Walking on the Sun by Smash Mouth

1. Walking on the SunbySmash Mouth Donna Kubik Spring, 2006

2. Walking on what? • What is the surface of the sun? • Although astronomers talk about the surface of the Sun, the Sun is so hot that it has no liquid nor solid material • It is comprised totally of gas that gets denser and denser toward the center • The Sun contains >99.85% of the total mass of the solar system

3. Walking on the photosphere? • The Sun appears to have a surface only because most of its visible light comes from one specific layer, called the photosphere. • The photosphere is the lowest of 3 layers comprising the Sun’s atmosphere • Because the upper 2 layers are transparent to most wavelengths of visible light, we see through them down to the photosphere. • We cannot see through the photosphere, so everything below the photosphere is called the Sun’s interior

4. Photosphere Chromosphere Corona Solar atmosphere

5. Photosphere • 400 km thick • 5800K

6. Limb darkening • Sun appears darker near the edges • This is called limb darkening

7. Limb darkening • At the edges, we are looking through more of the cooler atmosphere, so there is more absorption of the photons from the hottest (innermost) part of the photosphere • In the center we can receive more photons from the hotter part of the photosphere 4000 K 5800 K

8. Granulation • Granulation is the fine grain structure of the photosphere. • Individual granules are about 1000 km across. • The granulation is constantly changing, usually over time scales of minutes or less.

9. Granulation Each granule is a convective cell which consists of a bright, roughly-polygonal area of hot rising gas, and a cooler edge of descending gas The rising and descending is determined via Doppler shift of spectral lines Darker because cooler Brighter because hotter Convection in photosphere

10. Granulation The energy (E) and temperature (T) according to Stephan-Boltzman law: E~T4 So more photons per area emitted from hot regions Darker because cooler Brighter because hotter Convection in photosphere

11. Chromosphere • 2000 km thick • 4000K

12. The EUV Sun • The name, chromosphere “sphere of color” is misleading • The name suggests it is the layer we normally see • But the chromosphere’s light is swamped by that of the photosphere

13. The EUV Sun The chromosphere is only visible when the photosphere is blocked, as during a total solar eclipse, or when viewed at nonvisible wavelengths that the chromosphere is especially bright (as EUV), or when viewed through a filter (Ha) that blocks most of the photosphere’s light

14. The EUV Sun • Image taken at EUV wavelengths by SOHO (Solar and Heliospheric Observatory) operated by ESA and NASA. • The UV light originates from the lower regions of the chromosphere • These wavelengths also indicate active regions.

15. Supergranulation • The dark graininess seen in the image is due to supergranulation. • Supergranules contain ~ 900 granules • Typical diameter of a supergranule is slightly larger than the earth’s diameter • The source of this light is the chromosphere

16. Spicules Spicules Supergranules • High resolution images of the chromosphere, taken through an Ha filter, reveal numerous spikes, which are jets of gas called spicules • Spicules are usually located on the edge of supergranules • Spicules rise for several minutes at 45,000mph to a height of ~10,000km

17. Spicules • The image shows spicules on the limb of the Sun as imaged by the Big Bear Solar Observatory. • It shows a superposition of 11 limb images taken at different wavelengths

18. Sunspots inhibit formation of supergranules In these photos taken at the same time, there are no supergranules where there are sunspots.

19. Solar observatories • The Big Bear Solar Observatory is located in the middle of Big Bear Lake (in CA) to reduce the image distortion which usually occurs when the Sun heats the ground and produces convection in the air just above the ground • Turbulent motions in the air near the observatory are also reduced by the smooth flow of the wind across the lake instead of the turbulent flow that occurs over mountain peaks and forests. Big Bear Solar Observatory

20. Solar observatories • In addition to the atmospheric effects, solar telescopes suffer from heating by sunlight of the optics and the air within the telescope tube. • This causes the image to shiver and become blurred. • Modern solar telescopes are either vacuum telescopes, filled with helium or use careful control of the optic's temperature to reduce heating of the air in the telescope. Big Bear Solar Observatory telescopes

21. Solar observatories • The 65 cm and 25 cm telescopes are evacuated to avoid air turbulence inside the telescope tubes caused by the solar beam heating air molecules. • Special white paint used inside and outside the observatory diffusely reflects sunlight and radiates heat away to reduce turbulence due to solar heating. Big Bear Solar Observatory telescopes

22. Solar observatories • Some solar telescopes look very different from other optical telescopes Kitt Peak National Observatory

23. Solar observatories • Close to the ground the heating effect of the Sun causes a layer of hot, turbulent air, which makes images formed by mirrors near the ground unsteady, so the first mirror (heliostat) is often placed on a tall tower

24. Solar towers National Solar Observatory Sunspot, New Mexico National Solar Observatory Kitt Peak, Arizona

25. The Radio Sun • Radio image taken from Japan’s Nobeyama Radio Observatory • The most active regions are the most luminous. • The radio image provides information about the transition region between the chromosphere and corona.

26. Corona • Several million km thick • ~1-2 million K

27. Corona viewed during eclipse • The glow of the corona is a million times less bright than that of the photosphere • Like the chromosphere, the corona can only be seen when the photosphere is blocked by special filters, at non-visible wavelengths at which the corona is especially bright, or when the disk of the Sun is blocked during a total solar eclipse…….

