FG: Magnetic Reconnection: The Key to Understanding Earth's Space Environment

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Abstract

Magnetic reconnection is a fundamental plasma process that alters the magnetic field topology and facilitates energy transport. Over the decades, our understanding of magnetic reconnection has been evolving. Even today, with high-resolution observations, advanced simulation, data sciences, and significantly matured laboratory experiments, we are still discovering unknown aspects of reconnection. The Focus Group designated to unravel the mysteries of magnetic reconnection ended in 2024. We are proposing a new Focus Group that will continue the targeted study on magnetic reconnection and its related physical processes which are the main topics of several other GEM FGs. We anticipate collaboration opportunities with multiple FGs, and we expect to have fruitful scientific findings over the 4-year term.

Organizers

  • Yi Qi, University of Colorado, Yi.qi@lasp.colorado.edu. Expertise: In-situ observations of magnetic reconnection’s small-scale properties in terrestrial magnetosphere, Flux Transfer Events (FTEs), solar wind – magnetosphere coupling
  • John Dorelli, NASA, john.dorelli@nasa.gov, Expertise: Magnetic reconnection in Earth’s magnetosphere, global modeling of Earth’s magnetosphere, MMS and TRACERS
  • Katherine Goodrich, West Virginia University, katherine.goodrich@mail.wvu.edu, Expertise: Magnetic reconnection and plasma turbulence using electric field measurements. Electric field instrument calibration and data analysis via MMS and TRACERS. Co-I on TRACERS.
  • Chen Shi, University of California Los Angeles, cshi1993@ucla.edu, Expertise: MHD theory and simulations, dynamics of solar wind and solar corona
  • M. Hasan Barbhuiya, West Virginia University, mhb0004@mix.wvu.edu, Expertise: Theory and kinetic simulations of reconnection and turbulence, energy conversion processes in non-LTE phenomena
  • Krishna Khanal (Student representative), University of Alabama in Huntsville, kk0099@uah.edu, Expertise: Observation of situ and ionospheric signatures of reconnection with focus on spatiotemporal variability on a large scale.

Topic Description

Magnetic reconnection is a fundamental plasma process that enables magnetic field topological rearrangement and explosive energy conversion. In our heliosphere, it plays a vital role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, and the interaction of the solar wind with planetary magnetospheres. At Earth, magnetic reconnection is the primary pathway by which energy from the solar wind enters Earth’s magnetosphere. At the dayside magnetopause, reconnection creates an open magnetic field topology, providing the solar wind with direct access to the ionosphere through the polar cusps. As plasma flows across the polar cap boundary, magnetic energy builds up in the magnetotail lobes until it is explosively released by reconnection somewhere in the central plasma sheet. Magnetic reconnection also has a complex relationship with turbulence from the fluid to the sub-kinetic scale. The interplay between reconnection and turbulence leads to various effects that are still under investigation. It is unrealistic to cover all related topics in this one FG. Based on the discussion at the concluding session of the previous reconnection FG, we summarize four major topics that are most timely and interesting to the community.

Connecting kinetic and global-scale magnetic reconnection via combinations of MMS, TRACERS, and other spacecraft missions

The scale of magnetic reconnection is global (e.g. Dungey Cycle), but diffusion regions at the center of reconnection are much smaller, with an electron diffusion region embedded within a larger ion diffusion region. At the fluid level, electromagnetic forces cannot formally affect the internal energy of plasma species, but simulations and observations often detect strong EM fields alongside temperature enhancements, implying a fundamental gap in our understanding of energy conversion. This further indicates that on kinetic scales, it is largely unknown how electromagnetic forces influence the internal energy of plasma species, which further complicates the view of magnetic reconnection on a global scale as it is unknown how the energy partition/conversion at such scales, and what, if any, correlation exists with the spatiotemporal structure of reconnection in 2D and 3D. Over the last decade, MMS has provided the community with a tremendous opportunity to explore magnetic reconnection via multipoint measurements of species-specific 𝑱 ∙ 𝑬, electromagnetic energy and plasma kinetic energy at kinetic scales. These measurements have proved necessary in understanding reconnection and are crucial for properly understanding energy conversion/partition in collisionless space plasma. MMS cannot, however, provide insight into how these kinetic processes link to a more global process of magnetic reconnection. The Tandem Reconnection and Cusp Reconnaissance Satellites (TRACERS), to be launched in 2025, aims to observe the spatiotemporal variance of magnetic reconnection at a larger scale. Conjunction studies with MMS can then provide wonderful opportunities to connect the kinetic-scale physics that occurs within the diffusion regions to the dynamics in the magnetosphere’s cusp, its “final destination”. We will invite the community to advocate for and make use of data from both MMS and TRACERS. We will also invite the community to use other satellites within the HSO in order to probe the upstream solar wind, magnetopause and magnetotail reconnection, and low-altitude cusp responses at the same time, thus providing a truly global view of the magnetopause topology under a wide range of solar wind conditions.

