Substorm particle injections

spaceweb@oulu.fi - last update: 9 May 1997, 2030 UT (RR)

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 Sergeev, 1991).
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., 1983).
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 reconnection region, 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 affecting them.
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 meeting, 1998).
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.

References

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Arnoldy, R. L., and K. W. Chan, Particle substorms observed at the geostationary orbit, J. Geophys. Res., 74, 5019-5028, 1969.
Baker, D. N., et al., Observations and modelling of energetic particles at synchronous orbit on July 29, 1977, J. Geophys. Res., 87, 5917-5932, 1982.
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