Observational Evidence for Magnetic Reconnection in the Solar Corona

1. Observational EvidenceforMagnetic Reconnectionin theSolar Corona Len Culhane Mullard Space Science Laboratory University College London

2. SUMMARY • Change in field connectivity – the ultimate indicator of reconnection • Demonstrate by observing one or both foot points of a magnetic loop connect to different points after e.g a flare or other eruption • Dilemma: • Observe magnetic reconfiguration on disc but have difficulty in relating it to the eruptive consequences • Observe eruptive consequences on limb but have no knowledge of short-term magnetic field evolution • Review will focus on: • Flares; outflow and inflow • Coronal Jets • Quiet Sun network explosive events • Large-scale coronal reconnection • Evolution of photospheric magnetic field on disc • an incomplete list! • Data from current space missions -Yohkoh, SOHO, TRACE, RHESSI, will be presented • Conclude with an assessment of future observational prospects Magnetic Reconnection - Sun and Magnetosphere

3. Large Flare Observed by Yohkoh Pallavicini et al., 1977 proposed a flare classification that identified flares in: - large diffuse loop systems → two-ribbon flares Different models required? - compact loop structures → compact flares • Major two-ribbon limb flare (21-Feb-92) suggests CSHKP reconnection scheme • Yohkoh SXT data (Tsuneta, 1996)show: • - Intensity - Temperature • - Emission Measure - Pressure • Features include: • - cusp formation • - high T ridges at the outer loop • boundary • - ridges reach loop footpoints • - dense core at the loop top • - cooling channel between the • high T ridges • - channel temperature in range • 10 MK to 6 MK Magnetic Reconnection - Sun and Magnetosphere

4. Inferred Magnetic Structure • Based on the SXT image and parameter values, Tsuneta deduced a magnetic • reconnection scenario within the CSHKP framework • Features of Tsuneta scheme include: • - X point; h ~ 15 x 104 km above loop top • - Cooling fast downflow channel • - tcool gives X point height • - Slow shocks heat plasma on outer • reconnected field lines • - Heat conducted to the chromosphere • fills soft X-ray loops by evaporation • - Reconnection triggered spontaneously • by localised anomalous resistivity? • - Forbes and Acton (1996) discussed • reconnected loop evolution – shrinkage • - Note also rise of X-point and of post-flare • loops with separation of footpoints • Smaller h and larger B would yield : • - greater downflow velocity • - fast shock loop-top heating Magnetic Reconnection - Sun and Magnetosphere

5. Reconnection - Compact Flare/Loop-top Hard X-ray Source Grey scale: SXT Contours White: 14-23 keV Black: 23-33 keV Contours Thick: 23-33 keV Thin: SXT • Compact flares were thought due to magnetic reconnection between emerging flux and pre-existing • field in the corona (Heyvaerts, Priest, Rust,1977) → this topology appears later in another context • Masuda et al.,1995 showed that the CSHKP scheme could also apply to compact flares • Discovery of a hot loop-top source, with a higher hard X-ray source (13-Jan-92), strongly suggests • X-point reconnection above the lower-lying reconnected loops • Features include: • - High outflow velocity with • fast shock heating and • particle acceleration • - Shorter distance from loop- • top to reconnection region • and stronger B fields • emphasise fast shock role • - Hard X-ray source region • enhances non-thermal • electron acceleration • - X-ray burst timing shows high • electron acceleration site • Yohkoh observations gave strong morphological support for • reconnection but no agreement on formation of field geometry • or triggering of eruption - more observational details needed Magnetic Reconnection - Sun and Magnetosphere

6. Plasmoid Ejection and Compact Loop Flares • Plasmoid or loop ejection and current sheet formation result from reconnection • Shibata et al.,1995, observed ejections for the 13-Jan-92 event and for seven other • compact flares - faint ejected features (A and B) are shown • A) is loop-like and B) is jet-like • C) may show a bright footpoint • Difference image shows the A feature • propagating outwards • White represents expanding feature • Outline curve represents dark trailing edge • Ejection velocity range is 50km/s < v < 400 km/s • v < vA - high density of current sheet or ejecta mass? • SXT measurements of v, Te and emission measure • are very difficult for faint features Magnetic Reconnection - Sun and Magnetosphere

