Observational Evidence for Magnetic Reconnection in the Solar Corona Len Culhane Mullard Space Science Laboratory University College London.

Observational Evidencefor

Magnetic Reconnectionin the

Solar Corona

Len Culhane

Mullard Space Science Laboratory

University College London

Tuesday, 10th August 2004

Magnetic Reconnection - Sun and Magnetosphere

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

Large Flare Observed by Yohkoh

• 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

Pallavicini et al., 1977 proposed a flare classification that identified flares in:

Different models required? - compact loop structures → compact flares

- large diffuse loop systems → two-ribbon flares

Inferred Magnetic Structure• Based on the SXT image and parameter values, Tsuneta deduced a magnetic reconnection scenario within the CSHKP framework

• Smaller h and larger B would yield : - greater downflow velocity - fast shock loop-top heating

• Features of Tsuneta scheme include: - X point; h ~ 15 x 104 km above loop top - Cooling fast downflow channel - cool 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

Reconnection - Compact Flare/Loop-top Hard X-ray Source

• 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

• Yohkoh observations gave strong morphological support for reconnection but no agreement on formation of field geometry or triggering of eruption - more observational details needed

• 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

• 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

Grey scale: SXT

ContoursWhite: 14-23 keVBlack: 23-33 keV

ContoursThick: 23-33 keV Thin: SXT

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

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?

Downflows in the Flare Impulsive Phase

X4.8 Flare23-JUL-02S12o, E72o 17 GHz

50 – 100 keV

• Asai et al., 2004 have observed outflows above flare loops for the 23-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?

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

Image Spectra Downflow

SUMER Slit

Detectorartifact

• 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

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

X-ray Jets and Emerging Magnetic Flux

• Jets were first observed by Yohkoh SXT - see e.g. Shibata, 1999, for a review

• 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

Yohkoh SXT12-Nov-91

• 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

• Emerging flux interacts with pre-existing field: - a) Horizontal - b) Oblique

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

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

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 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

Connectivity Change - Evolution of AR0540

• 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.

White: +veBlack: -ve

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

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

New connections

Pre-flare connections

Individual TRACE and MDI ImagesPre-flare:TRACE C IV and MDI magnetogram images

Flare 1 start:Sigmoid formation and eruption - westernmost negative footpointlies 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 involvedin flare 2 (white) and flare 3connections (orange) are indicated in the lower left panel.

Future Prospects for Reconnection Studies

• The two STEREO spacecraft will provide 3-D imaging of particular relevance for studies of large coronal structures

• 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

• 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

Solar-B EIS Expected Accuracy of Velocity

Doppler velocity

Line width

Bright AR line Flare line

Photons (11 area)-1 sec-1

Photons (11 area)-1 (10sec)-1

Number of detected photons

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

END OF TALK

Observational Evidence for Magnetic Reconnection in the Solar Corona Len Culhane Mullard Space Science Laboratory University College London.

magnetic loop

hot loop

magnetic reconnection

reconnection region

evaporation reconnection

source compact flares

outer loop boundary

magnetic reconfiguration

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