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Space Science Reviews (2025) 221:19
https://doi.org/10.1007/s11214-025-01145-x
MagneticReconnectioninSpace:AnIntroduction
J.L. Burch1·Rumi Nakamura2,3
Received: 19 November 2024 / Accepted: 24 January 2025 / Published online: 12 February 2025
© The Author(s) 2025
Abstract
An International Space Science Institute (ISSI) workshop was convened to assess recent
rapid advances in studies of magnetic reconnection made possible by the NASA Magneto-
spheric Multiscale (MMS) mission and to place them in context with concurrent advances
in solar physics by the Parker Solar Probe, astrophysics, planetary science and laboratory
plasma physics. The review papers resulting from this study focus primarily on results ob-
tained by MMS, and these papers are complemented by reports of advances in magnetic
reconnection physics in these other plasma environments. This paper introduces the topi-
cal collection “Magnetic Reconnection: Explosive Energy Conversion in Space Plasmas”,
in particular introducing the new capabilities of the MMS mission used in majority of the
articles in the collection and briefly summarizing the advances obtained from MMS.
1ContextandPurpose
The study of magnetic reconnection is a relatively young branch of plasma physics, which
has recently garnered great interest as its importance in the laboratory, the Earth’s magne-
tosphere, the solar photosphere and corona and objects such as accretion disks surrounding
neutron stars and black holes and supernova remnants has become widely recognized. The
common occurrence of magnetic reconnection and its frequent explosive nature make it one
of the most important agents of energy transfer throughout the universe. A recent commen-
tary by Hesse and Cassak (2020) describes the universal importance of reconnection along
with prospects for its further understanding.
An International Space Science Institute (ISSI) workshop was convened to assess recent
rapid advances in this field made possible by the NASA Magnetospheric Multiscale (MMS)
mission (Burch et al. 2016a) and to place them in context with concurrent advances in solar
physics by the Parker Solar Probe (Drake et al. 2025, this collection), astrophysics (Guo
et al. 2024, this collection), planetary science (Gershman et al. 2024, this collection) and
J.L. Burch
jburch@swri.edu
R. Nakamura
Rumi.Nakamura@oeaw.ac.at
1Southwest Research Institute, San Antonio, TX, USA
2Space Research Institute, Austrian Academy of Sciences, Graz, Austria
3International Space Science Institute, Bern, Switzerland
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19 Page2of10 J.L.Burch,R.Nakamura
laboratory plasma physics (Ji et al. 2023b, this collection). The review papers resulting from
this study focus to a large extent on the results obtained by MMS, and these papers are com-
plemented by reports of advances in magnetic reconnection physics in these other plasma
environments.
Leading up to the launch of MMS in March 2015, significant progress in understanding
magnetic reconnection at the MHD and ion scales was made by the European Space Agency
Cluster mission (e.g., Eastwood et al. 2010) while theoretical predictions of electron-scale
phenomena by plasma simulations helped set the stage for future measurements (e.g., Hesse
et al. 2014; Bessho et al. 2014). As described by Burch and Drake (2009), four great myster-
ies of magnetic reconnection were: (1) What produces dissipation in a collisionless plasma,
allowing reconnection to occur? (2) What determines the aspect ratio of the dissipation re-
gion and the rate of release of magnetic energy? (3) What is the “spark” that causes the
magnetic energy that has built up over a period of time to be suddenly released? and (4)
What is the mechanism for efficient conversion of magnetic energy into the kinetic energy
of charged particles? The unprecedented temporal and spatial resolution of the MMS mea-
surements (Burch et al. 2016a) have led to significant progress toward solving these four
mysteries while at the same time revealing many unanticipated aspects of magnetic recon-
nection in the boundary regions of the Earth’s magnetosphere. Note that the above mysteries
of magnetic reconnection apply to many other plasma environments as discussed in Ji et al.
(2023a). Outstanding questions of magnetic reconnection in different plasma environments
are further discussed in Nakamura et al. (2025, this collection).
2 New Capabilities of MMS
By 2005 MMS had become the next NASA Solar-Terrestrial Probe. This event followed the
ever-increasing scientific focus on magnetic reconnection that had occurred in the previous
25 years. The International Sun-Earth Explorer mission (ISEE) had made the first mea-
surement of the reconnection ion exhaust at the dayside magnetopause (Paschmann et al.
1979), thereby establishing the existence of magnetic reconnection as an important mech-
anism of energy-transfer from the solar wind to the Earth’s magnetosphere. Subsequent
investigations by the WIND, Polar, Cluster and THEMIS missions formed a comprehen-
sive picture of how magnetic reconnection and other associated plasma processes operate at
the fluid and ion scales in the boundary regions of the magnetosphere (e.g., Øieroset et al.
