CC BY
67
DOI: http://dx.doi.org/10.20534/ESR-16-9.10-249-252
Soika Alexander Kuzmich, Military Academy of the Republic of Belarus, Professor, Department of Physics E-mail: [email protected]
Revisiting the nature of dark sunspots
Abstract: A new physical mechanism explaining the reason of darkness of sunspots is proposed in this work. It is based on the results of the experimental research of the effect of magnetic fields on the intensity of the light of incandescent electric lamps. These results indicate the magneto-optic nature of sunspots.
Keywords: magnetic fields, magneto-optic phenomena, dark spots on the Sun.
So far, extensive observational material about sunspots has been accumulated, but there is still no clear understanding of their physical nature [1; 2]. The question why the spots are actually dark remains open. There is only one answer to this question in reference literature: the spots are dark because they are colder than photosphere as their magnetic fields suppress the convection of photospheric plasma. However, the hypothesis about the magnetic refrigeration of spots cannot be acknowledged as either proved or convincing.
It has been known for long that the convection is not suppressed even in the biggest spots [1, 272]. Constant magnetic field conceptually cannot change mean kinetic energy of charged particles, because the force it affects them with is either perpendicular to the particle velocity or equal to zero. The transfer of energy in the photosphere of the Sun is primarily radiative and the role of convective heat exchange is comparatively small due to the low density (~40-7 g/cm 3) of the substance. Considering that the degree of ionization of photospheric gas is lower than 0,1%, it appears unlikely that the magnetic field, influencing the convection and not influencing the radiative heat exchange, can generate such considerable and sustainable refrigeration of the spots.
The model of a cold spot is inconsistent with a lot of observational data, for instance, such as sharpness of the spot borders and photosphere, shadow and half-shadow, possible bright bridges in the shadow, fiber structure of the half-shadow, delicate bright structure of the spot [2, 208-216] etc.
Photometric changes [1, 104-132, 144-146] causing different temperatures of the spots and photosphere are based on the laws of thermal radiation and presumption that both, photosphere and spots are similar to absolutely black bodies. But there are significant differences between spectra of spots and spectra ofphotosphere [1, 132-142], and they do not relate to each other as spectra of thermal radiation of same differently heated body, i. e. substances of the photosphere. This circumstance creates a big doubt that the presumption stated above corresponds to the facts.
The difference in the spectra of spots and photosphere is especially distinctive in the continuous radiation. In case of photosphere, it is clearly observed and described by the Planck function, i. e. it corresponds to the equilibrium thermal radiation. At the same time, in wide areas of the spectrum, the purely continuous radiation of the spots is not observed at all [1,133-134]. In [3], it is shown that averaged curve of radiation energy distribution in the spectra of the spots obtained according to the data of different observers does not agree with the curve of equilibrium thermal radiation under any circumstances. In [4], a two-component (according to the temperature) model of the shadow of the spot is proposed, because it is impossible to explain simultaneous observation of the lines with high and low
excitation potential in the spectrum of the shadow of the spot within the frames of temperature-homogenous model.
Let's emphasis another principally important fact [1, 140; 5, 233-234]. There are mutually excluding lines of titanium monoxide TiO and diatomic carbon C2 molecules of almost identical intensity in the spectra of the shadow of spots, which are not observed simultaneously in the spectrum of the Sun (and in the spectra of stars). It signifies that the spectrum of the shadow of spots cannot be referred to one of the certain spectral classes typical for thermal radiation sources.
A presumption that the darkness of the spots is caused by the effect of a non-thermal factor and cannot be explained based on the thermal radiation laws logically follows from the above said. According to the author, magnetic field, an attribute of sun spots, is such factor, but their darkness is determined not by cooling, but a direct effect of the magnetic field on the intensity of radiation. In other words, this means the extinction of thermal radiation of the photospheric gas with magnetic field. Herewith, in consequence of magnet-induced dissipation of the energy of electronic excitation of atoms and molecules of the photospheric gas, sun spots should be hotter than the environment surrounding their photospheres.
