Challenges in Understanding 3D Magnetic Reconnection in Observations and Simulations Session organizers: Peter Wyper (NASA Goddard Space Flight Center/Oak Ridge Associated Universities) Silvina Guidoni (NASA Goddard Space Flight Center/The Catholic University of America) Daughton et al., 2014 Wyper & Pontin, 2014 Kinetic scales MHD scales

Although we have a relatively complete understanding of reconnection in 2D, there are still many unanswered questions when considering 3D systems. Questions: Modeling: 1.What effects/physics is not captured by 2D modeling? 2.How do we define reconnection rate in 3D? Is this even a useful quantity? Observations: 1.What reconnection related phenomena (such as solar flares and jets, substorms, and FTEs) cannot be satisfactorily explained by 2D theories? 2.Can these observations help to further constrain theory and modeling efforts of 3D reconnection?

Magnetic Reconnection in General, Three- Dimensional, Geometries Michael Hesse – Magnetic Reconnection in General, Three- Dimensional, Geometries Discussions What does the reconnection rate help us to understand? int{Epar} gives the rate of flux swept out by flare ribbons. Difficult to measure this in practice? Related to DC particle acceleration. But is this the main acceleration mechanism? Q gives the instantaneous ribbon positions and is related to regions of high electric current. Qslip is a way of telling what field has/will reconnect. Differentiates within the maze of high Q layers. Only needs field lines & photospheric flows. Open questions: how are these connected to energy release? Should we be interested in “fast” or “slow” reconnection? All agree that “fast” reconnection brought on by sufficiently localised and thin current layers. The fast/slow relation to each measure was less clear. Int{Epar}, Q, Qslip

David McKenzie – How can we recognize tearing mode signatures when they occur? In non-steady systems MHD-scale self consistently formed 2D current layers should break up and form these islands. It still remains unclear how this works in 3D. Observations of “blobs”. Blobs edge on easy to see, but from the side the ray structures show little rope/blob-like signatures. Why is this? We see very few of them. Presumably formed by coalescence of many small ones. But the big ones are well resolved – where are the little ones? We also don’t see much of this coalescence. We still don’t know if a blob is an island! We discussed the island- like qualities. They are denser than the background and follow the outflow direction. What determines their out of plane length? Likely related to guide field.

David McKenzie – How can we recognize tearing mode signatures when they occur? Link to flare particle acceleration models. Observable blobs were suggested to be less effective at accelerating particles due their their increased density and collisions. Should there than be a sea of small unresolved islands that are accelerating particles? Drifting Pulsating Structures (DPS) in radio emission suggest they may exist. Models seem to need a “leaky bottle” that is not yet properly incorporated. Need to trap particles enough to accelerate them, but then let them out. How are they behaving in the 3 rd direction? The topics sparked a lot of discussion – needed more time! Future sessions: observations in particular had lots of interest. Special thanks to Doga. Good job! Thank you to the organisers!