2001; Mozer et al. 2002; Eastwood et al. 2010; Angelopoulos et al. 2008). The new frontier
for MMS was recognized to be the electron diffusion region, where magnetic fields in ad-
jacent regions (magnetosheath-magnetosphere, north-south tail lobes) become interlinked.
The primary advance that was needed was to increase the measurement cadence of the three-
dimensional electron distribution function by at least a factor of 100. This requirement was
reached through a simple consideration of the following estimates that were made at the
time: The electron diffusion region has a width of a few km; the magnetopause moves ra-
dially inward and outward at speeds of a few 10s of km/s; and at least three measurements
of the distribution function within the diffusion region are needed to characterize it. These
numbers led to a round-figure minimum time resolution for electrons of 30 ms. Because of
the larger size of the ion diffusion region, the time resolution for ions was relaxed to 150 ms.
This major advance in time resolution required a new approach to measuring plasmas over
4πsteradians (Pollock et al. 2016). Most previous missions used the spacecraft rotation to
sample the full sky with a 2D angular array of sensors, but the required time resolution for
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Magnetic Reconnection in Space: An Introduction Page 3 of 10 19
Table 1 Summary of measurements made on each MMS spacecraft.
Measurement Time Resolution Sensitivity/Energy Resolution
DC B 3D to ∼1ms <0.1 nT
DC E 3D to ∼1ms <0.5 mV/m
SC Potential (proxy for ne)to∼1ms
AC B Waveform, spectra to 6 kHz <2×10−5nT/Hz1/2@1kHz
AC E Waveform, spectra to 100 kHz <1×107V/m-Hz1/2@10kHz
Plasma Electrons 3D fe(v) every30ms;1eVto30keV 20%ΔE/E resolution
Electron beams at single E to 1 ms Poisson Statistics limited
Plasma Ions 3D fi(v) every 150 ms; 1 eV to 20 keV/q 20% ΔE/E resolution
Poisson Statistics limited
Plasma Ion 3D fi(v) every 10 s 20% ΔE/E resolution
Composition 10 eV/q to 30 keV/q for H+,He
++,He
+,O
+
Energetic Ions/Electrons 3D f(v)every 10 s; 20 keV to 1000 keV 35% ΔE/E resolution
Energetic Ions 3D fi(v) every 20 s; 40 keV to 1000 keV 35% ΔE/E resolution
For H+,He
++,He
+,O
+
MMS led to spacecraft rotation speeds that were not technically achievable. The new ap-
proach taken by MMS was to use multiple 2D arrays (tophats) with ±22.5° electrostatic
deflection of the azimuthal fields of view. This approach required accurate intercalibration
of eight electron and eight ion instruments on each spacecraft and across the four spacecraft.
Other key requirements were (1) to determine the reconnection electric field with high-
resolution three-axis electric field measurements, which had been difficult or impossible
to achieve in the previous missions (Torbert et al. 2016a; Ergun et al. 2016); (2) to mea-
sure plasma composition accurately in the presence of the high proton fluxes at the magne-
topause, which required a new instrument design (Burch et al. 2005; Young et al. 2016); (3)
to measure energetic electrons and ions produced by magnetic reconnection and associated
processes (Mauk et al. 2016); (4) to control the spacecraft potential to <4 V, as was done on
Cluster (Torkar et al. 2016); and (5) to maintain a tetrahedron of four spacecraft with separa-
tions in the range from 10 to 160 km for dayside and nightside observations of reconnection
(Tooley et al. 2016). The measurements made by the four MMS spacecraft are summarized
in Table 1.
Calibrated Level-2 data in physical units are provided to the public in common data
format (CDF) with 30-day latency through the MMS Science Data Center at https://lasp.
colorado.edu/mms/sdc/public/. To aid in MMS data plotting and analysis, an extensive li-
brary of IDL and Python codes are maintained in the Space Physics Environment Data
Analysis System (SPEDAS) (Angelopoulos et al. 2019).
3 Summary of Results from MMS
Regarding reconnection mystery (1), the plasma simulations of Hesse et al. (2014) suggested
that accelerated electrons could rapidly carry energy from the magnetic field out of the dif-
fusion region from laminar reconnection without the need for turbulence or the associated
anomalous resistivity. Early MMS measurements (Burch et al. 2016b) confirmed the exis-
tence of the crescent-type distributions predicted by Hesse et al. (2014) and by Bessho et al.