Three-dimensional magnetic reconnection (the spreading and extent of the x-line, relationship with turbulence)

The “standard model” of reconnection considers the field lines reconnecting in a quasi-2D plane, where the process is similar in each plane along the out-of-plane direction. However, the x-line cannot be infinitely long, and questions remain about how it spreads: How fast, in which direction relative to the reconnection plane, and what physical parameters determine its extent? Though reconnection can be simplified as a quasi-planar 2D process, real plasma systems are 3D. Understanding which 2D features apply to 3D systems is crucial. For instance, in plasma turbulence, intermittent structures may appear as small-scale current sheets, where reconnection may occur and dissipate turbulent energy. Yet, it is unclear in what fraction of these current sheets reconnection is triggered, and how it behaves differently in turbulent conditions. Conversely, reconnection may generate turbulence, affecting particle energization and energy dissipation.

Temporal variation of magnetic reconnection

Reconnection typically evolves through distinct phases. Including a slow linearly growing phase, an explosive phase where the reconnection rate increases rapidly, and steady phase where the reconnection rate saturates around ~ 0.1in normalized units as found in simulations and observations. Identifying these phases observationally is essential for understanding the broader impact of reconnection on its surroundings. However, it remains challenging to determine what phase of reconnection is being observed during in-situ satellite measurements, unlike in 2D numerical simulations. Given the current challenges and methods, how can we develop more reliable observational tools, preferably scalar quantities, to distinguish the different phases of reconnection during satellite crossings?

From Previous Proposal

Timeliness

In the past decade, our understanding of magnetic reconnection has been greatly advanced. Extensive studies have revealed that the spatiotemporal variation of magnetic reconnection, its multiscale coupling, and related energy conversion are far more complicated than we thought. While we have discovered more, there appear more open questions. The questions we mentioned in the topic description are key to understanding magnetic reconnection as a fundamental plasma process ubiquitous in our universe and plays a key role in the Earth’s and other planets’ space environment. It is crucial to develop high-resolution, strategically placed space observatories, next-generation numerical technologies, and a deeper theoretical understanding of the spatiotemporal dynamics of magnetic reconnection. With the rapid evolution of fluid, kinetic, and hybrid models, the application of data science techniques and lab facilities like “FLARE”, combined with in-situ observations obtained by MMS, other HSO missions, and future spacecraft missions like TRACERS, it is an excellent time for us to tackle the challenges and bring our understanding of magnetic reconnection to a new level. Future missions like HelioSwarm (to be launched in 2029) can also benefit from the knowledge we learn in this practice.

Relation to other FGs

  • The Impact of the Cold Plasma in Magnetospheric Physics: One of the goals of this FG is: “bring together theoretical and observational knowledge to assess the most important impacts associated with the cold plasma in the magnetospheric system”. Reconnection is a crucial process that impacts the whole magnetospheric system. The mass loading effect due to the cold ions impacts the dayside reconnection. We can collaborate and better understand this process.
  • Understanding the causes of geomagnetic disturbances in geospace for hazard analysis on geomagnetically induced currents: As mentioned on the FG’s wiki page, the common sources and driving mechanisms of GMDs are associated with large-scale geomagnetic activity, including storms and substorms. Since magnetic reconnection is the dominant driver of geomagnetic disturbances, its

better understanding will also shed light on the analysis of GMDs and GICs.

  • Mesoscale drivers of the nightside transition region: ionospheric and magnetotail evaluations (MESO): Plasma dynamics in the Nightside Transition Region (NTR) is essential to the M-I coupling. As a major driver for the geomagnetic disturbances, magnetic reconnections are directly related to bursty bulk flows, dipolarization fronts, etc., that feed into the NTR.
  • Magnetospheric Sources of Particle Precipitation and Their Role on Electrodynamic Coupling of Magnetosphere-Ionosphere-Thermosphere System: Magnetic reconnection relates to this FG as it is a fundamental process that leads to enormous dayside and nightside disturbances, flux convections, and wave generations, which can cause particle precipitation.
  • Comparative Planetary Magnetospheric Processes: The key comparative topics for the COMP FG include magnetotail dynamics and M-I coupling mechanisms. Magnetic reconnection in the planetary magnetosphere can be essential in particle acceleration, heating, and precipitation. Knowledge gained about reconnection on Earth is crucial for comparative studies.
  • Kinetic Plasma Processes in the Magnetotail during Substorm Dynamics: During the substorm expansion phase, reconnection releases the magnetic energy stored in the magnetotail during the previous substorm growth phase. The kinetic plasma processes in the magnetotail during the substorm is closely related to magnetic reconnection.