28. Coronagraph • …..or by using a special instrument called a coronagraph that artificially blocks the disk of the Sun so that it can image the region surrounding the photosphere

29. Coronagraph • A very common way to observe the corona is to cover the bright disk of the Sun. • This creates a sort of mini-eclipse and allows us to see the Sun's fainter outer atmosphere

30. What can you see with a coronagraph? • Streamers are structures formed by the Sun's magnetic field. They can last for months. • Sometimes streamers go unstable and erupt in huge magnetic bubbles of plasma known as coronal mass ejections (or CMEs) that blow out from the Sun's corona and travel through space at high speed.

31. Corona • It seems that the temperature should decrease as one rises through the Sun’s atmosphere (moving away from the apparent heat source • It does decrease from the photosphere (~5800K) to the chromosphere (~4000K), but then it rises to much higher temps in the corona (1-2 million K)!

32. Corona • Unexpected increase in temperature was discovered in about 1940 when Fe XIV (an iron atom stripped of 13 e-) was detected in the spectrum of the corona • Takes lots of energy to strip so many electrons from an atom, so the corona must be very hot

33. Corona • Astronomers have mounting evidence that the corona is heated by energy carried aloft and released there by the Sun’s complex magnetic fields (more on that later)

34. TRACE • TRACE (Transition Region and Coronal Explorer) is a NASA space telescope designed to provide high resolution images and observation of the photosphere and transition region to the corona. • The satellite, launched in April 1998

35. TRACE • The telescope is designed to take images in a range of wavelengths from visible light, through the Lyman alpha line to far ultraviolet. • The different wavelength passbands correspond to plasma emission temperatures from 4,000 to 4,000,000 K. • Sun-synchronous (98°) orbit of 600×650 km.

36. Sun-synchronous orbit • Sun-synchronous (98°) orbit of 600×650 km. • This type of orbit is designed to keep the satellite in full sun light for nine months a year. • The orbit moves the satellite to the west at the exact same rate that the sun appears to move across the Earth's surface.

37. Corona • If the temperature is to high, why doesn’t the corona outshine the photosphere? • According to according to Stephan-Boltzman law: E~T4 • So more photons should be emitted per area emitted from hot regions!? • But the density of the corona is very, very, very low, otherwise it would outshine the photosphere!

38. Solar wind • The Sun’s gravity keeps most of its atmosphere from escaping to space, but some of the gas in the corona is moving fast enough to escape. • This is the solar wind

39. Space weather • The solar wind is one aspect of space weather • You can view the current space weather conditions and the space weather forecast at http://spaceweather.com SPACE WEATHERCurrentConditions Solar Windspeed: 330.9 km/s density: 2.5 protons/cm3

40. Solar wind • The solar wind is comprised mostly of hydrogen and helium nuclei • Hydrogen nuclei are protons SPACE WEATHERCurrentConditions Solar Windspeed: 330.9 km/s density: 2.5 protons/cm3

41. Solar wind • The solar wind particles reach speeds up to 805 km/s • The wind achieves these high speeds in part by being accelerated by the Sun’s magnetic field SPACE WEATHERCurrentConditions Solar Windspeed: 330.9 km/s density: 2.5 protons/cm3

42. Solar wind • The Sun ejects a million tons of matter each second • Even at this rate of emission, the mass loss due to the solar wind will amount to only a few tenths of a percent of the Sun’s total mass throughout its lifetime SPACE WEATHERCurrentConditions Solar Windspeed: 330.9 km/s density: 2.5 protons/cm3

43. SOHO (Solar and Heliospheric Observatory), operated by NASA and ESA, is designed to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System. SOHO SOHO orbit is sunward of Earth Not to scale

44. All previous solar observatories have orbited the Earth, from where their observations were periodically interrupted as our planet `eclipsed' the Sun. A continuous view of the Sun is achieved by operating SOHO from a permanent vantage point 1.5 million kilometers sunward of the Earth SOHO SOHO orbit is sunward of Earth Not to scale

45. Lagrange points • The Italian-French mathematician Joseph-Louis Lagrange discovered five special points in the vicinity of two orbiting masses where a third, smaller mass can orbit at a fixed distance from the larger masses.

46. Lagrange points • Of the five Lagrange points, three are unstable and two are stable. • The unstable Lagrange points - labeled L1, L2 and L3 - lie along the line connecting the two large masses. • The stable Lagrange points - labeled L4 and L5 - form the apex of two equilateral triangles that have the large masses at their vertices. SOHO

47. Lagrange points • The L1 point of the Earth-Sun system provide an uninterrupted view of the sun and is the location of SOHO SOHO

48. Lagrange points • The L2 point of the Earth-Sun system is the location of WMAP and (perhaps by the year 2011) the James Webb Space Telescope. • The L1 and L2 points are unstable on a time scale of approximately 23 days, which requires satellites parked at these positions to undergo regular course and attitude corrections.

49. The quiet Sun vs. the active Sun • Quiet Sun • Granules, supergranules, spicules, and the solar wind occur continuously. They are features of the quietSun. • Active Sun • But the Sun’s atmosphere is periodically disrupted by magnetic fields that stir things up, creating the activeSun.

50. Solar magnetic field • In contrast to the Earth, the Sun has a very weak overall magnetic field (average dipole field). • However, the solar surface has very strong and tremendously complicated magnetic fields. • Because the surface magnetic fields are so complex, solar astronomers use computers to simulate the Sun's magnetic fields.