7. Outflows/Downflows • Plasma outflows late in a LDE event (20-Jan-99) were observed by McKenzie and • Hudson (1999) with Yohkoh SXT - image sequence shows motion of two dark voids • “Voids” are X-ray emitting – Te ~ 9 MK, ne ~ 109 cm-3, and • move downwards to the top of the flare loop arcade • Bright “rays” may be associated with arcade loop-top cusps • Data offer first evidence for high-speed downflows – v ~ 50 • – 500 km/s, above flare loops (see also Mckenzie, 2000) • Note that the “void” flows persist late in the decay phase • Reconnection continues long • after initial eruption? Magnetic Reconnection - Sun and Magnetosphere

8. X4.8 Flare 23-JUL-02 S12o, E72o 17 GHz 50 – 100 keV Downflows in the Flare Impulsive Phase • Asai et al., 2004have observed outflows above flare loops for • the23-Jul-02 X4.8 event • TRACE Fe XII flare images • show downflows to the • post-flare loops • – v ~ 100 – 250 km/s • Downflows • coincide with hard X-ray • and microwave bursts • have similar properties to • plasmoids? • - are correlated with • reconnection episodes? Magnetic Reconnection - Sun and Magnetosphere

9. SUMER Slit Detector artifact Image Spectra Downflow SUMER & TRACE - Spectroscopic Observations of Flare Arcade Downflows (Innes et al., 2003,a,b) • The TRACE imager and the SOHO SUMER spectrometer observed a flare • (X1.5/two ribbon; 21-Apr-02), on the west limb • Downflows were seen by TRACE (195 Å) across the whole arcade region • while spectra were obtained in the fixed SUMER slit • Analysis of emission line (Fe XII, Fe XXI) • and continuum spectra suggests: • - dark downflows or voids due to high Te low ne • plasma • - continuum asymetries near the Fe XXI • emission line indicate plasma flows at • v ~ 1000 km/s above the main arcade • If downflows are reconnected flux tubes, they • are probably driven by a fast shock • Tentative analysis with encouraging features • for the quantitative description of reconnection Magnetic Reconnection - Sun and Magnetosphere

10. Yohkoh SXT and SOHO EIT – Reconnection Inflow • Yokoyama et al., 2001 observed inflow in EIT images of a limb flare (18-Mar-99) • EUV “void” shows SXT emission • at T ~ 4 MK: • plasmoid ejection • X-point formation • movement of field lines towards X • point • Chen et al., 2004 question inflow velocity but support a reconnection scenario • High cadence imaging and spectroscopic observations are required to convincingly • identify both inflow and outflow Magnetic Reconnection - Sun and Magnetosphere

11. X-ray Jets and Emerging Magnetic Flux Yohkoh SXT 12-Nov-91 • Heyvaerts, Priest and Rust, 1977, developed an emerging flux model for flares • Initially applied to compact flares, current variants now have more relevance • for jets and coronal bright points • Jets were first observed by Yohkoh SXT - see e.g. Shibata, 1999, for a review • Emerging flux interacts with pre-existing field: • - a) Horizontal • - b) Oblique • Yohkoh observation by • Shibata et al.,1992 showed • a plasma jet expanding at • > 100 km/s to l~ 2.105 km • MHD reconnection simulations by Shibata et al., closely match the observed • jet properties Magnetic Reconnection - Sun and Magnetosphere

12. Quiet Sun Reconnection Events • Small-scale eruptive events – microflares, explosive events, occur continually at the • Chromospheric network cell boundaries • Innes et al., (1997) observed • bi-directional flows from • explosive events using small • scans, by SUMER on SOHO, • covering an area 9“ x 120“ at • Sun centre • Successive red and blue line • shifts indicate bi-directional • flows • Events last 2 – 5 min – “blue” flows are sustained for • greater distances than downward “red” flows • Photospheric motions draw oppositely directed field • lines towards the network cell boundaries • Reconnection results with vflow ~ vA≤ 150 km/s Magnetic Reconnection - Sun and Magnetosphere

13. Reconnection of Large-scale Coronal Fields • Loops that form over very large distances – and are often transequatorial, in the corona • suggest that reconnection is occurring on a large scale • Yohkoh image shows connections • between mature active regions with • no significant emergence of new flux • between the two ARs (Pevtsov, 2000) • For transequatorial loops in particular, • there have been no observations of • such loops emerging as pre-existing • flux tubes • However flaring has been observed in • such loops (Harra et al., 2003) – many • of their properties are similar to those • of single AR loops Magnetic Reconnection - Sun and Magnetosphere

