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Solar Eruptions: Flares and Coronal Mass Ejections

CSI 769/ASTR 769 Topics in Space Weather Fall 2005 Lecture 03 Sep. 13, 2005. Solar Eruptions: Flares and Coronal Mass Ejections. Aschwanden, “Physics of the Solar Corona” Chap. 16, P671-702 Chap. 17, P703-737

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Solar Eruptions: Flares and Coronal Mass Ejections

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  1. CSI 769/ASTR 769 Topics in Space Weather Fall 2005 Lecture 03 Sep. 13, 2005 Solar Eruptions: Flares and Coronal Mass Ejections • Aschwanden, “Physics of the Solar Corona” • Chap. 16, P671-702 • Chap. 17, P703-737 • Chap. 10, P407-463 • Chap. 11, P465-515

  2. 8010 + 17:17 17:40 18:03 G12 5 XRA 1-8A X17.0 2.6E00 0808

  3. 3-day Solar-Geophysical Forecast issued Sep 12 at 22:00 UTC Solar Activity Forecast: Solar activity is expected to be moderate to high http://www.sec.noaa.gov

  4. Magnetogram on Sep. 08 Sep. 12

  5. What is a solar flare? • A solar flare is a sudden brightening of a part of the chromosphere and/or corona, by traditional definition • Flare shows enhanced emission in almost all wavelength, from long to short, including radio, optical, UV, soft X-rays, hard X-rays, and γ-rays • Flare emissions are caused by thermal plasma heating, and non-thermal particle acceleration • Heating and particle acceleration are believed to be caused by magnetic reconnection in the corona • Flares release 1027 - 1032 ergs energy in a few to tens of minutes. • (Note: one H-bomb: 10 million TNT = 5.0 X 1023 ergs)

  6. Flare: Example in optical Hα Heating: temperature increase in Chromosphere Structure: Hα ribbons

  7. Flare: Example in EUV (~ 195 Å) TRACE Observation: 2000 July 14 flare • Heating: temperature increase in corona • Structure • EUV ribbons • Loop arcade • Filament eruption

  8. Flare: Example in soft X-rays (~ 10 Å) Heating: temperature increase in Corona Structure: fat X-ray loops

  9. Flare: Example in hard X-ray (< 1 Å) • RHESSI in hard X-rays (red contour, 20 Kev, or 0.6 Å) • and (blue contour, 100 Kev, or 0.1 Å) • Non-thermal emission: due to energetic electron through Bremsstrahlung (braking) emission mechanism

  10. Flare: Example in radio (17 Ghz) • Nobeyama Radioheliograph (17 Ghz, or 1.76 cm) • and (34 Ghz, or 0.88 cm) • Non-thermal emission • due to non-thermal energetic electron and magnetic field • emission mechanism: gyro-synchrotron emission

  11. Flare Frequency At solar minimum: 1 event/per day At solar Maximum: 10 events/per day GOES X-ray flare: 11696 events From 1996 Jan. To 2001 Dec.

  12. Flares: X-ray Classification

  13. Flare: Hα Classification • Brightness Importance • F: faint • N: Normal • B: Brilliant • e.g., 3B means covering >12.4 deg2 and exceptionally bright

  14. Flare: Temporal Property • A flare may have three phases: • Preflare phase: e.g., 4 min from 13:50 UT – 13:56 UT • Impulsive phase: e.g., 10 min from 13:56 UT – 14:06 UT • Gradual phase: e.g., many hours after 14:06 UT

  15. Flare: Temporal Property (Cont.) • Pre-flare phase: flare trigger phase leading to the major energy release. It shows slow increase of soft X-ray flux • Impulsive phase: the flare main energy release phase. It is most evident in hard X-ray, γ-ray emission and radio microwave emission. The soft X-ray flux rises rapidly during this phase • Gradual phase: no further emission in hard X-ray, and the soft X-ray flux starts to decrease gradually. • Loop arcade (or arch) starts to appear in this phase

