Magnetic Drivers of
CME Defection in the
Low Corona
C. Kay (Boston University)
M. Opher (Boston University)
R. M. Evans (NASA GSFC/ORAU
T. I. Gombosi (University of Michigan)
B. van der Holst (University of Michigan)
Abstract
Coronal mass ejection (CME) observations include cases where CMEs follow
a trajectory other than the radial path from the associated launch site. The
presence of a coronal hole (CH) can contribute to this defection. Using a
3D MHD model, the Space Weather Modeling Framework, we simulate the
propagation of a CME near a coronal hole. We establish a steady state
background solar wind starting with a magnetogram of Carrington
Rotation 2029 in which the solar wind is driven by Alfven waves. Our
model also includes the efects of surface Alfven wave and Kolmogorov-like
dissipation. We launch the CME by inserting a Titov-Démoulin fux ripe in
the region corresponding to active region 0958. Based on the orientation
of the coronal hole with the CME, we expect defection to occur mostly in
the equatorial direction. By tracking the position of the nose of the CME in
the plane containing the Sun's equator we measure an equatorial defection
of 10.7°. As the defection occurs low in the corona, a region of low plasma
beta, we expect magnetic forces to be responsible. We estimate the forces
from magnetic tension and magnetic pressure gradients and analyze the
magnitude of these forces over the CME's propagation. We see comparable
magnitudes between the coronal hole tension force and the diference
between the pressure gradients on opposite sides of the CME. Both forces
act to push the CME away from the coronal hole. From this we conclude
both forces should be considered when looking at CME defection near a
coronal hole.
Background
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CMEs impacting Earth fan have severe efects
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Satellite damage, power grid failure (Baker 2008)
Critical to CME's path from Sun
Observational studies fnd defections up to 50° within 50
Rsun (Macqueen 1986, Yashiro 2004, Byrne 2010, Liu 2010,
Rodriguez 2011)
CHs know to defect high latitude CMEs to ecliptic during
solar max (Kilpua 2009)
Magnetic pressure gradients possible cause of defection
(Gui 2009, Shen 2011)
Force vector describing CH infuence on CME points in
direction of observed defection (Cremades 2006,
Gopalswamy 2009)
Lugaz 2011 fnd magnetic forces cause some defection but
ultimately need hydrodynamic force to match observations
The Simulation
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Launch CME at AR 0758 in CR 2029 (April -May 2005)
Large CH on one side and streamer belt (SB) on other (Fig. 1)
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Use SWMF (Tóth 2011) – a 3D global MHD model
Modifed TD fux rope (Titov and Démoulin 1999)
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Expect mainly equatorial defection from orientation
No sub-photospheric charges or line current
Alfven wave-driven solar wind background including efects of
surface Alven wave and Kolmogorov-like dissipation (van der
Holst 2010, Evans 2012)
9 million cells with highest refnement (0.o23 Rsun) near the
CH and CME path
Start at base of corona (1.035 Rsun) and follow several Rsun
First study of CME defection with this background solar wind
Figure 1
Solar Surface
Coronal Hole
Direction of
Defection
←
Flux Rope
The solar surface is shown at 1.04 Rsun with contours of density
showing the location of the coronal hole (constant mass fux at
the boundary → high speed means low density). An isosurface of
current shows the fux rope in the position it is inserted at t=0.
We observe defection away from the coronal hole.
Magnetic Forces and Defection
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Split Lorentz force into magnetic curvature/tension and
pressure gradient
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CME pressure → outward force (Fig. 2, blue arrows) →
CME expansion
Background solar wind pressure gradients → force
pointing away from CH toward SB (red arrows)
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Include thermal pressure gradient as well
Acts against CH side expansion and with SB side expansion
Magnetic tension from curving CH feld lines also acts
against CH side expansion (black arrow)
Net result is unbalanced forces on the two sides causing an
uneven expansion which appears as a defection
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Moves CME away from initial radial path
Figure 2
Cartoon showing
defection resulting
from unbalanced
forces. The arrows
represent various
forces and are labeled
according to color.
The bottom panel
represents a time step
afer the top panel
when the CME has
expanded according to
the forces.
Measuring CME Position
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Wish to look at CME-pause → transition between fux rope
and sheath
Regions of diferent magnetic feld
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Expect change in orientation of the B feld
Look at angle
which shows the CME, shock and sheath (Fig 3)
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Use nose to defne direction of CME
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Look at change in solar wind speed from steady state → see
fows around CME (Fig 3)
Take stagnation point to be point on CME-pause where fow
changes directions
Figure 3
x
x
The lef panel shows thetaB and the right panel shows the
change of the velocity in the y-direction from the steady
state value. Both panels are from 15 minutes afer fux
rope insertion. The CME-pause is the transition from
green to yellow in the lef panel and we mark the nose
with an x in both panels.
Nose Defection
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Using the combination of thetaB and the change in y
velocity we track the position of the nose
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Can also use thetaB to get approximate edge of CME-pause
positions (points with largest and smallest y)
Look in equatorial plane (z=0 Rsun) as well as z=+0.25 and
z=+0.50 Rsun (Fig. 4)
Find a defection of 11° in the equatorial plane
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Compare fnal position in higher planes with x-axis
~12° for z=0.25 and ~14° for z=0.5
Figure 4
The top lef has the change in
nose position for the
equatorial plane, bottom lef
for z=+0.25 Rsun and bottom
right for z=+0.50 Rsun
Figure 5
We can look at the position of the edges in the equatorial
plane at the same time as the nose position. We fnd that
although the nose changes by 11° the edges do not change the
same.
Figure 6-Magnetic Forces
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Compare the force on the CH side and the SB side
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NOTE: these forces were estimated at a slightly
diferent position than the edges in Fig. 5 –>
matching forces coming soon!
Magnetic forces become negligible by ~1.5 Rsun
Figure 7-Importance of Tension
Find that the magnetic tension is similar in magnitude to the
diference between the pressure gradient on opposite sides
and should not be neglected in cases of defection near a
coronal hole.
Conclusions
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See a defection of 11° of the nose of the CME in
the equatorial plane
Find that magnetic forces become negligible
afer ~1.5 Rsun
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Other forces must infuence the CME beyond this
distance
The force from magnetic tension is comparable
to the net force from the diferent pressure
gradients on opposite sides of the CME
References and Acknowledgements
Baker et al. 2009, Severe Space Weather
Workshop Report
Byrne et al. 2010, Nature Communications
Cremades et al. 2006 Adv. Space Research
Evans et al. 2012, ApJ, under review
Gopalswamy et al. 2009, JGR
Gui et al. 2011, Solar Phys.
Kilpua et al. 2009, Solar Phys.
Liu et al. 2010, ApJ
Lugaz et al. 2011, ApJ
Macqueen et al. 1986, JGR
Shen et al. 2011, Solar Phys.
Rodriguez et al. 2011, Solar Phys.
Titov & Démoulin 1999, A&A
Tóth et al. 2011, JCP
van der Holst et al. 2010, APJ
Yashiro et al. 2004, JGR
This work is supported by the National
Science Foundation CAREER Grant ATM0747654 and SHINE AGS-1151422. The CME
simulations were performed on the NASA
AMES supercomputer Pleiades. The
background image originated from NASA.gov
and shows a CME from Dec 2 2003