Solar Activities:Flares and

Coronal Mass Ejections (CMEs)

CSI 662 / ASTR 769 Lect. 04, February 20 Spring 2007

References: •Aschwanden 10.5-10.6, P436-P463•Tascione 2.3-2.5, P18-P25

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-ray2. non-thermal energetic particles: radio, hard X-ray, γ-rays

Flare: Hα

Heating: temperature increase in ChromosphereStructure: 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 Intensity

(erg cm-2 s-1)

I (W m-2)

B 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 Model1. Magnetic reconnection occurs

at the top of magnetic loop2. 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

•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

Flare Model

Solar Activities:Flares and

Coronal Mass Ejections (CMEs)

CSI 662 / ASTR 769 Lect. 05, February 27 Spring 2007

References: •Aschwanden 10.5-10.6, P436-P463•Tascione 2.3-2.5, P18-P25

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.

•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

•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

Streamer Structure

A LASCO C2 movie, showing multiple CMEs

CME

CME Properties

H (height, Rs)

PA (position angle)

AW (angular width)

M (mass)

•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.

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 structure1. A bright frontal loop (or leading edge)

• Pile-up of surrounding plasma in the front2. A dark cavity (surrounded by the frontal loop)

• possibly expanding flux rope or filament channel3. A bright core (within the cavity)

• Composed of densely filament remnant material

CME Properties

CME Source Region

BBSO Hα Mt. Wilson Magnetogram

• Filaments always ride along the magnetic neutral line

•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 Source Region

•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 Structure

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

CME Eruption

• 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 model

CME models (cont.)

Antiocs’s 3-D CME eruption model•MHD numeric solution •Multi-polar•So-called break-out model

The End