  16. Flare: Spectrum Property • The emission spectrum during flare’s impulsive phase

  17. Flare: Spectrum Property (cont.) • A full flare spectrum may have three components: • Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV): • thermal Bremsstrahlung emission • Power-law distribution in hard X-ray energy range (e.g., 10 keV to 100 keV): • non-thermal Bremstrahlung emission • dF(E)/dE = AE–γ Photons cm-2 s-1 keV-1 • Where γ is the power-law index • Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV) • non-thermal Bremstrahlung emission • Nuclear reaction

  18. Bremsstrahlung Spectrum • Bremsstrahlung emission (German word meaning "braking radiation") • the radiation is produced as the electrons are deflected in the Coulomb field of the ions. Bremsstrahlung emission

  19. Flare Energy Source • Magnetic energy is slowly-built-up and stored in the corona, and then through a sudden release to produce flares, the so called storage-release model • Flare is believed to be caused by magnetic reconnection in the corona • Magnetic reconnection is a process that can rapidly release magnetic energy in the corona

  20. Magnetic Reconnection • MHD equations • (Aschwanden 6.1.36, P. 247) • Magnetic field diffusion time τdin the corona • τd = 4πσL2/c2 = L2/η • τd the time scale the magnetic field in size L dissipate away, • σ electric conductivity, η magnetic diffusivity, L the magnetic field scale size • In normal coronal condition, τd ~ 1014 s, or 1 million year (assuming L=109 cm, T=106 K, and σ =107T3/2 s-1) • To reduce τd, reduce L to an extremely thin layer

  21. Magnetic Reconnection: 2-D (cont.) • quick magnetic energy dissipates at the current sheet (a magnetic neutral sheet) that separates two magnetic regimes, which • have different directions (opposite directions • are forced to push together by a continuous converging motion • Analytic solutions are obtained by • Sweet-Parker model • Petschek model • (Aschwanden book, Chap. 10, p.408-412)

  22. Magnetic Reconnection: 2D (cont.) Sweet-Parker reconnection Petschek reconnection (Aschwanden Book, Fig. 10.2, P411)

  23. Magnetic Reconnection: 3-D (cont.) (Aschwanden Book, Fig. 10.10, P423)

  24. Magnetic Reconnection: 3-D (cont.) • Separatrix surface: 2D surface divides oppositely directed magnetic field in 3D volume • Separator line: 1D intersection lines of two 2D-separatrix surface • Nullpoint: nullpoint of the intersection of 1D separator lines • Magnetic reconnection occurs in these locations where current is concentrated and magnetic field is zero

  25. Flare Model: standard 2-D model • Aschwanden book. Fig. 10.20, P437

  26. Flare Model: standard 2-D model (cont.) • The initial driver of the flare process is a rising flux rope or filament above the neutral line (pre-flare phase) • At a critical point, the magnetic reconnection at the X-type region below the flux rope sets-in (major flare phase) • Newly reconnected field lines beneath the reconnection points have an increasingly larger height and wider footpoint separation (post-flare phase)

  27. Flare dynamic Model • A cartoon model by Gurman • Particle precipitation • Thermal conduction • Chromospheric Evaporation

  28. Flare Dynamic Model (cont.) • Loop structure of soft X-ray emission • Compact hard X-ray sources appear at two foot-points of soft X-ray loop • Hard X-ray sources appear at top of soft X-ray loops

  29. Coronal Mass Ejection and Filament Eruption • Both are large scale structural eruption in the corona • CMEs are observed in the outer corona by coronagraphs (since 1970s) • Filament Eruption are observed in Hα on the solar disk (since ~1900s) • CME eruptions are often associated with filament eruption

  30. Filament Eruption BBSO Hα Mt. Wilson Magnetogram

  31. Filament Eruption (cont.) • A filament always sits along the magnetic inversion line (magnetic neutral line) that separates regions of different magnetic polarity • A filament is supported by coronal magnetic field in a supporting configuration • Magnetic dip at the top of loop arcade (2-D) • Magnetic flux rope (3-D) • Helical or twisted magnetic structure is seen within filament

