Solar Activities : Flares and Coronal Mass Ejections (CMEs)

1. CSI 662 / ASTR 769 Lect. 04, February 20 Spring 2007 Solar Activities:Flares andCoronal Mass Ejections (CMEs) • References: • Aschwanden 10.5-10.6, P436-P463 • Tascione 2.3-2.5, P18-P25

2. Magneto-Hydrodynamics (MHD) References on MHD equations: • Aschwanden 6.1, P241-P247

3. Magnetic Reconnection References: • Aschwanden 10.1, P407-P414

4. Magnetic Reconnection • Magnetic reconnection is believed to be the physical process that explosively dissipate, or “annihilate”, magnetic energy stored in magnetic field • Magnetic reconnection causes violent solar activities, such as flares and CMEs, which in turn drive severe space weather

5. Magnetic Reconnection • Steady magnetic field diffusion time τdin the corona • τd = 4πσL2/c2 = L2/η • τd: the time scale the magnetic field in size L dissipates 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, and reduce the conductivity (increase resistivity, e.g., anomalous resistivity due to plasma turbulence)

6. Magnetic Reconnection • Magnetic fields with opposite polarities are pushed together • At the boundary, B 0, forming a high-β region. • Called diffusion region, since plasma V could cross B • Since E= -(V × B)/c, it induces strong electric current in the diffusion region,also called current sheet • Outside the diffusion region, plasma remains low β • Strong energy dissipation in the current sheet, because of high current and enhanced resistivity

7. Magnetic Reconnection • Sweet-Parker Reconnection (1958) Plasma Inflow Plasma Outflow Diffusion Region • Magnetic Reconnection Rate M = Vi/VO (in-speed/out-speed)

8. Solar Flare • A solar flare is a sudden brightening of solar atmosphere (photosphere, chromosphere and corona) • Flares release 1027 - 1032 ergs energy in tens of minutes. • (Note: one H-bomb: 10 million TNT = 5.0 X 1023 ergs) • A flare produces enhanced emission in all wavelengths across the EM spectrum, including radio, optical, UV, soft X-rays, hard X-rays, and γ-rays • Flare emissions are caused by • hot plasma: radio, visible, UV, soft X-ray • non-thermal energetic particles: radio, hard X-ray, γ-rays

9. Flare: Hα Heating: temperature increase in Chromosphere Structure: ribbons

10. Flare: in EUV (~ 195 Å) TRACE Observation: 2000 July 14 flare • Heating: temperature and density increase in corona • Structure • Ribbons • Post-eruption loop arcade • Filament eruption

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

12. Flare: 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

13. Flare: 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 • emission mechanism: gyro-synchrotron emission

14. Flares: X-ray Classification

15. Flare: Temporal Evolution • 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

16. Flare: Temporal Evolution • 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

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

18. Flare: Spectrum • 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

19. 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

20. Flare Model • Magnetic reconnection occurs at the top of magnetic loop • Energetic particles are accelerated at the reconnection site • Particles precipitates along the magnetic loop (radio emission) and hit the chromosphere footpoints (Hard X-ray emission, Hα emission and ribbon) • Heated chromspheric plasma evaporates into the corona (soft X-ray emission, loop arcade)

21. Flare Model • Post-eruption loop arcade appears successively high, because of the reconnection site rises with time • The ribbon separates with time because of the increasing distance between footpoints due to higher loop arcades

22. Flare Model • Coronal loop structure of soft X-ray sources • Compact hard X-ray sources appear at two footpoints of soft X-ray loop • Hard X-ray source appear at the top of soft X-ray loops

23. CSI 662 / ASTR 769 Lect. 05, February 27 Spring 2007 Solar Activities:Flares andCoronal Mass Ejections (CMEs) • References: • Aschwanden 10.5-10.6, P436-P463 • Tascione 2.3-2.5, P18-P25

24. CME • A CME is a large scale coronal plasma and magnetic field structure ejected from the Sun • A CME propagates into interplanetary space. Some of them may intercept the earth orbit if it moves toward the direction of the Earth • CME eruptions are often associated with filament eruption

25. 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

26. 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.

27. Streamer • 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.

28. Streamer Structure • Magnetic configuration • Open field with opposite polarity centered on the current sheet • Extends above the cusp of a coronal helmet • Closed magnetic structure underneath the cusp

29. CME A LASCO C2 movie, showing multiple CMEs

30. CME Properties H (height, Rs) PA (position angle) AW (angular width) M (mass)

31. CME Properties • 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 deviates from the real velocity in the 3-D space.

32. CME Properties • Whether a CME is able to intercept the Earth depends on its propagation direction in the heliosphere. • A halo CME (360 degree of angular width) is likely to have a component moving along the Sun-Earth connection line • A halo is a projection effect; it happens when a CME is initiated close to the disk center and thus moves along the Sun-Earth connection line. • Therefore, a halo CME is possibly geo-effective. 2000/07/14 C2 EIT

33. CME Properties • Three part CME structure • A bright frontal loop (or leading edge) • Pile-up of surrounding plasma in the front • A dark cavity (surrounded by the frontal loop) • possibly expanding flux rope or filament channel • A bright core (within the cavity) • Composed of densely filament remnant material

34. CME Source Region BBSO Hα Mt. Wilson Magnetogram • Filaments always ride along the magnetic neutral line

35. CME Source Region • 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

36. CME Structure • Twisted magnetic flux rope forms above the neutral line due to shearing motion of photospheric magnetic field • Flux rope carries strong electric current (Ampere’s Law), thus carries a large amount of free energy

37. CME 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

38. CME model • CME is caused by the eruption of twisted flux rope above the magnetic inversion line • Magnetic reconnection occurs underneath the flux rope, causing tether cutting • Tether cutting remove the overlying constraining force, allowing allows flux rope to escape

39. CME model • Lin’s 2-D CME eruption model • MHD analytic solution • Animation

40. CME model • Unified CME-flare model • CME: flux rope • Flare • Coronal loop arcade • Hα flare ribbon • Magnetic reconnection • Underneath the flux rope • Above the loop arcade • Current sheet • Reconnection inflow

41. CME models (cont.) • Antiocs’s 3-D CME eruption model • MHD numeric solution • Multi-polar • So-called break-out model

42. The End