THE NEW SOLAR CORONA

1. THE NEW SOLAR CORONA Ashwanden, Markus J., Arthur I. Poland, and Douglas M. Rabin Annual Review of Astronomy and Astrophysics 2001. 39:175-210 Presented by Joseph M. Vann PHYSICS 681 SOLAR PHYSICS AND INSTRUMENTATION December 6, 2005

2. “The Sun is New each day.” “Everything Flows.” Heraclitus (ca. 530-575 BC) “Alles Vergängliche Ist nur ein Gleichnis.” (“All that is transitory is but a symbol.”) J. W. von Goethe (1749-1832) Faust, II, 12104-12105

3. PHYSICAL MODELING BEGAN WITH GRAVITATIONALLY STRATIFIED LAYERS QUESTIONS ARE POSED WITHIN THIS SYSTEM: How hot is the corona? What are its radiative, conductive and advective losses? What heats the gas? How strong is the magnetic field? WHILE STILL BASIC, WE NOW KNOW NO SIMPLE ANSWERS TO SUCH QUESTIONS ARE POSSIBLE. THE SOLAR CORONA IS MORE DYNAMIC AND COMPLEX IN STRUCTURE THAN ONCE REALIZED.

4. THE RISE AND FALL OF CORONIUM • DURING THE 1869 TOTAL ECLIPSE OF THE SUN YOUNG AND HARKNESS FOUND A BRIGHT EMISSION LINE AT 530.3 nm – THE “GREEN LINE” – UNKNOWN IN LABORATORY SPECTRA. MANY OTHER LINES, INCLUDING A RED LINE WOULD BE OBSERVED. THE NEW LINES WERE THOUGHT TO BE THOSE OF A NEW ELEMENT, YET UNDISCOVERED ON EARTH. • LIKE HELIUM, A NEW ELEMENT HAD BEEN DISCOVERED ON THE SUN AND IT WAS NAMED CORONIUM, BUT IT, LIKE NEBULIUM, BASED ON SIMILILARY UNIDENTIFIED LINES FOUND IN GASIOUS NEBULAS, WOULD NEVER MAKE IT TO THE PERIODIC TABLE.

5. CORONLIUM AND NEBULIUM –THE SAGA CONTINUES IN THE 1930s AND 1940s LINES FOR CORONIUM AND NEBULUM WERE REPRODUCED USING EXTREMELY HOT SPARKS (~ 106 K). GROTRIAN (1939) DETERMINED THAT THE RED LINE WAS ACTUALLY 14-TIMES IONIZED IRON – FE XV AND EDLÉN (1942) DETERMINED THAT THE “GREEN LINE” WAS 13-TIMES IONIZED IRON – FE XIV. EDLÉN ALSO IDENTIFIED 19 OTHER LINES AS BEING MULTIPLY- IONIZED ATOMS OF ELEMENTS SUCH AS IRON AND NICKEL. DUE TO THE HIGH IONIZATION POTENTIALS INVOLVED, IT WAS NOW KNOWN THAT TEMPERATURES IN THE CORONA WERE OF THE ORDER OF 106 K.

6. THE CORONA IS HOTTER THAN THE CHROMOSPHERE A NEW QUESTION ARISES: HOW IS THE CORONA HOTTER THAN REGIONS CLOSER TO THE CORE? CORONA TEMPERATURES ARE TYPICALLY 106 TO 2X106 K – EVEN HIGHER IN LOCAL REGIONS. CHROMOSPHERE TEMPERATURES ARE TYPICALLY 4400 TO 20,000 K. THE ANSWER IS STILL NOT COMPLETE BUT COMPLEX MAGNETIC FIELD GEOMETRIES AND CLOSED LOOPS ARE CLEARLY INVOLVED.

7. THE MULTITEMPERATURE STRUCTURE OF THE SOLAR CORONA

8. STRUCTURAL ELEMENTS OF THE CORONA • K CORONA (CONTINUOUS, GERMAN: KONTINUIERLICH) – PRODUCES WHITE LIGHT EMISSION FROM SCATTERING OF PHOTOSPHERIC LIGHT FROM FREE ELECTRONS. NO FRAUNHOFER LINES ARE OBSERVED. THE REGION IS LOCATED MOSTLY BETWEEN 1-2 SOLAR RADII. • F CORONA (FRAUNHOFER) – PRODUCED BY SCATTERING OF PHOTOSPHERIC LIGHT FROM DUST GRAINS BEYOND 2-3 SOLAR RADII. FRAUNHOFER LINES ARE VISIBLE. F CORONA MERGES INTO THE ZODIACAL LIGHT. • E CORONA (EMISSION) - SOURCE OF EMISSION LINES FROM HIGHLY IONIZED IONS IN CORONA. OVERLAPS K AND F REGIONS.