  32. Filament Eruption (cont.) • One of filament models: filament is supported by twisted magnetic flux rope

  33. Filament Eruption (cont.)

  34. Filament Eruption TRACE 195 Å, 1999/10/20 Filament eruption and loop arcade TRACE 195 Å, 2002/05/27 A failed filament eruption TRACE 195 Å, 1998/07/27 Filament dancing without eruption

  35. Filament Eruption Model • model of Martens & Kuin • A unified model of • Filament and • Flare

  36. CMEs • A CME is a large scale coronal plasma (and magnetic field structure) structure ejected from the Sun, as seen by a coronagraph • A CME propagates into interplanetary space. Some of them may engulf the earth orbit and cause a series of geo-space activities, such as geomagnetic storms. • A CME disturbs the solar wind, drives shock in interplanetary space, and produce energetic particles at the shock front.

  37. Coronagraph • Coronagraph • A telescope equipped with an occulting disk that blocks out light from the disk of the Sun, in order to observe faint light from the corona • A coronagraph makes artificial solar eclipse • The earliest CME observation was made in early 1970s

  38. Coronagraph: LASCO • C1: 1.1 – 3.0 Rs (E corona) (1996 to 1998 only) • C2: 2.0 – 6.0 Rs (white light) (1996 up to date) • C3: 4.0 – 30.0 Rs (white light) (1996 up to date) C1 C2 C3 • LASCO uses a set of three overlapping coronagraphs to maximum the total effective field of view. A single coronagraph’s field of view is limited by the instrumental dynamic range.

  39. Coronagraph (cont.) • White light corona has three components: • K (continuum) corona, caused by scattering of photospheric light of rapidly-moving coronal electrons (so called Thomson scattering); the major component of white-light corona • F (Fraunhofer corona), caused by scattering of photospheric light off dust in interplanetary space between the orbits of Mercury and Earth • E (Emission corona), caused by emission of radiation by highly-ionized species in the actual corona, so called forbidden emission lines, e.g., at 5302 Å from Fe XIV (coronal green line)

  40. Coronagraph (cont.) • K corona: continuum • F corona: continuum plus absorption • E corona: emission lines

  41. Coronagraph (cont.) • K corona brightness is less than 10-5 that of disk • Space observation is necessary • Special instrumental design to reduce scattering of light in instrument

  42. Streamer Several streamers in a typical coronagraph image

  43. Streamer (cont.) • A streamer is a stable large-scale structure in the white-light corona. • It has an appearance of extending away from the Sun along the radial direction • It is often associated with active regions and filaments/filament channels underneath. • It overlies the magnetic inversion line in the solar photospheric magnetic fields. • When a CME occurs underneath a streamer, the associated streamer will be blown away • When a CME occurs nearby a streamer, the streamer may be disturbed, but not necessarily disrupted.

  44. CME: transient phenomenon (example) A LASCO C2 movie, showing multiple CMEs

  45. CME Property: Measurement H (height, Rs) PA (position angle) AW (angular width) M (mass)

  46. CME Property: velocity • Velocity is derived from a series of CME H-T (height-time) measurement • A CME usually has a near-constant speed in the outer corona (e.g, > 2.0 Rs in C2/C3 field) • Note: such measured velocity is the projected velocity on the plane of the sky; it is not the real velocity in the 3-D space.

  47. CME Property: velocity distribution • CME velocity ranges from • 50 km/s to 3000 km/s • Average velocity: 400 km/s • Peak velocity: 300 km/s • Median velocity: 350 km/s 6300 LASCO CMEs from 1996 to 2002

  48. CME Property: size AW = 80 degree AW = 360 degree, halo CME

  49. CME Property: size • Broad distribution of CME apparent angular width • Average width 50 degree • A number of halo CMEs (AW=360 degree), or partial halo CMEs (AW > 120 degree) • Halo CMEs are those likely to impact the Earth orbit

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