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CSI 662 / ASTR 769 Lect. 04, February 20 Solar Activities: Flares and Coronal Mass Ejections (CMEs) References: •Aschwanden 10.5-10.6, P436-P463 •Tascione 2.3-2.5, P18-P25 Spring 2007 Magneto-Hydrodynamics (MHD) References on MHD equations: • Aschwanden 6.1, P241-P247 Magnetic Reconnection References: • Aschwanden 10.1, P407-P414 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 Magnetic Reconnection •Steady magnetic field diffusion time τd in 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) 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 Magnetic Reconnection • Sweet-Parker Reconnection (1958) Plasma Inflow Plasma Outflow Diffusion Region • Magnetic Reconnection Rate M = Vi/VO (in-speed/out-speed) 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 1. hot plasma: radio, visible, UV, soft X-ray 2. non-thermal energetic particles: radio, hard X-ray, γ-rays Flare: Hα Heating: temperature increase in Chromosphere Structure: ribbons 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 Flare: in soft X-rays (~ 10 Å) Heating: temperature increase in Corona (~ 10 MK) Structure: fat X-ray loops 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 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 Flares: X-ray Classification Class B Intensity I (W m-2) (erg cm-2 s-1) 10-4 10-7 C 10-3 10-6 M 10-2 10-5 X 10-1 10-4 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 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 Flare: Spectrum • The emission spectrum during flare’s impulsive phase Flare: Spectrum • A full flare spectrum may have three components: 1. Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV): • thermal Bremsstrahlung emission 2. 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 3. Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV) • non-thermal Bremstrahlung emission • Nuclear reaction 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 Flare Model 1. Magnetic reconnection occurs at the top of magnetic loop 2. Energetic particles are accelerated at the reconnection site 3. Particles precipitates along the magnetic loop (radio emission) and hit the chromosphere footpoints (Hard X-ray emission, Hα emission and ribbon) 4. Heated chromspheric plasma evaporates into the corona (soft X-ray emission, loop arcade) 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 Flare Model •Coronal loop structure of soft X-ray sources •Compact hard X-ray sources appear at two footpoints of soft Xray loop •Hard X-ray source appear at the top of soft X-ray loops CSI 662 / ASTR 769 Lect. 05, February 27 Solar Activities: Flares and Coronal Mass Ejections (CMEs) References: •Aschwanden 10.5-10.6, P436-P463 •Tascione 2.3-2.5, P18-P25 Spring 2007 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 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 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. 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. 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 CME A LASCO C2 movie, showing multiple CMEs CME Properties H (height, Rs) PA (position angle) AW (angular width) M (mass) CME Properties •Velocity is derived from a series of CME H-T (heighttime) measurement •A CME usually has a nearconstant 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. 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 CME Properties • Three part CME structure 1. A bright frontal loop (or leading edge) • Pile-up of surrounding plasma in the front 2. A dark cavity (surrounded by the frontal loop) • possibly expanding flux rope or filament channel 3. A bright core (within the cavity) • Composed of densely filament remnant material CME Source Region BBSO Hα Mt. Wilson Magnetogram • Filaments always ride along the magnetic neutral line 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 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 CME Eruption TRACE 195 Å, 1999/10/20 Filament eruption and loop arcade TRACE 195 Å, 2002/05/27 TRACE 195 Å, 1998/07/27 A failed filament eruption Filament dancing without eruption 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 CME model Lin’s 2-D CME eruption model • MHD analytic solution •Animation 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 CME models (cont.) Antiocs’s 3-D CME eruption model •MHD numeric solution •Multi-polar •So-called break-out model The End