Kanekal and Miyoshi Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00413-y(2021) 8:35

REVIEW

Progress in Earth andPlanetary Science

Open Access

Dynamics of the terrestrial radiation
belts: a review of recent results during the
VarSITI (Variability of the Sun and Its
Terrestrial Impact) era, 2014–2018
Shrikanth Kanekal1*

and Yoshizumi Miyoshi2

Abstract
The Earth’s magnetosphere is region that is carved out by the solar wind as it flows past and interacts with theterrestrial magnetic field. The inner magnetosphere is the region that contains the plasmasphere, ring current, and theradiation belts all co-located within about 6.6 Re, nominally taken to be bounding this region. This region is highly
dynamic and is home to a variety of plasma waves and particle populations ranging in energy from a few eV torelativistic and ultra-relativistic electrons and ions. The interplanetary magnetic field (IMF) embedded in the solar wind
via the process of magnetic reconnection at the sub-solar point sets up plasma convection and creates themagnetotail. Magnetic reconnection also occurs in the tail and is responsible for explosive phenomena known as
substorms. Substorms inject low-energy particles into the inner magnetosphere and help generate and sustain
plasma waves. Transients in the solar wind such as coronal mass ejections (CMEs), co-rotating interaction regions(CIRs), and interplanetary shocks compress the magnetosphere resulting in geomagnetic storms, energization, andloss of energetic electrons in the outer radiation belt nad enhance the ring current, thereby driving the geomagnetic
dynamics. The Specification and Prediction of the Coupled Inner-Magnetospheric Environment (SPeCIMEN) is one of
the four elements of VarSITI (Variability of the Sun and Its Terrestrial Impact) program which seeks to quantitatively
predict and specify the inner magnetospheric environment based on Sun/solar wind driving inputs. During the past
4 years, the SPeCIMEN project has brought together scientists and researchers from across the world and facilitatedtheir efforts to achieve the project goal. This review provides an overview of some of the significant scientific advancesin understanding the dynamical processes and their interconnectedness during the VarSITI era. Major space missions,with instrument suites providing in situ measurements, ground-based programs, progress in theory, and modeling arebriefly discussed. Open outstanding questions and future directions of inner magnetospheric research are explored.
Keywords: Inner magnetosphere, Energetic particles, Plasma waves, Wave-particle interactions, Radiation belts,
Plasmasphere, Ring current*Correspondence: shrikanth.g.kanekal@nasa.gov
NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greeneblt 20771,
MD, USA
Full list of author information is available at the end of the article
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Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

1 Introduction
The term “magnetosphere” was introduced by Gold in
1959 (Gold 1959) to describe the region above the ionosphere and coincided with James Van Allen’s discoveryof the radiation belts (Van Allen et al. 1959). The solar
wind (Parker 1958), flows past the Earth forming themagnetopause which is the outer boundary of the magnetosphere. The solar wind is supersonic and forms thebow shock just ahead of the magnetopause. Co-locatedwith the radiation belts is a region containing low-energy
plasma which is home to a variety of plasma waves. The
radiation belts comprise an outer belt and an inner belt
separated by the so-called slot region. The outer belt is
populated mostly by electrons ranging in energy from a
few hundred keV to tens of MeV and is highly dynamical
in nature varying on time scales ranging from minutes toyears (Baker and Kanekal 2008). The inner belt comprises
mostly of protons and is relatively stable varying slowly
over time scales of years and is presumed to be predominantly the result cosmic ray albedo neutron decay (Hess1959).
In the recent past major missions such as NASA’s Van
Allen Probes (Mauk et al. 2014; Sibeck et al. 2012),
JAXA’s Arase (Miyoshi et al. 2018), THEMIS (Angelopoulos 2008), and others vastly improved our understandingof the magnetosphere, particularly the inner magnetosphere which includes the radiation belts and the plasmasphere. New observations such as the discovery of along-term storage ring or the “third belt” (Baker et al.
2013) to the first direct observation of wave-particle interaction (Fennell et al. 2014; Kurita et al. 2018; Kasahara etal. 2018) are prime examples of new observational in situ
measurements enabled by advances in instrumentation.
Theoretical understanding of the importance of waveparticle interactions have also enabled understanding thephysics behind the particle energization and loss (see, forexample, chapter by Bortnik et al. in Balasis et al. (2016)).
The Variability of the Sun and Its Terrestrial Impact(VarSITI) program (Shiokawa and Georgieva 2020) commenced in 2014 with an aim to promote international
collaboration in data analysis, modeling, and theory tounderstand how the solar variability affects the Earth. The
program comprises four scientific elements. The SPeCIMEN (Specification and Prediction of the Coupled InnerMagnetospheric Environment) element’s main goal is “thequantitative prediction and specification of the Earth’s
inner magnetospheric environment based on Sun/solar
wind driving inputs”.

2 Review

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and delineates it from the surrounding interplanetary
space. The Sun produces many transient eruptions, ofwhich coronal mass ejections (CMEs), co-rotating interaction regions (CIRs), and interplanetary shocks affect theterrestrial magnetosphere. The consequences, i.e., magnetospheric responses, encompass phenomena such as
substorms, geomagnetic storms, and aurore and affectplasma waves and energetic particle populations. The
time scales of magnetospheric response, particularly the
inner magnetosphere, range from minutes to years (Baker
and Kanekal 2008).
2.1.1 Coronal mass ejections

Coronal mass ejections are large eruptions of plasma
together with the embedded magnetic field from the Sunthat are now understood as major drivers of geomagnetic activity and were first observed as late as the 1970s(Gopalswamy 2016). CMEs propagate at different speeds
through the solar wind and often are preceded by an interplanetary shock. They may contain well-ordered magnetic
fields, e.g., magnetic flux ropes and clouds (Klein and
Burlaga 1982), and the interplanetary shock ahead of
the CME is a prime acceleration site for solar energetic
particles SEP (Desai and Giacalone 2016). A schematic
illustrating the topology of a CME is shown in Fig. 1(reproduced from Zurbuchen and Richardson (2006)).
2.1.2 Corotating Interaction Regions and High-Speed
Streams