Goals and deliverables

We will encourage interactions between communities, and utilize tools and resources between different focus groups. With workshop-style collaborations, we will foster an in-depth understanding of magnetic reconnection, and advance the development of observational, numerical, and laboratory techniques.

  1. Primary Community Goal: To foster collaborative research and interdisciplinary dialogue within the space physics and plasma physics communities, aiming to advance understanding of multiscale coupling, three-dimensional processes, and energy conversion in magnetic reconnection through the integration of satellite data (e.g., MMS, TRACERS, HelioSwarm, etc.) and simulation-based approaches. The focus group will promote community-driven efforts to interpret in-situ observations, support the development of advanced observational tools, and encourage a global understanding of reconnection dynamics in planetary magnetospheres through joint sessions with other focus groups.
  2. Primary Science Goal: To address fundamental questions about magnetic reconnection by investigating:
    1. The multiscale coupling between kinetic and global processes, particularly the impact of small-scale dynamics on large-scale structures.
    2. The three-dimensional nature of magnetic reconnection, including the spreading of the x-line and the interplay between magnetic reconnection and turbulence.
    3. The temporal evolution of magnetic reconnection, and the development of reliable observational tools for distinguishing various phases of reconnection.
    4. The mechanisms of energy conversion and partition, particularly the effect of electromagnetic forces on plasma species' internal energy (temperature).

The deliverables include but are not limited to: 1) A shared list of current theories and methods on the topics mentioned in earlier sections. 2) A list of events for which there is good simultaneous coverage of a) solar wind conditions, b) dayside magnetopause condition, c) magnetotail condition, and d) Ground and Low Earth Orbit (LEO) observations to be simulated by global modelers and compared with observations. 3) An open repository that organizes and shares results from the comparative study. 4) A database containing results of different simulations of reconnection, including local/global, fluid/hybrid/PIC simulations.

  1. Community Deliverables:
    1. Collaborative Data Analysis Initiatives: Establish a framework for joint analysis of data from missions like MMS, TRACERS, and other HSO satellites, facilitating conjunction studies and multiscale investigations, and thus providing a global view of reconnection events under varying solar wind conditions.
    2. Shared Tools and Methodologies: Develop and share observational and analytical tools that can help the community reliably identify different phases of reconnection from satellite data, using various scalar quantities and new diagnostics.
    3. Educational and Outreach Materials: Create educational resources that facilitate a broader understanding of magnetic reconnection’s impact on heliophysics and planetary magnetospheres, contributing to the training of the next generation of researchers.
  2. Science Deliverables:
    1. Multiscale Coupling Analysis: Provide new insights into the coupling between small kinetic and global scales in reconnection, using conjunction data from MMS, TRACERS, and other satellites to investigate how small-scale processes impact larger-scale magnetic topologies.
    2. 3D Reconnection Characterization: Develop a detailed understanding of x-line spreading, and how reconnection interacts with and generates turbulence, by quantifying the impact of reconnection has in turbulent plasma environments on particle energization and energy dissipation.
    3. Reconnection Phases Identification: Refine and develop new methods to observationally identify the distinct phases of magnetic reconnection, improving our ability to detect these phases using in-situ satellite data under varying solar wind conditions.
    4. Energy Conversion Formalism: Advance the theoretical framework for understanding energy conversion in reconnection, in particular focusing on the conversion between internal energy of the particles and electromagnetic energy of the fields.

Terms

4 years (2025-2029)

Research areas

Primary: Solar Wind - Magnetosphere Interaction (SWMI); Secondary: Global System Modeling (GSM)

Expected Activities, Session Topics and Challenges

  1. Year 1 - Initial workshop-style meeting focusing on establishing collaboration between missions and individual groups. We will identify key observational challenges in multiscale coupling, the 3D nature of reconnection, the interplay between reconnection and turbulence, reconnection phases, and energy conversion mechanisms in reconnection, setting the stage for workshops in future years.
  2. Year 2 - A review highlighting the findings in the past year. We will solicit state-of-the-art methods and technologies to tackle the previously identified challenges, and decide on the list of events (magnetopause and magnetotail) for different groups to work together. Comparative analysis of different reconnection regions with a focus on the temporal phases of reconnection and the influence of EM forces and particle energy.
  3. Year 3 - A review highlighting the findings in the past year. We will investigate the 3D aspects of reconnection, focusing on the x-line spreading and the interplay between reconnection and turbulence through joint simulation-observation workshops.
  4. Year 4 - A review highlighting findings from the past 3 years and we will present our current understandings and deficiencies/gaps. We will isolate the challenges and possible remedies.
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