Transcript Lecture 4 The Formation and Evolution of CMEs

Lecture 4
The Formation and Evolution of CMEs
Coronal Mass Ejections (CMEs)
• Appear as loop like features that breakup helmet
streamers in the corona.
• Three part structure:
– Bright outer rim
– Dark cavity behind rim
– Bright inner core of erupted prominence material
Scales of CMEs
• Gray areas are
covered by codes
• Micro – inertial
length, Larmor radius
• As CME propagates
out through the solar
system – both time
and spatial scales
increase (apparent
shrinking is do to log
log plot)
Prominences
• Suspensions of cool (T~104K), dense (n~1010-1011 cm-3)
chromospheric material surrounded by the hot (T~106K)
and tenuous (n~107-109 cm-3) corona
minences
inences are dense relatively cool clouds of material suspended
e the surface of the Sun
by loops of magnetic
field. (Density
1016Prominence
Magnetic
Field
m-3, temperature 5,000-8,000K)
• Magnetic
field
foundthe
to same
be approximately
aligned
inences
and filaments
areis
actually
things except that
inences the
are seen
projecting out above the limb, or edge, of the
filament.
• Highly sheared field.
filaments and prominences can remain in a quiet or quiescent
• Called
a filament
when viewed
from
above.
for months.
However,
as the magnetic
loops that
support
them
y change,
filaments
and prominences
erupt and rise off of the
• Can
be stable
for days orcan
weeks.
over the course of a few minutes or hours.
ption of June 4,
6 with blue
th for scale
ption speed ~
km/s
minence would
ss the US in 15
onds
with
Fundamental Questions
• How are CMEs initiated?
– Widely accepted that the energy of CMEs is stored in coronal
magnetic fields – the strongly sheared field of a filament
(prominence) channel.
– The CME is thought to be the catastrophic disruption of the force
balance between the upward magnetic pressure of the filament
and the downward tension of the overlying field.
– How this disruption occurs is the main unanswered question in
CME initiation.
• Flux cancellation models
• Breakout models
• Flux injection model
Flux Cancellation Models
• Flux cancellation is the
disappearance of magnetic fields of
opposite polarity at the neutral line
separating them.
• Flux cancellation at the neutral line
of a sheared arcade causes the flux
rope that supports prominence
material.
• Equilibrium breaks down if flux
cancellation continues after the flux
rope is formed.
• A new equilibrium forms farther out.
• In reality the solar wind pulls the flux
rope out and forms a current sheet
at which reconnection occurs.
A Simulation Study of the Eruption of a CME
(Linker et al., 2003)
• The initial configuration –azimuthal symmetry
–
–
–
–
–
Build a model of a helmet streamer
Use a spherically symmetric MHD solar wind solution
Use a potential magnetic field
Integrate until an equilibrium results.
To create a source of free magnetic energy put a shear flow near
the neutral line of the streamer – specify the tangential E field.
Formation of a Flux Rope
• (top) Projected field lines (shading) and azimuthal field
(color)
• (middle) Current density out of the plane.
• (bottom) Polarization brightness that a coronagraph
would observe.
• Flux cancellation forms a stable flux rope within the
helmet streamer (1350τA)
• Once the configuration is beyond a stability threshold
halting the flux cancellation cannot stop the eruption
(1390τA).
• Prominence formation is part of the flux cancellation
mechanism.
Change in Magnetic Energy and Kinetic Energy
• Magnetic energy closed flux
(top-bottom), kinetic energy
(bottom)
• During formation of helmet
streamer B2/2μ0 increases
15% (t<600)
• Energization of streamer
(600>t>1300)
• Flux rope formation (1320)
• Eruption (1380)
• Energy of open flux (toptop) – eruption occurs when
closed energy ~ open
energy
• Half of energy goes into flux
rope.
Add azimuthal asymmetry
• Azimuthal models – entire coronal field must be opened
and flux rope is detached from the Sun.
• 3D model allows azimuthal asymmetry.
• Reduce magnetic flux only in one sector of Sun.
• Creation of flux rope and eruption occur as before.
Structure of flux rope
• Isosurface of density
• Field lines in flux rope after it has propagated away
from the Sun
• Note that both ends of the flux rope are attached to the
Sun.
The Breakout Model
• Reconnection occurs external to the filament channel –
quasi-potential overlying flux and neighboring flux
systems.