A stream interaction region is formed when slow solar
wind is overtaken by the fast solar wind which originatesfrom coronal holes. The coronal holes move down tolower heliospheric latitudes during the descending phaseof the solar cycle and usually persist for periods longer
than the solar rotation period of about 27 days (as seenfrom the Earth). During these times, the stream interaction regions are somewhat stable and rotate with the
Sun and are termed CIRs. Figure 2 shows a conceptual
schematic of a CIR (adapted from Owens and Forsyth(2013)).
2.1.3 Interplanetary shocks

Interplanetary shocks are known to accelerate charged
particles (Jones and Ellison 1991) and are also responsible
for accelerating electrons in the outer zone on very shorttime scales of the order of a few drift times (Blake et al.
1992). The acceleration and injection are quite distinct (Liet al. 2018) from either radial transport or wave-particle
interactions (see the Section 2.5 section).

2.1 Solar drivers of magnetospheric dynamics

2.2 The inner magnetosphere2.2.1 Morphology and structure

It is by now well established that the Sun is the ultimate driver of magnetospheric phenomena; indeed, it is
the steady solar wind that carves out the magnetosphere

Until recently, observations of the Van Allen belts indicated that there are two belts, an outer belt comprised
mostly of electrons and an inner belt mostly comprised

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Fig. 1 A schematic illustrating the topology of a CME (reproduced from Zurbuchen and Richardson (2006)). The interplanetary shock ahead of the
CME is followed by a turbulent sheath region. Closed magnetic loop lines have counter-streaming particles and may have amagnetic cloud topology

of protons. These two regions were separated by the socalled slot region which was a region that was normallybereft of energetic electrons. Other particle populations
include transiently trapped solar particles and somewhat more long-lasting trapped anomalous cosmic rays

(Cummings et al. 1993). However, after the launch of the
Van Allen Probes, new observations by the REPT (Relativistic Electron Proton Telescope) (Baker et al. 2014a)
showed that a third belt or “storage ring” was formed soonafter the passage of a CME effectively “splitting” the outerFig. 2 A conceptual schematic of a CIR (adapted from Owens and Forsyth (2013)). Forward (reverse) shock is shown as a dashed blue (red) line

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

belt. Figure 3 reproduced from Baker et al. (2013) shows
the logarithm of relativistic differential electron fluxes
at 3 distinct energy ranges, viz., 3.2 to 4.0 MeV, 4.0 to
5.0 MeV, and 5.0 to 6.2 MeV in panels a, b, and c respectively. The time period is from 1 September to 4 October
2012 with the ordinate showing the L* parameter (for a
detailed discussion of L*, McIlwain L value, and radiation
belt coordinates, see Roederer and Lejosne (2018)) withthe fluxes are color coded according to the color bar atright.
After about September 4, when the belts were enhanced,
the plasmapause was located at very high L* values (> 4.5)
which may have accounted for the stable existence of the
newly formed ring. It has been suggested that the electron
decay lifetimes which are due to interaction with plasmaspheric hiss inside the plasmapause are long (Thorne etal. 2018). Other competing theories have also been proposed and include rapid storm-time outward ULF wave
transport (Mann et al. 2016) and lack of resonant scattering of ultra-relativistic electrons by chorus and hiss
(Shprits et al. 2013).
Another hitherto unnoticed morphological feature of
the outer electron belt at relativistic and at ultrarelativistic energies is that of the so-called impenetrable
barrier (Baker et al. 2014a). The observations of highenergy electron fluxes by the REPT instrument onboard
Van Allen Probes showed that over prolonged periods ofmany years. Figure 4 reproduced from Baker et al. (2014a)
shows in a format similar to Fig. 3 color-coded logarithmof differential electron fluxes of energies ranging from
2.0 to 8.8 MeV (panels a through e) for a period ranging
from 1 September 2012 to 1 May 2014. It is seen that theelectrons do not “penetrate” to L shells lower than 2.8
across all energies for this entire period.

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This remarkable feature may be due to extremely slow
inward diffusion coupled with pitch angle scatteringremoving those electrons that reach these low L values.
However, there has been some speculation that the “barrier” could also potentially arise from particle scattering
by human produced waves by radio transmitters.
Thus, in the recent past, measurements have revealed
hitherto unobserved morphological features of the radiation belt in the inner magnetosphere made possible by
new missions with state-of-the-art instrumentation
2.3 Inter-connectedness and cross scale/cross-energy
coupling

The influence of the plasmasphere on radiation belts
via wave-particle interactions is well known (see the“Wave-particle interactions” section) and arises due to theplasmasphere being home to plasmaspheric hiss, while
outside the plasmapause injection of low-energy electrons and ions from the magnetotail and their anisotropy
results in EMIC and chorus waves (Li et al. 2019;
Magnetosphere-Ionosphere Coupling in the Solar System, 2016; See Chapter 9 by Thorne. R., et al. 2016).
The plasma waves generated by the low-energy particles (so-called source populations; see Jaynes et al.
(2015)) transfer energy to intermediate energy particles (“seed” populations) energizing them. The waves
also pitch angle scatter electrons de-trapping themand ultimately leading to particle loss. Thus, a crossscale coupling exists that connects the lowest energy
to the highest energy particle populations in the innermagnetosphere, via the intermediary plasma waves.
Figure 5 reproduced from Miyoshi et al. (2018) shows
schematically the inter-connectedness and cross-energy
coupling.