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19 Page4of10 J.L.Burch,R.Nakamura
(2014) while also confirming the prediction of Cassak and Shay (2007) that the dissipation
occurs Earthward of the X line for asymmetric reconnection at the magnetopause. Torbert
et al. (2016b) showed further that the non-ideal electric field in generalized Ohm’s law was
produced mainly by the divergence of the electron pressure tensor with a much smaller con-
tribution from electron inertia and a significant residual, which could represent anomalous
resistivity. The primary conclusion from these early studies is that at the magnetopause the
reconnection electric field is produced by non-isotropic electron pressure, that the electron
crescent distributions carry the out-of-plane current and the dissipation, as measured by
J·E, is caused completely by the electron dynamics. Norgren et al. (2025, this collection)
provide a detailed discussion of electron dynamics in the electron diffusion region (EDR),
while further results on the generalized Ohm’s Law and reconnection rate from theory and
measurements are reviewed by Liu et al. (2025, this collection).
As noted by Burch and Drake (2009), there is the possibility that “with turbulence, the
field lines would be strongly twisted so that multiple ones could reconnect simultaneously,
vastly increasing the reconnection rate.” The Earth’s magnetosheath is a region that is often
turbulent by virtue of the dynamical interaction of solar-wind plasma with the bow shock.
While reconnecting magnetosheath current sheets have been observed by Cluster (Retino
et al. 2007) and THEMIS (Øieroset et al. 2017), the electron dynamics were not observable
owing to their 4-s and 3-s time resolutions, respectively. With MMS, turbulent reconnection
is observed routinely in the magnetosheath at the electron scale with the significant result
that often there is only an electron diffusion region, which is not embedded within an ion
diffusion region as occurs in the standard reconnection model (Phan et al. 2018).
In the MMS era, the magnetosheath has become an important laboratory for the study
of kinetic plasma turbulence and its relationship with magnetic reconnection (Wilder et al.
2017; Chasapis et al. 2018; Bandyopadhyay et al. 2021). Stawarz et al. (2024, this collec-
tion) cover the latest advances in turbulent reconnection by MMS in the magnetosheath and
bow shock as well as in Kelvin-Helmholtz vortices in the flank magnetopause. In the magne-
tosheath, MMS revealed that reconnection between interlinked flux tubes originating from
multiple X-lines leads to the formation of flux ropes or flux transfer events (FTEs). This
3D FTE formation process occurs predominantly when the interplanetary magnetic field
has a strong east-west component, a condition that leads magnetic field lines to collide and
reconnect. These phenomena are covered in detail by Hwang et al. (2023, this collection).
Regarding reconnection mystery (2), the rate of release of magnetic energy, or the re-
connection rate, can be measured by determining the aspect ratio of the electron diffusion
region, by measuring the inflow rate of electrons into the diffusion region, and by measuring
the reconnection electric field. Theoretical estimates summarized by Cassak et al. (2017)
are that the normalized reconnection rate is near 0.1. MMS data have been used for all three
approaches with the aspect ratio being determined for a magnetotail reconnection event by
Nakamura et al. (2019); the electron inflow velocity being measured by Burch et al. (2020,
2022); and the reconnection electric field being determined by Nakamura et al. (2018)and
Genestreti et al. (2018). These measurements have yielded normalized reconnection rates in
the range of 0.05 to 0.25 with the highest value being for the Phan et al. (2018) electron-only
reconnection event. Further discussion of the latest theoretical and experimental advances
on the reconnection rate appears in Liu et al. (2025, this collection).
For reconnection mystery (3), the magnetotail is an opportune region for determining
what initiates reconnection because there is a growth phase during which the magnetic flux
is transferred from the dayside to the nightside magnetosphere leading eventually to the
rapid onset of magnetic reconnection across the tail neutral sheet and the initiation of a
magnetospheric substorm (Angelopoulos et al. 2008). A recent result by Genestreti et al.
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Magnetic Reconnection in Space: An Introduction Page 5 of 10 19
(2023) showed how a thinning magnetotail current sheet triggered by a solar-wind pressure
pulse develops multiple X lines with low reconnection rates until one of them reaches lobe
field lines. At this time, rapid reconnection is initiated, a primary X line is formed, and
the other X lines are swept down the tail within the exhaust of the primary X line. This
observation of the initiation of rapid reconnection and its implications are discussed further
by Nakamura et al. (2025, this collection).
Mystery (4) stems from the observation of electrons and ions with energies of hundreds
of keV during magnetotail reconnection events. These energies exceed the full potential drop
across the magnetotail and far exceed the energies that can result from the reconnection elec-
tric field of a few mV/m or the energy of reconnection outflows, which proceed at near the
Alfvén speed. While some energetic particles, particularly on the day side of the magneto-
sphere can be explained by leakage of radiation-belt particles through the magnetopause,
this process does not apply in the mid to distant magnetotail. Betatron and Fermi acceler-
ation, along with acceleration by parallel electric fields have been predicted and observed
by MMS to be important in the Earthward flows, reconnection exhaust and separatrices,
respectively (Turner et al. 2016;Maetal.2022). Particularly intriguing is the turbulent ac-
celeration of ions and electrons near the reconnection X line and the surrounding diffusion
region (Ergun et al. 2022). These results are discussed in detail by Oka et al. (2023,this
collection).
