Sept. 12, 2006

Relationship Between Particle Acceleration and Magnetic

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:– Monthly meetings (first wed each month)– Next Meeting: SAO/CfA

• Introductions (Affiliation/Interest)

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Agenda• 10:00-10:10 AM -- Congregate in room 500. 

• 10:10-10:25 AM -- Introducttion

• 10:25 - noon -- Science Discussion (1 - 2 slide discussion)

• Noon - 1 PM -- Lunch will be served (Pizza, Salad, Sandwiches) Over lunch we will continue discussion

• 1 - ?? PM - Continued Discussion?

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Summary from Meeting 3 (p. 1)

• Magnetic Reconnection plays a critical role in the evolution of large- scale magnetic topologies, the rapid heating of plasmas and possibly the acceleration of high energy particles. The microphysics of reconnection is certainly complex and remains an area of active research. Despite this complexity, the "Axford Conjecture" puts forth a relatively simple condition that reconnection takes place at an average rate determined by external boundary conditions. In other words, the microphysics may adjust to the macrophysical constraints imposed on the system. If so, the quantitative effects of reconnection may be relatively straightforward to predict in diverse astrophysical environments. Comparing the effects of reconnection in disparate plasmas may provides a means to test the Axford Conjecture. A good example is found in the plasmoids released in the magnetotail during substorms. Is this phenomenon a direct parallel to coronal mass ejections, in which magnetic buoyancy plays the central in the release of a disturbance (plasmoid or CME)? If so, this comparative example would provide support for the Axford conjecture.

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Summary Meeting 3 (p. 2)

• Magnetic reconnection also appears to be intermittent. Is this a result of the external boundary conditions? In the case of CMEs, when does the system become unstable, and what makes it erupt. This same question may be asked of plasmoids in the magnetotail. Intermittency also appears in the laboratory experiments that achieve magnetic reconnection. At first glance, intermittency seems to be a result of the creation of thin current sheets, suggesting the release of energy of very small spatial and temporal scales. In this respect, it may be natural that the dissipation of thin current sheets channels significant quantities of energy into a minority of plasma particles that participate in the dissipation of thin current sheet.

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Previous Discussions

• Is the Axford conjecture correct?• Micro-physics vs. a driven system• 3-Dimensionality is critical• Examples:• Substorms - Convective Transport• Flares. Microflares, Nanoflares• CMEs, plasmoids in the downtail• Lifetime of the flare ribbons, relaxation process• Sequence of events in substorms (Cluster)• Find the method to distinguish paradigms (e.g., test the Axford

conjecture)• Intermittency of Reconnection• Observed in the laboratory• Non-linear microphysics - energy buildup, release 6

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Microphysics

• Reconnection requires plasma evacuation on small scales– electron spatial scales (gyro-radius) much

smaller– association with Whistler modes between

proton and electron kinetic scales– electrons the last to leave the sites of

reconnection– preferential electron acceleration?

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Acceleration within magnetic islands

• Electron and ion heating within magnetic islands

• Does not seem to be associated with acceleration cavities

Drake, 2006

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Electron Dynamics in magnetic islands

• Electrons follow field lines and drift outwards due to E× B drift– Eventually exit the magnetic island

• Gain energy during each reflection from contracting island– Increase in the parallel velocity

• -Electrons become demagnetized as they approach the x line– - Weak in plane field and sharp directional change– Scattering from parallel to perpendicular velocity

• Sudden increase in Larmorradius• ?Isotropic distribution consistent with observations

Drake, 2006

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Conclusions - Drake, 2006

• Acceleration of high energy electrons during reconnection may be controlled by a Fermi process within contracting magnetic islands

• Reconnection in systems with a guide field involves the interaction of many islands over a volume

• Remains a hypothesis based on our 2-D understanding

• Averaging over these islands leads to a kinetic equation describing the production of energetic electrons that has similarities to diffusive particle acceleration in shocks

• Power law distributions of energetic electrons• Energy going into electrons is linked to the magnetic energy released• Feedback on reconnection must be included• Spectral distribution depends strongly on the initial electron Low leads to hard spectra• High suppresses island contraction and electron acceleration 1

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Ion Beam Hypothesis

• We propose a process in which initially the ions are heated and also provide the free energy for electron heating and tail formation

• [Krauss-Varban & Welsch, 2006].

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Ion Beam Hypothesis (Krauss-Varban)

• Low beta kinetic reconnection leads to:

• bulk heating of the ions (scaling: βpi-1, m)

• bi-directional ion beams• bi-directional fast/magnetosonic waves• energetic ion tails• presumably, efficient electron heating and

acceleration due to transit-time damping (e.g., Lee & Völk, 1975, Miller et al., 1996)

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Questions• Is Axford conjecture correct

– what are links between large-scale and micro-scale dynamics?

• Are electrons preferentially accelerated at reconnection sites?

• What controls Intermittency?• Wave energy (Ion Beams, Poynting

Flux) vs more direct energy release?

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• http://www.lesia.obspm.fr/~aulanier/29_2006_Slip.Running.pdf#search=%22%20site%3Awww.lesia.obspm.fr%20pariat%20demoulin%203D%20reconnection%22

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