14. Reconnection of Coronal Interconnecting Loops • In March, 1992, Yohkoh SXT (Tsuneta, 1996) observed a developing system of loops that • connected ARs on opposite sides of the equator • Loops and an X-shaped structure • are in panel (a) • Temperature distribution in panel (b): • → 2 – 3 MK for N/S; upstream • → 4 – 7 MK for E/W; downstream • suggests reconnection • Corresponding sunspot groups in • panel (c) • Magnetic field configuration in • panel (d) • Tsuneta believes this situation is an • example of large-scale reconnection • Upstream and downstream • temperature difference allows a • magnetic field estimate of ~ 20 G • for an inflow speed of ~ 10 - 20 Km/s Magnetic Reconnection - Sun and Magnetosphere

15. Magnetic Breakout Model • Developed by Antiochos et al. 1999 • and Aulanier et al., 2000 • Model exemplifies an approach • where surface observables may • be related to evolution in magnetic • topology Magnetic Reconnection - Sun and Magnetosphere

16. Connectivity Change - Evolution of AR0540 White: +ve Black: -ve • Work underway on a geo-effective flare and CME (Harra et al.) • MDI magnetogram movie covers 13-Jan-04 23:59 to 23-Jan-04 19:11 UT – flares observed on 20-Jan-04 • Notable change - movement of emerging negative polarity in the positive region, negative polarity eventually removed. Magnetic Reconnection - Sun and Magnetosphere

17. Flare Ribbons on 20th Jan • TRACE C IV images show 3 “ribbon” components to this flare: 1→ 07:29UT flare at sheared neutral line (spiral flare ribbon) 2→ ~ 07:39 UT (TRACE misses start) quadra-polar flare 3→ 08:00UT long duration predominantly two ribbon flare Magnetic Reconnection - Sun and Magnetosphere

18. Individual TRACE and MDI Images Pre-flare: TRACE C IV and MDI magnetogram images Flare 1 start: Sigmoid formation and eruption - westernmost negative footpoint lies in the middle of flare 2 region. Field opening may have de-stabilized field lines above flare 2 site. Flare 2: The pre-flare connections involved in flare 2 (white) and flare 3 connections (orange) are indicated in the lower left panel. Flare 3 – not shown: Similar quadrapolar configuration, almost identical footpoints - but flares 2 and 3 occurred along different inversion line segments (red) + ve footpoints - ve footpoints Pre-flare connections New connections Magnetic Reconnection - Sun and Magnetosphere

19. Future Prospects for Reconnection Studies • Important new observations will come from Solar-B (2006), STEREO (2006) • and Solar Dynamics Observatory (SDO; 2008) • Solar-B will have: • Vector magnetographic and photospheric imaging capability with 0.25 arc • sec resolution for Active Region-sized fields of view • X-ray imaging and EUV spectroscopic capability with 1 – 2 arc sec • resolution and velocity measurement to ± 5 -10 km/s • The two STEREO spacecraft will provide 3-D imaging of particular relevance for studies of large coronal structures • SDO will include: • Full-sun imaging and vector magnetograph data with 1 arc sec resolution and • 40s time cadence • Full-sun TRACE-style imaging (1 arc sec) in 10 EUV bands with10 s cadence • Solar-B and SDO operating together will enable: • Tracking of faint features e.g. inflows, outflows, with 10s time resolution • Full-sun magnetograms (1 arc sec/50s) and AR magnetograms (0.25 arc sec/60s) • Velocity measurements to ~ ± 5 km/s for selected features at ~ 30 - 60s cadence Magnetic Reconnection - Sun and Magnetosphere

20. Solar-BEISExpected Accuracy of Velocity Flare line Bright AR line Photons (11 area)-1 sec-1 Photons (11 area)-1 (10sec)-1 Doppler velocity Line width Number of detected photons Magnetic Reconnection - Sun and Magnetosphere

21. CONCLUSIONS • Observations with the Yohkoh SXT and HXT strongly support the view that magnetic reconnection is a widespread solar phenomenon • Understanding is being further enhanced by observations with SOHO instruments, TRACE EUV imaging and more recently with RHESSI • Quantitative observational data for detailed comparison with reconnection models are difficult to obtain - a start has been made • Specific observational predictions from 3-D reconnection models are required for comparison with data from future missions • Important for 3-D models to offer testable predictions – particularly when observables differ signifcantly from those related to 2-D or 2.5-D models • Perhaps emphasis needs to be placed on different aspects of models • Helicity for large-scale coronal behaviour • Energy content for smaller-scale phenomena • Solar-B (2006), STEREO (2006), SDO (2008) will provide much relevant data but will need careful planning of multi-instrument observations Magnetic Reconnection - Sun and Magnetosphere

22. END OF TALK Magnetic Reconnection - Sun and Magnetosphere