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 Sattarov

Tashkent State Pedagogical University, 103 Yusif Kxos Kxojib str.,Tashkent 100070,

Uzbekistan

email: [e-mail]

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 umbral

dots, 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 groundbased

telescopes. 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’s

slit 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 the

Sattarov

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 umbral

dots 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 intergranular

spaces 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, Lites

et 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

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