1. Advances in contemporary physics and astronomy --- our current understanding of the Universe Lecture 7: the Sun and the solar system May 21 th, 2003

2. 7 “Planets” Sun Moon Mars Mercury Jupiter Venus Saturn Sunday Monday Tuesday Wednesday Thursday Friday Saturday Sonntag Montag Dienstag Mittwoch Donnerstag Freitag Samstag Dimanche Lundi Mardi Mercredi Jeudi Vendredi Samedi Domingo Lunes Martes Miércoles Jueves Viernes Sábado 星期日 星期一 星期二 星期三 星期四 星期五 星期六 Discovery of Uranus by Herschel in 1781 – 7 planets!

3. 12 Houses 12 lunar orbits per year. 12 Hours in the day. 12 Months in the year. 12 Signs of the zodiac. 7 and 12 were very important numbers. 12 Tribes of Israel. 12 Disciples of Jesus. 12 Members of the British jury.

4. The solar system

5. Eratosthenes and the size of the Earth Eratosthenes 276 –194 BC Radius of the Earth ~ 6,300 kilometers

6. Hipparchus and the distance to the Moon Hippaarchus (190-120 BC) the Moon was 59 times the radius of the Earth.

7. Aristarchus and the distance to the Sun Aristarchus of Samos Born: about 310 BC in Greece Died: about 230 BC in Greece Aristarchus was trying to calculate the distance between the Earth and the Sun by calculating angle of Sun-Earth-Moon when it is Half Moon. His angle, 87º is about 1.5º off the real value, 88.5º. His estimation is about 20 times smaller.

8. The Sun(1) Solar luminosity: 3.9 33 ergs/s—requires converting 600 tons of H to He per second. Nuclear burning happens below 0.2 r . Radiation layer: between 0.2 and 0.7 r . Takes ~ 10 million yrs. Convection layer: from 0.7 r  out. Eddy structures. Energy Flux

9. The Sun(2) temperature at the center reaches 15 million Kelvin degree. sharp drop near the photosphere, reaching temperature of 5870 degrees Kelvin. the temperature as a function of r is decided by the strength of energy currents, via radiation (below 0.7 ) and convection (above 0.7 ) separately. Temperature

10. The Sun(3) density at the center reaches is ~ 150 g/cm 3. sharp drop near the photosphere, becomes one tenth of a millionth g/cm 3, 10000 times less dense than air. The density within the solar interior is determined by the balance between the gravity, and the gas pressure gradient. Density

11. The Sun(4) The Sun rotates, first noticed by Galileo through observing sun spots. The rotations in the convection zone is different from the equatorial region ~ 25.5 days to the pole ~ 35 days. The rotation has very important consequences to the sun’s magnetic field. Rotation

12. The Sunspot sunspots are black dots on surface of the sun, often appear in groups, forming action region. sunspots are darker than surroundings, suggesting a lower temperature. The white region surrounding sunspots are called plages. The center of a sunspot is called Umbra and the outer ring is called penumbra.

13. Solar magnetogram pink-red-yellow: magnetic fieldlines pointing away from the Sun. purple-blue-light blue: magnetic fieldlines pointing toward the Sun. strongest magnetic fields, ~ 1000 - 4000 Gauss (corresponding to the yellow and light blue region on the magnetogram) always coincide with sunspots. Moderate magnetic fields ~ 100 Gauss for plages. Magnetic field strength at Earth’s surface is ~0.5 Gauss.

14. Filaments and prominences The Sun in H  : this image is taken through a filter centered on a spectral line of Hydrogen (H, wavelength = 6563 Å) Filaments: dark string-like structures visible on the disk. Prominences, bright structures extending outward over the limb. Prominences are just sideviews of filaments. Both are condensations of cooler gas high up in the solar atmosphere. Filaments can be long- lived, and often move across the disk.

15. X-ray image of the Sun To generate X-ray, temperature must exceed 2 million degrees. Flares: short lived increases in brightness. reconnection is believed to be the energy source of solar flares. very dark regions are called coronal holes.

16. Solar corona corona is the result of sunlight scattered by hot (1 - 2 million degrees Kelvin), tenuous ionized gas streaming away from the Sun. The streaming away gas is called solar wind. corona usually extends outward many solar radii above the solar surface. Taken 27 days apart, the morphology of helmet streamers are similar on both images, indicating that the large-scale corona has evolved very little during this time interval.