Fig. 3 Formation of the third belt or “storage ring” soon after the passage of a CME effectively “splitting” the outer belt observed by the REPT
instrument on Van Allen Probes; reproduced from Baker et al. (2013)

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Fig. 4 The “impenetrable barrier” for relativistic electrons as observed by the REPT instrument on Van Allen Probes (reproduced from Baker et al.
(2014a)). At all energies, inward radially diffusing electrons do not penetrate below L ≈ 2.8

That the plasmasphere itself may be affected by solar
influences was demonstrated recently by observations
from Van Allen Probes of the cold plasma density andelectric field in the inner magnetosphere (Thaller et al.
2019). Using long-term observations from Van Allen
Probes, Thaller et al. (2019) demonstrated that the outerplasmasphere and the plasmapause boundary were driven
by modulations in the solar wind. Furthermore, theirfindings indicated that these modulations were stronger
during the declining phase of the solar cycle, possibly due
to the prevalence of CIRs, and were substantial in amplitude. As mentioned earlier, the processes in plasmasphere
are intimately connected to the electron populations in
the outer radiation belts. Thus, these modulations of theplasmasphere potentially could be important in understanding the driver dependence of outer radiation belt
dynamics

2.4 Inner magnetosphere observatories

The VarSITI era saw several major space missions thatwere either directly dedicated to the study of the innermagnetosphere, e.g., NASA’s Van Allen Probes (Mauk et
al. 2014; Sibeck et al. 2012) and JAXA’s Arase (Miyoshi etal. 2018), or were able to contribute significantly to scientific questions pertaining to the inner magnetosphere.
These include THEMIS (Angelopoulos 2008) and MMS(Burch et al. 2016). The VarSITI era also witnessed thematuration of the paradigm shifting technology of CubeSats and associated inexpensive access to space.
In the following, we provide a very brief overview ofthese major missions and illustrate the CubeSat approach
using as an example the CeREs mission (Kanekal et al.
2018).
The Van Allen Probes mission launched in 2012
comprises two identically instrumented spacecraft eachKanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

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Fig. 5 Diagram illustrating the interconnectedness and cross-scale coupling between particles, waves, and underlying physical processes(reproduced from Miyoshi et al. (2018)). The energy and spatial scales are indicated on the ordinate and abscissa, respectively. The processes andcoupling are shown as colored arrows

carrying a comprehensive suite of instruments measuring plasma waves, magnetic fields, and energetic particles(electrons, ions, and ion composition) over a wide energy
range. The goal of the mission was to “provide understanding, ideally to the point of predictability, of how
populations of relativistic electrons and penetrating ionsin space form or change in response to variable inputs ofenergy from the Sun.” The mission and its science goals
are fully described by Mauk et al. (2014).
The ERG project is a Japanese geospace exploration
project, which consists of three different research teams:
Arase (ERG) satellite, ground-based observations, andmodeling simulation. The main science target of the
project is understanding of transportation, acceleration,and loss of energetic particles in radiation belts anddynamics of geospace during space storms. As a key
observation of the ERG project, Arase was launched in2016 and has started the regular observations since March2017. Arase has nine science instruments to measureplasma/particles and field and waves. The orbit of Araseis similar to Van Allen Probes, but Arase measures higher
magnetic latitudes up to 40 degrees and higher L-shell up
to 10. Overview of the project and science goals are fully
described by Miyoshi et al. (2018).
The THEMIS mission was launched in 2007 andcomprises five identical spacecraft whose main science
goal was the study of substorms in the magnetotail.
Each spacecraft carries onboard particle instruments thatmeasure low-energy electrons and ions as well as electricand magnetic fields. While the mission’s main focus is on
the magnetotail, since the spacecraft travers through theradiation belts, the mission also contributes to the studyof the inner magnetosphere. The details of the mission
are described by Angelopoulos (2008). Since 2010, twoof THEMIS spacecraft orbit around the Moon while theremaining three remain in Earth orbits.
2.4.1 The CubeSat/SmallSat paradigm

In the recent past, CubeSats, which comprise 10 cm cubes
called 1U, have moved from being a teaching and academic tool to becoming contenders for doing significant
science (National Academies of Sciences, Engineering,and et al. 2016). In particular, low earth orbit (LEO) is well
suited for CubeSats, since currently due to their power
and volume limitations, data transmission rates are somewhat of an issue. However, innovative new instrumentation utilizing advances in detector technology, electronics,
and rad-hard processors have brought the capabilities
of instruments onboard major missions in the past tothese compact space platforms. For example, the MERiT(Kanekal et al. 2019) instrument onboard the CeREs(Kanekal et al. 2018) combines avalanche photodiodes(APD) and solid-state detectors (SSD) to provide electron
measurements from a few keV up to ten MeV. Figure 6
shows the CeREs CubeSat with the MERiT instrument
at the top. The CubeSat, which was launched DecemberKanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

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and O’Brien 2016) and spectra (Crew et al. 2016). CubeSat
missions are being considered for interplanetary regions
as well, for example, the CUSP (Desai et al. 2019) CubeSat
expected to launch early 2020 will measure supra-thermaland solar energetic protons. Figure 7 shows a synoptic
view measurements made by the FireBird (Crew et al.
2016) together with those made by the MagEIS (Blakeand et al. 2013) instrument onboard Van Allen Probes
illustrating the complementary nature of contributions toradiation belt science by CubeSats and major missions.
One of the main advantages that CubeSats bring to thestudy of inner magnetosphere is the capability to do multipoint measurements. Detailed in situ measurements ofparticles and waves in the inner magnetosphere have beenmostly confined to single point measurements or at mostmissions comprising two spacecraft. Large major missions
are expensive and take many person-years of effort. CubeSats, taking advantage of recent developments in both
inexpensive launch vehicle systems and instrumentation,
can fill this lacuna and provide valuable scientific insightsthat arise from multi-point measurements, e.g., resolving
spatio-temporal ambiguities.
2.5 Electron dynamics in the outer belt

Fig. 6 Photograph of the CeREs (Compact Radiation belt Explorer)
CubeSat. CeREs is a typical 3U CubeSat carrying a single instrument,
the MERiT (Minaturized Elecgron pRoton Telescope) as the science
payload2018, however soon lost contact with the ground station
only about a week into the mission. CubeSats remain arisky proposition while affording the possibility of goodscience at a low cost.
Another 3U CubeSat, the Colorado Student Space
Weather Experiment CubeSat, CSSWE has however beensuccessful and made some significant science contributions (Schiller et al. 2014), such as establishing that energetic electrons from cosmic ray neutrons get trapped in
the radiation belts (Li et al. 2017). Other CubeSat missions
that are studying terrestrial radiation belts include ELFIN(Shprits et al. 2017), AC6 (Blake and O’Brien 2016), and
FireBird (Spence et al. 2012). The latter two have studied electron microburst precipitation and have provided
information regarding microburst spatial extent (Blake

