FG: Magnetic Reconnection in the Magnetosphere

Abstract Magnetic reconnection underlies the dynamics in many aspects of magnetospheric physics and is critical to the development of a predictive understanding of space weather phenomena. It importance spans from the dayside where solar wind-magnetospheric coupling occurs to the nightside where storm and substorm phenomena occur. Many existing Focus Groups treat reconnection in the context of a particular application, but a Focus Group specifically on reconnection is being proposed to develop an understanding that could not be attained from studies on particular applications. Broadly, the proposed Focus Group will address kinetic physics of reconnection and how it couples to macroscales, reconnection at the dayside and its relation to solar wind-magnetospheric coupling, three-dimensional reconnection, reconnection onset and transient behavior and implications for transport, and particle acceleration in reconnection. The proposed Focus Group is timely as simulations have matured to the point where they can offer realistic comparisons to observations, and the existing THEMIS/ARTEMIS and upcoming MMS missions include reconnection physics as a key feature of their goals. Reconnection is by nature cross-disciplinary and offers many opportunities for interaction with existing Focus Groups. The proposed reconnection Focus Group is for five years, and will produce deliverables of import for many aspects of GEM goals.

Topic Description

During magnetic reconnection, a change in magnetic topology facilitates the conversion of magnetic energy to kinetic and thermal energy and allows mixing of plasmas on either side of current sheets. It is the driver of many dynamical behaviors in the magnetosphere that are of crucial importance to space weather. At the dayside magnetopause, reconnection plays a critical role in solar wind-magnetospheric coupling and recent evidence suggests it may even control the coupling efficiency. This process is the driver of magnetospheric convection, evolution of the radiation belts, and loading of energy in the magnetotail. The release of this energy in geomagnetic storms and substorms is a result of reconnection in the magnetotail. Reconnection also produces energetic particles that can damage satellites. At present, magnetic reconnection is studied separately under the auspices of particular applications within separate GEM Focus Groups. The proposed Focus Group will address the physics of reconnection as a distinct physical process which is important for magnetospheric applications in ways that cannot be addressed in applied settings. There is a long history of success in GEM on reconnection (including the highly-cited GEM Reconnection Challenge). The proposed Focus Group will foster interaction between observations, simulations, and models. The broad goals of the Reconnection Focus Group are to contribute to the understanding of (1) the kinetic microphysics of reconnection and how it couples to the magnetosphere at macroscales, (2) how and where dayside reconnection occurs and how it contributes to solar wind-magnetospheric coupling including the role of flux transfer events, (3) how three-dimensional reconnection proceeds, especially at the dayside, (4) the onset of reconnection and the physics of transient reconnection events and their relation to bursty bulk flows, dipolarization fronts, and entropy bubbles, and (5) how reconnection produces energetic particles. Recent advances in fluid and kinetic simulations both locally at reconnection sites and globally containing the whole magnetosphere have led to a point where realistic numerical models can be compared directly with satellite observations. The Focus Group will benefit from a wealth of observational data from existing satellites, as well as new highly resolved multi-point observational data from the THEMIS/ARTEMIS mission and the upcoming Magnetospheric MultiScale (MMS) mission. Strong ties with existing Focus Groups studying applications of reconnection will be made.

Timeliness of the Focus Group

A GEM Focus Group on magnetic reconnection is particularly timely. NASAs Magnetospheric MultiScale (MMS) Mission, with a planned launch date of October of 2014, "will use Earth's magnetosphere as a laboratory to study magnetic reconnection" (http:// mms.gsfc.nasa.gov/about_mms.html), focussing on the kinetic-scale microphysics. This requires spacecraft to be closer together than previous missions and will provide unprecedented observational resolution. The three-probe THEMIS constellation will also be reconfigured to form a larger scale tetrahedron with MMS at one node to provide the macroscopic context for the microscopic measurements by MMS. In addition, the ARTEMIS satellites repurposed from the THEMIS mission have recently moved into a lunar orbit and will provide information on the mid-tail region where reconnection is expected to occur and serve as a solar wind monitor on the dayside. Therefore, near-future in-situ observations will provide measurements at a wide scale range from the electron kinetic scale to the global scale. In the run-up to launch of MMS, a GEM Focus Group on reconnection emphasizing numerical and observational studies will play a central role in preparing for the mission by investigating what signals should be expected and in what locations of the magnetosphere. After the launch of MMS, the GEM Focus Group will be an avenue for analyzing and interpreting the data with comparisons to simulations and for putting the newly obtained knowledge in context of larger-scale magnetospheric behavior.

Relation to Existing GEM Focus Groups

As magnetic reconnection underlies dynamics in every GEM Research Area, this Focus Group has enormous potential for interaction with existing Focus Groups. It is the goal of this Focus Group to both support existing efforts in other Focus Groups and to drive new avenues of research through discovery achieved in this Focus Group. A non-exhaustive discussion follows for ways in which a reconnection Focus Group is synergistic with existing Focus Groups. For each example listed, the Co-Chairs of the existing Focus Group have been contacted and agree that interdisciplinary activities with a reconnection Focus Group would be desirable.

1. Plasmasphere-Magnetosphere Interactions (2008-13) - The PMI Focus Group is an

example where previous incarnations of reconnection studies at GEM have successfully had interdisciplinary activities. The previous reconnection focus group produced studies on asymmetric reconnection and how it may occur at the dayside magnetosphere. Recent studies showed that solar wind-magnetospheric coupling is less efficient when high density plasmaspheric drainage plumes reach the dayside reconnection site. This is consistent with reconnection becoming less efficient when asymmetric reconnection occurs with high density plasma. A presentation on this was given in a PMI session. Though the PMI Focus Group will end soon, it is expected that the previous interdisciplinary activities will continue.