17. Corona mass ejection

18. Solar Wind proposed by E. N. Parker. Verified by noticing the comet tail. Charateristics: wind speed, density, magnetic field strength and direction. two types of wind: 1) less variation fast solar wind --in solar maximum and at higher latitude. 2) Turbulent slow speed, -- in solar minimum and at lower latitude.

19. Solar cycle the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle. The positions of sunspots as a function of heliospheric latitude also shows a 11 year cycle. The sunspots start from higher latitude, and gradually move toward lower latitude. Upon reaching the equator, the sunspots start emerge from higher latitude again.

20. Sun-Earth connection

21. Earth’s magnetosphere Earth has its own magnetic field. The fly-by solar wind will push the magnetic field line of the earth so that it get compressed toward the Sun and stretched against the Sun. There is little mixture between solar magnetic field carried by solar wind and the earth’s magnetic field. The cavity inside of which the earth’s magnetic field is dominant is called magnetosphere.

22. Radiation belt When there is magnetic field present, charged particles, like electrons and positive ions will rotate around magnetic field lines, while at the same time slide along those lines, resulting a spiral trajectory. When electrons and ions move from a region of weaker magnetic field to a region of stronger magnetic field. They can reverse their motion. Thus electrons and ions can remain trapped for a long time, bouncing back and forth from one hemisphere to the other –forming the so called radiation belt.

23. Planets Mercury – cratered – no atmosphere – magnetic field - polar ice caps? 1) Terrestrial – Mercury, Venus, Earth, Moon, Mars (asteroid belt). NASA Earth – eroded craters – tectonics, volcanoes – atmospheric, oceanic erosion – magnetic field.

24. 2) Gas giants – Jupiter, Saturn, Uranus, Neptune. Planets 1) Terrestrial – Mercury, Venus, Earth, Moon, Mars (asteroid belt). NASA Gaseous, solid core. H, He envelopes (Jupiter, Saturn). Rings, many satellites. Magnetic field. CNO envelopes (Uranus, Neptune). Rings, many satellites. Magnetic field.

25. 2) Gas giants – Jupiter, Saturn, Uranus, Neptune. Planets 1) Terrestrial – Mercury, Venus, Earth, Moon, Mars (asteroid belt). 3) Pluto + Kuiper Belt objects. Pluto - Charon - Small, icy - Transient atmosphere.

26. Planets Origin : Disk of gas and dust with central condensation (sun). Hot region within 3 AU – refractory dust only. NASA Cool region beyond 3 AU – icy grains. Dust and grains coagulate to form ever larger objects - Moon sized in inner region. - Comet - Pluto sized in outer region. Origin of planets

27. Planets Origin : NASA Inner region - growth through inelastic collisions - sometimes drastic (Moon, asteroid belt) - oceans/atmospheres added by collisions with comets. Timescale ~ million  orbital period Earth ~ 10 6 years Jupiter ~ 10 7 years Neptune ~ 10 8 years. Outer region – growth through inelastic collisions - supplemented by gas accretion (Jupiter, Saturn) - drastic collisions ( Uranus). Planets origin

28. Outer heliosphere Termination shock Beyond 40 AU, there is no planet in solar system. The property of solar wind is the deciding factor of what is happening outside 40 AU. Solar system terminates at the boundary between the interstellar medium and the solar system. The termination is decided by the balance of the pressure of the interstellar medium from the pressure of the solar wind itself.

29. Solar system and local environment

30. References Space physics tutorial: http://space.umd.edu/IACG_c4/c4_tutorials.html http://spaceboy.nasda.go.jp/ (in Japanese) An introductory site about solar physics, http://www.hao.ucar.edu/public/education/slides/slides.html NASA’s site on sunspots: http://science.nasa.gov/ssl/pad/solar/sunspots.htm Explorations by various spacecrafts: http://www.solarviews.com/eng/craft2.htm Another good site about solar physics: http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html An introduction to solar system (very resourceful): http://www.solarviews.com/ss.html The universe revealed, by Pam Spence (Cambridge)

31. Planets Origin : NASA Inner region - growth through inelastic collisions - sometimes drastic (Moon, asteroid belt) - oceans/atmospheres added by collisions with comets. Timescale ~ million  orbital period Earth ~ 10 6 years Jupiter ~ 10 7 years Neptune ~ 10 8 years. Outer region – growth through inelastic collisions - supplemented by gas accretion (Jupiter, Saturn) - drastic collisions ( Uranus). Planets origin