For the purpose of experimental check of the hypothesis about magnetic extinction of thermal radiation, the effect of magnetic fields on the intensity of light of electric lamps was researched. Such choice of the object of research is determined by the fact that the working temperatures of tungsten wires of filament lamps are within the range (2200-2900) K, which is close (in respect of the order of value) to the temperature of the photosphere substance.
Quartz-halogen lamp for the microscopes of HL-WS4-TGL-11381-20W-6V type and regular flashlight lamp of 3,5 Bx0,26 A type were used in the experiments. The intensity of the light spread in the direction along the force lines of the magnetic field affecting the hot filament of the lamp was registered.
The impulses of the magnetic field in the form of subsiding sinusoid with the frequencies around 100 and 380 Hz and the amplitudes of induction up to 5 and 12 T respectively were applied, which were obtained by the discharge of capacitor bank with the capacity of 3,46 mF and voltage up to 5 kV through copper coils with the induction of approximately 50 and 985 ^HY. The charge was commutated by air trigatron [6].
The experiment was conducted as follows. Electric lamp was fixed inside the coil so that the hot filament was in its center. In the magnetic fields with the frequency of 380 Hz, a halogen lamp powered with alternate current with the force of 2 A and frequency of 50 Hz, and a lamp with the frequency of 3,5Vx0,26 A in the fields of 100 Hz, the source of supply ofwhich was a battery with the voltage of 3 V were used.
The light from the lamp fell on the end face (diameter 3 mm, length 2 m) and arrived at the entrance window ofphoto-electronic multiplier FEU-36 through another face, the signal of which was registered by a two-beam memory oscilloscope C1-42. The anode of the PEM was neutralized and high voltage of negative polarity (~1150 V) was sent to cathode. The PEM signal had negative polarity and was sent to the entrance (direct or inverse, closed or open) of oscillograph operating in the waiting mode. The sweep of oscillograph was started by external impulse, after which to the controlelec-trode of the trigatron was supplied high voltage pulse, initiating its ignition. The second beam of the oscillograph was supplied with a signal from Rogowski loop with RC-integrator that registers time travel of current (magnetic field) in the coil.
Before the application of magnetic field (and soon after it) on the lamp, a certain level of the constant component of the PEM signal was established and measured according to the intensity of oscillogram. It should be noted that the pulsation of brightness of halogen lamp at used duration of the sweep of oscillograph (1 ms/div) was not found. Relative error of the measurements of the amplitudes of signals did not exceed ± 10%.
The PEM was working in the linear mode (bleeder current exceeded anode current by not less than 20 times). The optical channel of the PEM was soundly protected from foreign light. The PEM had a dual copper-iron screen and distanced from the coil of the magnetic field to the distance of ~ 1,5 meters. The impulse magnet, including capacitor bank, was fully closed from the PEM with an iron screen with the thickness of walls of 3 mm.
The main result of the experiment lies in the establishment of the fact of partial or complete extinction of the intensity of light of
filament lamps during the application of the filament of the impulse magnetic field on them. Threshold values of the field induction provoking noticeable (registered) reduction of the light intensity, and critical values of the field induction, at which full extinction of lamp luminance is achieved, are different for different impulses of the magnetic field depending on the amplitude of induction, duration of the impulse and velocity dB/dt of the change of field induction in time.
The dynamic picture of changes of the light intensity of researched lamp in alternating magnetic fields is quite fully and appropriately reflected by oscillograms presented in Fig. 1 and Fig. 2.
According to these oscillograms, one can assess that the critical values of induction account for about 7 T for the field with the frequency of 380 Hz and ~ 0,6 T for the field with the frequency of 100 Hz, and threshold values — ~ 0,3 and ~ 0,1 T respectively. A big difference of critical values of the induction of magnetic fields, oscillating with less than one order of different frequencies, is determined, apparently, by a strong dependence of the efficiency of magnetic extinction on the velocity dB/dt of the change of field induction in time.
