THEMIS Radiation Belt Science
Introduction
One of the most interesting and important questions in current solar-terrestrial physics research
concerns the acceleration of electrons to relativistic speeds. The fundamental mechanisms
proposed to explain the dynamics, energization and loss of these particles are numerous, and
which dominate remains largely unknown (see e.g., the review by Friedel et al. (2002)). Likely
the most influential acceleration mechanisms are resonance with VLF lower band chorus, which
operates through violation of the first adiabatic invariant (e.g., Meredith et al., 2003; Chen et al.,
2007, and resonance with ULF waves which typically operates through violation of the third (e.g.,
Falthammer 1966; Schultz and Lanzerotti, 1974; Elkington. et al 1999; Mathie and Mann, 2000).
Figure 1 (left panel), from Mathie and Mann (2000), shows clear correlations between Pc5 ULF
power and both solar wind speed and > 2MeV electron flux at geosynchronous orbit (GEO) for 6
months of the declining phase of the solar cycle in 1995. Any acceleration process is also
continuously competing with co-incident loss processes, scattering of electrons into the loss cone.
EMIC waves have been identified as a potentially dominant loss process for MeV electrons from
the outer zone radiation belt, through a Doppler shifted cyclotron resonance and scattering into
the loss cone [e.g., Horne and Thorne, 1998; Friedel et al., 2002; Meredith et al., 2003; Summers
and Thorne, 2003].
THEMIS has an excellent capability for supporting studies of long period ULF and
electromagnetic ion cyclotron (EMIC) wave related radiation belt acceleration and loss processes
in the time domain, using SST measurements as well as the combined 3D electric and magnetic
fields from the EFW, and FGM and SCM. Longer term changes in radiation belt morphology due
to the stochastic combination of ULF (Pc3-5 and EMIC) and VLF acceleration and loss processes
through studies of the dynamics of radial profiles of electron phase space density (PSD) up to
energies of 900 keV are also possible with the SST. During the extended mission phase, new
satellite separations which have not previously been available during the THEMIS prime mission
configuration will enable new discoveries in understanding the radiation belts at energies up to
900 keV. The broader response of the radiation belts can also be monitored using partner
measurements within the Great Observatory from Cluster, HEO, SAMPEX, LANL and GOES
satellites. THEMIS studies will enable the analysis of the key processes driving radiation belt
dynamics in advance of the more extensive studies probing higher electron and proton energies to
be completed with the NASA Radiation Belt Storm Probes.
Key Science Targets in the Extended Mission Phase
During mission year 3, further coverage in the inner magnetosphere will be provided as the solar
cycle approaches maximum. This will increase the number of suitable intense storm events
which can be studied with the THEMIS constellation. THEMIS multiple satellite sweeps through
the inner magnetosphere will enable both the characteristics of the ULF wave resonance particle
Poincare maps to be developed [cf. Elkington et al., (2003); Degeling et al., (2007) for application
to the electron radiation belts], and the structure of the particle acceleration response to discrete
frequency Pc5 ULF wave modes to be analyzed (cf. right panel of Fig. 1). This will enable the
role of discrete frequency ULF waves in radiation belt dynamics upto 900 keV energy to be
analysed [cf. Degeling et al., 2007] as well as non-diffusive features which may develop due to
electron orbit changes in response to changes in the magnetic field in the inner magnetosphere
[e.g, Ukhorskiy et al., 2006].
The energy, W, of a radiation belt electron drifting through perturbing electric and magnetic fields
will change at a rate given by,
dW
M b
 qE  Vd 
dt
 t
where M 
2
p
. Here
2m p B
p is the perpendicular component of the particle’s relativistic
momentum, (γmpV┴), mp is the mass of the particle, γ is the relativistic correction factor (γ=(1-V 2/
c 2) -1/2 ) where V is the total speed of the particle), and B is the magnitude of the magnetic field at
the location of the particle (e.g., Brizzard and Chan, 2001). Observation and supporting modeling
within the THEMIS team will enable the relative importance of poloidal and toroidal electric field
polarisation, as well as compressional magnetic wave power, in driving radiation belt radial
transport and acceleration to be determined [cf. Elkington et al., 2003; Degeling et al., 2007].
Similarly, along the THEMIS orbit, high temporal cadence PSD(L) maps will be produced and
compared to profiles of Pc5 and EMIC waves seen in-situ and with the THEMIS GBO network.
Fig. 1. Left Panel: Figure from Mathie and Mann, GRL, (2000) showing the strong correlation
between ground-based Pc5 ULF wave power and the flux of >2 MeV electons at GEO. Right
panel: Time-domain relationships between ground magnetic fields, and energetic particles
measurements from two LANL and the CRRES satellites during the March 1991 superstorm.
