Coronal Mass Ejections without photospheric / chromospheric signatures

1. Coronal Mass Ejections without photospheric/chromospheric signatures • Session organizers: Alexei Pevtsov (NSO) and VasylYurchyshyn (BBSO) • Discussion leaders: Yuhong Fan (HAO) and Benjamin Lynch (UCB)

2. “Stealth” CMEs STEREO-A LASCO STEREO B Robbrecht et al, 2009 Ma et al, 2010 Pevtsov et al 2011

3. O. Panasenco

4. It is not “stealth” if I can see it! • Terminology (“stealth” or “without source region” or better (?) • “CMEs with hard-to-detect on-disk signatures”) – we do see some changes in low corona if we know what to look for; there is always a source region, if we know where to look at. Traditionally, we expect flares, filament eruption, coronal waves and dimmings associated with CMEs. “Stealth” CMEs do not have these attributes and so, are harder to detect on the disk. But once they erupt, we can see them well in higher corona. Why we should care? • A small but significant fraction of CMEs comes from unidentifiable sources (Jie Zhang, major geostorms DST < -100; 1998-2005, 10 out of 88 CMEs had no clear source on the disk).

5. What are “stealth” CMEs? • Do they represent a different class of CMEs (same/different magnetic topology)? • How common/rare they are? (specific phase of solar cycle)? • What makes them different from “regular” CMEs?

6. Properties of “stealth” CMEs • Are typically slow CMEs • Are more massive than “typical” CMEs • More than half of the “stealth CMEs” show coronagraph signatures indicative of the standard flux rope eruption scenario Jan 1 through Aug 31, 2009 – 1/3rd of CMEs are “stealth” Ma et al, 2010 B. Lynch

7. Courtesy of T. Berger (LMSAL)

8. “Stealth” CMEs originate from long high-corona flux ropes • (very stable, slow rising which allows: • filament material to drain off completely prior to the eruption) – O. Panasenco observed several cases when filament channels had emptied prior to “stealth” CMEs • Torus instability to develop (Y. Fan model). • To collect significant amount of mass from streamer belt (M. Kramar, A. Vourlidas) • “Stealth” CMEs originate from areas of weak magnetic field • (but in a weak field environment, the flux rope may form lower in atmosphere) • Some CMEs are “stealth” because of limitation of our observations (not looking at the right wavelength, insufficient column density to make on-disk features visible etc.

9. Why we do not see signature of eruption in low corona? • If the reconnection rules: • 1. if it is a weak field environment - much less energy is released • 2. bulk of energy goes to kinetic energy and very little - to particle acceleration • 3. because of the height of the erupting flux rope, the low corona signatures are minor (or distant). • - “stealth” CMEs do not require magnetic reconnection (Jie Zhang) A. Vourlidas no dimming dimming

10. How about forecasting of “stealth” CMEs? • “Stealth” CMEs are not driven from below (photosphere/ chromosphere) – potentially could make easier to forecast? • Flux rope is formed prior to eruption, current sheet is formed below the flux rope, and filament had drained off – could these properties be used for forecasting? • (are we looking in the right place?) shear/stress along the neutral line or helicity build up in the corona, – could any of thesebe useful for forecasting? • Limb observations seems to be essential if the flux rope has the right orientation (but limb observations of transequatorial “loops” may give a hint of “stealth” CME configuration).

11. What is next? • We need to develop forecasting tools for “stealth” CMEs based on their properties • Properties of “stealth” CMEs suggest that they could be easier to model than “regular” CMEs. • Case study? Potential candidates: 13 June 2010 (west limb), 12 June 2010 (east limb), 3 Mar 2011?