THE SUN

1. Lecture 10 THE SUN This set of slides was compiled by Prof. Jeff Forbes of the Aerospace Engineering Department, University of Colorado, Boulder (It is used here with his permission, which I received at CDG Airport, Paris, France, on 4/12/03)

2. THE SUN • GENERAL CHARACTERISTICS • • Descriptive Data • Electromagnetic Radiation • Particle Radiation • ENERGY GENERATION AND TRANSFER • • Core  Radiation Zone  Convection Zone  Solar Atmosphere • REGIONS OF THE SOLAR ATMOSPHERE • • Photosphere, Chromosphere, Corona • FEATURES OF THE SOLAR ATMOSPHERE • • Coronal Holes, Flares, Sunspots, Plages, Filaments & Prominences • THE SOLAR CYCLE • 6 . SOLAR FLARES AND CORONAL MASS EJECTIONS • • Description and Physical Processes • Classifications • 7. OPERATIONAL EFFECTS OF SOLAR FLARES • a) radio noise b) sudden ionospheric disturbances • c) HF absorption c) PCA events

3. Our Sun • Our Sun is a massive ball of gas held together and compressed under its own gravitational attraction. • Our Sun is located in a spiral arm of our Galaxy, in the so-called Orions arm, some 30,000 light-years from the center. • Our Sun orbits the center of the Milky Way in about 225 million years. Thus, the solar system has a velocity of 220 km/s • Our galaxy consists of about 2 billion other stars and there are about 100 billion other galaxies • Our Sun is 333,000 times more massive than the Earth . • It consists of 90% Hydrogen, 9% Helium and 1% of other elements • Total energy radiated: equivalent to 100 billion tons of TNT per second, or the U.S. energy needs for 90,000 years - 3.86x1026 W • Is 5 billions years old; another 5 billion to go • Takes 8 minutes for light to travel to Earth • The Sun has inspired mythology in many cultures including the ancient Egyptians, the Aztecs, the Native Americans, and the Chinese.

4. OTHER SUN FACTS • radius 6.96 x 105 Km109 RE • mean distance from earth (1 AU) = 1.49 x 108 Km215 RS • mass 1.99 x 1030 Kg330,000 ME • mean density 1.4 x 103 Kg m-31/4 rE • surface pressure 200 mb1/5 psE • mass loss rate 109 Kg s-1 • surface gravity 274 ms-2 28 gE • equatorial rotation period 26 days • near poles 37 days • inclination of sun's equator to ecliptic 7°23.5° for Earth • total luminosity 3.86 x 1026 W1368 Wm-2 @ Earth • escape velocity at surface618 km s-1 • effective blackbody temperature 5770 K

5. REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE p-modes g-modes (See Fig. 5.1)

6. The Sun radiates at a blackbody temperature of 5770 K A blackbody is a “perfect radiator” in that the radiated energy depends only on temperature of the body, resulting in a characteristic emission spectrum. insulation radiated energy max  1/T In a star heating element T2 The radiation reacts thoroughly with the body and is characteristic of the body T1>T2 radiated energy T1 In the laboratory area a T4 wavelength

7. Radiation Laws

8. ELECTROMAGNETIC RADIATION The Sun emits radiation over a range of wavelengths

9. The wavelengths most significant for the space environment are X-rays, EUV and radio waves. Although these wavelengths contribute only about 1% of the total energy radiated, energy at these wavelengths is most variable

11. PARTICLE RADIATION The Sun is constantly emitting streams of charged particles, the solar wind, in all outward directions. Solar wind particles, primarily protons and electrons, travel at an average speed of 400km/s, with a density of 5 particles per cubic centimeter. The speed and density of the solar wind increase markedly during periods of solar activity, and this causes some of the most significant operational impacts

12. 2. ENERGY GENERATION AND TRANSFER The core of the Sun is a very efficient fusion reactor burning hydrogen fuel at temperatures ~1.5 x 107 K and producing He nuclei: 4 H1  He4 + 26.73 MeV This 26.73 MeV is the equivalent of the mass difference between four hydrogen nuclei and a helium nucleus. It is this energy that fuels the Sun, sustains life, and drives most physical processes in the solar system. (See eqs 5.1 to 5.5 for details)

13. Between the radiation zone and the surface, temperature decreases sufficiently that electrons can be trapped into some atomic band states, increasing opacity; convection then assumes main role as energy transfer mechanism. ( If radiation came straight out, it would take 2 seconds; due to all the scatterings, it takes 10 million years !) Near the surface, in the photosphere, radiation can escape into space and again becomes the primary energy transport mechanism. The photosphere emits like a black body @ 5770 K.

