Solar-cycle variations of the internetwork magnetic field

Faurobert, M.; Ricort, G.

doi:10.1051/0004-6361/201526298

Cited by 17 publications

(18 citation statements)

References 14 publications

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“…However, when the large-scale field is stronger than the equipartition value, we observe that the generation of the smallscale field is quenched by the large-scale field-producing a weak anticorrelation between small-and large-scale fields. This reciprocal correlation between the small-scale field and the global dynamo cycle is in agreement with observations of the small-scale magnetic field at the solar surface (e.g., Hagenaar et al 2003;Jin et al 2011;Jin & Wang 2012Faurobert & Ricort 2015). In particular, Jin et al (2011) have analyzed separately the small-scale elements based on their magnetic flux contents and the number of magnetic elements and found that their cyclic variations for magnetic elements with fluxes in the range 3 ( -30 10 Mx…”

Section: Discussionsupporting

confidence: 86%

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Karak

Brandenburg

2015

ApJ

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The small-scale magnetic field is ubiquitous at the solar surface-even at high latitudes. From observations we know that this field is uncorrelated (or perhaps even weakly anticorrelated) with the global sunspot cycle. Our aim is to explore the origin, and particularly the cycle dependence, of such a phenomenon using three-dimensional dynamo simulations. We adopt a simple model of a turbulent dynamo in a shearing box driven by helically forced turbulence. Depending on the dynamo parameters, large-scale (global) and small-scale (local) dynamos can be excited independently in this model. Based on simulations in different parameter regimes, we find that, when only the large-scale dynamo is operating in the system, the small-scale magnetic field generated through shredding and tangling of the large-scale magnetic field is positively correlated with the global magnetic cycle. However, when both dynamos are operating, the small-scale field is produced from both the small-scale dynamo and the tangling of the large-scale field. In this situation, when the large-scale field is weaker than the equipartition value of the turbulence, the small-scale field is almost uncorrelated with the large-scale magnetic cycle. On the other hand, when the large-scale field is stronger than the equipartition value, we observe an anticorrelation between the smallscale field and the large-scale magnetic cycle. This anticorrelation can be interpreted as a suppression of the smallscale dynamo. Based on our studies we conclude that the observed small-scale magnetic field in the Sun is generated by the combined mechanisms of a small-scale dynamo and tangling of the large-scale field.

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Section: Discussionsupporting

confidence: 86%

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Karak

Brandenburg

2015

ApJ

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“…In other words, the quiet Sun looks the same regardless of the observer's line of sight (Martínez González et al 2008b;Lites et al 2008;Orozco Suárez & Katsukawa 2012). Moreover, their magnetic properties either do vary with the activity cycle, or this variation is very weak (Faurobert et al 2001;Buehler et al 2013;Lites et al 2014;Faurobert & Ricort 2015). Finally, the degree of spatial coherence of the signals increases with the polarimetric signal.…”

Section: Introductionmentioning

confidence: 99%

Inference of magnetic fields in the very quiet Sun

González

Yabar

Lagg

et al. 2016

A&A

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Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution. Aims. We present high-precision spectro-polarimetric data with high spatial resolution (0.4 ) of the very quiet Sun at 1.56 µm obtained with the GREGOR telescope to shed some light on this complex magnetism. Methods. We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component. Results. Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak (∼150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area ∼50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised.

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“…To select INR, we used the method described in Faurobert & Ricort (2015), which relies on line-integrated unsigned circular polarization maps of the quiet Sun measured simultaneously with the intensity maps with the SOT/SP instrument. We selected 98 INR of 10 × 10 located around local minima of the polarization signal.…”

Section: Measurement Methodsmentioning

confidence: 99%

“…This would also affect the apparent scale of the granulation seen in the continuum. Faurobert & Ricort (2015) showed that the power spectrum of the granulation is unchanged between the two dates except for the effect of a small defocus (0.6-0.7 mm) of the 2007 images with respect to 2013. A defocus mainly decreases the contrast of the images; this does not affect the phase of the image crossspectrum, and the effect on the pixel size is negligible.…”

Section: Measurement Of the Image Formation Depthmentioning

confidence: 97%

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Variation of the temperature gradient in the solar photosphere with magnetic activity

Faurobert

Raman

Ricort

2016

A&A

Self Cite

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Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient or the photospheric radius, might affect the irradiance. Aims. We intend to investigate possible variations of the photospheric temperature gradient with magnetic activity. Methods. We used high-resolution center-to-limb observations of the FeI 630.15 nm line profile in the quiet Sun performed onboard the Hinode satellite on 2007, December 19, and on 2013, December 7, that is, close to a minimum and a maximum of magnetic activity, respectively. We analyzed samples of 10 × 10 internetwork regions. The wings of the FeI 630.15 nm line were used in a non-standard way to recover images at roughly constant continuum optical depths above the continuum formation level. The image formation height is derived from measuring its perspective shift with respect to the continuum image, both observed away from disk center. The measurement relies on a cross-spectral method that is not limited by the spatial resolution of the SOT telescope and does not rely on any radiative transfer computation. The radiation temperature measured in the images is related to the photospheric temperature at their respective formation height. Results. The method allows us to investigate the temperature gradient in the low photosphere at altitudes of between 0 and 60 km above the 500 nm continuum formation height. In this layer the internetwork temperature gradient appears steeper in our 2013 sample than in the sample of 2007 in the northern hemisphere, whereas we detect no significant change in the southern hemisphere. We argue that this might be related to some strong hemispheric asymmetry of the magnetic activity at the solar maximum of cycle 24. Conclusions. Structural changes have been observed in numerical simulations of the magneto-convection at the surface of the Sun where the increase of the ambient sub-surface magnetic fields leads to some steepening of the temperature gradient in the internetwork. Hemispheric asymmetry of the activity has been reported for the last solar cycles with successive dominant north and south hemisphere during the activity maximum. Our results seem consistent with this global physical picture, but need to be confirmed by additional studies.

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Solar-cycle variations of the internetwork magnetic field

Cited by 17 publications

References 14 publications

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Inference of magnetic fields in the very quiet Sun

Variation of the temperature gradient in the solar photosphere with magnetic activity

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