9. SPACE EXPLORATION OF THE CORONA THE TEMPERATURE OF THE CORONA IS SO HIGH THAT MUCH OF THE RADIATION IS IN THE FORM OF UV AND X-RAYS WHICH CANNOT BE OBSERVED ON EARTH. X-RAY EMISSIONS WERE FIRST DETECTED BY BURNIGHT IN 1949 USING A PINHOLE CAMERA ABOARD A ROCKET. MANY OBSERVATIONS WERE MADE IN THE 1960s AND 1970s BY US AND SOVIET ROCKETS, ESPECIALLY USEFUL DATA CAME FROM THE 1973-74 NASA SKYLAB MISSION. THE IMAGES REVEALED A COMPLEX OF LOOPS WHICH EVOLVED GREATLY OVER THEIR SEVERAL DAY LIFETIMES. THERE WERE LARGE-SCALE LOOPS WHICH REACHED OVER THE POLES, ALMOST TO THE EQUATOR, ENDING IN AREAS OF DEPLETED EMISSIONS, CORONAL HOLES. CORONAL HOLES ARE DARK REGONS OF THE CORONA WITH STRONGLY-REDUCED EMISSIONS OF UV AND X-RAYS. THEY ARE ASSOCIATED WITH THE SOLAR WIND.

10. LATER SPACECRAFT ADDED TO KNOWLEDGE OF THE CORONA NASA SOLAR MAXIMUM MISSION (1980-1989) YOHKO (“SUNBEAM”) – LAUNCHED 1991 SOHO (SOLAR AND HELIOSPHERIC OBSERVATORY) – LAUNCHED 1995 TRACE (TRANSITION REGION AND CORONAL EXPLORER) – LAUNCHED 1998

11. MAGNETIC ORGANIZATION IT IS DIFFICULT TO MEASURE MAGNETIC FIELDS IN THE CORONA. METHODS INCLUDE : IMAGING SPECTROSCOPY AT RADIO WAVELENGTHS ABOVE ACTIVE REGIONS. ZEEMAN SPECTROPOLARIMETRY IS USEFUL FOR BOTH COHERENT AND INCOHERENT LIGHT BUT IS HANDICAPPED IN THE CORONA BY LOWER FIELD STRENGTHS AND HIGHER TEMPERATURES (THUS THERMAL LINE WIDTHS) BUT IS USED FOR CORONAL INVESTIGATIONS OF IR. HANLE EFFECT (MAGNETICALLY-INDUCED RADIATIVE SCATTERING OF COHERENT LIGHT RESULTING IN DEPOLARIZATION AND ROTATION OF THE PLANE OF POLARIZATION) IS USED FOR STUDYING PROMINANCES. THE HANLE EFFECT IS SENSITIVE TO MUCH WEAKER FIELDS THAN THE ORDINARY ZEEMAN EFFECT. HOWEVER, THE HANLE EFFECT IS INSENSITIVE TO VERTICAL MAGNETIC FIELDS AND IS USELESS IN MEASURING FLUX TUBE FIELDS. THE HANLE EFFECT CAN ONLY DETERMINE TWO COMPONENTS OF THE STOKES VECTOR, WHILE THE ZEEMAN EFFECT, IN PRINCIPLE, GIVES THE ENTIRE VECTOR. THE HANLE EFFECT IS USEFUL IN MEASURING TANGLED MAGNETIC FIELDS WHERE THE POLARARITIES ARE MIXED WHILE THE ZEEMAN EFFECT IS USELESS FOR SUCH FIELDS.

12. PROPAGATING WAVES IN THE CORONA PASSING OF WAVE FRONT DISTURBS LOCAL DENSITY BUT WITHOUT BULK FLOW THREE TYPES OF PROPAGATING WAVES IN THE CORONA: ALVENIC – PERIODIC MOTION PERPENDICULAR TO MAGNETIC LINES OF FORCE MAGNETOSONIC WAVES FAST SLOW SHOCK WAVES P-MODE OSCILLATIONS OF THE SOLAR INTERIOR ARE THE STRONGEST DRIVERS OF WAVE MOTION. THEY CAN EXCITE UPWARD PROPAGATING WAVES IN GRAVITATIONALLY STRATIFIED FLUX TUBES PASSING THROUGH THE PHOTOSPHERE AND CHROMOSPHERE.