14. GRANULES

15. HOW DO WE INFER THE INTERNAL PROPERTIES OF THE SUN ?

16. is the study of the interior of the Sun from observations of the vibrations of its surface. HELIOSEISMOLOGY In the same way that seismologists use earthquakes and explosions to explore Earth’s crust, helioseismologists use acoustic waves, thought to be excited by turbulence in the convection zone, to infer composition, temperature and motions within the Sun. Another way of inferring the corresponding upward and downward motions of the surface is by measuring the Doppler shifts of spectral lines. By subtracting two images of the Sun’s surface taken minutes apart, the effects of solar oscillations are made apparent by alternating patches in brightness that result from heating and cooling in response to acoustic vibrations of the interior.

17. REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE p-modes g-modes

18. 3. REGIONS OF THE SOLAR ATMOSPHERE: THE PHOTOSPHERE The photosphere is the Sun’s visible “surface”, a few hundred km thick, characterized by sunspots and granules The solar surface is defined as the location where the optical depth of a  = 5,000 Å photon is 1 (the probability of escaping from the surface is 1/e) The photosphere is the lowest region of the solar atmosphere extending from the surface to the temperature minimum at around 500 km. 99% of the Sun’s light and heat comes out of this narrow layer.

19. THE CHROMOSPHERE The chromosphere is the ~ 2000 km layer above the photosphere where the temperature rises from 6000 K to about 20,000 K. At these higher temperatures hydrogen emits light that gives off a reddish color (H-alpha emission) that can be seen in eruptions (prominences) that project above the limb of the sun during total solar eclipses. When viewed through a H-alpha filter, the sun appears red. This is what gives the chromosphere its name (color-sphere). In H-a, a number of chromospheric features can be seen, such as bright plages around sunspots, dark filaments, and prominences above the limb. 6563 Å

20. THE CORONA The coronais the outermost, most tenuous region of the solar atmosphere extending to large distance and eventually becoming the solar wind. The most common coronal structure seen on eclipse photographs is the coronal streamer, bright elongated structures, which are fairly wide near the solar surface, but taper off to a long, narrow spike.

21. UV solar emission lines and corresponding regions and temperatures

22. The corona is characterized by very high temperature (a few million degrees) and by the presence of a low density, fully ionized plasma. Here closed field lines trap plasma and keep densities high, and open field lines allow plasma to escape, allowing much lower density regions to exist called coronal hoes. TRANSITION REGION At the top of the chromosphere the temperature rapidly increases from about 104 K to over 106 K. This sharp increase takes place within a narrow region, called the transition region. The heating mechanism is not understood and remains one of the outstanding questions of solar physics

23. 4. FEATURES OF THE SOLAR ATMOSPHERE:SUNSPOTS Sunspots are areas of intense magnetic fields. Viewed at the surface of the sun, they appear darker as they are cooler than the surrounding solar surface - about 4000oC compared to the surface (6000oC).

24. SUNSPOTS ARE REGIONS OF INTENSE MAGNETIC FIELDS The video below depicts regions of negative (black) qnd positive (white) magnetic polarity (like a magnet).

25. CHROMOSPHERIC FILAMENTS & PLAGES Filaments are the dark, ribbon-like features seen in Ha light against the brighter solar disk. The material in a filament has a lower temperature than its surroundings, and thus appears dark. Filaments are elongated blobs of plasma supported by relatively strong magnetic fields. Plages are hot, bright regions of the chromosphere, often over sunspot regions, and are often sources of enhanced 2800 MHz (10.7 cm) radio flux Ha, 6563 Å

26. SOLAR PROMINENCES Prominences are variously described as surges, sprays or loops. Filaments are referred to as prominenceswhen they are present on the limb of the Sun, and appear as bright structures against the darkness of space.

30. CORONAL HOLES One of the major discoveries of the Skylab mission was the observation of extended dark coronal regions in X-ray solar images. Coronal holes are characterized by low density cold plasma (about half a million degrees colder than in the bright coronal regions) and unipolar magnetic fields (connected to the magnetic field lines extending to the distant interplanetary space, or open field lines). Near solar minimum coronal holes cover about 20% of the solar surface. The polar coronal holes are essentially permanent features, whereas the lower latitude holes only last for several solar rotations.

31. 5. THE SOLAR CYCLE Maunder Minimum The number of sunspots (‘Zurich’ or ‘Wolf’ sunspot number -- see Intro) on the solar disk varies with a period of about 11 years, a phenomenon known as the solar (or sunspot) cycle.

32. Sunspot latitude drift The remarkably regular 11-year variation of sunspot numbers is accompanied by a similarly regular variation in the latitude distribution of sunspots drifts toward the equator as the solar cycle progresses from minimum to maximum.

34. Evolution of the Sun’s X-ray emission over the 11-year solar cycle

35. 6. CMEs & SOLAR FLARES • Flares and CMEs are different aspects of solar activity that are not necessarily related. • Flares produce energetic photons and particles. • CMEs mainly produce low-energy plasma. • CMEs and flares are very important sources of dynamical phenomena in the space environment. • The triggering mechanisms for CMEs and flares, and the particle acceleration mechanisms, are not understood beyond a rudimentary level. However, this knowledge is essential for development of predictive capabilities.

