The Eruption and Propagation of Geoeffective Coronal Mass Ejections (CMEs)

1. The Eruption and Propagation of Geoeffective Coronal Mass Ejections (CMEs) Joan Burkepile National Center for Atmospheric Research ASP Space Weather Colloquium June 7, 2005

2. Outline Review Basic Properties of the Corona What are Coronal Mass Ejections (CMEs)? CMEs in the Solar Wind (ICMEs) What Solar Wind Conditions are Geoeffective? Determining Magnetic Structure at/near Sun Outstanding Questions Joan Burkepile June 7, 2005

3. Properties of the Corona The corona is an interesting astrophysical laboratory that can be studied up close. Atmosphere is structured by gravity and magnetic field: magnetically ‘closed’ and ‘open’ regions. The Corona is continuously expanding -dP/dr + -(GMr/r2) = ru du/dr (No magnetic field) Optically thin White light reveals density structure The solar corona on June 9, 2000 taken by the MK4 K-coronameter at Mauna Loa Solar Observatory. Image is an average of images acquired over ~1 hour. Joan Burkepile June 7, 2005

4. Properties of the Corona Highly conducting (electrically, thermally) Low b = ratio of thermal plasma pressure /magnetic pressure b = 2nekTe / (B2/8p) << 1.0 in low corona Values of some Basic Properties: Avg. B ~10 Gauss Density ~ 108 particles / cm3 Temp = 1 to 2 million OK Scale ht = ~ 0.1 Rsun ~ 7 x 104 km Vsound = (gpth/r)1/2 = (g2nekTe /r)1/2 ~175 km/sec Valfven = B/(mor)1/2~500-1000 km/sec Escape velocity at surface = 618 km/sec Joan Burkepile June 7, 2005

5. EUV and X-ray Corona Characteristic thermal emission of a body at 1-2 million degrees is in X-ray wavelengths (~30 Angstroms) Reveals Thermal State of the Corona (also proportional to r2) EIT 195 Angstroms Aug 16, 1999 Yohkoh Soft X-rays Aug 16, 1999

6. Global Properties of Corona Continually Expanding  Solar Wind Reverse Direction of Magnetic Field every ~11 years New Magnetic Flux and Helicity transported into atmosphere. Large variation in flux over solar cycle (factor ~10) Strong hemispheric preference for a given sign of helicity that does not alter with the 11-year solar cycle.

7. New emerging magnetic flux in the atmosphere interacts with existing magnetic structures Magnetic Interactions creates stresses alters and creates currents alters the state of the field Solar Activity is a response to stresses in the corona Non-Linear System

8. What is a CME? A CME is a ‘sudden’ expulsion of magnetized plasma into the solar wind from regions initially magnetically closed. Joan Burkepile June 7, 2005

9. CME Observations Form in Low Corona (below ~2 Rsun) CME Rates: Activity minimum: avg. ~0.4 per day Activity maximum: avg. ~ 4 per day CME Latitude Distribution LASCO C2 Feb 18, 2003 Joan Burkepile June 7, 2005

10. CME Observations Avg. Width 45o-50o (SMM Mission) 72o (LASCO Mission) Avg. Speed 400 km/sec (SMM projection corrected) Avg. Acceleration ~500 m/s2 (peaks below 3 Rsun) <10 m/s2 (3-30 Rsun) Avg. Mass ~5 x 1015 grams (from below ~2. Rsun) Energy (Kin + Pot) 1031 to 1032 ergs Projection effects, differences in telescopes, interpretative nuances Expect greater inaccuracies in LASCO observations due to projection effects – LASCO records many more disk-centered events than other white light telescopes Joan Burkepile June 7, 2005

11. Relation of CMEs to Other Forms of Solar Activity - Prominences At least ~75% of CMEs associated with active or erupting prominences which form over magnetic polarity inversion lines Joan Burkepile June 7, 2005

