Simulation of dispersionless injections and drift echoes of energetic electrons associated with substorms

Abstract

Abstract. The term "dispersionless injection" refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes-in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

Particle Transport and Energization Associated with Disturbed Magnetospheric Events

“…We examine substorm energetic particle injection by computing the particle flux and comparing with geosynchronous satellite observations. Our results show that for pulse parameters leading to consistency with observed flux values, the bulk of the injected particles arrive from distances less than 9 R E , which is closer to the Earth than the values obtained by the previous model [1] and is also closer to the distances obtained by the injection boundary model. …”

“…This modification of particle flux after the particles are swept to the geosynchronous location is to simulate the subsequent loss (mainly by pitch-angle diffusion due to cyclotron instabilities, as shown in [15]). This practice was also used in previous calculations [1]. We also note that in our model particles interact with the pulse only once because the drift period at r geo is about 11 min for 600 keV particles and is much larger than the time (about 2 min) needed for all three parts of the pulse to pass through the geosynchronous orbit.…”

“…While test particle simulations based on this model have been extensively performed [1,6], in this paper we seek greater physical insight by providing analytical solutions for non-relativistic particle motion. Our field model and particle orbit solutions are qualitatively different from the previous simulations [1,6] in that by using pulse fields obtained in the cylindrical symmetry, we obtain that enhanced energetic particle fluxes ob-served at geosynchronous orbit during substorms are due to particles coming mainly from distances less than 9 R E away from the Earth, which is much closer to the Earth than the distance obtained by the previous simulation studies.…”

Substorm associated magnetotail energetic electrons pitch angle evolutions and flow reversals: Cluster observation

“…2) The injected energetic electrons tend to form a ''pancake'' distribution [Baker et al, 1978;Li et al, 1998] because the near-perpendicular pitch angle electrons are energized more efficiently than the near parallel pitch angle [Schulz and Lanzerotti, 1974;Sergeev et al, 1992].…”

Pitch angle dispersion of ion injections

“…Specifically, it would be interesting to study the dependence of pitch angle dispersion with ring current intensity and other factors that are parameterized separately in more recent generations of the Tsyganenko models [Tsyganenko, 1995]. For detailed event studies it may be necessary to perform modeling calculations in time-dependent model fields either of MHD model runs or of heuristic models as has recently been done for electrons [Li et al, 1993[Li et al, , 1998]. are correct when the orbits are wrong, the effects of this scattering on the average drift needed to be included in the present calculations.…”