Presentation on theme: "High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is operated by the University."— Presentation transcript:
High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation. An Equal Opportunity/Affirmative Action Employer. The Eruption and Propagation of Geoeffective Coronal Mass Ejections (CMEs) Joan Burkepile National Center for Atmospheric Research ASP Space Weather Colloquium June 7, 2005
Outline Joan BurkepileJune 7, 2005 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
Properties of the Corona Joan BurkepileJune 7, 2005 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 + -(GM /r 2 ) = u 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.
Properties of the Corona Joan BurkepileJune 7, 2005 Values of some Basic Properties: Avg. B ~10 Gauss Density ~ 10 8 particles / cm 3 Temp = 1 to 2 million O K Scale ht = ~ 0.1 Rsun ~ 7 x 10 4 km V sound = ( p th / ) 1/2 ( n e k e / ) 1/2 ~175 km/sec V alfven = B/( ) 1/2 ~500-1000 km/sec Escape velocity at surface = 618 km/sec Highly conducting (electrically, thermally) Low = ratio of thermal plasma pressure /magnetic pressure n e k e in low corona
EUV and X-ray Corona EIT 195 Angstroms Aug 16, 1999 Yohkoh Soft X-rays Aug 16, 1999 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 2 )
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.
New emerging magnetic flux in the atmosphere interacts with existing magnetic structures Magnetic Interactions alters the state of the field alters and creates currents creates stresses Non-Linear System Solar Activity is a response to stresses in the corona
What is a CME? Joan BurkepileJune 7, 2005 A CME is a ‘sudden’ expulsion of magnetized plasma into the solar wind from regions initially magnetically closed.
CME Observations Joan BurkepileJune 7, 2005 Form in Low Corona (below ~2 Rsun) CME Rates: Activity minimum: avg. ~0.4 per day Activity maximum: avg. ~ 4 per day LASCO C2 Feb 18, 2003 CME Latitude Distribution
CME Observations Joan BurkepileJune 7, 2005 Avg. Width 45 o -50 o (SMM Mission) 72 o (LASCO Mission) Avg. Speed 400 km/sec (SMM projection corrected) Avg. Acceleration ~500 m/s 2 (peaks below 3 Rsun) <10 m/s 2 (3-30 Rsun) Avg. Mass ~5 x 10 15 grams (from below ~2. Rsun) Energy (Kin + Pot) 10 31 to 10 32 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
Relation of CMEs to Other Forms of Solar Activity - Prominences Joan BurkepileJune 7, 2005 At least ~75% of CMEs associated with active or erupting prominences which form over magnetic polarity inversion lines
Relation to Other Forms of Solar Activity - Flares Joan BurkepileJune 7, 2005 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
Relation to Other Forms of Solar Activity - Flares Joan BurkepileJune 7, 2005 Many CMEs begin before or near the onset time of the flare. Does the flare ‘drive’ the CME? Observations suggest this is not the case.
Interpretation Joan BurkepileJune 7, 2005 CME – Closed field region becomes magnetically open. Hundhausen, in Cosmic Winds, Jokipii, Sonett and Giampapa (eds.), 1997
Various forms of solar activity do not form in isolation but in concert as a result of changes in magnetic field Joan BurkepileJune 7, 2005 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
Physical Causes of CMEs Joan BurkepileJune 7, 2005 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
Physical Causes of CMEs Joan BurkepileJune 7, 2005 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
CMEs in the Solar Wind (ICMEs) Joan BurkepileJune 7, 2005 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 Fast CMEs will drive shocks energetic particles From Reames et al., Astrophys. J., 466, 473, 1996
CMEs in the Solar Wind (ICMEs) Joan BurkepileJune 7, 2005 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 strong magnetic fields Temperature enhancements and depressions
Joan BurkepileJune 7, 2005 Geoeffective Condition – Strong southward B field Red line at B z = 0 N/S Component of solar wind B-field Geomagnetic Indices ap and DsT DsT < -100 ap > 100 Begin sustained southward B
Geoeffective Conditions What produces Bz? CMEs may contain strong southward fields or generate them via CME-driven shocks, which deflect and compress ambient solar wind flow
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)
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
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