Magnetic Drivers of CME Defection in the Low Corona

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1 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)

2 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 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 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.

3 Background CMEs impacting Earth fan have severe efects 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

4 The Simulation Launch CME at AR 0758 in CR 2029 (April -May 2005) Large CH on one side and streamer belt (SB) on other (Fig. 1) Use SWMF (Tóth 2011) a 3D global MHD model Modifed TD fux rope (Titov and Démoulin 1999) 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

5 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.

6 Magnetic Forces and Defection Split Lorentz force into magnetic curvature/tension and pressure gradient CME pressure outward force (Fig. 2, blue arrows) CME expansion Background solar wind pressure gradients force pointing away from CH toward SB (red arrows) 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 Moves CME away from initial radial path

7 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.

8 Measuring CME Position Wish to look at CME-pause transition between fux rope and sheath Regions of diferent magnetic feld Expect change in orientation of the B feld Look at angle which shows the CME, shock and sheath (Fig 3) Use nose to defne direction of CME 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

9 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.

10 Nose Defection Using the combination of thetab and the change in y velocity we track the position of the nose 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 Compare fnal position in higher planes with x-axis ~12 for z=0.25 and ~14 for z=0.5

11 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

12 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.

13 Figure 6-Magnetic Forces Compare the force on the CH side and the SB side 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

14 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.

15 Conclusions 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 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

16 References and Acknowledgements Baker et al. 2009, Severe Space Weather Workshop Report Byrne et al. 2010, Nature Communications Cremades et al 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 ATM and SHINE AGS The CME simulations were performed on the NASA AMES supercomputer Pleiades. The background image originated from NASA.gov and shows a CME from Dec