Fri., February 15, 2008

BU

Comparative Aspects of Reconnection

Welcome

• Purpose: – To facilitate interaction among colleagues in

space science in the New England Area (UNH, SAO/CfA, BU, MIT, Hanscom/AFRL, Haystack, Dartmouth )

– To leverage these interactions for initiating new, cross-disciplinary and far-reaching projects

• Meetings:– quarterly meetings

• Introductions (Affiliation/Interest)

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Questions• To what extent can reconnection account for heating otherwise

attributed to turbulent dissipation in the solar wind?• To what extent is the rate of reconnection controlled by macroscopic

boundary conditions as opposed to kinetic-scale processes?• To what extent do observations of reconnection in the solar wind bear

upon the issue of coronal heating?• To what extent does quasi-steady reconnection occur in the

magnetosphere and the solar atmosphere?• Does reconnection necessarily produce particle (as opposed to plasma)

acceleration? If not, why not• In what essential ways is guide field reconnection fundamentally

different from anti-parallel reconnection?• What causes extended X-lines?• What distinguishes "explosive" reconnection?• Why does reconnection prefer low plasma beta?

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Discussion

• To what extent can reconnection account for heating otherwise attributed to turbulent dissipation in the solar wind?

• To what extent do observations of reconnection in the solar wind bear upon the issue of coronal heating?– How do observed events contribute to a heting rate?– We are not seeing all the events?– The finer the data, the more events are found (but are they

real reconnection events)– What heating rate do we need? (couple times 10^3 J/(kg s)– High speed wind heated more than slow wind (Vasquez

paper , 2007)– DO all the particles find their way to the current sheet

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Discussion

• Run S/C through simulations to see how they compare to observations

• How much of the heating occurs at unresolvable scales?

• We are within an order magnitude of the observational scales we need to resolve the recon. Structures– Rapid plasma measurements on the oder

of the gyrofrequency (convected) .. 10 Hz?

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Discussion• Relation of outward/inward propagating modes• Fast wind mostly outward propagating .. This may partially

explain why we see fewer rec. events in fast wind• Braiding• Most of the discontinuities/current sheets are not

reconnecting– Is it possible we see current sheets, but not the exhaust– What is the length of the diffusion region

• What is special about the reconnection events we observe– Beta is important (beta typically low in reconnection events)

• What do we know about the rate at which reconnection proceeds from the Sun– Is fast wind just less evolved

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Discussion

• Tpar larger than Tper in exhaust• Slowing of the jet gives rise to heating• Evidence of wave-particle interaction

(resonance .. Higher Tper) in the corona condradicts heating from reconnection

• Rate of holes rises exponentially below beta = 1 (firehose instability may play a limiting role for beta > 1)

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Discussion

• How does reconnection in the solar wind bear upon reconnection in the corona?– Steps: current sheets occur in closed fields

• 5-10 Rs .. How important are current sheets there

– Borovsky structures in solar wind appear to be related to source structure (granulation/supergranulation) .. Or is this in situ development .. Fossil coronal structure

– Do we see source structures from the Sun– How can we connect them?

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Discussion

• Is the rate of reconnection set by the micro or macrophysics? (which is the dog and which is the tail?)– What determines the explosiveness of the

event?– What controls the intermitence?– Length of the current sheet is a telling

factor– Both macro and microphysics contribute to

setting the timescale (rate) of reconnection

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Discussion

• CME - plasmoid comparison demonstrates that the onset scenario can operate in different regimes– Comparison of length scales (ion intertial, plasma

and gyroradii are very different)– The similarity of phenomena (plasmoids,

Supraarchade downflows, bursty bulk flows) suggests that the driving conditions are fundamental to the overall response of the conditions

– Tearing mode instability (enhanced) may be key

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Discussion

• Thermal Electrons do not show evidence of heating in the diffusion region [Egdal]. This is consistent with Jack’s observations

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Discussion

• Is reconnection always bursty .. Or can it be steady? [Forbes]– Gosling has shown us a lot of steady reconnection

events– Flux Transfer Events non-steady [1 every 8 minutes]– Bursty Bulk Flows non-steady– Is the burstyness controlled by variability in the

driving or the microphysics?– Takes an Aflven wave about 10 hours to move

along an observed X-line [6 Rs .. Va ~ 50 km/s]– Is reconnection inherently patchy? Global

reconnection rate sum over patches• Jack believes the answer is no

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Looking Forward

• Multi-spacecraft studies• What are the possibilities for imaging?• Can we make more of our existing

observations– Theory and observations .. Bringing these

together more tightly

• Instrumentation capable of higher resolution measurements .. Electrons, ions, composition

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Panel

• Chuck Smith• Terry Forbes• Jimmy Raeder• John Dorelli• Aad Van Ballegooijen

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Today’s Agenda

– 11-11:15 congregate– 11:15-12:15 Reconnection Observed:

Gosling– 12:15-1:30 Lunch– 1:30-3 PM Panel Discussion – 3- 5 PM Informal Discussion/Meeting

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Special Section