1. Current Sheets from WL and UV data: open questions Alessandro Bemporad INAF – Arcetri Astrophysical Observatory 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad

2. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad SUMMARY FIRST PART: introduction CS & CME flux rope model CS observations in WL: some open problems CS observations in UV: some open problems SECOND PART: our studies CS evolution CME early evolution Conclusions

3. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad FIRST PART: introduction

4. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS IN CME MODEL (from Lin et al., 2004, ApJ, 602, 422) CME flux rope models predict that, after the eruption of the flux rope (top) and the formation of the current sheet below the expanding bubble, magnetic reconnection, starting from the chromospheric level, relaxes the open configuration into a closed. Magnetic reconnection heats plasma (bottom) converting magnetic energy into kinetic and thermal energies. How do observations compare with this model?

5. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad WL OBSERVATIONS OF CSs (from Webb et al., 2003, JGR, 108, 1440) Post-CME CSs in white light images appear as thin radial persistent features which are: observable only in ~½ of CMEs brighten ~4 hours after the CME observed sometimes within multiple ray-like structures (which one is the CS?) ~ 6 x 10 4 km thick, constant with time ≥ 8 hours long These observations leave at present some open questions, i.e.: 1)Why CSs are not observable in all CMEs? (only orientation?) 2)Why CSs brighten late? (Density increase? Thickness increase?) 3)How can we explain the CSs long duration? (at present reproduced using an empirical density model) → time dependent theory of driven reconnection needed! SMM

6. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad UV OBSERVATIONS OF CSs Ciaravella et al., 2002, ApJ, 575, 1116 Ko et al., 2003, ApJ, 594, 1068 Post-CME CSs identified also in EUV coronal emission: Strong Fe XVIII emission → CS plasma at high temperatures (~ 5 ּ10 6 K) CS have long (~days) lifetime Elemental abundance fractionation similar to that of the ambient corona (FIP effect) → CS material brought in from its sides In the low corona rising post-CME loops are located at the lower end of the CS These observations leave at present some open questions, i.e.: 1)Why some CS are observed in UV, not in WL (Ciaravella et al. 2002)? 2)How can we explain such high T? (only magnetic reconnection?)

7. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad SECOND PART: our studies

8. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS EVOLUTION On November 2002 UVCS observed a post-CME coronal reconfiguration over ~2.3 days → ideal case for the study of the CS evolution UV images of the low corona show a post-CME rising loop system (in agreement with CME models); these bridge over the solar limb → CS ║ to the plane of the sky The UVCS slit FOV cuts across the CS; selected spectral ranges include lines from “low temperature” (10 5.9 K) and high temperature (10 6.7 K) ions. What is the observed CS line intensity evolution? (from Bemporad et al., 2006, ApJ, 638, 1110) LASCO/C2EIT Fe XII

9. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS EVOLUTION At CME latitudes a strong persistent Fe XVIII emission is observed during the whole observation interval Fe XVIII (T max =10 6.7 ) line intensity increases reaching its maximum about 21h after the CME initiation, then decreases, while Fe XV (T max =10 6.3 ) intensity increases continuously By a comparison of intensities of these two lines from different ionization states of the same element we infer the CS temperature evolution, while with a different technique we estimate the CS density.

10. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS EVOLUTION: T e and N e Results: 1.CS temperature > 8 ּ10 6 K immediately after the CME; then in the following days the CS plasma slowly cools down 2. CS density keeps approximately constant (~7 times larger than the ambient coronal density) In the computation we assumed a constant CS thickness (as observed in WL data).

11. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS EVOLUTION: PLASMA HEATING Plasma heating in the CS can be provided by ohmic heating, adiabatic compression and other processes (wave heating, shocks, etc…) Given density & temperature inside and outside the CS, we check whether the CS plasma is heated via adiabatic compression. In this case: → At a late stage of the event adiabatic compression can provide for the high CS temperatures, while at earlier times other processes should be invoked.

12. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS EVOLUTION: FURTHER QUESTIONS AND FUTURE WORK Which heating processes dominate the CS evolution? Is there any theoretical prediction on T e and N e evolution in CS? Which are the physical CS plasma conditions in its early stages? How long the CS thickness can be considered really constant? So far each CS has been observed in UV at a single altitude: are there data that allow us to check on the CS behaviour at different altitudes? How does density that we found from UV compare with CS densities derived from WL? Is it possible to provide physical CS parameters along it length using, e.g., UVCS, Mauna Loa, LASCO/C2/C3? FUTURE WORK: make a comparative UV – WL study of a CS! needed a model describing temporal evolution of CS physical parameters!

13. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad EARLY CME EVOLUTION In CS/CME flux rope models the high temperature plasma heated in the CS is injected in the outer shell of the CME bubble. At the same time, mainly because of adiabatic expansion, internal shells cool down. As a consequence a temperature decrease from the CME outer shell to the CME core (built up of “cool” chromospheric material) is expected. However we never observed in in situ data a high-low-high temperature variation when crossing an ICME…why? (geometry?)

14. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad EARLY CME EVOLUTION On January 2000 the early evolution of a slow CME has been observed both in UV and WL. This CME showed at this early stages the typical three-part structure → ideal case for the study of the CME temperatures across the bubble. Mauna Loa 19:23 – 19:32 EIT 19:25 – 18:36 Mauna Loa 19:23 EIT 19:25 (Difference images) (Normal images) Y point CME core UVCS FOV (from Bemporad et al., 2007, ApJ, in press)

15. LINE INTENSITY EVOLUTION N e peaks at the core, whileThe core moves faster than Ly  (  N e ) and OVI intensities the front, but Doppler dim- (  N e and N e 2 ) peak at the frontming it’s not enough We need a front → core T e increase SiXII intensity (  N e 2 ) does not increase at the core 10 6.40 10 6.45 10 6.40 10 6.30 10 6.40 10 6.45 10 6.40 10 6.30 T e higher than the Si XII emissivity peak (10 6.3 K) 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad

16. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CME TEMPERATURES From a comparative WL – UV data analysis we derived that across the CME bubble temperature maximizes at the CME core (10 6.45 K) and increases between 1.6 and 1.9 R  ; the front (at 10 6.30 K) may be heated by adiabatic compression of the ambient coronal plasma (at 10 6.15 K). This core behaviour is in agreement with laboratory experiments on free expansion of plasma spheromacs (conservation of magnetic helicity and dissipation of magnetic energy → plasma heating > adiabatic cooling) High in situ temperatures are really due only to CS?

17. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CONCLUSIONS & QUESTIONS We observed the typical three-part structure of a CME both in UV and WL data at early stages of the event Temperature increases from the outer shell to the CME core Which physical processes can account for the observed temperature behaviour? This CS is observed only in UV and not in WL. Why? Photospheric fields show this CME to originate in a quadrupolar configuration. CS formations are envisaged both in catastrophe and breakout models: which observable features may allow us to distinguish between these two models?

18. 1° ISSI Group Meeting October 23-27, 2006, Bern “CS from WL and UV data: open questions” A. Bemporad CS: SWEET & PARKER MODEL In a region where B reverses the Lorentz force j x B pushes toghether the oppositely field lines driving reconnection. Reconnection inflow opposes the CS resistive diffusion. A stationary state implies a balance between inflow and diffusion that gives v in =  /l. Large stresses in the vicinity of the null-point result in a magnetic catapult that ejects plasma along the CS with velocity ~ v A. (from Boyd & Sanderson, “The physics of plasmas”, 2003)