• Axisymmetric system with two spatial dimensions and
three velocity dimensions (MacNeice et al., 2004).
• Imposed shear flow at the equatorial neutral line –
generates a Bφ which produces an upward magnetic
pressure (50251s, 70680s)
• As fluxrope expands outward – downward tension on
overlying field lines increases – stretch radially the field
near the null.
• Reconnection begins at the top of the expanding rope
(79008)
• Vertical current sheet forms deep inside (85185) –
reconnects.
0
50251s
70680s
79008s
85185s
95020s
Mass Density and Radial Velocity of Flux
Rope
The Breakout Model and Observations
• The main feature of the
breakout model
– “Flare” reconnection does
not initiate the eruption,
– Multipolar pre-eruption
topology
• Density difference shows
changes in density.
• The three part structure
seen in coronagraph
images is found in the
simulations.
Lynch et al., 2004
Energetics of the Breakout Model
• Azimuthal magnetic energy
– solid line
• Azimuthal magnetic energy
below 1.5RS – dashed-dotted
line
• Change in non-azimuthal
energy – dashed line
• The kinetic energy – dotted
line with triangles.
• Initially about half of the
azimuthal magnetic energy is
converted into kinetic energy.
By the end of the simulation
all magnetic energy about
1.5RS has been converted.
Flux Injection Model (Chen, 1989, 1996)
• The underlying magnetic field of a CME is that of a
three-dimensional flux rope.
• While all models end up with flux ropes this one starts
with them.
• The flux rope is determined by the Lorentz force,
pressure gradients and drag on the coronal plasma.
• It is difficult to distinguish between the flux cancellation
models and the flux injection model since they evolve in
the same way once the flux rope emerges.
CME Evolution and Propagation (Forbes et al.,
2006)
• A CME propagates through the interplanetary medium as
an ICME.
• Assume a flux tube (the CME) circles the Sun like in the
symmetric simulation.
• Under excess internal pressure the flux tube expands –
that expansion is resisted by the inertial reaction of the
medium into which it expands.
• The excess total pressure (particle plus magnetic)
causes the flux tube to accelerate into the medium over
coming gravity and drag.
Basic Interaction
• The interaction can be written in words
– Expansion
• (Ambient mass density) X (Rate of expansion)2= Delta Pressure
(inside – outside)
– Acceleration
• (Mass of CME + Virtual mass) X Acceleration = Force of gravity +
Delta (outside magnetic and particle pressure on lower surface area
– outside magnetic and particle pressure on upper surface area) –
Drag
• Virtual mass allows us to correct for the force necessary to move all
the ambient medium away = volume of cylinder time the mass
density of the ambient medium.
– Problem is in turning this into equations.
• Standard drag term CDAρ abs (VCME –Vsw) (VCME –Vsw) where CD is
a drag coefficient, and (VCME –Vsw) is relative velocities of CME and
solar wind.
• How to do it is controversial.
MHD simulation of CME propagation in the
heliosphere (Riley et al., 2003)
• Combined a CME eruption model (flux cancellation) with
a solar wind model.
• Flux cancellation model was used as input to solar wind
model.
– Flux cancellation model assumes g=1.05 to mimic nearisothermal corona.
– Solar wind model has g=5/3.
– Discontinuity at interface is “harmless” affecting only temperature
slightly.
– Corotation enforced at the boundary between the models
B'  B  2Br sin  r / vr
– Plasma and magnetic field parameters were set at the outer
boundary of the heliospheric simulation
Evolution of the CME out to 0.5AU
• Number density – black; Vr – color contours; Magnetic
field lines – blue
• White line is boundary between solutions.
• Flux rope becomes circular and then “pancake” shaped
– kinematic expansion as ejecta expands and then
collision with surrounding material.
• Shock and flux rope develop concave deformations.
Evolution of CME out to 5AU
• Density – red, field lines – black and velocity
shading.
• Ejecta becomes more distorted with distance.
• Acceleration related to post eruption reconnection.
Comparison with spacecraft observations
• Ejecta was traveling faster than
ambient solar wind –shock
(both)
• Speed profiles are similar
• Simulated B does not have
peak in sheath (ACE))
• Magnetic discontinuity not
found in simulation (Ulysses)
• B modeled better at Ulysses.
• Strong magnetic structure at
Ulysses but not at ACE –
Ulysses passed near center of
flux rope – ACE near the flank