12. Relation to Other Forms of Solar Activity - Flares Most (all?) CMEs accompanied by some level of X-ray emission (but X-ray intensity can vary greatly - spans 5 orders of magnitude). There are many more flares than CMEs Not a 1 to 1 correspondence between CMEs and flares Joan Burkepile June 7, 2005

13. Relation to Other Forms of Solar Activity - Flares Does the flare ‘drive’ the CME? Observations suggest this is not the case. Many CMEs begin before or near the onset time of the flare. Joan Burkepile June 7, 2005

14. Interpretation CME – Closed field region becomes magnetically open. Hundhausen, in Cosmic Winds, Jokipii, Sonett and Giampapa (eds.), 1997 Joan Burkepile June 7, 2005

15. Various forms of solar activity do not form in isolation but in concert as a result of changes in magnetic field Prominence Eruptions – nearly always see CME Flares – Many more flares than CMEs but many high intensity flares associated with CMEs Radio Emission Dimmings, Transient Holes Atmospheric Waves Shibata et al. 1995 Joan Burkepile June 7, 2005

16. Physical Causes of CMEs General Agreement: CMEs are magnetically driven Stressed magnetic fields in the corona generate current systems that store free magnetic energy to drive solar activity. No general agreement that explains the process by which magnetic energy initiates CMEs. Models assume significantly different initial conditions Low and Hundhausen, Axisymmetric Flux Rope Joan Burkepile June 7, 2005

17. Physical Causes of CMEs Breakout Model of Antiochos, DeVore and Klimchuk, 1999 ApJ From Gary and Moore ApJ, 2004 No general agreement that explains the process by which magnetic energy initiates CMEs Joan Burkepile June 7, 2005

18. CMEs in the Solar Wind (ICMEs) What we might expect 1. Magnetic field structure, density distribution, temperature, composition, charge states of CME may be very different from ambient solar wind 2. Interactions between CME and ambient wind such as: Momentum transfer (CME speed will eventually approach the solar wind speed) Compressions/rarefactions, heating/cooling From Reames et al., Astrophys. J., 466, 473, 1996 Fast CMEs will drive shocks  energetic particles Joan Burkepile June 7, 2005

19. CMEs in the Solar Wind (ICMEs) Common ICME signatures Time scales (hours to a few days – spatial scale) Counterstreaming suprathermal electrons (interpreted as magnetically connected to sun) Unusual charge and composition states (produced by conditions at source) Rotation of magnetic field (i.e. magnetic clouds) presence of flux ropes Low b (strong magnetic fields) Temperature enhancements and depressions Joan Burkepile June 7, 2005

20. Joan Burkepile SHOCK CME June 7, 2005

21. Geoeffective Condition – Strong southward B field N/S Component of solar wind B-field Begin sustained southward B Red line at Bz = 0 Geomagnetic Indices ap and DsT ap > 100 DsT < -100 Joan Burkepile June 7, 2005

22. Geoeffective ConditionsWhat produces Bz? CMEs may contain strong southward fields or generate them via CME-driven shocks, which deflect and compress ambient solar wind flow

23. Frequency of Large Events Severe Storms: DsT < -200 1978 through May 2005 Last 3 solar maxima 41 storms Avg. 1 to 2 per year Over half (56%) occurred at maximum activity (+/- 1 year)

24. Determining the Magnetic Structure of CMEs Current method for obtaining coronal B fields: Photospheric B provides boundary conditions for models. Interpolate into corona. Need coronal magnetic field observations. Methods – resonant scattering, Zeeman and Hanle effects, gyroresonant emission … coming of age Determine impacts of ICME on ambient solar wind B-field. Need to know SW sector structure + CME/shock location, speed

25. Some Space Weather Science Goals/Needs • We want to know: • How CMEs are produced and what is their magnetic structure • How CMEs and shocks interact with solar wind • How particles are accelerated at shocks • Additional observations needed • Some new observations coming on-line: coronal magnetic fields: COMP, Solar-C, additional lines-of-sight: STEREO • Coupled Modeling Efforts: e.g. CISM, MURI