Predicting Deflections of Coronal Mass Ejections Using ForeCATpeople.bu.edu/ckay/SDposter.pdf ·...
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Predicting Deflections of Coronal Mass Ejections Using ForeCAT 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 10 15 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. CME Mass Speed Size Mag. Field Solar Mag. Field Density Deflection Drag Propagation Expansion CME Trajectory ForeCAT The Model ForeCAT: Forecasting a CME’s Altered Trajectory ‣ 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 Contact: [email protected] ForeCAT Results Test Case Input parameters: mass 10 15 g, final propagation velocity 475 km s -1 , cross-sectional radius 0.15 R s , and internal magnetic field 10 G 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 5x10 15 g faster CMEs deflect less ‣ CMEs with more expansion deflect less ‣ Initially large cross-sectional radius have less deflection ForeCAT Model Details Deflection Use both components of Lorentz force (JxB) to calculate deflection 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 R s ‣ Rapid acceleration - linear acceleration between 1.5 R s and 3 R s ‣ Propagation - assume constant radial propagation beyond 3 R s 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 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. 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 (CME)? ‣ An eruption of solar material (10 15 to 10 16 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 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 Image from Solar Dynamics Observatory Image from National Solar Observatory 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
Transcript of Predicting Deflections of Coronal Mass Ejections Using ForeCATpeople.bu.edu/ckay/SDposter.pdf ·...
Predicting Deflections of Coronal Mass Ejections Using ForeCATC. Kay (BU), M. Opher (BU), R. M. Evans (NASA/GSFC/ORAU)
AbstractThe 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.
CMEMassSpeedSize
Mag. Field
SolarMag. Field
Density
Deflection
Drag
Propagation
Expansion CME Trajectory
ForeCAT
The Model ForeCAT: Forecasting a CME’s Altered Trajectory‣ 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
Contact: [email protected]
ForeCAT ResultsTest CaseInput parameters: mass 1015 g, final propagation velocity 475 km s-1, cross-sectional radius 0.15 Rs, and internal magnetic field 10 G
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 SpaceVary 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
ForeCAT Model DetailsDeflectionUse both components of Lorentz force (JxB) to calculate deflection
Magnetic Tension" " " " " " Magnetic Pressure Gradients
PropagationThree-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
ExpansionUse hydrodynamic solution for expanding overpressure region but replace thermal overpressure with magnetic overpressure similar to Melon-Seed-OverExpansion model of Siscoe 2006
DragExpect 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 ModelUse 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
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.
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 (CME)?‣ An e rup t ion o f so la r 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 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
Image from Solar Dynamics Observatory
Image from National Solar Observatory
ReferencesByrne et al. 2010, Nature Let.Liu et al. 2010, ApJPlunkett et al. 2001, JASTPSiscoe et al. 2006, Solar Phys.Toth et al. 2011, J. Computational PhysZhang et al. 2001, ApJ
