1446 Introductory Astronomy II

1. 1446 Introductory Astronomy II Chapter 10A The Sun’s Atmosphere R. S. Rubins Fall 2011

2. The Size of the Sun • i. The Moon’s orbit could easily fit within the Sun. • ii. The Sun’s radius is about 700,000 km; i.e. RSun ≈ 109 REarth. • iii. Just over a million Earths could fit inside the Sun.

3. About the Sun • The Sun is a glowing ball of gas and plasma, containing no solid or liquid matter. • Plasma, the 4th state of matter, is a high temperature mixture of electrons and positive ions (atoms that have lost one or more electron). • The Sun appears whitish from space, but yellowish from the Earth, because of the preferential scattering of the blue end of the visible spectrum by the Earth’s atmosphere. • The Sun is a smallish star near the lower end of the middle range of main- sequencestars. • The Sun is almost half way through its 10 billion-year lifetime as a main-sequence star.

4. Some Basic Statistics Mass:MSun ≈ 2 x 1030 kg ≈ 333,000 MEarth. Average distance from Earth: 1 AU ≈ 93 million miles ≈ 8.3 light minutes. Surface temperature:Approximately 5800 K. Core temperature:10 - 15 million K (~ 107 K). Composition by mass:74% H, 25% He, 1% other. Composition by number of molecules: 92% (H, H2), 8% (He), 0.1% (other). Rotational period: From 25 days (equator) to 36 days (poles). Orbital period around Galaxy:Approx. 220 milliony. Orbital speed around the Galaxy: Approx. 220 km/s.

5. Layers of the Sun 1

6. Layers of the Sun 2

7. The Photosphere • The photosphere, at about 700,000 km from the Sun’s center, is a very thin layer, less than 500 km in width. • It is defined as the Sun’s surface, since almost all of the Sun’s visible light is emitted from the photosphere. • The Sun’s spectrum corresponds approximately to a thermal radiator (blackbody) at 5800 K. • The density of the photosphere is about 1% of that of the air we breathe. • The photosphere has a blotchy appearance, produced by an effect known as solar granulation. • Each light-colored granule is surrounded by a darker boundary.

8. Solar Granulation • The photosphere is covered by millions of granules, each averaging about 1000 km in diameter. • Granules form, disappear, and reform over a period of minutes. • Doppler effect measurements show that hot gas rises at the center of each granule, where it emits electromagnetic radiation (sunlight), and cools. • Pushed out by the hotter gas below, the cooled gas drops back into the Sun’s interior along the darker boundary of the granule. • The boundary of the granule is about 100 K cooler than its center. • This method of heat transfer is known as convection, and is analogous to the process taking place on a pan of water heated by a flame.

9. Convection in a Solar Granule 1 Solar granule Cooler gas Hotter gas

10. Convection in a Solar Granule 2

11. The Chromosphere 1

12. Bohr Theory and H Spectra The Balmer emission lines are transitions to the n=2 level. 12 12

13. The Chromosphere 2 • The chromosphere is observable as a pinkish ring (due to the red emission line of H), but only when the photosphere is blocked out, as occurs in a total solar eclipse. • Its light is dim because of its very low density, which is less than a billionth of the density of the air we breathe. • The chromosphere is only about 1500 km thick, lying between the photosphere and the corona. • Moving outwards from the photosphere, the temperature in the chromosphere drops from about 5800 K to 4500 K. • At any instant, large numbers of gas jets, known as spicules, rise from about 5000 km to 10,000 km above the surface of the chromosphere.

14. Spicules • Spiculesare gas jets about 500 km across, which appear dark, and last for about 5 -10 minutes.

15. The Transition Zone • Situated between the chromosphere and the corona, the transition zone is thought to be from hundreds to thousands of kilometers wide. • Moving outwards in the transition zone, the temperature rises rapidly, from under 5000 K in the chromosphere to over a million K at the corona, and the pressure drops rapidly. • Only above 10,000 K is the temperature alone sufficient to remove an electron from an atom; i.e. to an ionize an atom. • In the photosphere, only 1 atom in 10,000 is ionized. • However, the transition zone (and the corona) contain a plasma, the 4th state of matter, which consists of positively charged ions and electrons in equilibrium with each other.

16. About the Corona • It is the outermost region of the Sun’s atmosphere, observable only when the photosphere is blocked out. • Its density starts at about a ten billionth of the air we breathe, and decreases rapidly with height above the photosphere. • At sunspot minima, the corona surrounds the Sun almost uniformly, to a depth of about a half-million km. • At sunspot maxima, the corona is irregular, streaming in some directions for several million km. • The streaming effect is a flow of charged particles, known as the solar wind, which leaves the corona at about 400 km/s. • The corona has surprisingly high temperatures of more than a million K, deduced from the observed absorption line of the ion Fe XIV, which is an iron atom stripped of thirteen electrons.

17. Solar Coronas at Sunspot Extrema Sunspot minimum Sunspot maximum

18. The Solar Wind • The solar windis a continuous outward plasma flow through the corona, consisting mainly of electrons, protons and alpha particles (helium nuclei). • The solar wind takes several days to reach the Earth, causing i. comet-tails to point away from the Sun; ii. auroras near the Earth’s poles, which are produced when the solar wind, trapped by the Earth’s magnetic field, strike the molecules of the upper atmosphere. • Although the Sun ejects about a million tons of matter each second into space, less than one percent of its mass will be lost in its 10 billion year main-sequence lifetime.