1. Substorm Expansion Onset: The First 10 Minutes (2008-13) - This is another

example where there has been cross-participation in the past. The physics of dipolarization fronts and how they can be used to understand reconnection onset in substorms is common to both groups, as it was recently determined that they occur at the onset of reconnection. Kinetic simulations have been very useful to understand their properties.

1. Magnetosheath (2010-14) - The magnetosheath is strongly linked to dayside

reconnection physics. Many issues of the structure of the magnetosheath are related to how reconnection proceeds, such as the appearance or lack of flux pile-up at the magnetopause and the propensity for flux transfer events. Understanding how kinetic effects impact dayside reconnection and the march to global kinetic modeling, and where reconnection at the dayside occurs as a function of solar wind conditions, are both synergistic topics.

1. Metrics and Validation (2011-15) - This group addresses challenges of model-data

comparisons and how to assess them. In the past, a barrier to performing careful metrics and validation studies of magnetic reconnection has been a lack of sufficient observations, but the MMS mission is expected to change this. Therefore, a potential avenue for collaboration with the Metrics and Validation Focus Group is comparisons of kinetic modeling and observational data from MMS.

1. Tail-Inner Magnetosphere Interactions (2012-16) - This Focus Group addresses

modes of transport that cause heating in the magnetotail and inner magnetosphere. It is thought that transient events such as bursty bulk flows, entropy bubbles, and dipolarization fronts play a potentially important role. As these events are associated with magnetotail reconnection, there is strong potential for cross-disciplinary interactions with this Focus Group.

Goals and Deliverables

The goals of the Reconnection Focus Group are to understand aspects of magnetospheric reconnection with an eye to applications in other Focus Groups. This list is not exhaustive, but it has been compiled using input from the GEM community.

1. What is the physics of reconnection at the kinetic scale and how does it couple to the

magnetosphere at macro-scales? Particular topics include the role of the extended electron diffusion region, pressure anisotropies, normal magnetic fields for magnetotail applications, asymmetries in density and magnetic field, and whether or how the microphysics of reconnection can be incorporated into fluid models in geospace general circulation models (GGCMs). Examples of deliverables for simulation and modeling work include answers to the following questions - What sets the scale of the extended electron diffusion region and what is its effect on reconnection? (This will be important both before and after the launch of MMS for locating and analyzing reconnection events.) How do asymmetries effect the kinetic signatures of reconnection? Under what conditions do pressure anisotropies arise? Observational deliverables include THEMIS/ARTEMIS and MMS results, which will be compared to numerical simulations.

1. How does reconnection proceed at the dayside and how does it contribute to solar

wind-magnetospheric coupling? Topics include the role of asymmetries and shear flow in setting the efficiency of reconnection and the role of flux transfer events. Deliverables include quantitative predictions of dayside reconnection efficiency as a function of solar wind conditions that can be fed into solar wind-magnetospheric coupling functions as in the Borovsky coupling function and expected properties of flux transfer events in kinetic simulations and observations.

1. How does three-dimensional reconnection proceed, especially at the dayside?

Numerical deliverables include answering these questions - What is the location of reconnection on the dayside and its efficiency as a function of solar wind conditions? What differences are there between in 2D and fully 3D reconnection? Observational deliverables include signatures of the dissipation region in fully 3D settings and their meso- and macro-scale ramifications.

1. How does reconnection onset and what is the physics of transient reconnection

events such as bursty bulk flows, dipolarization fronts, entropy bubbles, and flux transfer events? Deliverables include how their properties depend on magnetospheric conditions, their role in energy and mass transport, how they expand and spread as a function of time, and comparisons with observational data from the THEMIS/ARTEMIS mission.

1. How are energetic particles produced during reconnection? This includes

mechanisms based on reconnection electric fields and secondary islands. Deliverables include the relative importance of various particle acceleration mechanisms and the observational signatures that result, and observations of these events from THEMIS/ARTEMIS and MMS.

Co-Chairs

• Paul Cassak, West Virginia University, Paul.Cassak@mail.wvu.edu (Expertise - Local and global fluid simulations, local kinetic simulations)
• Andrei Runov, University of California, Los Angeles, arunov@igpp.ucla.edu (Expertise - Magnetotail observations)
• Homa Karimabadi, University of California, San Diego, homakar@gmail.com (Expertise - Local and global kinetic simulations)

Term of Focus Group

Five years (2013-2017)

Expected Activities, Session Topics, and Challenges

The following are a possible set of activities and session topics for the duration of the proposed Focus Group. This list is not intended to be binding, nor is it intended to be noninclusive to research in areas planned for other years, but is offered as a guide to the direction of the Focus Group. Year 1, Signatures of Kinetic Scale Reconnection Physics (2013) - This is important in the runup to the launch of the MMS mission. The state-of-the-art in kinetic simulations will be gathered and discussed in relation to existing observational data to make predictions for MMS. Year 2, Dayside Reconnection and Solar Wind-Magnetospheric Coupling (2014) - This activity will collect progress in determining how reconnection occurs at the magnetopause and how it impacts solar wind-magnetospheric coupling. Cross-disciplinary activities with the Magnetosheath group are possible. Year 3, Kinetic Physics of Reconnection and Particle Acceleration (2015) - With early results from MMS expected, this session will focus on analyzing this data and comparing it to numerical simulations. It is expected that particle acceleration will be a strong component of the observations. Cross-disciplinary activities with the Metrics and Validation group are possible. Year 4, Reconnection Onset and Transients (2016) - This session will address the role of transients and reconnection onset. Cross-disciplinary activities with the Tail-Inner Magnetosphere Interactions group are possible. Year 5, Understanding Three-dimensional Reconnection (2017) - Efforts on the challenging and largely unknown physics of three-dimensional reconnection will be gathered and discussed.