The Earth’s outer radiation belt comprising mostly of electrons is a very dynamic region with electron intensities
varying by orders of magnitude in time scales ranging
from minutes to days (see, for example, Kanekal (2006);
Miyoshi et al. (2018)) which has been well established byobservations going back to the nineties (Baker et al. 1994).
Figure 8 shows the measurements of energetic electrons
spanning nearly a decade by SAMPEX and Polar spacecraft. The dynamic nature of the outer zone is evident
from the figure, which shows many electron enhancements as well as loss over a nearly a solar cycle. Furthermore, the global coherent nature (Kanekal et al. 2001) ofelectron energization is also well illustrated in Fig. 8. Notethat SAMPEX was in low Earth orbit while Polar was ina high-inclination, high-altitude orbit during this period.
It has long been established that the causative agents forelectron energization are high solar wind speeds (Paulikas
and Blake 1979; Baker et al. 1989) and a persistent southward component of the interplanetary magnetic field(Blake et al. 1997). Electron losses are important as it
is currently known that the net flux of energetic electrons is a “delicate balance” between energization and loss
processes. Figure 9 illustrates this by showing observations of three storms of similar geomagnetic intensities
wherein net energization, loss, and no change in pre- andpost-storm electron fluxes are seen.
In the following sections below, we discuss and review
the current understanding of electron energization andloss processes in the context of focusing on recent results
during the SPECiMEN era.

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Fig. 7 Measurements of > 985 keV electrons by sensors onboard FireBird and differential flux by MagEIS for the 893 keV channel (reproduced from
Crew et al. (2016). The top panel shows the Dst index, the middle panel FIREBIRD-integral counts, and the bottom panel MagEIS measurements

2.5.1 Electron energization processes

Radial transport Electrons moving radially inward gain
energy due to the conservation of the first adiabatic invariant; this process is termed radial diffusion and has been
established several decades ago (Falthammar 1965; Schulz

and Lanzerotti 1974); similarly outward radial motion
tends to decrease particle energy and more importantly
to loss via escape through magnetopause. Radial diffusion is caused by magnetic and electric field perturbations(Falthammar 1965), and more recent work has shown that

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Fig. 8 Energetic electron intensities measured by instruments onboard SAMPEX (top) and Polar (bottom) satellites. Daily averaged fluxes spanningover a decade (1992 to 2003) are shown for 2.0 to 6.0 MeV and > 2.0 MeV electrons. The electron intensities are smoothed with a 30-day runningbox-car average. Also shown as a blue trace model calculated location of the plasmapause multiplied by a factor of 1.3. Figure is reproduced from
Kanekal (2006)

enhanced ULF wave activity speeds up this process considerably (Elkington et al. 1999; Shprits et al. 2008; Su andet al. 2015). Jaynes et al. (2018) have used REPT observations of ultra-relativisitic electrons during the 17 March
2015 event to show that electrons are energized to ashigh as ≈ 8 MeV in energy on rather short time scales.
Figure 10 shows measurements of 6.3 MeV electron fluxes
by the REPT instrument for the month of March 2015.
The top panel of the figure shows a clear inward motion
on electrons, and Jaynes et al. (2018) calculate a diffusion
rate DLL of about 0.3 day−1 for these ultra-relativistic electrons. Another study by Zhao et al. (2018) demonstrated
that inward radial diffusion was responsible for energizing electrons to ultra-relativistic energies, again using datafrom the REPT instrument.
Another study by Ozeke et al. (2020) used ground-based
measurements of ULF waves to characterize the global
distribution of the ULF wave power and simulated radial

diffusion effects during March 2015 and March 2013 onelectron fluxes in the outer zone. They compared theirsimulation results with observations made by the REPT
instrument onboard Van Allen Probes and concluded thatULF wave-driven radial diffusion was an important mechanism for both energization and loss via magnetopause
shadowing (see Fig. 4 of Ozeke et al. (2020)).
It is clear from these and other studies that enhanced
radial diffusion plays an important role in radiation belt dynamics while energization and pitch angle
scattering by wave particle are also an equally significant
mechanism.

Wave-particle interactions Wave-particle interactions
play an important role in energizing radiation belt
electrons. There are two different processes in the
wave-particle interactions: (1) drift resonance between
drifted electrons and MHD fast mode waves and (2)Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

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Fig. 9 Radial diffusion of ultra-reltivistic electrons (6.3 MeV) shown in the top panel. Also shown in the top panel is the comparatively slower radial
diffusion during October 2012. Compare to much slower diffusion seen in previous strong storm in October 2012, black dashed lines show the slopeof inward drift. The rest of the panels show Dst, Kp, AL, and solar wind parameters. Figure is reproduced from Jaynes et al. (2018)

Fig. 10 Radiation belt response during geomagnetic storms of similar strength. The net flux of energetic electrons may increase, decrease, or staythe same between the main and recovery phase illustrating the important dynamic between energization and loss. Figure is reproduced fromReeves et al. (2013)

