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Solar Photosphere Magnetization Véronique Bommier A&A 634, A40 (2020) Astronomy https://doi.org/10.1051/0004-6361/201935244 & c V. Bommier 2020 Astrophysics Solar photosphere magnetization Véronique Bommier LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France e-mail: [email protected] Received 11 February 2019 / Accepted 14 December 2019 ABSTRACT 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. Key words. magnetic fields – plasmas – Sun: magnetic fields – Sun: photosphere – sunspots – stars: solar-type 1. Introduction provided that it can be proved that what is measured by the Zeeman effect is H and not B. This is the object of Sect.5. The problem of the large magnitude difference between the To establish this point it is necessary to go back to the micro- observed horizontal and vertical magnetic field gradients in and scopic scale of an atom embedded in the medium magnetized around sunspots has been known for a long time. Balthasar(2018) at the macroscopic scale. In addition, we study the atom poten- wrote a detailed review of observations, where it is shown that tial magnetic energy. As introduced in Sect.4, in this paper we typical values of 3 G km−1 and 0.3 G km−1 are obtained for the denote as B the magnetic induction and as H the magnetic field, vertical and horizontal gradients of the magnetic field, respec- which are related by the law B = µ0 (H + M), where M is the tively, regardless of the telescope, spectral line(s), and mea- magnetization. surement interpretation method used. Using these values would However, the photosphere magnetization is negligible when surprisingly lead to a non-zero divergence of the observed mag- evaluated via the usual models. In Sect.6 we propose a model netic field, which is a priori not acceptable. In his review that is able to increase magnetization in the photosphere. It is “Sunspots: An overview”, Solanki(2003, p. 184) expressed the considered that in the solar interior, at 0.5 R , the electron ther- problem as follows: No satisfactory solution has been found as yet mal velocity of 12 Mm s−1 largely surpasses the escape velocity for the unexpectedly small vertical gradients obtained by applying of 850 km s−1; however, this is not the case for protons whose the div B = 0 condition [to the observed horizontal gradients]. thermal velocity is 290 km s−1, only due to their much higher Balthasar(2018) tries to explain the discrepancy by simu- mass. A similar effect occurs in the solar corona (Meyer-Vernet lating unresolved magnetic structures. This does not manage to 2007). Following Allen(1973), the mass inside the sphere of fully explain the observations. In Sect.2 we rule out the e ffect of 0.5 R radius is 0.94 of the total solar mass, and the tempera- measurement inaccuracies. In Sect.3, we rule out a pure e ffect ture is 3:4 × 106 K there. This leads to the above velocity values. of spatial resolution on the basis of two mathematical theorems. Thus, gravity separates the charges, in the sense that electrons We discuss the difference between derivatives and finite differ- escape when protons do not. More precisely, the underlying pro- ences (the observations), and we show that these mathematical tons do not completely prevent electrons from escaping. The theorems prove that the observed non-zero divergence reveals result is a free electron accumulation and a space charge at the existence of at least one non-zero contribution from an ele- the Sun’s surface. We show that the electron migration is a ment of the averaged region. quasi-static process. As a result, the photosphere magnetiza- In this paper we explain the observations by applying tion due to the plasma diamagnetism, which is itself due to the Maxwell relation in magnetized media B = µ0 (H + M), these charges, could become non-negligible. Under the usual A40, page 1 of 11 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A&A 634, A40 (2020) Fig. 1. Histograms of inversion results of noised theoretical polarization profiles. The introduced noise level is 1:5 × 10−3, in terms of polarization degree. The method used is a Milne-Eddington inversion, but including a magnetic filling factor α (see text). As a result, the inaccuracy can be estimated to be 20 G for the x and y components (left and middle plots), whereas it is 10 G for the z component (right plot). The Oz-axis lies along the line of sight. The assumed line is Fe i 6302.5 Å. These histograms were published (in spherical coordinates) in Fig. 5 of Bommier et al.(2007). photospheric conditions (the VALC model; Vernazza et al. measurements, leading to the vertical gradient value of 3 G km−1, −4 1981), the magnetization would be about 10 lower than B/µ0, as reported by Balthasar(2018), the 10 G inaccuracy on the due to the low electron and ion densities (Bommier 2015). The longitudinal field is much smaller than the finite difference of accumulation of escaping electrons could increase the magneti- the measurements. The horizontal gradient is found to be only zation up to a non-negligible level. about 0.3 G km−1, which corresponds to the reversal of a 1500 G In Sect.7 we present the most direct electron density typical horizontal component from one side to the other of measurements in the solar photosphere to our knowledge. We a sunspot with a typical diameter of 10 000 km. Assuming a propose that these measurements show evidence of an electron 0.3 arcsec pixel, which is 250 km wide, the observed horizontal overdensity in the solar photosphere. We conclude in Sect.8. field variation is about 75 G, which is not very large, yet larger than the 20 G inaccuracy on the transverse field. As a result, the difference between the observed vertical and horizontal gradi- 2. Measurement inaccuracies ents cannot be simply assigned to measurement inaccuracies. Balthasar(2018) published a review of observations of the mag- netic field gradients in and around sunspots. The measurement 3. Some mathematical theorems inaccuracy obviously depends on the method and the instrument used. However, we find it interesting to publish here the his- In this section we determine whether the lack of spatial resolu- tograms of magnetic inaccuracy obtained within the UNNOFIT tion is able to explain the observed discrepancy. First of all we inversion method that we recently developed (Bommier et al. note that if the lack of spatial resolution were able to explain 2007), in magnetic field cartesian coordinates. These histograms the effect, we would expect the non-zero value observed for the can be found in Fig. 5 of the above-mentioned paper, but in divergence to depend on the pixel size. As far can be seen in the spherical coordinates, which prevents us from clearly discrimi- review by Balthasar(2018), this is not the case. The review deals nating between longitudinal and transverse field components. In with various instruments, and therefore various pixel sizes, but Fig.1 of the present paper, we publish the histograms for each the results are homogeneous, even though obtained using differ- of these components. As described in Bommier et al.(2007), ent active regions. In order to investigate this question in a more these histograms were obtained from inversion of theoretical quantitative manner, we considered the spectropolarimetric data noised polarization profiles. The noise level was assumed to be of NOAA 10953 acquired by the SOT/SP experiment on board 1:5 × 10−3, in terms of polarization degree.
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