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Soika Alexander Kuzmich, Military Academy of the Republic of Belarus, professor, Department of Physics E-mail: [email protected]
REVISITING THE NATURE OF DARK SUNSPOTS: A MODEL OF HOT SPOTS
Abstract: The empirical and physical inconsistency of the cold sunspot model is shown. It is shown that a dark sunspot can only be hot. Based on the results of an experimental study of the effect of a magnetic field on the intensity of visible light emitted by a tungsten filament lamp, a hot sunspot model was proposed. The heating and darkness of sunspots are due to the dissipation of the energy of electronic excitation of negative hydrogen ions induced by the magnetic field of the spot.
Keywords: dark spots on the Sun, quenching of radiation by a magnetic field, negative hydrogen ion.
Introduction
This work is a continuation of the work [1; 2], in which the darkness of sunspots is explained by the influence of the magnetic field of the spot on the intensity of the radiation of the photospheric gas, as a result of which the spots must be hot, and not cold. Such an explanation follows from the results of experimental studies of the effect of magnetic fields on the intensity of visible light of tungsten filaments of electric incandescent lamps, described in detail in [1; 2].
Problems of the model of a cold sunspot
In the list of unresolved problems in the physics of the Sun, questions about the temperature of matter of sunspots and the cause of their darkness are not listed. It is believed that the spots are dark, because they are cold, and cold, because their magnetic fields prevent convective heat transfer to the surface of the Sun. However, almost all the observational data on the Sun, with the exception of visible dark spots, do not agree with the model of a cold sunspot or contradict it. In addition, no convincing mechanism for magnetic spot cooling has been presented so far [3, P. 249-251].
In the context of the foregoing, it is appropriate to quote one quotation from the well-known monograph on sunspots [4, P. 265]: "The first magnetic theories of sunspot cooling were based on the suggestions that convection within the spot umbra is suppressed (Biermann) or diluted (Hoyle) by the field (cf. Section 8.3.2). However, observations of granulation in sunspot umbrae, described in Section 3.6, show that the convection is not suppressed; for this and other reasons both Biermann's and Hoyle's theories are no longer tenable. Nevertheless, it is almost certain that the coolness of a spot is due to a reduction in the amount of energy convected upwards, resulting in some way from the influence of the magnetic field".
These words, written more than half a century ago, sound surprisingly modern and relevant, because even today the visible darkness of sunspots is an indisputable proof of their coldness. Moreover, the a priori statement that "the spots are dark, because they are cold and cold, because their magnetic fields impede convection", consisting of two circulus vitiosus,
not subjected to either empirical or theoretical verification, is already actually elevated to the rank of the postulate of solar physics. The negative aspect of this circumstance is that new ideas and alternative hypotheses about the nature of sunspots are not considered and, apparently, are generally not allowed to be discussed in the scientific literature.
Meanwhile, the key role of spots in all manifestations of solar activity is obvious. Without a clear understanding of the processes taking place in the sunspot and causing its darkness in visible light, without precise data on the actual temperature of the photospheric gas in spots, the solution of many other problems of solar physics is hardly possible.
If we compare the data of the Sun's cosmic observations with the predictions made on the basis of the concept of cold sunspots, then a paradoxical or even absurd picture of the processes and phenomena occurring on the surface of the Sun and in its atmosphere is obtained. It turns out that the "cold" spots shine brightly against the background of the photosphere in ultraviolet (1 ~ 10 nm) and X-rays, and the hotter faculae around the spots look very dark (X-ray brightness of the spots is 100 times greater than the photosphere). The most powerful and fastest movements of the photospheric gas (the Evershed flows) take place precisely in the "cold" spots, and solar flares emerge in them. It is over the "cold" spots that the hottest areas of the chromosphere and the corona are located, in which multiply ionized atoms of oxygen, argon, magnesium, silicon, iron and other elements emit bright spectral lines in the extreme ultraviolet and X-ray wavelength bands.
In this connection, another interesting question arises: how could the aforementioned atoms be in the Sun's corona? This issue is also not discussed in scientific articles, although the discovery of these atoms in the corona of the Sun looks from the point of view of the concept of cold spots exactly as if on Earth ordinary road dust was found at the same altitudes above sea level as the volcanic eruption ash.
There are many other facts and phenomena observed on the Sun, different in scale, but equally paradoxical, mysterious, or at least strange, if we explain them in terms of the concept
of cold sunspots. It follows that the model of cold sunspots leads to a deadlock, since empirical data on the Sun can not be explained within the framework of this model.
Insurmountable difficulties, and even of a fundamental nature, arise when considering the compatibility of the phenomenon of magnetic cooling of sunspots with the second law of thermodynamics.
