The statistical distribution of the magnetic-field strength in G-band bright points

Criscuoli, S.; Uitenbroek, H.

doi:10.1051/0004-6361/201322909

Cited by 20 publications

(18 citation statements)

References 26 publications

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“…Only isolated, small-scale magnetic field concentrations of kG strengths appear bright in the solar photosphere, as was shown by Criscuoli & Uitenbroek (2014) and Riethmüller et al (2014). However, the influence of the telescope point-spread function (PSF) leads to a smoothing of the "true" magnetic field strength to lower values, so that recent observations display a distribution of field strengths from small fractions of kG up to 1.5 kG (see, e.g., Beck et al 2007;Utz et al 2013c and references therein).…”

Section: Introductionsupporting

confidence: 55%

THE FORMATION AND DISINTEGRATION OF MAGNETIC BRIGHT POINTS OBSERVED BYSUNRISE/IMaX

Utz

Iniesta

Rubio

et al. 2014

ApJ

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The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First, we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next, we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4 km s −1 set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4 m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBP's evolution. Also, only relatively weak downflows are found on average at this stage of the evolution. Only 16% of the features display upflows at the time that the field weakens, or the MBP disappears. This speaks either for a very fast evolving dynamic process at the end of the lifetime, which could not be temporally resolved, or against strong upflows as the cause of the weakening of the field of these magnetic elements, as has been proposed based on simulation results. It is noteworthy that in about 10% of the cases, we observe in the vicinity of the downflows small-scale strong (exceeding 2 km s −1 ) intergranular upflows related spatially and temporally to these downflows. The paper is complemented by a detailed discussion of aspects regarding the applied methods, the complementary literature, and in depth analysis of parameters like magnetic field strength and velocity distributions. An important difference to magnetic elements and associated bright structures in active region plage is that most of the quiet Sun bright points display significant downflows over a large fraction of their lifetime (i.e., in more than 46% of time instances/measurements they show downflows exceeding 1 km s −1 ).

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Section: Introductionsupporting

confidence: 55%

THE FORMATION AND DISINTEGRATION OF MAGNETIC BRIGHT POINTS OBSERVED BYSUNRISE/IMaX

Utz

Iniesta

Rubio

et al. 2014

ApJ

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“…If there are N funnel funnels of open flux on the coronal hole‐covered Sun, then the total open flux produced at the Sun by the open funnels is

F_{sun} = N_{funnel} {d_{normalbp}}^{2} B_{bp},

where d bp is the diameter of a bright point (funnel base) and B bp is the magnetic field strength in the bright point. Typical values are d bp ~ 150 km [ Utz et al ., ; Yang et al ., ] and B bp ~ 1500 G [ Ishikawa et al ., ; Criscuoli and Uitenbroek , ]; with those two parameters, expression yields F sun = N funnel 3.38 × 10 7 G km 2 for the total open flux coming out of the funnels. Equating the two above expressions F sw = F sun yields N funnel = 2.34 × 10 5 for the number of funnels required to yield the heliospheric total open flux (if the whole sun was covered by a single coronal hole).…”

