Magnetic Reconnection as a Universal Plasma Process: Investigating Onset, Energy Release and Particle Acceleration

Principal subject of the research: Magnetic reconnection is a universal and fundamental plasma process that transfers energy and momentum between the magnetic field and the plasma itself. It occurs in a wide variety of circumstances - in planetary magnetospheres it is responsible for magnetic storms, in solar and stellar atmospheres it causes flares, and it is thought to be important in several astrophysical phenomena such as active galactic nuclei, pulsars, gamma ray bursts and magnetars. Understanding magnetic reconnection is also of practical importance since it is one of the key physical processes at the heart of space weather [Eastwood, Phil. Trans. R. Soc. A, in press, 2008]. As such, this proposal helps answer the STFC science questions 'How does the Sun affect the Earth?' and 'What are the origins and properties of the energetic particles reaching the Earth?' Key aims: Question 1: When and how is reconnection triggered? To ultimately predict explosive reconnection events such as solar and stellar flares, substorms and other astrophysical phenomena, it is necessary to understand how reconnection is triggered. I will use in-situ spacecraft observations of reconnection onset in the Earth's magnetotail to answer the key question of whether it is controlled mainly by local instabilities or external driving. Question 2: What controls the structure and dynamics of reconnection? Magnetic reconnection in planetary magnetospheres and stellar coronae is typically bursty whereas reconnection in the solar wind can persist over large scales for long times. I will compare new and archived observations of the magnetotail's distant X-line (an intermediate reconnection regime) to determine when and why reconnection is either bursty or quasi-steady. I will also use multi-point observations to test new theories of asymmetric reconnection. Question 3: The role of slow shocks in magnetic reconnection Slow shocks attached to the central diffusion region are a crucial part of fast reconnection. Recently, reconnection has been discovered in the solar wind, under conditions that are ideal for slow shock formation; our preliminary study has confirmed the existence of slow shocks. I will identify and examine the solar wind reconnection events in all available spacecraft observations and perform a statistical study to fully evaluate their occurrence rate and detailed experimental properties for the first time. Question 4: How does reconnection produce energetic particles? The generation of energetic particles by reconnection, especially electrons, is a major unsolved problem in plasma physics. Several different theories have been developed to explain how this occurs. By examining reconnection in the magnetotail, where energetic particles are generated during magnetic substorms, I will use magnetotail observations to determine statistically where particle acceleration occurs during reconnection and establish when different mechanisms tend to dominate. Where and how the research would be undertaken: The research will be undertaken at Imperial College because of its strength in observing space plasmas and because the success of the project depends on the opportunity to work with a wide range of researchers across disciplines. I will address these questions by analysing observations made in situ by a variety of spacecraft in different regions of space. Who else would be involved I will work in collaboration with plasma simulation groups at the University of Maryland and the University of Delaware who are engaged in separately funded studies of reconnection. Results from simulations will be used to inform the analysis of the data, and data will be used to develop theory. This research will also involve collaboration with mission specific teams, in particular Cluster, STEREO and THEMIS.