Cooling a sunspot below the temperature of the photosphere means a spontaneous decrease in the entropy of the gas in the spot without removing heat from outside and without the work of an external force, which just prohibits the second law of thermodynamics. The removal of heat from the spot is impossible, since the photosphere is in a state of local thermodynamic equilibrium and the horizontal temperature gradients are practically zero. The magnetic field does not change the kinetic energy of the charged and neutral gas particles and can not cool it, but at the expense ofJoule heat it is capable of heating. In general, no adiabatic process can cool the gas in an unchanged volume, you can only heat it.
Photometric measurements of spot temperature [4, Chap. 4], indicating their coldness, can not be considered obj ective-ly reliable. Doubt is not the measurement technique complicated by many artifacts (diffused light, etc.), but the legitimacy of the application of the laws of thermal radiation to measure the temperature of the spots. The point is that the radiation of the spots has a nonequilibrium character and can not be approximated by thermal radiation, especially in the visible region of the spectrum, where the true continuous emission of umbra spots is absent altogether [4, P. 133-134].
The direction of thermal processes occurring in sunspots can not contradict the law of increasing entropy, from which it follows that heating and cooling of matter are fundamentally asymmetric and unequal processes. The heating of any body increases its entropy, and cooling reduces, and if there is no compensating increase in the entropy of other bodies, cooling is impossible. It is for this reason that in nature, for example, there exist only compression shock waves associated with heating the gas at the wave front, and shock waves of rarefaction associated with cooling the gas are impossible (Zemplen's theorem, 1905), although the solutions of the gas-dynamic equations allow the existence of shock waves ofboth types. Here it is important to note that the Zemplen theorem is also true for magnetohydrodynamic shock waves [5].
Model of hot sunspot
In the first approximation, let us consider the radiant heat exchange between the gas of a spot and the gas of an unperturbed photosphere, neglecting thermal conductivity and convection, since the role of these processes in the energy transfer in the photosphere is comparatively small. However, we take into account the important fact that the photosphere
of the Sun is in a state of radiant equilibrium and for the gas in it the Kirchhoff law for a black body is fulfilled.
Since the magnetic field does not interfere with the propagation of light, the volume of the spot is filled with an equilibrium thermal radiation emanating from the gas particles in the spot and from the particles of the photosphere located in some boundary layer of gas along the entire surface separating the spot and the unperturbed photosphere. The local temperatures of sufficiently small gas regions in the spot and in the boundary layer are proportional to the depths of their immersion in the photosphere and are initially identical both in the spot and in the photosphere. If the magnetic field does not affect the absorption and emission of photons by gas particles in the whole spectrum of its radiation, then an equilibrium thermal radiation will emerge from the spot through its outer surface, which will hardly differ from radiation emanating from neighboring sections of the surface of a quiet photosphere. In other words, under radiant equilibrium conditions, the radiative cooling of a sunspot is impossible, and it can not be dark.
To change the temperature of the gas in the spot, at least the Kirchhoff law for the blackbody must be violated in the photosphere region occupied by the spot, which is equivalent to breaking the detailed balance between the absorption and emission of photons by the gas particles in the spot.
However, if we now assume that the magnetic field has disturbed the radiant equilibrium in the spot, which is indeed quite possible, then it is not difficult to see that the consequence of this violation can only be the heating of the spot. This conclusion again dictates the second law of thermodynamics, since cooling the spot would mean a decrease in the entropy of the gas without removing heat from outside by internal processes alone, which is impossible.
The same conclusion, provided that the magnetic field of the spot has violated Kirchhoff's law, necessarily follows from the most famous observational fact, attesting to the almost total darkness of the umbra of sunspots in visible light. This darkness means that the processes of absorption of photons of visible light dominate in the gas of the spots, and the processes of their emission are almost completely suppressed by the magnetic field. It follows that photons of visible light heat the gas of the spot, since it is opaque to them. We note that for radiation volume cooling of gas it is necessary that photons are produced due to its internal energy and the gas for them is transparent. Consequently, dark sunspots can only be hot, and out of them come out those photons, the radiation process of which is not affected by the magnetic field of the spot, as well as photons that radiate gas spots due to its heating. The latter circumstance, in particular, explains the brightness of sun-spots in the extreme ultraviolet and X-ray wavelength ranges.
Negative hydrogen ion in the magnetic field of the spot
The question arises, what is the possible mechanism for violating Kirchhoff's law by the magnetic field of the sunspot, at least in the initial stage of its appearance?
