1. Lecture 10 THE SUN “Surface” Layers

2. Announcements Lunar Eclipse this Saturday, March 3 rd. –Observe at sunset and write up what you see. Nobel laureate lecture Saturday, March 24 –4 pm, McM campus “Mabee” room in student center. –Trebuchet demo about 2:30 Homework 5 is due now. Assignment for this week follows:

3. Homework Assignment 6 Due Monday March 5, 2007 Unit 49: RQ 3, P 3, TY 1, 3 Unit 50: P3, TY 1, 2, 3 Unit 51: RQ 2, 6, P 1, TY 3

4. The Sun’s Physical Properties Distance from Earth = 1 AU (149,597,892 km) Diameter = 1.4 x 10 6 km (109 times Earth) Mass = 1.99 x 10 30 Kg (330,000 times Earth) Density = 1.4 gm/cm 3 (water = 1.0 gm/cm 3 ) Luminosity = 3.8 x 10 26 J/s (watts) Surface Temperature = 5,800 K Core Temperature = 15,000,000 K Rotation Period = 25 days at the equator & 29 days near the pole Composition = 99% hydrogen and helium State = gaseous (plasma)

5. The Solar Atmosphere Photosphere Chromosphere Corona The solar wind

6. Photosphere

7. Properties Of The Photosphere Where most of the Sun's visible light comes from Absorbs and re-emits radiation produced in the solar interior Temperature = 5,800 K Thickness = 500 km Its density is very low – about 3,000 times less dense than the air you breathe

8. Photosphere Mottled, granular appearance because of convection cells Individual granules are typically about as big as Texas. Sunspots appear as dark (cooler) regions. The dark (absorption) spectral lines are formed in the photosphere.

9. Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through convection Bubbles of hot gas rising up Cool gas sinking down ≈ 1000 km Bubbles last for ≈ 10 – 20 min. Solar Convection

10. Solar Granulation Consequence of convection Last 10 – 20 minutes Continuously replaced Rising & sinking gases, 0.4 km/sec

11. A sideways view of “hills” (granules) and “valleys” near the edge of the Sun This image highlights the three- dimensional nature of the solar surface when seen at a low angle.

12. Supergranules Revealed Spectroscopically Huge granules: 30,000 km across Very slowly rising gas currents Last 1-2 days Mark the “transition” between the photosphere and the convection zone.

13. Chromosphere

14. Properties Of The Chromosphere Visible as a reddish layer above the photosphere during a total solar eclipse 1,000 times fainter than photosphere Color comes from the red line of hydrogen (H-alpha line) 10,000 km thick Temperature minimum at 4,000 K, but then rises to hundreds of thousands of Kelvins

15. Chromosphere Extremely low density - less dense than photosphere Images show details if viewed with an H-alpha filter Convection may twist magnetic field lines and heat the chromosphere to hundreds of thousands of degrees

17. Spicules And Supergranules Thin “jets” of gas that flow from the lower chromosphere to the upper chromosphere. Not well understood – related to energy flow out of the sun. Outline the Supergranules.

18. Corona

19. Properties Of The Corona The outer solar atmosphere - merges with the solar wind Only visible well during a total solar eclipse Very hot (million K) but very diffuse (10 -14 times the Earth's atmosphere)

21. The Magnetic Carpet of the Corona Corona contains very low-density, very hot (1 million K) gas Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”) Computer model of the magnetic carpet

22. Solar Activity Magnetism is the key to understanding the dynamics of the Sun. Magnetism is produced in the Sun by the flow of electrically charged ions and electrons.

23. Solar Magnetogram

24. Sunspots Dark regions in the photosphere that are about 2,000 K cooler Typically about the size of the Earth The centers of strong localized magnetic fields that inhibit the flow of energy from the interior

27. Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic Field Lines

28. Prominences Huge clouds of gases extending into the chromosphere & corona Can be 100,000 km or more above the photosphere Most often associated with sunspot (active) regions Can be loops, hovering, or eruptive

32. Solar Flares Violent short-lived eruptions Temperatures can rise to tens of millions of degrees for brief periods Vast quantities of charged particles & high energy radiation are emitted Energy released is equivalent to 10 8 times the largest nuclear bomb

34. The Solar Activity Cycle Solar physicists can accurately predict the Sun's temperature, luminosity, and internal structure, but fail to predict the solar activity cycle. The theories of solar structures are just not sophisticated enough to model accurately the solar activity cycle.  SUNSPOTS  PROMINENCES  FLARES

37. Origin of Sunspots They may be caused by the effect of differential rotation on the Sun's general magnetic field. They usually come in pairs with opposite magnetic polarity. The leading spot in each hemisphere always has the same polarity. Polarity reversals occur every 11 years, therefore there is also a 22-year magnetic cycle on the Sun.

43. The Maunder Minimum There is now evidence that correlates sunspot numbers (solar activity) with terrestrial climate changes. The prolonged Maunder sunspot minimum (1645-1715) correlates with a period of extreme cold in northern Europe and North America known as the "Little Ice Age". Also a number of extreme sunspot maxima correlate with unusually warm spells.

45. Solar Activity And Droughts Also in North America the 22-year sunspot cycle correlates with droughts in the western U.S. and Canada, but no one has been able to determine the cause for these reported correlations. Earth-orbiting satellites have recorded small variations in the solar constant, but the long- term effects of this variation is not known.

46. For Next Time Read unit 50. Wednesday we will discuss what happens below the photosphere (solar interior) and above the corona (solar wind, space weather).