Are the umbral dots, penumbral grains, and G band bright points formed by the same type of magnetic flux tubes?

The Physics of Sun and Star Spots
Proceedings IAU Symposium No. 273, 2010
D.P. Choudhary & K.G. Strassmeier, eds.
_c International Astronomical Union 2011
doi:10.1017/S1743921311015699
Are the umbral dots, penumbral grains, and
G band bright points formed by the same
type of magnetic flux tubes?
Isroil SattarovTashkent State Pedagogical University, 103 Yusif Kxos Kxojib str.,Tashkent 100070,
Uzbekistan
email: isattar@astrin.uz
Abstract. Today’s Solar Physics comes across of different type of fine structures in solar atmosphere
including umbral dots and penumbral grains in sunspots, and G-band bright points in
quiet Sun. In this report, we present evidence that umbral dots, penumbral grains, and, possibly,
G band bright points are related to a common type of features in solar atmosphere magnetic
flux tubes.
Keywords. Umbral dots, penumbral grains, sunspots, G-band bright points
1. Introduction
Stratospheric balloon-borne observations of sunspots in USA under leadership of M.Schwarzchild in 1959 (Danielson 1964), and balloon experiments in USSR under leadership
of V. Krat (Krat et al. 1972) excited great interest and put forward many problems
in physics of sunspot fine structure. The mechanism of emergence of so-called umbraldots, their relation with penumbral filaments are examples of questions raised by these
observations. These earlier stratospheric observations had turned out to be insufficient
for study the dots evolution, which require long-term observations; and the author was
among the first observers, who begun studying the sunspots’ fine structure using groundbasedtelescopes. Today’s Solar Physics comes across different types of fine structures in
solar atmosphere. In this report we present evidence to support that umbral dots, penumbral
grains, and, possibly, G-band bright points are related to the same type of features
magnetic flux tubes in solar atmosphere.
2. The telescope and observations
Horizontal solar telescope ATSU-5 used by us is one of the serial instruments (D=44
cm, F=1700 cm, where D is diameter of main mirror, and F is its focal length) developed
by Russian optician V.N. Ponamarev 55 years ago. Ten years later, the majority of Soviet
Union observatories were equipped with such telescopes. In 1964, ATSU-5 telescope was
mounted in Tashkent Astronomical Observatory in the city of Tashkent. The territory of
Uzbekistan is very sunny. In Tashkent, the average sunshine time is near 2900 hours per a
year with a consistently very good seeing from June 20 to the beginning of October. The
very good seeing takes place during windless and hot weather, frequently for several days
before a cyclone. White light observations of sunspots, discussed in this article, were
carried out in Cassegrain focus (diameter of solar disk is 60 cm) using high-speed 35
mm photographic camera (exposure time 0.01 sec). Bursts of images were taken during
the periods of best seeing, based on visual control by the observer (the author of this
article). Spectral observations were conducted in Newton (prime) focus of the telescope
IAU273: Umbral dots, penumbral grains, and G band bright points

Figure 1. Two prints (a 0650 UT, b 0828 UT) of umbra of the unipolar sunspot observed 8 August
1981 at the ATSU-5 in Tashkent. Vertical touch on Fig. 1b shows position of spectrograph’sslit at receiving spectrum presented on Fig. 3.