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

cyclotron resonance between gyrating electrons andWhistler mode/magnetosonic mode waves. The first process has been recognized as an adiabatic process due
to conservation of the first and second invariants, while
the second process has been recognized as non-adiabatic
process through violation of all invariants.
Since the late 1990s, theories have suggested that interactions through cyclotron resonance causes accelerationsof relativistic electrons (Summers et al. 1998), and several observational studies indicate that interactions with
Whistler mode chorus contribute to flux enhancement ofrelativistic electrons. Measurements on the radial profileof the phase space density are essential to discriminate
the processes (Green and Kivelson 2004). Several studies carried out prior to the Van Allen Probes mission
reported on the observation of a peak in the phase spacedensity inside the outer belt likely arising from internal
electron energization. However, detailed and comprehensive observations by Van Allen Probes provided definitive
evidence for local electron energization as distinct fromenergization due to radial transport (Reeves et al. 2013).
Reeves et al. (2013) showed growing peak of thephase space density, which indicates that generation of
MeV electrons occur inside the outer belt. Thorne etal. (2013) showed variations of the plasma wave dataand electron data from Van Allen Probes and comparedwith the quasi-linear simulation. They concluded that
Whistler mode waves actually cause acceleration of MeV
electrons.
During the Van Allen Probes era, there are a numberof reports that whistler mode waves accelerate electrons
and growing peak of the phase space density inside theouter belt (Reeves et al. 2013), and it has been establishedthat internal acceleration through resonance with whistler
mode waves should be a key process on the formation of
MeV electrons of the outer belt. The elementary process

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of accelerations has also been identified by both Van Allen
Probes and Arase (Fennell et al. 2014; Kurita et al. 2018).
Another important aspect of the wave-particle interactions is understanding the non-linear wave-particle
interactions. Whistler mode chorus waves are generated
through non-linear interactions with electrons throughformation of the electron hole/hill in the phase space(Omura et al. 2009). Evidence of non-linear interactions
has been reported by comparing theories (Foster et al.
2017; Kubota and Omura 2018).
Generation of chorus waves and acceleration of relativistic electrons are the result of an interplay among different plasma/particle populations via wave-particle interactions. Cross-energy coupling process (Miyoshi et al.
2018) is a key concept to understand enhancement of theouter belt electrons. Figure 11 (Jaynes et al. 2015) showsa schematic diagram of how different plasma/particle
populations contribute to generate of plasma waves andsubsequent enhancement of outer belt electrons. These
processes occur associated with substorm activities. Sustained substorm activities for a few days driven by thesmall amplitude of southward IMF are essential to cause
the large flux enhancement through this process (Miyoshi
et al. 2013).

Rapid injection by IP shocks Interplanetary shocks caninject energetic electrons into the magnetosphere very
rapidly on time scales of a few minutes, i.e., on electron
time drift time scales; the most dramatic such an event
was first observed by the CRRES spacecraft (Blake et al.
1992). The observations showed very high energy electrons with energies > 13 MeV were injected deep into
the magnetospheric slot region ( 2 < L  3). Blake
et al. point out that the spectrum continues up to almost
50 MeV. The injected electrons were stably trapped in theFig. 11 Diagram illustrating the interplay and contribution of low-energy electrons (source) in generating plasma waves which act upon a seedpopulation to energize electrons to relativistic energies (reproduced from Jaynes et al. (2015))

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

inner zone and lingered for many years as observed by the
SAMPEX spacecraft (Looper et al. 1994). The process of
injection is fairly well understood and has been modeled(Li et al. 1993); (Hudson et al. 1997); Wygant et al. (Wygant
and et al. 1994)) and results from the electrons “surfing”
a magnetosonic pulse electric field while conserving theirfirst invariant.
During the VarSITI era, multiple IP shocks have resulted
in rapid injections of electrons energized during theirtransport from outer regions of the magnetosphere (Foster et al. 2015; Kanekal and et al. 2016; Schiller et al. 2016).
The injected electrons often lead to enhanced bunches
electrons as they drift around (drift echos). Figure 12

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shows the radiation belt electron response to the IP shock
event of March 2015 (Kanekal and et al. 2016). The figure
shows pitch angle-resolved electrons of several energies
as measured by the REPT instrument. Rapid injection followed by velocity dispersed drift echos are seen, with thearrival of the shock indicated by a vertical black line. Electron enhancements are seen to occur within about 5 minof the shock arrival. Due largely to the paucity of measurements, studies relating various shock properties tocharacteristics of electron injections have not been wellestablished. For example, the March 2015 shock event
showed a more harder spectrum (as compared to electron
spectrum just prior to injection) than the September 2017

Fig. 12 REPT measurements (Van Allen Probe A) of pitch angle-resolved energetic electrons. Relativistic electron intensities at energies of 2.6, 3.4,
4.2, and 6.3 MeV are shown from the top to bottom panels, respectively. The red vertical line shows the IP shock arrival time (reproduced from
Kanekal and et al. (2016))

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

event which was a much stronger shock (Kanekal 2020).
A study by Schiller et al. (2016) studied the relationshipbetween shock characteristics and the highest energy of
the injected electrons and found that shock mach number correlated best and suggest that shock strength plays
an important role. The studies mentioned in the previous
paragraph have established underlying physical processes
that result in these injections; however, we emphasize noconclusive relationship between IP shock parameters andelectron properties such as spectral hardness has been
established.
The space weather implications of rapid energization
due to IP shocks are significant, since, as the March 1991
event demonstrated, very high-energy electrons can beinjected deep unto the magnetosphere and persist forseveral years. Persistent high fluxes of electrons pose a
critical threat to spacecraft.

Solar driver dependence Relativistic electrons of theouter belt show different responses associated with thesolar wind disturbances. As mentioned previously, largescale solar wind structures such as CMEs and CIRs cause
geomagnetic storms, and the outer belt electron flux andspatial distributions also change during the period. For
geomagnetic storms, intense southward IMF is essential
to cause severe magnetic storm through the enhancementof the cross-polar cap potential. On the other hand, magnetic storms do not always cause large flux enhancementof the outer belt electrons (Reeves et al. 2013). Several
studies showed some cases of strong outer belt relativistic
electron enhancements during non-storm times (Schiller
et al. 2014), and these studies pointed out increasing ofchorus wave power during the flux enhancement event.
It has been indicated that the high-speed coronal holestream following CIR is more effective for the large flux
enhancement at GEO rather than CME-driven storms andthe southward IMF embedd in the high-speed coronalhole stream is important for the large flux enhancement(Miyoshi and Kataoka (2005), (2008); Kilpua et al. 2015).
Li et al. (2011) has shown that the solar wind speed is nota necessary condition for the flux enhancements at GEO.
These studies indicate the importance of the southward
IMF besides the solar wind speed that has been considered as a primary parameter for the outer belt electrons,
and prolonged substorm activities are essential to cause
the flux enhancement.
From the analysis of 200 stream interface events insolar cycle 23, Miyoshi et al. (2013) have shown that thesouthward IMF in the high-speed stream that is controlled
by the Russell-McPherron effect (Russell and McPherron 1973) affects key parameters of electron acceleration by chorus waves. These parameters include source
electron population, thermal plasma density, and chorus