Let's comment on the oscillograms of the change oflight intensity of the lamp 3,5 Bx0,26 A in the magnetic field, oscillating with the frequency of 100 Hz, which are presented with curves below in Fig. 2 a), 2 b) and 2 c).
The oscillogram in Fig. 2 a) was received at the open inverse entrance of the oscillograph and shows full PEM signal, including its constant component. Already at the beginning of the half-wave of the magnetic field (amplitude 1,2 T), there is full extinction of the observed luminance of the lamp, after which, the flash of light appears only in the first minimum of the magnetic field.
a)
b)
Figure 1. — a) Oscillogram of the light intensity of the halogen lamp in the magnetic field; the entrance of the oscillograph is inverse, closed, vertically 2 V/div; constant component of the signal 5,6 V. — b) Oscillogram of the current (magnetic field) of the coil, vertically 3 T/div, amplitude of induction В = 11,2 T. The sweep for both oscillograms is 1 ms/div
The oscillogram of the intensity of the light of the same lamp in Fig. 2 b) was obtained with bigger (than in Fig. 2 a)) duration of the sweep at the closed direct entrance of the oscillograph and shows alternating component of the MEP signal. The complete extinction of the lamp's light takes place at the very beginning of the front edge
of the first half-wave of the magnetic field (amplitude 2,4 T), and future flashes of light (of different brightness) are observed starting from the second minimum of the field, which is, possibly, related to the manifestation of the effect of consequence of the magnetic field on the electronic structure of the tungsten [7].
a)
c)
Figure 2. — a) Upper beam — oscillogram of the current (magnetic field) of the coil, vertically 1,5 T/div, amplitude of induction Bm = 1,2 T, lower beam — oscillogram of the intensity of light of the lamp 3,5 Bx0,26 A in the magnetic field, the entrance of the oscillograph is inverse, opened, vertically 0,5 V/div, constant component of the signal 0,7 V, sweep 1 ms/div. — b) Upper beam — oscillogram of the current (magnetic field) of the coil, vertically 3 T/div, amplitude of induction Bm = 2,4 T, lower beam — oscillogram of the intensity of light of the lamp 3,5 Bx0,26 A in the magnetic field, the entrance of the oscillograph is direct, closed, vertically 0,5 V/div, constant component of the signal 0,8 V, sweep 2.5 ms/div. — c) Upper beam — oscillogram of the current (magnetic field) of the coil, vertically 3 T/div, amplitude of induction Bm = 3,6 T, lower beam — oscillogram of the intensity of light of the lamp 3,5 Bx0,26 A in the magnetic field, the entrance of the oscillograph is direct, closed, vertically 0,5 V/div, constant component of the signal 0,8 V, sweep 5 ms/div
The oscillogram of the intensity of light of the lamp (see Fig. 2 c)) was obtained in the magnetic field with the amplitude of 3,6 T. The duration of the sweep allows tracking the activity of the field during the time of about 0,05 s. As in previous cases, the lamp goes down at the very beginning of the first half-wave of the field and future flashes are observed starting from the third minimum of the field. (It should be noted that noticeable drift up of the zero line of the oscillograms of the intensity of light in Fig. 2 b) and 2 c) is explained by the fact that the constant of time of the closed entrance of the oscillograph is equal to approximately 20 ms.)
Thus, the results of the experiment allow stating a new physical phenomenon that lies in the fact that the magnetic field can reduce the intensity of the visible light of heated bodies till complete extinction. Due to the fact that the experiment in relation to the photosphere is modeled, this phenomenon cannot be directly transferred to the processes in sunspots, but there is also no basis to deny its applicability to them only on the basis of the fact that the source of radiation is a burning hot metal and not photospheric gas. One can assume that the extinction of radiation of photospheric gas should not be accompanied by any effects of the consequence of the magnetic field possible only in condensate environments.
It is interesting that the threshold (~ 0,1 T) and critical (~ 0,6 Ta) values of the induction of magnetic fields with the frequency of100 Hz for the luminance of tungsten wire agree well with the values of the lowest and biggest induction of magnetic fields in sunspots [2, 227].
