rubio, L. R. Bellot; Schlichenmaier, R.; Sütterlin, P.

doi:10.1051/0004-6361:20065620

Cited by 81 publications

(107 citation statements)

References 38 publications

(51 reference statements)

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“…2 for the three regions follow a similar log-normal distribution to those found in previous studies (Berger et al 1995;Beck et al 2007;Crockett et al 2010;Feng et al 2013;Keys et al 2013). Berger et al (1995) suggest that this distribution is the result of a fragmentation mechanism, similar to that found in sunspot umbrae (Bogdan et al 1988).…”

Section: Area Distributions and Bp Numberssupporting

confidence: 86%

Dynamic properties of bright points in an active region

Keys

Mathioudakis

Jess

et al. 2014

A&A

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Context. Bright points (BPs) are small-scale, magnetic features ubiquitous across the solar surface. Previously, we have observed and noted their properties for quiet Sun regions. Here, we determine the dynamic properties of BPs using simultaneous quiet Sun and active region data. Aims. The aim of this paper is to compare the properties of BPs in both active and quiet Sun regions and to determine any difference in the dynamics and general properties of BPs as a result of the varying magnetic activity within these two regions. Methods. High spatial and temporal resolution G-band observations of active region AR11372 were obtained with the Rapid Oscillations in the Solar Atmosphere instrument at the Dunn Solar Telescope. Three subfields of varying polarity and magnetic flux density were selected with the aid of magnetograms obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Bright points within these subfields were subsequently tracked and analysed. Results. It is found that BPs within active regions display attenuated velocity distributions with an average horizontal velocity of ∼0.6 km s −1 , compared to the quiet region which had an average velocity of 0.9 km s −1 . Active region BPs are also ∼21% larger than quiet region BPs and have longer average lifetimes (∼132 s) than their quiet region counterparts (88 s). No preferential flow directions are observed within the active region subfields. The diffusion index (γ) is estimated at ∼1.2 for the three regions. Conclusions. We confirm that the dynamic properties of BPs arise predominately from convective motions. The presence of stronger field strengths within active regions is the likely reason behind the varying properties observed. We believe that larger amounts of magnetic flux will attenuate BP velocities by a combination of restricting motion within the intergranular lanes and by increasing the number of stagnation points produced by inhibited convection. Larger BPs are found in regions of higher magnetic flux density and we believe that lifetimes increase in active regions as the magnetic flux stabilises the BPs.

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Section: Area Distributions and Bp Numberssupporting

confidence: 86%

Dynamic properties of bright points in an active region

Keys

Mathioudakis

Jess

et al. 2014

A&A

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“…This uncertainty introduces a random error with a σ B about 20 to 60 G, depending on the wavelength range and the dispersion of the spectra. The value is comparable to the formal error attributed to field strength in inversions of the Fe i line at 1564.8 nm (e.g., Beck et al 2007). …”

Section: Error Estimatesupporting

confidence: 69%

Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

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Aims. We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods. We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years. Results. Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (±16) G yr −1 , while it increases in the rising phase of cycle 24 by about 138 (±72) G yr −1 . The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (±0.3)% of I c yr −1 toward the end of cycle 23 and decreases with a rate of 3.8 (±1.5)% of I c yr −1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4 G yr −1 . If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss. Conclusions. The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future.

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“…We aligned the specklereconstructed G-band images taken during the TIP maps with the same methods as described in Beck et al (2007); the TESOS spectra were not considered in the present study. Figure 1 shows overview maps of the observations: the aligned speckle-reconstructed G-band data, the continuum intensity I c at around 1565 nm, the maximum linear polarization signal L max of the 1564.8 nm line, the maximum circular polarization signal V max , the polarization degree p, and finally, the polarity of the magnetic fields that indicates whether the field lines were (anti)parallel to the line of sight (LOS).…”

Section: Observationsmentioning

confidence: 99%

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Beck¹,

Rezaei²

2009

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Context. Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun. Aims. We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 μm, where the sensitivity to magnetic fields is higher than in the visible. Methods. We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10 −4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux. Results. We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (γ > 45 • ) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable. Conclusions. During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in 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 emphasize 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 be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating.

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Magnetic properties of G-band bright points in a sunspot moat

Cited by 81 publications

References 38 publications

Dynamic properties of bright points in an active region

Dynamic properties of bright points in an active region

Variation in sunspot properties between 1999 and 2014

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

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