Page 13 of 22

wave activity itself. They showed that prolonged substorm activity is essential for the large flux enhancement.
Their model as depicted in Fig. 13 (Miyoshi et al. 2013)shows how solar wind speed and the prolonged southward IMF control relativistic electron flux enhancement
through wave-particle interactions. More recently, Zhao
et al. (2017) showed increase of electron PSD associatedwith solar wind/geomagnetic parameters as well as theimportant role played by substorms.
It is generally accepted that CMEs predominantly occur
during the ascending phase and CIRS during the descending phase of the solar cycle; however, that is not always thecase. Kanekal et al. (2015) studied an interesting period
during November 2013 when the radiation belts were
driven concurrently by weak CMEs during an ongoing
HSS event. They used Van Allen Probes measurementsof electron intensities to calculate phase space densities(PSD) and found that the lower-energy electrons wereaccelerated by radial diffusion while the higher-energy
electrons were energized by wave-particle interactions.
Figure 14 shows the radial profile of PSD obtained using
MagEIS and REPT measurements onboard Van Allen
Probe A (similar results were found for Probe B). The profiles for two values of K, the second adiabatic invariant andfor multiple values of the first, i.e., μ show that at lower
values of the latter, the profiles are monotonic whereas at
higher values exhibit a peak.
Another recent study by Pandya et al. (2019) comparedradiation belt responses to 28(27) CME(CIR) events. They
examined the dependence of electron flux variability uponinterplanetary parameters such as solar wind speed, IMF
BZ . Their findings emphasize the different nature of electron responses to these two types of solar drivers. The
distinct response of the radiation belts to solar drivers has
also been demonstrated by Baker and et al. (2014c) who
studied an event during March 2013 when a high-speed
stream was followed by a coronal mass ejection. They
showed that this resulted in energization due to prolonged
gradual radial diffusion which was followed by a much
more rapid enhancement. More recently, Baker et al.
(2019) used relativistic electron measurements spanning
6 years from the REPT instrument onboard Van Allen
Probes. Their long-term study confirmed the importantrole played by solar wind speed in energizing electrons,
specifically a threshold speed of 500 km/s being required
to enhance relativistic electron intensities. Long periods
when the solar wind speed did not exceed the threshold showed no detectable high-energy electrons. As theauthors point, this fact, despite being known for decades(see, for example, Baker et al. 1979; Paulikas and Blake
1979), still remains unexplained as to the physical processes leading to the existence of such a threshold value.
From these studies, prolonged southward IMF, highspeed solar wind, and low solar wind dynamic pressureKanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

Page 14 of 22

Fig. 13 Schematic of the ideal setup and sequence for strong enhancement of outer belt electrons through chorus wave-particle interactions in thehigh-speed streams (Miyoshi et al. 2013)

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

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Fig. 14 Radial profiles of electron phase space density obtained from measurements by Van Allen Probe A of pitch angle-resolved energetic
electrons. The panels show for two values of the second adiabatic invariant, K multiple values of PSD for μ, the first invariant as color coded lines.
The numerical values are listed in the legend accompanying the figures (reproduced from Kanekal et al. (2015)

have been recognized as the main contributions to cause
the large flux enhancement. These situations are readily found in trailing edge of the stream interface withthe toward/away sector polarities in spring/fall throughRussell-McPherron effect. During the period, intense chorus waves are observed (Miyoshi et al. 2013; Li et al. 2014).
It is important to note that prolonged substorm activities
with/without magnetic storms are essential to cause thelarge flux enhancements. This is an important paradigm
to indicate the need for more accurate space weather
forecasting models.
2.5.2 Electron loss processes

Magnetopause shadowing An important mechanism ofloss of electrons from the radiation belts is the so-called
magnetopause shadowing, where drifting electrons areon trajectories that encounter the magnetopause, andas a consequence, they escape the magnetosphere into
interplanetary space. During disturbed times, as for example when a CME compresses the magnetosphere, themagnetopause moves inward and electrons which wereon closed drift shells now encounter the magnetopause

and escape. Studies suggest electron loss occurs associatedwith enhancements of the solar wind dynamic pressure,suggesting that the magnetopause shadowing leads toelectron loss (Turner and Ukhorskiy 2020).
If the phase space density of electrons in the outer partof the outer belt decreases, negative spatial gradient of thephase space density is expected, and subsequent outward
radial diffusion takes place (Miyoshi et al. 2003; Shprits etal. 2006), which contributes to electron loss of the wholeouter belt. Simulations using the Fokker-Planck equation
have assumed that electron phase space density decreases
if the last-closed drift L-shell is larger than the daysidemagnetopause.
On the other hand, actual trajectories of electrons in
the dayside magnetopause are complicated, and drift-shellbifurcation causes the pitch angle scattering. Several test
particle simulations have investigated these electron trajectories in detail (Kim et al. 2008, 2010, Saito et al.
2010). As mentioned by Kim and Lee (2014) and Mauket al. (2016), it is difficult for electrons to cross the field
line of the dayside magnetopause because gyro-radius ofelectrons are too small, and further detailed analysis lb

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

considering actual structure of the dayside magnetopause
is necessary.

