The Problem of the Height Dependence of Magnetic Fields in Sunspots

Balthasar, H.

doi:10.1007/s11207-018-1338-x

“…Recently, Balthasar (2018) presented an extensive review about the unsolved problems of the estimation of the magnetic field gradient in sunspots, exploring the different techniques employed and the reasons for the discrepancy of the results presented in the literature. For example, Rueedi et al (1995) found that the vertical gradient of the magnetic field decreases outwards in the sunspot, with values of 0.1-0.3 G km −1 in the outer penumbra, and with the height in the atmosphere.…”

Section: Introductionmentioning

confidence: 99%

Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere

Murabito

¹

,

Ermolli

²

,

Giorgi

³

et al. 2019

ApJ

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We studied the physical parameters of the penumbra in a large and fully-developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the NICOLE code, we obtained the three-dimensional structure of the magnetic field in the penumbra from the bottom of the photosphere up to the middle chromosphere. We analyzed the azimuthal and vertical gradient of the magnetic field strength and inclination. Our results provide new insights on the properties of the penumbral magnetic fields in the chromosphere at atmospheric heights unexplored in previous studies. We found signatures of the small-scale spine and intra-spine structure of both the magnetic field strength and inclination at all investigated atmospheric heights. In particular, we report typical peak-to-peak variations of the field strength and inclination of ≈ 300 G and ≈ 20 • , respectively, in the photosphere, and of ≈ 200 G and ≈ 10 • in the chromosphere. Besides, we estimated the vertical gradient of the magnetic field strength in the studied penumbra: we find a value of ≈ 0.3 G km −1 between the photosphere and the middle chromosphere. Interestingly, the photospheric magnetic field gradient changes sign from negative in the inner to positive in the outer penumbra.

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“…4 of Khomenko & Collados (2007). As the field strength there is on the order of one thousand gauss, with a difference of about 300 G between the two line measurements, leading to the vertical gradient value of 3 G km −1 , as reported by Balthasar (2018), the 10 G inaccuracy on the longitudinal field is much smaller than the finite difference of the measurements. The horizontal gradient is found to be only about 0.3 G km −1 , which corresponds to the reversal of a 1500 G typical horizontal component from one side to the other of a sunspot with a typical diameter of 10 000 km.…”

Section: Introductionmentioning

confidence: 64%

“…The problem of the large magnitude difference between the observed horizontal and vertical magnetic field gradients in and around sunspots has been known for a long time. Balthasar (2018) wrote a detailed review of observations, where it is shown that typical values of 3 G km −1 and 0.3 G km −1 are obtained for the vertical and horizontal gradients of the magnetic field, respectively, regardless of the telescope, spectral line(s), and measurement interpretation method used. Using these values would surprisingly lead to a non-zero divergence of the observed magnetic field, which is a priori not acceptable.…”

Section: Introductionmentioning

confidence: 99%

“…In his review "Sunspots: An overview", Solanki (2003, p. 184) expressed the problem as follows: No satisfactory solution has been found as yet for the unexpectedly small vertical gradients obtained by applying the div B = 0 condition [to the observed horizontal gradients]. Balthasar (2018) tries to explain the discrepancy by simulating unresolved magnetic structures. This does not manage to fully explain the observations.…”

Section: Introductionmentioning

confidence: 99%

“…8. Balthasar (2018) published a review of observations of the magnetic field gradients in and around sunspots. The measurement inaccuracy obviously depends on the method and the instrument used.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Solar photosphere magnetization

Bommier

¹

2020

A&A

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Context. A recent review shows that observations performed with different telescopes, spectral lines, and interpretation methods all agree about a vertical magnetic field gradient in solar active regions on the order of 3 G km−1, when a horizontal magnetic field gradient of only 0.3 G km−1 is found. This represents an inexplicable discrepancy with respect to the divB = 0 law. Aims. The objective of this paper is to explain these observations through the law B = μ0(H + M) in magnetized media. Methods. Magnetization is due to plasma diamagnetism, which results from the spiral motion of free electrons or charges about the magnetic field. Their usual photospheric densities lead to very weak magnetization M, four orders of magnitude lower than H. It is then assumed that electrons escape from the solar interior, where their thermal velocity is much higher than the escape velocity, in spite of the effect of protons. They escape from lower layers in a quasi-static spreading, and accumulate in the photosphere. By evaluating the magnetic energy of an elementary atom embedded in the magnetized medium obeying the macroscopic law B = μ0(H + M), it is shown that the Zeeman Hamiltonian is due to the effect of H. Thus, what is measured is H. Results. The decrease in density with height is responsible for non-zero divergence of M, which is compensated for by the divergence of H, in order to ensure div B = 0. The behavior of the observed quantities is recovered. Conclusions. The problem of the divergence of the observed magnetic field in solar active regions finally reveals evidence of electron accumulation in the solar photosphere. This is not the case of the heavier protons, which remain in lower layers. An electric field would thus be present in the solar interior, but as the total charge remains negligible, no electric field or effect would result outside the star.

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Analysis of photospheric magnetic fields in AR 12546: a case study

Abdel-Kawy

¹

,

Shaltout

²

2023

Astrophys Space Sci

1

0

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We investigate high-resolution observations with the spectropolarimeter (SP) aboard the Hinode satellite of the Solar Optical Telescope (SOT) of a positive polarity sunspot of an active region (AR) (NOAA 12546). We present a case study for the properties of the thermal, magnetic field, and plasma flows as a function of the optical depth from the inversion of the observed Stokes profiles, covering a wide field of view area. Particular attention is paid to the examination of the net circular polarization (NCP) and area asymmetry of spectral lines in sunspots. We detect a large red-shifted velocity of 10 km sec−1 localized with the presence of a strong magnetic field corresponding to the NCP best fit of the inverted profiles. In addition, the comparison between the observed and calculated NCPs or Stokes V area asymmetries of spectral lines fitted well for most pixels in the field of view region, with a significant indication of a single-component inversion. We study the vertical gradients of temperature, magnetic strength, inclination field, and LOS velocity in the deeper and higher layers of the photosphere. The difference in the penumbral temperature between the two atmospheric layers is −130 K. The penumbral regions reveal a magnetic field gradient of $\Delta B \sim -0.5$ Δ B ∼ − 0.5 to −0.9 G km−1. The inclination gradient in the limb-side penumbra is between $(\Delta \gamma /\Delta z) = -9 \times 10^{-3}$ ( Δ γ / Δ z ) = − 9 × 10 − 3 deg km−1 to $-3.5 \times 10^{-2}$ − 3.5 × 10 − 2 deg km−1. In particular, we find a higher positive inclination gradient in the disk center-side penumbra $(\Delta \gamma /\Delta z)= 0.2 - 1.5 \deg \text{ km}^{-1}$ ( Δ γ / Δ z ) = 0.2 − 1.5 deg km − 1 . The velocity gradient is $(\Delta V_{LOS}/\Delta \log \tau )= -0.13$ ( Δ V L O S / Δ log τ ) = − 0.13 to −0.30 km sec−1 in the limb-side penumbra, and the disk center-side penumbra is given by $(\Delta V_{LOS}/\Delta \log \tau )= 0.11$ ( Δ V L O S / Δ log τ ) = 0.11 to 0.29 km sec−1.

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The Problem of the Height Dependence of Magnetic Fields in Sunspots

Cited by 17 publications

References 78 publications

Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere

Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere

Solar photosphere magnetization

Analysis of photospheric magnetic fields in AR 12546: a case study

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