An important and unexpected result from MMS is the ubiquitous occurrence of recon-
nection wherever thin current sheets co-exist with either laminar or turbulent magnetic-field
reversals. Examples include thin current sheets embedded within the vortices of Kelvin-
Helmholtz MHD instability, which often occur along the flanks of the magnetotail (Eriks-
son et al. 2016; Hwang et al. 2023, this collection); and the bow shock, which also has been
found by MMS to contain reconnecting thin current sheets on a regular basis (Stawarz et al.
2024, this collection).
The reconnection diffusion region has been found to be the site of a wide variety of
plasma waves with frequencies from below the ion cyclotron frequency to above the elec-
tron plasma frequency, which often develop from plasma instabilities that result from the
accumulation of free energy. Advances from MMS concerning these waves and the instabil-
ities that cause them are reviewed by Graham et al. (2025, this collection).
During the time over which rapid advances in reconnection physics have been made by
MMS, similar advances have been made in the plasma simulations that predicted many of
the experimental results and were later used to provide deeper theoretical understanding of
unexpected measurements. The development of plasma simulations during the MMS era is
summarized by Shay et al. (2025, this collection). Important advances have also been made
in methods for analyzing the multi-spacecraft MMS data including reconstruction of the
magnetic-field surrounding reconnection sites. A comprehensive review of these analysis
methods is provided by Hasegawa et al. (2024, this collection).
4 Advances in Theory and Modeling
As reviewed by Shay et al. (2025, this collection), a wide variety of plasma simulation tech-
niques have been developed to explore the kinetic physics of reconnection diffusion regions,
the transfer of particles and energy to the region closely surrounding the diffusion regions,
as well as the macroscale and global effects of reconnection. Because of limitations on com-
puter speed and memory, complete simulation of kinetic physics in the diffusion region is
still not feasible at realistic ion-to-electron mass ratios or with time-resolution sufficient
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19 Page6of10 J.L.Burch,R.Nakamura
to track electron-scale wave phenomena. However, evolution of computer capabilities is
steadily improving this situation.
Before the launch of MMS, plasma simulation was somewhat more advanced than the
measurements because they could access electron kinetic scales with the most common
technique being 2.5-D particle-in-cell (PIC) simulations. With this technique electric and
magnetic fields and plasma distribution functions are resolved in 3D but are only allowed to
vary along a 2D grid. While limited in scope, these simulations nevertheless made important
predictions, some of which have been verified with MMS data. A classic example is the
electron crescent distribution, which is described by Genestreti et al. (2025, this collection).
As noted by Shay et al. (2025, this collection), mesoscale, macroscale and up to global
simulation can be handled by embedding PIC codes into MHD codes. A similar approach is
used in solar flare physics where explicit particle-acceleration codes are embedded in MHD
codes as described by Drake et al. (2025, this collection).
5 Summary and Conclusions
Considering the many advances in magnetic reconnection physics that have been made by
MMS, it is possible to identify further advances that can be made by this remarkable mission
or by subsequent missions that may focus on specific targets for further study. These poten-
tial advances are discussed in the outlook paper by Nakamura et al. (2025, this collection).
One example is the investigation of cross-scale coupling through which the many microscale
reconnection phenomena identified by MMS propagate to the successively larger meso and
macro scales. Early attempts at understanding the first stages of cross-scale coupling are
being made by MMS through the use of asymmetric tetrahedron and string-of-pearls forma-
tions. Ultimately, the determination of how magnetic reconnection affects global magneto-
spheric dynamics will require coordinated measurements at the various scales from micro to
global. Such a large undertaking would no doubt require significant multi-national coopera-
tion, which would be well justified based on the extensive advances made at the microscale
by MMS. Outstanding questions about the coupling of the MMS-observed kinetic processes
and large-scale magnetospheric processes are reviewed by Fuselier et al. (2024, this collec-
tion).
Acknowledgements The authors are indebted to the International Space Science Institute for supporting the
workshop on which this collection of review papers is based, to their authors, and to all who participated in
the discussion. The authors thank J. F. Drake, B. L. Giles, M. Hesse, M. Hoshino, B. Lavraud, and R. B.
Torbert for co-convening the workshop. We acknowledge the important contributions of Rudolf von Steiger
and Maurizio Falanga of ISSI.
Funding Open access funding provided by Österreichische Akademie der Wissenschaften. The work of JLB
was supported by NASA Contract NNG04EB99C at SwRI. The work of RN was supported by Austrian
Science Fund (FWF): https://doi.org/10.55776/P32175.
Declarations
Competing Interests The first author (JLB) is a member of the Space Science Reviews Editorial Board. There
are no other competing interests to report.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence,
and indicate if changes were made. The images or other third party material in this article are included in the
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