Particle precipitation Pitch angle scattering is anotherimportant process to cause loss of relativistic electrons
from the outer belt. Whistler mode waves and EMIC
waves are important for the pitch angle scattering of relativistic electrons (Millan and Thorne 2007). From late of
the 2000s, observational evidence has shown that EMIC
waves actually cause the pitch angle scattering of MeV
electrons (Miyoshi et al. 2008; Rodger et al. 2015; Usanova
et al. 2014), which has been theoretically predicted in 1971
(Thorne and Kennel 1971). Since EMIC waves are generated by a temperature anisotropy of hot ions, ring current
ions affect MeV electrons through wave-particle interactions. Several studies (Kubota and Omura 2017) indicatedthat rapid precipitations are possible through non-linear
interactions between EMIC waves and MeV electrons.
Whistler mode waves also contribute to the loss ofelectrons. Hiss waves cause the pitch angle scattering of
MeV electrons, and the flux of MeV electrons gradually
decreases. Interactions with hiss waves are a primary loss
process of MeV electrons in the slot region. Recent studies (Zhao et al. 2019) showed that interactions with hisswaves produce reversed energy spectra in the slot region.
Chorus waves also cause precipitation of electrons. The
Arase observations identified for the first time that flux
enhancements of tens keV electrons inside loss cone correspond to enhancement of chorus waves (Kasahara etal. 2018), which is definite evidence that pitch angle
scattering and subsequent precipitation by wave-particle
interactions actually occur. Figure 15 reproduced from
Kasahara et al. (2018) illustrates schematically the interaction between chorus waves at the equator and bouncing
electrons which are pitch angle scattered ultimately into
the loss cone and precipitate. Figure 16 also reproduced
from Kasahara et al. (2018) shows ERG/Arase measurements illustrating the simultaneous enhancement of wave
power (top panel) and increased intensities of nearly fieldaligned electrons clearly indicating the causal connection
between the waves and precipitation of these electrons. If
chorus waves propagate to higher latitudes, chorus waves
can resonate with high-energy electrons and cause thepitch angle scattering of sub-relativistic/relativistic electrons (Miyoshi et al. 2015; Miyoshi et al. 2020), which
may contribute tothe loss of MeV electrons. One of thesignificant phenomena about particle precipitation is the
microbursts of relativistic electrons. Combined observations by balloon, FIREBIRD-II, AC-6 Cubesat identified the spatial and temporal evolution of a microburst
region (Anderson et al. 2017). The relationship between
microbursts and chorus waves as observed by FIREBIRDII and Van Allen Probes (Breneman et al. 2017) indicatesthat microbursts of MeV electrons result from non-linear

Page 16 of 22

wave-particle interactions between electrons and chorus
waves (Horne and Thorne 2003; Saito et al. 2012; Wangand Shprits; Miyoshi et al. 2020).
While it has been confirmed that both Whistler mode
waves and EMIC waves cause significant precipitation ofenergetic electrons from the outer belt, quantitative estimation to discriminate dominant loss process of the outerbelt has still been open problem.
2.6 Ground-based observations and conjunction studies

During solar cycle 24, multi-spacecraft have observed
Geospace, which enable us to realize multi-point coordinated observations. Many conjunction studies among
Van Allen Probes, Arase, THEMIS, MMS, and other satellites are achieved. One of the coordinated observations
from multi-spacecraft is the radial profile of the phasespace density. Boyd et al. (2018) investigated the phasespace density profile up to L = 7.5 using both Van Allen
Probes and THEMIS satellite, and they showed the peakof PSD beyond apogee altitude of Van Allen Probes.
Multi-spacecraft observations could discriminate spatialtemporal variations of particle and field/waves. For example, Kurita et al. (2018) showed loss of MeV electron flux
on timescales 30 min using both Van Allen Probes andArase satellites and argued that EMIC waves cause thepitch angle scattering. Teramoto et al. (2019) identified
modulations of MeV electrons by the MHD-fast mode
waves occurring over the limited azimuthal longitudes
using Van Allen Probes and Arase satellite.
Another great advantage for the inner magnetosphereresearch is the development of ground-based network
observations. Several multi-point network observations
have been developed, SuperDARN radars, magnetometers, optical imagers, riometers, and VLF/LF radio wave
receivers. There have been many opportunities for conjugate observations between satellites and ground-based
observations, which identified latitudinal/local time distributions.
2.7 Open questions and future directions

During the VarSITI era, new discoveries (e.g., multiple
belt structure) and new observations (e.g., direct observations of wave-particle interactions) advanced the current understanding of the dynamics of the inner magnetosphere, in particular, that of energization and lossof radiation belt electrons. This was made possible byimproved instrumentation and dedicated missions such as
Arase, Van Allen Probes, MMS, and THEMIS. For example, Arase and van Allen Probes carried comprehensive
instrument suites that characterized electric and magnetic
fields, plasma waves, and energetic particles.
Nevertheless, many open questions remain, and indeed,some of the recent discoveries may have opened up new
frontiers. For example, while the role of wave-particle

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

Page 17 of 22

Fig. 15 Interaction of electrons with Chorus waves leading to their precipitation Figure reproduced from Kasahara et al. (2018)

interactions in energization and loss has long been recognized (for a review see for example Thorne (2010)), theimportance of non-linear wave-wave and wave-particle
interactions remains an open question. Recent theoretical and modeling (e.g., see Omura et al. 2015) suggest thatthey play a crucial role. Another important open questionis in regard to electron loss from the radiation belts, viz.,the relative importance of magnetopause shadowing andwave-particle precipitation (Turner et al. 2012). Of criticalimportance is the role of energetic particle precipitation in
affecting the chemistry of the upper atmosphere (Turunen
et al. 2016), an important open question that needs tobe answered regarding climate change. The discovery of
the so-called impenetrable barrier (Baker et al. 2014a)
raises the question of its possible anthropic origin via
ground-based VLF transmitters. Another open questionis regarding the immediate injections resulting from IP
shocks, i.e., what are the IP shock properties and internal

magnetospheric conditions that result in the most intense
injections? An event similar to the March 1991 (Blake etal. 1992) could pose a serious hazard for spacecraft andspace weather consequences such as induced currents in
power grids. Quantitative understanding of the origin of
the highest energy electrons also remains an important
open question, despite some recent progress (Zhao et al.
2019).
The need for multi-point measurements in advancing
our quantitative understanding of the inner magnetospheric dynamics has long been recognized. This is a
key direction in future scientific endeavors. The adventof CubeSats has made this a real and tangible goal.
Innovations in space instrumentation and low-cost accessto space have further spurred interest in the community,and proposed missions include constellations of CubeSats to study radiation belts, ring current, and plasma
waves that drive particle dynamics (Kanekal et al. 2018,

Kanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

Page 18 of 22

Fig. 16 Figure reproduced from Kasahara et al. (2018)2019; Li et al. 2013). These CubeSat missions also play acomplementary role to major “flag ship” type missions as
the latter often have capabilities that are comprehensive(Fox and Burch; Miyoshi et al. 2018). Increasing societal reliance upon space-based assets for communication,
Earth observation, and global climate issues are also driving the need for future mission to comprehensively understand the near-Earth environment. Flagship type missions
that address the Sun-Earth, inner heliosphere as an integrated system are another important future trend that isbeing recognized as necessary to understand not only thesystem-as-a-whole but also the constituent parts such as
the radiation belts and ring current.

3 Conclusions
During the varSITI era, much progress has been madein understanding the dynamics of the inner magnetosphere, and this review has focused particularlyon the dynamics of the outer zone electrons. Major
missions such as Arase, THEMIS, Van Allen Probes,
and CubeSats and high-altitude balloons have contributed
to the progress. New discoveries, such a multi-beltmorphology, existence of a putative prevention ofcontinued inward motion of energetic electrons (so-called

impenetrable barrier mentioned earlier), and direct observation of wave-particle interactions, are some of the significant observational findings during the VarSITI era.
In addition, our understanding of the underlying physical processes of electron energization and loss has also
advanced considerably, e.g., in delineation of the importance of source, seed populations that lead to energization. Other areas of advances in understanding electronenergization include the complementary role played bywave-particle and radial diffusion in producing ultrarelativistic electrons. Conjunctions between spacecraft
and ground-based observations have been used to understand particle precipitation and the role of various plasma
waves in general in regard to electron energization as
well as loss. Contribution to space weather aspects, suchas detailed observations of rapid injection of multi-MeV
electrons by interplanetary shocks, has re-emphasized
the need for continuous monitoring of radiation in
geospace.
Yet, despite the impressive achievements during the
VarSITI era, not unexpectedly, the new findings andadvancements have opened up new vistas with many
new scientific questions. This era has also seen the
coming-of-age of CubeSats and advances in theKanekal and Miyoshi Progress in Earth and Planetary Science(2021) 8:35

miniaturization of space instrumentation, which havefor the first time made multi-point in situ measurements possible. We believe that the future is brightfor further progress in our understanding of the innermagnetosphere, especially in the light of technological innovations regarding space platforms suchas CubeSats, low-cost launches and sophisticatedinstrumentation.
Abbreviations
CeREs: Compact Radiation belt Explorer; CME: coronal mass ejections; CIR:
co-rotating interaction regions; CRRES: Combined Release and Radiation
Effects Satellite; CSSWE: Colorado Student Space Weather Experiment
CubeSat; EMIC: Electromagnetic Ion Cylcotron Waves; FIREBIRD: Focused
Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics;
GEO: Gysynchronous Earth Orbit; IMF: interplanetary magnetic field; JAXA:
Japan Aerospace eXploration Agency; MagEIS: Magnetic Electron Ion
Spectrometer; MMS: Magnetospheric Multi Scale mission; RBSP: Radiation Belt
Storm Probes; REPT: Relativistic Electron Proton Telescope; SAMPEX: Solar
Anomalous Magnetic Explorer; SPeCIMEN: Specification and Prediction of the
Coupled Inner-Magnetospheric Environment; THEMIS: Time History of Eventsand Macroscale Interactions during Substorms; VarSITI: The Variability of the
Sun and Its Terrestrial Impact
Acknowledgements
SK gratefully acknowledges the travel support to attend the VarSITI closing
workshop at Nagoya by the Institute for Space-Earth Environmental Research,
Nagoya University. He graciously acknowledges the hospitality and support
extended by professor Kazuo Shiokawa, Nagoya University.
Authors’ contributions
SK has contributed toward reviewing the results from Van Allen Probes. YM ischiefly responsible for the results from Arase. Both authors have read all
sections and approved the final manuscript.
Authors’ information
Shrikanth G. Kanekal is a research astrophysicist at NASA Goddard Space Flightcenter and is the deputy mission scientist on Van Allen Probes. He is also a Co-I
on the ECT suite on Van Allen Probes and instrument scientist for the REPT
instrument. He is the PI of the CubeSat CeREs. He has been the science teammember of NASA missions SAMPEX and Polar. His research interests include
the study of radiation belt dynamics, solar energetic particles, space
instrumentation, and space weather. He is also an adjunct associate professor
in the Physics Department at the Catholic University if America.
Yoshizumi Miyoshi is a professor at the Institute for Space-Earth Environmental
Research, Nagoya University, and a visiting professor, ISAS/JAXA, Graduate
School of Science, Tohoku University. He is also the project scientist for the
Arase mission. His research interests include data analysis and computer
simulation on radiation belts, wave-particle interactions, fast aurora
observations with EMCCD/s-CMOS cameras, data assimilation of radiation
belts, space weather forecast, and X-ray emission from Geospace. He is amember of several academic societies including JpGU and AGU. he has servedon the editorial board of several journals such as Annales Geophysicae and the
Journal of Geophysical Research.
Funding
SK was partly funded by the NASA Van Allen Probes mission.
Availability of data and materials
Not applicable.
Competing interests
The authors declare that they have no competing interest.
Author details
1 NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greeneblt 20771,
MD, USA. 2 Institute for Space-Earth Environmental Research, Nagoya
University, Furocho, Chikusa Ward, Nagoya, Aichi 464-8601, Japan.

Page 19 of 22

Received: 31 August 2020 Accepted: 2 February 2021

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