The parameters of the Sun's rotation ( Eng. Solar rotation ) depend on the latitude of the place. The sun is not a solid, it consists of a gaseous plasma . Points at different latitudes rotate with different periods, that is, the rotation of the Sun is differential . The reason for the differential rotation is currently one of the issues of solar astronomy [1] . Rotation speed is greatest at the equator of the Sun (latitude = 0 ° ) and decreases when moving to the poles. The rotation period of the Sun is 24.47 days at the equator and almost 38 days near the poles.
Rotation Equation
The differential rotation speed can be described by the equation
where ω is the angular velocity expressed in degrees per day, φ is latitude, A, B and C are constants. The values of A, B, and C differ depending on the method used to measure and also on the magnitude of the observation period. [2] The following mean values are currently used [3] :
-
day
day
day
Sidereal rotation
At the equator, the rotation period of the Sun is 24.47 days. This value is called the sidereal period of rotation, it should not be confused with the synodic period of rotation equal to 26.24 days and representing the period of time after which for the observer on Earth the detail of the surface of the Sun will repeat its position. The synodic period exceeds the sidereal one, because when the position of the part is repeated on the surface, the Sun makes not only one revolution, but also a rotation by a small additional angle, compensating for the Earth's displacement in its orbit. Note that in the astrophysical literature the rotation period at the equator is usually not used; instead, the Carrington rotation is determined: the synodic rotation period is 27.2753 days, the sidereal period is 25.38 days. Such values of the period correspond to a direct rotation at a latitude of 26 ° north or south of the equator, which is a characteristic value for the region of occurrence of sunspots and manifestations of periodic solar activity. When observed from the north pole of the ecliptic, the Sun rotates counterclockwise. If a person is at the North Pole of the Earth, then it will seem to him that the sunspots are moving from left to right along the disk of the Sun.
Bartels number
The Bartels rotation number is a serial number characterizing the number of revolutions of the Sun when observed from the Earth. Used to track repetitive or shifting manifestations of solar activity. It is assumed that each revolution lasts 27 days, which is close to the Carrington synodic period. Julius Bartels , as the starting point for the number of revolutions, adopted the date of February 8, 1832. The ordinal number of revolutions may be a kind of calendar consistent with the repetition periods of solar and geophysical parameters.
Carrington rotation
Carrington rotation is a system for comparing the positions of parts on the surface of the Sun, separated by a certain period of time, which allows you to track the evolution of groups of sunspots or flares.
Since the parameters of the rotation of the Sun vary with latitude, layer depth, and over time, such comparison systems are approximate. In the case of the Carrington rotation model, the solar rotation period is taken to be 27.2753 days. Each revolution of the Sun in such a scheme has its own number, the starting point of which is November 9, 1853. (The Bartels number [4] is constructed according to a similar scheme, but the circulation period is assumed to be 27 days, the reference point is February 8, 1832.)
The heliographic longitude of the part on the surface of the Sun corresponds to the angular distance from the object to the central meridian, that is, to the line from the Sun to the Earth. The Carrington longitude of the part is the angular distance relative to the fixed point whose position Carrington indicated.
Richard Carrington determined the speed of rotation of the Sun from the data on sunspots at low latitudes in the 1850s, according to his estimates the sidereal period of revolution of the Sun is 25.58 days. Sidereal rotation is measured relative to distant stars, but since the Earth rotates around the Sun, then for the Earth observer the rotation period of the Sun will be 27.2753 days.
You can build a chart in which the longitude of the spots is plotted on the horizontal axis, and the time on the vertical. Longitude is measured at the intersection time of the central meridian and is based on the Carrington model of rotation. If we draw on this diagram the position of sunspots after each revolution, then most of the new points will be strictly below the points from previous revolutions. Over long time intervals, small shifts to the right or left are possible.
