Predicting Deflections of Coronal Mass Ejections Using ForeCAT
Contact: ckay@bu.edu
C. Kay (BU), M. Opher (BU), R. M. Evans (NASA/GSFC/ORAU)
Abstract
The accurate prediction of the path of coronal mass ejections (CMEs) plays an important role in
space weather forecasting, and knowing the source location of the CME does not always suffice.
During solar minimum, for example, polar coronal holes (CHs) can deflect high latitude CMEs
toward the ecliptic, and when CHs extend to lower latitudes deflections in other directions can
occur. To predict whether a CME will impact Earth, the effects of the background on the CME's
trajectory must be taken into account. We develop a model, ForeCAT (Forecasting a CME's Altered
Trajectory), of CME deflection close to the Sun where magnetic forces dominate. ForeCAT
includes CME expansion, a three-part propagation model, and the effects of drag on the CME's
deflection. Given the background solar wind conditions, the launch site of the CME, and the
properties of the CME (mass, final propagation speed, initial radius, and initial magnetic strength),
ForeCAT predicts the deflection of the CME due to magnetic forces. For a background where the
CME is launched from an active region located between a CH and streamer region, the strong
magnetic gradients cause a deflection of 39.6° in latitude and 22.6° in longitude for a 1015 g CME
propagating out to 1 AU. ForeCAT does not include the effects of variations in the background
solar wind speed or interactions with other CMEs, which could cause additional deflection.
ForeCAT can also be used to better understand the space weather in extrasolar planetary systems.
Magnetic deflection will play an important role in M stars that have strong magnetic fields that
exceed those of Sun.
The Model ForeCAT: Forecasting a CME’s Altered Trajectory
CME
Mass
Speed
Size
Mag. Field
Deflection
Propagation
Expansion
Solar
Mag. Field
Density
Solar Terminology
‣ Active region - area of strong magnetic
field on the surface of the Sun
‣ Coronal hole - region of open magnetic
field lines
‣ Streamer belt - band of closed magnetic
field lines along the polarity inversion line
separating opposite magnetic polarity
hemispheres
What is a Coronal Mass Ejection
Image from National Solar Observatory
(CME)?
‣ An eruption of solar
material (1015 to 1016 g) and
magnetic field that travels
100-1000s of km s-1 through
the solar system
‣ Interacts with encountered
objects (planets, satellites ...)
‣ At Earth CMEs drive aurora
and cause geomagnetically
induced currents which have
Image from Solar Dynamics Observatory
the potential to damage
power grids
‣ Shocks driven by CMEs create energetic particles which can damage
satellites and harm astronauts
➡ Knowing the path of a CME is key for space weather prediction
Space Weather?
Solar driven changes in ambient conditions (plasma, magnetic fields,
radiation) in the near-Earth or interplanetary space environment
A Brief History of CME
Deflection
‣ Latitudinal deflections first
observed in single coronagraph
images
‣ Reconstruct full 3D motion from
multiple viewpoints
‣ See deflections up to 30° within
10-20 Rs (Byrne 2010, Liu 2010)
Left image from Plunkett 2001: deflection
of a CME within a coronagraph
Bottom image from Byrne 2011: 3D
reconstruction and deflection of a CME
CME
Trajectory
Drag
ForeCAT
‣ Combine new analytic model for magnetic
deflection with existing analytic and empirical
models to describe the path of a CME
‣ CME input parameters determined from
observations
‣ ForeCAT calculates deflection within a “deflection
plane” defined using magnetic gradients and initial
CME position
‣ Contains maximum deflection
‣ Sum forces on edges of CME cross-section
within deflection plane
‣ Ignore non-magnetic sources of deflection
‣ No interactions with other CMEs, varying
background solar wind speeds or reconnection
ForeCAT Model Details
ForeCAT Results
Deflection
Use both components of Lorentz force (JxB) to calculate deflection
Test Case
Input parameters: mass 1015 g, final propagation velocity 475 km s-1, cross-sectional
radius 0.15 Rs, and internal magnetic field 10 G
Magnetic Tension" "
"
"
"
"
Magnetic Pressure Gradients
Propagation
Three-part propagation model similar to that of Zhang 2001
‣ Slow rise - assume a constant velocity until 1.5 Rs
‣ Rapid acceleration - linear acceleration between 1.5 Rs and 3 Rs
‣ Propagation - assume constant radial propagation beyond 3 Rs
Expansion
Use hydrodynamic solution for expanding overpressure region but replace thermal
overpressure with magnetic overpressure similar to Melon-Seed-OverExpansion
model of Siscoe 2006
Drag
Expect interaction between radially outflowing solar wind and CME → CME’s
deflection motion should be impeded by drag force
‣ Effects of radial drag already incorporated into empirical propagation model so do
not explicitly calculate radial drag
Magnetic Field Model
Use steady state solar wind results from an magnetohydrodynamic simulation (Space
Weather Modeling Framework - Toth 2011)
‣ Take results from ring at 1.15 Rs within deflection plane and fit polynomial
‣ Scale to other radii using r-3 for active regions and r-2 elsewhere
‣ Φ is polar angle within deflection plane
a. Trajectory of CME below ~10 Rs: the line represents the diameter of the circular
cross-section parallel to to the y-axis so the figure shows both the position and
size of the CME
b. Central position angle of the CME out to 1 AU: the majority of the deflection
occurs before 3 Rs and the CME maintains a relatively constant position out to 1
AU. CME deflects 44.9° → 39.6° in latitude and 22.6° in longitude
Sampling Parameter Space
Vary two input parameters and hold all others constant to explore parameter space
Trends for input
parameters:
‣ More massive
deflect less
‣ Below 5x1015 g
faster CMEs
deflect less
‣ CMEs with more
expansion deflect
less
‣ Initially large
cross-sectional
radius have less
deflection
Our newly developed model, ForeCAT, predicts the trajectory of a CME away from the Sun. Sets of
ForeCAT runs covering a wide range in input parameter space produce a range of possible
deflections from a given source location, which could prove useful for space weather forecasting
as well as insights into the space weather of extrasolar planetary systems. ForeCAT produces
results on the order of minutes, faster than the 2-5 days a CME typically takes to travel to 1 AU.
References
Byrne et al. 2010, Nature Let.
Liu et al. 2010, ApJ
Plunkett et al. 2001, JASTP
Siscoe et al. 2006, Solar Phys.
Toth et al. 2011, J. Computational Phys
Zhang et al. 2001, ApJ