Exploring Our Sun: Characteristics, Energy Generation, and Solar Flares

1. Lecture 6 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)

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

9. 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

10. Radiation Laws

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

12. 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

14. 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

15. 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.

16. 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.

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

18. 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.

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

20. 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.

23. 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 an 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

24. 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.

25. UV solar emission lines and corresponding regions and temperatures

26. 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 holes. 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

27. 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).

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

30. 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.

32. SDO Observation: Prominence

33. 5. THE SOLAR CYCLE Maunder Minimum The number of sunspots on the solar disk varies with a period of about 11 years, a phenomenon known as the solar (or sunspot) cycle.

34. 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.

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

37. 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.

38. CORONAL MASS EJECTIONS (CMEs)

40. 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

42. 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.

43. The Solar Cycle: Physical Description • Sunspots start at relatively high latitudes and move towards the equator. • During the solar cycle the latitude of emergence moves towards the equator. • The magnetic polarity of the Sun reverses during the 11 year solar cycle so that it takes time (22 years) for the Sun’s magnetic field to get back to its original state. • Sunspots frequently are observed in bipolar groups with the leading spot (in the direction of apparent motion) having the same polarity as the hemisphere it appeared in while the following spot has the opposite polarity. The bipolar groups in opposite hemispheres have opposite magnetic orientation and this orientation reverses in each new solar cycle. • The Sun rotates faster at its equator and slower at the poles.

44. The Solar Cycle: Physical Description • During solar minimum the magnetic field is poloidal. • As the Sun rotates the equatorial portion of the field lines in the Sun are pulled ahead of the polar portions and wrapped around the Sun forming a toroidal field (see below). • Velocity shears in the convection zone cause the field to wrap into flux ropes. • The field in the flux ropes becomes strong and buoyant ( ) • When the tube breaks through the surface it creates a pair of sunspots from which the field expands as a small dipole. • The polarity of the dipole is determined by the direction of the torodial field. • The preceding spot will have the same polarity as the polar field for that hemisphere.

45. The Solar Cycle: Physical Description

46. The Solar Cycle: Physical Description • The latitude of the first appearance of sunspots depends on the differential rotation and magnetic field strength. • When the first sunspots emerge at high latitudes the magnetic pressure is reduced. The process then moves to lower latitudes leading to motion toward the equator. • The preceding spots from the two hemispheres merge (reconnect). • The trailing spots merge with the polar field. • Close to sunspot maximum the polar fields reverse as the field from the trailing spots dominate. • Near minimum the field returns to a dipole-like field with the poles reversed. • The last step of this process is poorly understood. In an alternate model as an element of toroidal flux rises it is twisted by Coriolis forces so as to create a new poloidal field of the opposite sense.