19. Sunspots 1

20. Sunspots 2 • Reported by Chinese astronomers 2000 years ago, sunspots are dark regions on the surface of the Sun, often as large in diameter as the Earth (about 10,000 km). • A sunspot consists of a dark spot, the umbra (at about 4300 K), surrounded by a ring, the penumbra (at about 5000 K), which is not quite as dark. • The magnetic field in a sunspot is about 1000 times greater than in the surrounding photosphere. • Sunspot often occur in clusters, known as sunspot groups, which are sometimes visible to the unaided eye. • Individual sunspots last from hours to weeks, while sunspot groups may be visible for months. • CAUTION Do not look at the Sun!

21. Zeeman EffectThe Zeeman effectis the splitting of certain spectral lines in the presence of a magnetic field.

22. Sunspot Group

23. Sunspot Pairs • Sunspots often occur in pairs aligned in an east-west direction, with the magnetic field leaving through one member of the pair and returning through the other. • The polarity is the same for all pairs in the Northern hemisphere, and reversed in the Southern hemisphere.

24. Sunspot Maximum 1979

25. Sunspot Minimum 1986

26. Sunspots Return 2010

27. Sunspot and “True” Solar Cycles • The period of the sunspot cycle, which is the time between sunspot maxima (or between minima), is roughly 11 years. • Recent maxima occurred in 1990 and 2000, with the next expected in 2011. Minima occurred in 1996 and 2007. • The Sun’s magnetic field, reverses direction every 11 years, so that the “true” solar cycle, which includes both the number of sunspots and magnetic field direction, is roughly 22 years. • There is a large variation in the number of sunspots visible at a maximum, while few or no sunspots are visible at a minimum. • By following the movement of sunspots across the Sun, Galileo (in 1609) discovered that the Sun took roughly a month to rotate once about its axis.

28. Sunspot Cycle and Butterfly Diagram

29. New Sunspot Formation • The average latitude of new sunspot formation occurs at roughly 30o north and south of the equator just after each sunspot minimum, and then forming increasingly closer to the equator as the cycle progresses. • Note: individual sunspots do not change latitude once they have formed. • A plot of the latitudes of sunspot formation with time is known as the butterfly diagram, because of its shape. • The eleven year sunspot cycle is not set in stone: in the last few years, we had a very extended minimum which lasted about 12 years, which is the longest minimum of the last 100 years.

30. The Sun’s Rotation • Like the Earth, the Sun rotates from west to east, but unlike the Earth, there is a differential rotation of the photosphere, with the equator rotating faster than the polar regions. • Deep within the Sun, all latitudes rotate with the same period of about 27 days. • The rate of rotation of the photosphere for latitudes above 30o north and south is obtained by measuring the Doppler blueshift at the western edge of the Sun, and redshift at its eastern edge.

31. Rotation Sequence 1

32. Features of the “Active” Sun 1 • All the features mentioned below, like sunspots, are linked with strong magnetic fields, and so are most likely to occur at sunspot maxima. • Solar flaresare violent eruptions of high energy particles and X (and UV) rays from the Sun’s corona, which occur when energy stored in twisted magnetic fields is suddenly released. • They usually last less than an hour, and reach temperatures of about 5 million K. • When reaching the Earth, solar flares interfere with power grids, satellite links, and mobile phone networks. • Coronal mass ejectionsare huge balloon-shaped masses of high-energy particles, containing typically about 2 trillion tons of gas, which are expelled from the Sun in a few hours.

33. Solar Flares

34. Coronal Mass Ejection 2

35. Features of the “Active” Sun 2 • Loop prominencesare huge loops (or arches) of glowing gas, which break through the photosphere with the magnetic fields passing through sunspots. If particularly energetic, the loop may break free of the magnetic field shaping it, and be ejected from the Sun. Loop prominences may have10 times the diameter of the Earth and reach temperatures of 50,000 K. • Filamentsare loop prominences viewed from above, and appear as dark lines above the photosphere. • Coronal holesare cooler regions of the corona, which appear dark in X-ray photographs, and act as a passageway for gases to flow out of the Sun.

36. The Active Sun The image was made using a filter, which passed only a wavelength corresponding to a Balmer line of hydrogen.

37. The photo shows gases emitting energy as they move along magnetic field lines. The loops reach heights of 160,000 km. To show the size of the effect, the blue sphere represents the Earth. Loop Prominences

38. Loop Prominence 2010

39. Magnetic Field Diagram Loop Loop prominence The importance of sunspot pairs in producing loop prominences is shown below.

40. A Loop Prominence Breaks Free • Taken at the wavelength of an He II emission line, image (a) shows the loop to be at a temperature of about 50,000 K. • Image (b) shows that the gas in the loop was energetic enough to break free of its confining magnetic field.

41. X-Ray image of a Coronal Hole

42. The Earth’smagnetic field (or magnetosphere) deflects most of the charged particles approaching the Earth. Coronal Mass Ejection and Earth’s Field Coronal mass ejection Earth’s magnetosphere

43. The Sun’s Magnetic Field The solar magnetic field outside the Sun is mapped by following the paths of particles in solar flares.

44. Colored Flashes on the Sun Green flash Blue flash Colored flashes, often green or blue which may be observed above the Sun very briefly just before sunrise or after sunset were first reported by Joule in 1869. This phenomena is a result of the differential refraction of the colors of the rainbow.