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Observational Evidence for Magnetic Reconnection in the Solar Corona

Observational Evidence for Magnetic Reconnection in the Solar Corona. Len Culhane Mullard Space Science Laboratory University College London. SUMMARY. Change in field connectivity – the ultimate indicator of reconnection

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Observational Evidence for Magnetic Reconnection in the Solar Corona

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  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

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