13. EIT IMAGES OF MORETON-LIKE WAVES MORETON WAVES ARE WAVES EXCITED BY ERUPTIONS IN THE CHROMOSPHERE AND CORONA WHICH PROPAGATE BOTH ALONG THE SOLAR SURFACE AND UPWARD THROUGH THE CORONA INTO INTERPLANETARY SPACE.

14. EVOLUTION OF THE CORONA CARTOON • INCREASED KNOWLEDGE, MUCH OF IT GAINED BY SPACECRAFT, HAS REVOLUTIONIZED OUR KNOWLEDGE OF THE STRUCTURE AND DYNAMICS OF THE SOLAR CORONA.

15. PLASMA β IS THE RATIO OF GAS PRESSURE TO MAGNETIC PRESSURE: β = 2µP B2 MAGNETIC PRESSURE EXCEEDS STATIC GAS PRESSURE UP TO 100Mm IN THE CORONA.

16. TRACE OBSERVATIONS OF ACTIVE-REGION LOOPS 6NOV1999 MOST OF THE LOOPS ARE FILLED WITH PLASMA UP TO THE LOOP TOPS (~150Mm) SO LOOPS NOT IN HYDROSTATIC EQUILIBRIUM – THEY REQUIRE DYNAMIC MODELS.

17. MAGNETIC STRUCTURES REVEALED BY YOHKO COLLIDING AND RECONECTING FLUX TUBES PRODUCE A CASCADE OF SHOCK WAVES THAT PROPAGATE RAPIDLY UPWARD AND DUMP KINETIC ENERGY INTO THE CORONA VIA ACOUSTIC WAVES. MHD SIMULATIONS DEMONSTRATE THAT SHOCK WAVES OF COLLIDING CHROMOSPHERIC FLUX TUBES EXCITE SURFACE AND BODY ALFVEN WAVES SUITABLE FOR CORONAL HEATING.

18. CORONAL MASS EJECTION OF 6 NOV1997 AS OBSERVED BY SOHO CORONAL MASS EJECTIONS (CMEs), FLARES, AND ERUPTIVE FILAMENTS ARE EXAMPLES OF PHENOMENA ASSOCIATED WITH RAPID, LARGE-SCALE MAGNETIC FIELD DESTABILIZATIONS. THEY OCCUR EITHER SEPARATELY OR TOGETHER, IN ANY COMBINATION. IT IS USELESS TO ARGUE DOES ONE CAUSE ANOTHER – RATHER THEY ARE ALL THE RESULT OF MAGNETIC REORGANIZATION.

19. TRACE IMAGE OF FILAMENT ACTIVATION 29SEPT2000 FILAMENTS (CALLED PROMINANCES WHEN OBSERVED ON THE SOLAR LIMB) ARE CLOUDS OF RELATIVELY COOL MATERIAL SUSPENDED ABOVE THE SOLAR PHOTOSPHERE. EMERGENCE OF NEW FLUX IN THE REGION OF A FILAMENT SEEMS TO BE IMPORTANT FOR THE FORMATION OF AN INSTABILITY – IT MAY LEAD TO THE FILAMENTS DISAPPEARANCE, TO EJECTION, OR TO HEATING AND EJECTION OF A CME.

20. TRACE OBSERVATIONS OF THE 14JULY1998 1255UT FLARE SOLAR FLARES ARE THE MOST ENERGETIC (1029 – 1032 ERGS) AND RAPID (MILLISEC) PHEONOMENA OBSERVED ON THE SOLAR CORONA. RADIO BURSTS ARE EMITTED IN THE SHOCK WAVE PRODUCED BY FLARES AND ERUPTIVE PROMINANCES.

21. SECOND BASTILLE DAY FLARE OBSERVED BY YOHKO 14JULY2000 SOLAR FLARES ARE ASSOCIATED WITH CME’S BUT ARE NOT THEIR “CAUSE.” THE QUESTION IS MORE COMPLICATED – FLARES, ERUPTIVE FILAMENTS, AND CME’S ARE ALL THE RESULT OF MAGNETIC REORGANIZATION. WE STILL DON’T KNOW HOW MAGNETIC ENERGY IS DISSIPATED SO QUICKLY INTO HEATING AND PARTICLE ACCELERATION. THE DOMINANT MECHANISM APPEARS TO BE GYRO-RESONANT WAVE- PARTICLE INTERACTION.

22. OUTLOOK • We still do not have a reliable diagnostic of physical processes that heat the corona. • We do not know what accelerates the solar wind. • We have fewer reasons than ever to believe that these questions have a single answer. 4. A new fleet of spacecraft has been developed to add to our knowledge.