Section: Discussion: Expectations For Structure Scalesmentioning

confidence: 99%

The plasma structure of coronal hole solar wind: Origins and evolution

Borovsky

2016

JGR Space Physics

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Whereas slow solar wind is known to be highly structured, the fast (coronal hole origin) wind is usually considered to be homogeneous. Using measurements from Helios 1 + 2, ACE, Wind, and Ulysses, structure in the coronal hole origin solar wind is examined from 0.3 AU to 2.3 AU. Care is taken to collect and analyze intervals of “unperturbed coronal hole plasma.” In these intervals, solar wind structure is seen in the proton number density, proton temperature, proton specific entropy, magnetic field strength, magnetic field to density ratio, electron heat flux, helium abundance, heavy‐ion charge‐state ratios, and Alfvenicity. Typical structure amplitudes are factors of 2, far from homogeneous. Variations are also seen in the solar wind radial velocity. Using estimates of the motion of the solar wind origin footpoint on the Sun for the various spacecraft, the satellite time series measurements are converted to distance along the photosphere. Typical variation scale lengths for the solar wind structure are several variations per supergranule. The structure amplitude and structure scale sizes do not evolve with distance from the Sun from 0.3 to 2.3 AU. An argument is quantified that these variations are the scale expected for solar wind production in open magnetic flux funnels in coronal holes. Additionally, a population of magnetic field foldings (switchbacks, reversals) in the coronal hole plasma is examined: this population evolves with distance from the Sun such that the magnetic field is mostly Parker spiral aligned at 0.3 AU and becomes more misaligned with distance outward.

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“…To that end, we calculated time-series of magnetoconvection simulations using the Copenhagen STAGGER radiation magnetohydrodynamics code. The quality of those simulations was confirmed in follow-up studies, where the snapshots were tested for thermodynamical and polarisation properties (Beck et al 2013, as well as being employed for associating G-band bright points with magnetic field strengths of a few kG (Criscuoli & Uitenbroek 2014), and for comparing numerically-predicted and observationally-derived velocity fields in solar convection via Fourier power spectra (Yelles Chaouche et al 2014). Our 3D magnetohydrodynamical (MHD) models were also used for the interpretation of solar irradiance measurements, whereby the predictions of the simulations may provide a possible explanation for the counter-phase variation of the spectral irradiance at visible and IR wavelengths with the solar activity cycle (Criscuoli & Uitenbroek 2014).…”

Section: Introductionmentioning

confidence: 94%

CONTINUUM INTENSITY AND [O i] SPECTRAL LINE PROFILES IN SOLAR 3D PHOTOSPHERIC MODELS: THE EFFECT OF MAGNETIC FIELDS

Fabbian

Moreno‐Insertis

2015

ApJ

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The importance of magnetic fields in three-dimensional magnetoconvection models of the Sun's photosphere is investigated in terms of their influence on the continuum intensity at different viewing inclination angles, and on the intensity profile of two [O i] spectral lines. We use the RH numerical radiative transfer code to perform a posteriori spectral synthesis on the same time-series of magnetoconvection models used in our publications on the effect of magnetic fields on abundance determination. We obtain a good match of the synthetic disc-centre continuum intensity to the absolute continuum values from the FTS observational spectrum; the match of the centre-to-limb variation (CLV) synthetic data to observations is also good, thanks, in part, to the 3D radiation transfer capabilities of the RH code. The different levels of magnetic flux in the numerical time-series do not modify the quality of the match. Concerning the targetted [O i] spectral lines, we find, instead, that magnetic fields lead to non-negligible changes in the synthetic spectrum, with larger average magnetic flux causing the line to become noticeably weaker. The photospheric oxygen abundance that one would derive if instead using non-magnetic numerical models would thus be lower by a few to several centidexes. The inclusion of magnetic fields is confirmed to be important for improving the current modelling of the Sun, here in particular in terms of spectral line formation and of deriving consistent chemical abundances. These results may shed further light on the still controversial issue regarding the precise value of the solar oxygen abundance.

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The statistical distribution of the magnetic-field strength in G-band bright points

Cited by 20 publications

References 26 publications

THE FORMATION AND DISINTEGRATION OF MAGNETIC BRIGHT POINTS OBSERVED BYSUNRISE/IMaX

THE FORMATION AND DISINTEGRATION OF MAGNETIC BRIGHT POINTS OBSERVED BYSUNRISE/IMaX

The plasma structure of coronal hole solar wind: Origins and evolution

CONTINUUM INTENSITY AND [O i] SPECTRAL LINE PROFILES IN SOLAR 3D PHOTOSPHERIC MODELS: THE EFFECT OF MAGNETIC FIELDS

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