Download Sun: Solar Activities -- Flares, CMEs

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