36. CORONAL MASS EJECTIONS (CMEs)

37. Size of Earth Relative to Solar CME Structure • The Earth is small compared to the size of the plasma “blob” from a Coronal Mass Ejection (CME). • The image shows the size of a CME region shortly after “lift off” from the solar corona. • The CME continues to expand, as it propagates away from the Sun, until its internal pressure is just balanced by the magnetic and plasma pressure of the surrounding medium. CME Earth

38. Optical Classification of Flares The optical (as seen in Hydrogen-alpha light) classification of a flare is made using a two-character designation. For example, a 1B designation indicates a ``brilliant” intensity flare covering a corrected area between 100 and 249 millionths of the solar hemisphere. FLARE BRIGHTNESS CATEGORIES: F: FAINT N: NORMAL B: BRILLIANT The most common optical flare intensity or ``brilliance” classification is based on the doppler shift of the H-alpha line. This doppler shift is a measure of the ejected gas particle velocity and is used by observers to make a subjective estimate of flare intensity.

39. frequency of optical solar flares during cycles 20-21

40. X-Ray Classification of Flares The most common x-ray index is based on the peak energy flux of the flare in the 1 to 8 Å soft x-ray band measured by geosynchronous satellites. These measurements must be made from space, since the Earth’s atmosphere absorbs all solar x-rays before they reach the Earth’s surface. The left categories are broken down into nine subcategories based on the first digit of the actual peak flux. For example, a peak flux of 5.7 x 10-2 ergs/cm2-sec is reported as a M5 soft x-ray flare. Classification X-Ray Flux (ergs/cm2-sec) C 10-3 M 10-2 X 10-1

42. The Bastille-day flare was ‘X-class’ and accompanied by one of the largest solar energetic proton events ever recorded c3714

43. 7. OPERATIONAL EFFECTS OF SOLAR FLARES

44. Solar Effects on Radio Wave Reception Radio Noise Storms. Sometimes an active region on the Sun can produce increased noise levels, primarily at frequencies below 400 MHz. This noise may persist for days, occasionally interfering with communication systems using an affected frequency. Solar Radio Bursts. Radio wavelength energy is constantly emitted from the Sun; however, the amount of radio energy may increase significantly during a solar flare. These bursts may interfere with radar, HF (3 – 30 MHz) and VHF (30 – 300 MHz) radio, or satellite communication systems. Radio burst data are also important in helping to predict whether we will experience the delayed effects of solar particle emissions.

45. Solar Effects on Radio Wave Reception Systems in the VHF through SHF range (30 MHz to 30 GHz) are susceptible to interference from solar radio noise. If the Sun is in the reception field of the receiving antenna, solar radio bursts may cause Radio Frequency Interference (RFI) in the receiver, as depicted here.

46. Ionospheric Plasma A plasma is a gaseous mixture of electrons, ions, and neutral particles. The ionosphere is a weakly ionized plasma. -- + -- + -- If, by some mechanism, electrons are displaced from ions in a plasma the resulting separation of charge sets up an electric field which attempts to restore equilibrium. Due to their momentum, the electrons will overshoot the equilibrium point, and accelerate back. This sets up an oscillation. + -- + -- + + -- -- + + + -- -- E + The frequency of this oscillation is called the plasma frequency, = 2f = (Nee2/me)1/2, which depends upon the properties of the particular plasma under study.

47. Radio Waves in an Ionospheric Plasma A radio wave consists of oscillating electric and magnetic fields. When a low-frequency radio wave (i.e., frequency less than the plasma frequency) impinges upon a plasma, the local charged particles have sufficient time to rearrange themselves so as to “cancel out” the oscillating electric field and thereby “screen” the rest of the plasma from the oscillating E-field. This radio wave (low frequency) cannot penetrate the plasma, and is reflected. For a high frequency wave (i.e., frequency greater than the plasma frequency), the particles do not have time to adjust themselves to produce this screening effect, and the wave passes through. MUF LUF

48. Radio Waves in an Ionospheric Plasma The critical frequency of the ionosphere (foF2) represents the minimum radio frequency capable of passing completely through the ionosphere. N(cm-3)=1.24x104 f2 (MHz)

49. Ionospheric Disturbances Ionospheric disturbances occur when the Earth’s ionosphere (50 – 500 km) experiences a temporary fluctuation in degree of ionization. This variation can result from geomagnetic activity (and the influences of the neutral atmosphere), or it can be the direct result of X-rays and EUV produced by a solar flare. A Sudden Ionospheric Distrurbance (SID) is a disturbance that occurs almost simultaneously with a flare’s X-ray emission (generally constrained to dayside).

50. When collisions between oscillating electrons and ions and neutral particles becomes sufficiently frequent (as in the D-region, 60 – 90 km), these collisions “absorb” energy from the radio wave leading to what is called radio wave absorption. Short Wave Fade (SWF) is a particular type of SID that can severely hamper HF radio users (up to 20 – 30 MHz) by causing increased ionization and signal absorption which may last for up to 1-2 hours.