In mission years 4 and 5, the change to P3-5 probe configuration to provide a closely spaced
constellation at apogee will have the added benefit that the probes will transform into a closely
spaced string of pearls as they cross the radiation belt. In the dawn-dusk sector, separations of
~0.1-1.0 Re will allow characterization of the long period Pc3-5 waves excited in the Earths
magnetospheric waveguide on the flanks, multiple satellites traversing regions of enhanced
magnetic and electric field power and fine structure such as at Pc5 field line resonances [e.g., Rae
et al, 2005], and the resulting energetic particle dynamic response. Recent studies have also
suggested that eastward propagating moderate azimuthal mode waves (m~20-40), driven by driftbounce resonance with ~few 100 keV O+ ions outside a depleted plasmapause, can also energise
MeV electrons via drift resonance at L~4 (Ozeke and Mann, 2007). Given there is an ample
supply of energy in the ring current, such a mechanism is attractive for radiation belt electron
acceleration. THEMIS case studies can be used to validate these important concepts and define
the relationship between the plasmasphere, ring current, and radiation belts.
A very surprising recent observation is the correlation of the inner edge of the radiation belt with
the plasmapause [e.g., Li et al., 2006]. The unusual MeV electron penetration into the slot region
during first day of the Halloween 2003 storms [e.g., Baker et al., Nature, 2004] was shown by
Loto’aniu et al., [2006] to be consistent with enhanced ULF wave radial diffusion occurring in
response to ULF wave penetration to anomalously low-L. On the 29th October 2003, a rapid
decrease in eigenfrequency was observed using the cross-phase technique [cf. Menk et al., 2004;
Dent et al., 2006], most likely due to the injection of O+ ions from the ionosphere, enabling ULF
wave energy to penetrate much more deeply than usual [Loto’aniu et al., 2006; Kale et al., 2007].
Tantalisingly, this suggests that cold (eV energy) plasma might play a critical role in the
dynamics of the apparently totally separate MeV energy radiation belt particle population, 6
orders of magnitude away in energy, via the intermediary of ULF waves. In addition, EMIC wave
growth rates are also modulated by the ambient density, and the intensity of VLF wave particle
interactions in the radiation belts are also influenced by total mass density. The suite of
instruments on-board the THEMIS probes allow the relationship between wave-particle
interactions and electron density to be determined using in-situ EFW spacecraft potential.
Dayside total mass density can also be monitored conjugate to the probes using ground-based
magnetometer cross-phase applied to the THEMIS GMAGs and supporting additional stations
such as from the Canadian CARISMA array (www.carisma.ca).
The THEMIS string of pearls configuration can also be used to examine the spatial and temporal
structure of EMIC wave regions and their role in radiation belt electron loss. Depending on ion
composition, EMIC waves typically occur in three bands below the hydrogen, helium and oxygen
ion gyrofrequencies. EMIC waves in the inner magnetosphere are believed to be preferentially
excited in a spatially localized zone along the high density dusk-side plasmapause [Horne and
Thorne, 1993, Kozyra et al., 1997; Jordanova et al., 2001]. Although EMIC waves are present
even during relatively quiet geomagnetic conditions, the waves occur most frequently and are
most intense during magnetic storms. THEMIS will study the structure of free energy in ion
distribuitions driving the EMIC waves, most likely through temperature anisotropy. The
dependence of the excitation on the ambient cold plasma density properties in the plasmasphere,
plasmatrough, and in plasmaspheric plumes can be examined using electron density proxy from
EFW spacecraft potential as well as data from the ESA.
Interestingly, recent observations from the THEMIS cruise phase show that intense structured
EMIC emissions can be localized in L-shell to regions less than ~0.5 Re. Fig. 2 shows THEMIS
and conjugate ground-based observations (from CARISMA ISLL station at L=5.5) of EMIC
emissions triggered during an enhancement of dynamic pressure. THEMIS C, D and E, all
traverse similar structured emissions as they each cross an EMIC active region. During the
extended mission phase in years 4 and 5, on the nightside, on the dayside, and on the flanks,
closer THEMIS probe separations will occur than were available in the cruise phase, sometimes
in a tetrahedron. This will enable the phase relationships between the EMIC electric and magnetic
fields, as well as between ring current ions and radiation belt electrons, to be determined since
closer spaced multiple probes may be embedded in the same EMIC emission at the same time.
An additional important EMIC controversy is that Pc1 waves on the ground only seem to occur
during storm recovery phase, and not during the main phase when most MeV electron loss occurs
(Mark Engebretson, Personal Communication, 2007). The apparent lack of EMIC waves on the
ground might be explained by ion cross-over absorption due to storm-time heavy ions [e.g.,
Thorne and Horne, 1994], however it may also represent a lack of waves in the magnetosphere at
the times when most MeV electron loss is observed. In general, waves below the He+
gyrofrequency are believed to be the most efficient for MeV electron scattering. The THEMIS
probes and the GBOs provide the ability to complete case and superposed statistical surveys.
Monitoring EMIC wave power on the ground and in space, together with radiation belt energetic
electron flux as a function of L during storms, can be used to address this question.
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