Figure 2. Drawing of penumbral grains and umbral dots (short filigree) of the large sunspot
exhibiting counter-clockwise rotation for 14(a), 15(b) and 16(c) September 1977.
using ASP-20 horizontal spectrograph with linear dispersion of 0.6 ˚A / mm at optical
wavelength. The spectra were recorded using photographic plates and film with exposure
time of 1-3 seconds. The observations were used to study the magnetic field, fine structure
and evolution penumbral filaments and umbral dots.
3. The results of sunspots fine structure studies
It was found that in most studied cases the umbral dots persist for more than 30
minute: 43 % of dots persist more than 60 minutes and 10% - more than 90 minutes.
In their evolution, bright dots increase the brightness reaching the maximum during a
relativity short time, and after that, gradually become weaker and expand. Finally, the
dots break up and disappear (Sattarov 1981a). Most of bright dots are observed at the
disk center side of penumbra umbra boundary (as in right side of umbra on Fig. 1 b).
In September 1977, a rotation of an unipolar sunspot with two umbrae (small northern
and large southern umbra) was observed. During 13-16 September the spot rotated
by 700 around its northern small umbra in counter-clockwise direction. The rotation
affected the orientation of penumbral filaments (grains) and umbral fine structure, twisting
them in clockwise direction. In the course of rotation, the main (large) umbra was
deformed; its east boundary was “attacked” by brushes of penumbral grains, which appear
as “peninsula” intruding into the umbra. The dots in front of “peninsula” were
transformed into the bright penumbral grains. On the west boundary of main umbra,
the opposite affect took place, i.e. the penumbral west boundary joined the umbra and
the bright penumbral grains turned into the umbral dots (Sattarov 1981b). It appears
that the magnetic flux tube that forms the large unipolar sunspot rotates, indeed. The
rotation may be caused by interaction of the magnetic tube forming unipolar sunspot
and the magnetic tubes of new sunspot group developed near the southern boundary
of the penumbra of unipolar sunspot. The magnetic field of sunspot had southern (S)
polarity, and the sunspot of leading (N) polarity of this new group had developed near
it later (Sattarov 1981b). Between the white light observations we took spectrograms of
sunspots by scanning their umbrae in selected (narrow or wide) spectral bands. In theSattarov
Figure 3. A part of the central section (noted in Fig. 1b by vertical stroke) spectrum with the
line Fe I 6302.5 of the umbra printed on Fig. 1.
spectrograms we have identified individual bright points or their clusters. It was found
that the magnetic field in umbral dots (or clusters of them) is more longitudinal than
the field strength in the darkest regions of umbra. The field strength in umbral dots is
about 20% lower than the field strength in the darkest regions of umbra (Sattarov 1982,
Litvinov & Sattarov 1989).As one can see on Fig. 3 spatial resolution of spectral observations is lower than the
resolution of the white light images (compare Fig. 1 with Fig. 3). It is the result of blurring
effect, which is larger near the boundary of umbra. In our estimate, the contribution of
scatter light in brightness of sunspot is not significant. Fig. 3 shows the spectral line Fe I
6302.5 with telluric lines of both sides. On Fig. 3, one can see the simple doublet Zeeman
splitting (middle white band on Fig. 3) on bright dot’s cluster (Fig. 1b). If the scatted
light was strong, we would see strong central component, which is not present on the
middle white band. The observations of clusters of dots do not show measurable Doppler
displacement. This implies that if there were plasma flows in dots the velocity of these
flows should not exceed 100 m/s (Litvinov & Sattarov 1989).
4. Discussion and conclusions
Bright dots were observed near the disk side of penumbra-umbra boundary in the “intrusion”
of penumbral material into the umbra. Rotation of the sunspot had transformed
umbral dots into penumbral grains and penumbral grains into umbral dots. Umbral dots
are associated with vertical magnetic tubes and, it seems, the rotation deflected umbraldots flux tubes from its vertical direction to more horizontal orientation prior to formation
of penumbral peninsula in umbra. Magnetic field in penumbral grain flux tubes have
more horizontal direction and sunspot rotation had deflected penumbral flux tubes on
the opposite (limb) side of the umbra from its horizontal direction to more vertical position.
Umbral dots and penumbral grains are the very small structural elements of solar
atmosphere. The movie of a newly developing sunspot (Fig. 4 presents one of images of
this movie) shows how the penumbral grains are formed. They develop in intergranularspaces from low contrast small photosphere features (it seems, from photosphere bright
points), but not from granules themselves. Plasma flow velocities in umbral dots are
as small as in intergranular spaces in the photosphere(Litvinov & Sattarov 1989, Liteset al. 1991). The umbral dots live longer than the photospheric granules, and it seems
they may have a different mechanism of flaring up than granulation As it appears from
our study, the granules cannot transform to the penumbral grains (as was suggested by
some models of sunspot penumbra). The granules show plasma flows of sufficiently larger
velocity directed vertically upward, whereas the dots have not show such strong plasma
flows. Intergranular lanes exhibit bright points - a very small structural element of solar
atmosphere. A sunspot movie (Nemirooff & Bonnel 2000) shows how the formation of
IAU273: Umbral dots, penumbral grains, and G band bright points

Figure 4. Small new sunspot observed at 8 August 1981 in ATSU-5 in Tashkent.
grains and photosphere bright points takes place near outside boundary of penumbra;
both features go away: the grains move to umbra, while the bright points move to the
surrounding photosphere. The grains and the bright points have opposite magnetic polarity.
On sunspot images of low resolution the grains present inner and the photosphere
bright points outer rings of Secchi. Therefore, we propose that the umbral dots, penumbral
grains, and photosphere bright points are associated with the feature - magnetic flux
tubes - which float to the solar surface in outside parts of sunspot penumbra.
Acknowledgments
The presentation of this paper in the IAU Symposium 273 was possible due to partial
support from the National Science Foundation grant numbers ATM 0548260, AST
0968672 and NASA - Living With a Star grant number 09-LWSTRT09-0039.
References
Danielson, R. E. 1964, Astrophys. J. 139, 45
Krat, V. A., Karpinsky, V. N., & Pravdjuk, L. M. 1972, Solar Phys. 26, 305
Sattarov, I. 1981a, Morphology and Cyclicity of Solar Activity, 122
Sattarov, I. 1981b, Morphology and Cyclicity of Solar Activity, 107
Sattarov, I. 1982, Sun and Planetary System, 96, 129
Sattarov, I. 1980, Soviet Astron. 24, 352
Litvinov, O.V., & Sattarov, I. 1989,Magnetic Fields and Corona, (Moscow, Nauka), 210
Lites, B. W., Bida, T. A., Johannesson, A., & Scharmer, G. B. 1991, Astrophys. J. 373, 683
Nemirooff, R.& Bonnel, J. 2000, Astronomy Picture Of the Day, 23/02/2000.