Substorm particle injections
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The sharp increases of energetic particle fluxes in the
near-Earth tail, known as particle injections, are among the most
important and well-known manifestations of magnetospheric substorms. Although being known since late 1960s
(Arnoldy and Chan, 1969; Winckler, 1970), they are still not
explained in a satisfactory way. They have been extensively
studied using the geostationary
and other spacecraft (e.g., Walker at al., 1976; Baker et al.,
1982; Belian et al., 1978; Sauvaud and Winckler, 1980).
Some observations concernign the injections:
- Injections are considered to be one of the most common
and reliable indicators of substorm onset. They are
observed in association with nearly every substorm
identified by other means.
- Both electrons and ions (mainly protons) from about tens
to hundreds of keV are injected. The lower energies are
not affected similarly (energy cutoff; e.g.
Birn et al., 1997a).
- When enhanced fluxes at all energies are observed
simultaneously, injection is referred to as a
dispersionless injection. The region of space where
dispersionless injections are seen is called the
injection region. Also term "injection
boundary" (McIlwain, 1974; Mauk and McIlwain, 1974;
Reeves et al., 1991) has been used for the inner boundary.
- In general, the injection regions are located at
distances ranging from x = -4.3 Re to x = -15 Re (Friedel
et al., 1996). It seems probable that at least some
injection regions propagate towards Earth, with inward
propagation speed of about 24 km/s (Reeves et al., 1996).
- Injections are more typical in the premidnight sector.
They have a limited longitudinal extent which corresponds
to the sector occupied by SCW
(see, e.g., the statistical results by Vagina et al.,
1996). Some observations indicate that the regions of
electron and proton injections are slightly separated in
longitudinal direction, ions (electrons) being shifted westward
(eastward) of the center longitude (Birn et al., 1997a).
- Outside the injection region one observes particles that
have drifted out of it, and which thus show energy
dispersion due to different magnetic drift speed of
particles of different energy. In addition to energy
dispersion, also pitch angle dispersion is sometimes
observed (Walker et al., 1978; Greenspan et al., 1985).
Note that the energy dispersed flux increases can be used
to evaluate the original longitudinal position and time
of injections by tracing back the magnetic drift of
particles (e.g., Reeves et al., 1991; Shukhtina and
- Injections are often related to local magnetic field
dipolarizations, especially when the injections are
dispersionless. This magnetic field change is associated
with strong induced electric field (e.g., Aggson et al.,
- As many other substorm signatures, also injections
exhibit temporal finestructure
(e.g., Belian et al., 1984)
The main questions relating to the injections are the location
and means of particle acceleration. It seems obvious that the
dipolarization related induced electric fields play some role in
the particle acceleration (Lezniak and Winckler, 1970). Also the
inward, adiabatic drift may play role in some injection events.
However, there is most likely more to it:
- The dispersionless character of the energetic particle
flux increases seem to provide evidence for their local
(or near local) acceleration. This acceleration could be
due to any of the instabilities suggested by substorm models that favour near-Earth
initiation (cross-field current instability and
ballooning models; see, e.g., Lopez et al. (1990)). Also
induced electric field related to magnetic field
dipolarization can accelerate particles locally.
- Remote sites, like the current disruption (SCW) and near-tail
could accelerate particles by radiating
fast magnetosonic waves
(e.g., Morioka and Oya, 1996; Sergeev et al., 1998).
Also here acceleration is "local" since the
particles are not moving, only the electric field
- The third possibility is that particles are indeed moving
inward, perhaps from several Re away. In the ''convection
surge'' model (Quinn and Southwood, 1982; Mauk, 1986;
Delcourt et al., 1990) particles are energized by the
dipolarization process, and subsequently convected
earthward by the inductive electric field. Even long
drift paths could be possible, if the magnetic field
change propagates with the particles and cancels the
normal radial magnetic field gradient that otherwise
might separate particles of different energies (Li, ICS-4
- The reconnection process can also produce some of the
acceleration (Birn et al., 1997b).
Particle acceleration and adiabatic earthward displacement may
not always produce flux increases (Sergeev et al., 1998).
The resulting flux variation is a compromise between
the flux increase due to acceleration (depending on how soft the
energy spectrum is) and the density of energetic particles at the
point where they are taken from (if we have nothing, we will get
nothing). If the initial flux is low and the energy spectrum
flat, one may get a flux decrease instead of an increase. The
drifting electron holes (DEHs) are an
extreme example of this effect.
Finally, the strongest injections may be responsible for the
storm effects (enhanced ring current). However, the connection
between storms and substorms is not quite settled yet.
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