In the solar photosphere, photons ofvisible light are most strongly absorbed and emitted by negative hydrogen ions (Н-), in which there is one stable singlet state (S = 0, the spins of electrons are paired), the ionization energy ofwhich is 0.75 eV [3, P. 149-150]. Apparently, all the excited states of the ion Н- belong to the continuous spectrum of the energy values, when one electron of the Н- ion remains in the discrete spectrum, and the other electron in the continuous spectrum. We note that a negative hydrogen ion is capable of absorbing and emitting photons in both bound-free and free-free transitions.
If there is not a sufficiently strong magnetic field in the photospheric gas, then the absorption by the ion of the Н-photon is completed by the formation of a loosely bound quantum system "hydrogen atom + electron", which remains in the singlet state for some time, since an electronic transition does not change the spin (AS = 0). The electron, being in the field of radiation near the hydrogen atom, polarizes it, thereby stimulating the reverse process - the recombination of the Н- ion with the emission of a photon, as the Kirchhoff law demands. Naturally, the formation of the Н- ion must occur before the time determined by the lifetime of the "hydrogen atom + electron" system, when the distance between its particles is already excessively large.
If a sufficiently strong magnetic field is present in the photospheric gas, the course of events indicated above may be completely different. The spin of an electron or atom directed against a magnetic field is unstable, and for a short time it is reversed with an orientation along the field direction. For free electrons in a magnetic field B < 1 T, the spin flip time at room temperature does not exceed 10-10 s, while the duration of optical processes is ~ 10-8 s.
Thus, with a high degree of probability it can be asserted that in a time shorter than the lifetime of the "hydrogen atom + electron" system, the magnetic field will change the spin state of the electron, which means the transition of the system
to a triplet state (S = 1). In this case, a photon with an energy of ~ 10-5 eV (the range of centimeter radio waves), which is associated with the spin flip of an electron in a magnetic field, will be emitted. The triplet state of the "hydrogen atom + electron" system is unstable, since the approach of particles with equally directed spins leads to a continuous increase in repulsive forces. The formation of an Н- ion with the emission of a photon becomes impossible, and the energy of the electronic excitation of the "hydrogen atom + electron" system becomes kinetic energy of the translational motion of its particles. In other words, there is a nonradiative transition of the energy of a quantum system into the thermal energy of its particles.
It is interesting to note that back in 1919, W. Steubing described in detail the observation of the quenching by the magnetic field of the fluorescence of iodine vapor [6], which later was called the phenomenon of magnetic predissociation of iodine molecules in the gas phase and has long been included in textbooks. The interesting thing is that the mechanism of magnetic quenching of iodine molecules emission is practically analogous to the mechanism described above for suppressing by the magnetic field of the radiation of negative hydrogen ions in the solar photosphere. In work [7], quenching of laser ruby luminescence by a strong pulsed magnetic field due to stimulation of nonradiative deactivation of optically excited Cr3+ ions by a magnetic field in the transition between energy levels responsible for the ruby laser radiation is described.
Conclusion
There is no doubt that the magnetic field in optical phenomena can play the same role as frictional forces in mechanical phenomena. An optically conservative photospheric gas becomes a dissipative medium in a sunspot, which is a direct consequence of the irreversibility of the absorption of photons by negative hydrogen ions in the magnetic field of the spot.
Dark spots on the surface of the Sun are original, giant in size and power analogs of volcanoes on the surface of the Earth, and the actual temperatures and speeds of movements of their dark "lava" have yet to be determined. It is the spots that are the sources of the energy that heats the Sun's corona.
References:
1. Soika A. K. Revisiting the nature of dark Sanspots, European Science Review,- No. 9-10,- 2016. September - October. -P. 249-252.
2. Soika A. K. Magnetic Quenching of the thermal Radiation and the Nature of dark Spots on the Sun, Materials of the XV International Research and Practice Conference, December 14th - 15th, Munich, Germany, 2016.- P. 20-28.
3. Foukal P. V. Solar Astrophysics, WILEY-VCH Verlag Gmb H. & Co. KGaA, Weinheim,- 2004.- 466 p.
4. Bray R. J., and Loughhead R. E. Sunspots, London, Chapman and Hall,- 1964.- 303 p.
5. Iordanskii S. V. Zemplen's theorem in magnetic hydrodynamics, Dokl. Akad. nauk SSSR,- Vol. 121.- No.4.- 1958.-P. 610-612.
6. Steubing W. Spektrale Intensitätsverschiebung und Schwächung der Jodfluoreszenz durch ein magnetisches Feld, Annalen der Physik,- Vol. 363.-No.1.- 1919.- P. 55-104.
7. Boiko B. B., Soika A. K. The influence of a strong magnetic field on the ruby luminescence, Dokl. Akad. nauk BSSR,-Vol. 22.- No. 2.- 1978.- P. 1072-1074.