If it is granted that the darkness of the spots is determined by magnetic extinction of the radiation ofphotospheric gas, then a lot of observational data, which is difficult to explain based on the model of cold spot, gains natural and clear interpretation. Here are several examples on this topic, which are formulated in the form of statements.
Inhomogeneity of the brightness of the spots is the reflection of inhomogeneity of their magnetic fields. Bright bridges in the shadow are the sites of the spots, where magnetic field is lower than the threshold one; they can be fairer than photosphere, because they are hotter than it. Long-living pores, which the formation of spots starts from, do not have half-shadow because their magnetic field is quite homogenous and strong. The energy of Evershed motions is the energy of radiation dissipated into the heat by the magnetic field, i. e. the shadow of the spot is the hottest area of the photosphere. Such examples may be continued (see [2, 262-264]).
To conclude, let's note that the detection of magnetic extinction of thermal radiation in simple experimental conditions is not an accidental event and has a long pre-history.
In 1913, Steubing discovered magnetic extinction (till extinguishing) of the luminescence of iodine vapors [8]. This phenomenon remained unclear for long, until 1932, when Van Vleck explained it as a result of magnet-induced pre-dissociation of iodine molecules found later in other mainly two-atom molecular gases [9].
In [10], the ph enomenon of magnetic extinction of the luminescence of laser ruby at room temperature in the impulse fields with 50 T is described, which was studied at all possible mutually perpendicular and parallel orientations of the magnetic field, optic axis of the ruby sample and direction of the observation of its luminescence. Either partial (threshold induction 24 T) or complete magnetic extinction of the luminescence of the ruby (critical induction from 40 T and higher) was observed.
Within the experiment described above, the issues related to the spectral and spatial selectivity of the influence of magnetic fields on the intensity of thermal radiation of different substances remain unexplained. Additional research is required to answer these questions.
References:
1. Bray, R. J., and Loughhead, R. E. Sunspots, London, Chapman and Hall, - 1964. - 303 p.
2. Solanki, S. K. Sunspots: an overview. Astron. Astrophys. Rev., - 11, - 2003. - 153-286.
3. Mattig, W., Schröter, E. H. Is There Radiative Equilibrium in Sunspots?, Astrophysical Journal, - Vol. - 140. - 1964, - 804-807.
4. Makita, M. Physical States in Sunspots, Publ. Astron. Soc. Japan, - Vol. - 15, - № 2, - 1963. - 145-176.
5. Richardson, R. S. An investigation of molecular spectra in Sunspot, Astrophysical Journal, - Vol. 73, - 1931. - 216-249.
6. Soika, A. K., Sologub, I. A. Generation of Nondestructive Unipolar Strong Pulsed Magnetic Fields, Instruments and Experimental Techniques, - Vol. 53, - No. 1, - 2010. - 124-127. Pleiades Publishing, Ltd.
7. Soika, A. K., Sologub, I. O., Shepelevich, V. G., Sivtsova, P. A. Magnetoplastic Effect in Metals in Strong Pulsed Magnetic Fields, Physics of the Solid State, - Vol. 57, - No. 10, - 2015. - 1997-1999. - Pleiades Publishing, Ltd.
8. Steubing, W. Spektrale Intensitätsverschiebung und Schwächung der Jodfluoreszenz durch ein magnetisches Feld, Annalen der Physik, - Vol. 363, - № 1, - 1919. - 55-104.
9. Зельдович, Я. Б., Бучаченко, А. Л., Франкевич, Е. Л. Магнитоспиновые эффекты в химии и в молекулярной физике, Успехи физических наук, - Т. 155, вып. 1, - 1988. - 3-45.
10. Бойко, Б. Б., Сойка, А. К. Влияние сильного магнитного поля на люминесценцию рубина, Доклады Академии наук БССР, -Т. 22. - № 12. - 1978. - 1072-1074.