Using Sunspots to Measure Rotation
The constants in the rotation model were determined by measuring the motion of various parts of the surface of the Sun. The most famous such details are sunspots. Although spots have been observed since ancient times, but only with the invention of the telescope it became clear that they rotate with the Sun, so you can determine the period of rotation of the Sun. English researcher Thomas Harriot is probably the first to observe sunspots with a telescope, as evidenced by sketches in a notebook dated December 8, 1610. The results of the observations of Johann Fabricius , who systematically observed the spots for several months, were published in June 1611 under the heading “De Maculis in Sole Observatis, et Apparente earum cum Sole Conversione Narratio” ("Description of sunspots observed on the Sun and their visible rotation together with the Sun "). This work can be considered the first observational evidence of the rotation of the Sun. Christopher Scheiner (“Rosa Ursine sive solis”, book 4, part 2, 1630) was the first to measure the speed of the Sun at the equator and noticed that rotation at high latitudes occurs at a lower speed than at low, so Shayner can be considered a pioneer differential rotation of the sun.
Each measurement gives a slightly different result from the previous ones, which leads to the appearance of a standard error (indicated after +/-). S. John (1918) was probably the first to collect published estimates of the speed of rotation of the Sun and came to the conclusion that it is difficult to explain the difference in results only by observer errors and local disturbances on the Sun; the differences are likely due to variations in rotational speed. Hubrecht (1915) pointed out that the two hemispheres of the sun rotate somewhat differently. The study of magnetographic data gave a synodic period of 26.24 days at the equator and almost 38 days at the poles. [five]
The inner rotation of the sun
Before the era of helioseismology , the study of solar oscillations, very little was known about the internal rotation of the sun. It was assumed that the differential profile of the surface rotation extends to the inner part of the Sun. [6] According to helioseismology, it is known that the rotation of the Sun does not occur according to this scheme. A rotation profile was obtained; on the surface, the Sun rotates more slowly at the poles and faster at the equator. Such a rotation mechanism exists in the convection zone. In the tachocline region, the rotation regime sharply changes to solid-state rotation in the region of radiant transfer . [7]
Notes
- ↑ Zell, Holly Solar Rotation Varies by Latitude . NASA (March 2, 2015). Date of treatment February 14, 2019.
- ↑ Beck, J. A comparison of differential rotation measurements (Eng.) // Solar Physics . - 2000. - Vol. 191 . - P. 47-70 . - DOI : 10.1023 / A: 1005226402796 . - .
- ↑ Snodgrass, H .; Ulrich, R. Rotation of Doppler features in the solar photosphere (Eng.) // The Astrophysical Journal : journal. - IOP Publishing 1990. - Vol. 351 . - P. 309-316 . - DOI : 10.1086 / 168467 . - .
- ↑ Bartels, J. (1934), " Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923-1933 ", Terrestrial Magnetism and Atmospheric Electricity T. 39 (3): 201–202a , DOI 10.1029 / TE039i003p00201
- ↑ 5. Astronomy and Astrophysics, vol. 233, no. 1, July 1990, p. 220-228. http://adsabs.harvard.edu/full/1990A%26A...233..220S
- ↑ Glatzmaier, GA Numerical simulations of stellar convective dynamos III. At the base of the convection zone (Eng.) // Solar Physics : journal. - 1985. - Vol. 125 . - P. 1-12 . - DOI : 10.1080 / 03091928508219267 . - .
- ↑ Christensen-Dalsgaard J. The Solar Tachocline: Observational results and issues regarding the tachocline. - Cambridge University Press , 2007. - P. 53–86.
Literature
- Cox, Arthur N., Ed. "Allen's Astrophysical Quantities", 4th Ed, Springer, 1999.
- Javaraiah, J., 2003. Long-Term Variations in the Solar Differential Rotation. Solar Phys., 212 (1): 23-49.
- St. John, C., 1918. The present condition of the problem of solar rotation, Publications of the Astronomical Society of the Pacific, V.30, No. 178, 318-325.