Particle energization by substorm dipolarizations Konstantin Kabin Royal Military College of Canada Eric Donovan, German Kalugin, and Emma Spanswick University of Calgary

Introduction Substorm injection “creates” a population of electrons with energies of tens to hundreds of keV during the substorm expansion phase To model the energization process we use a simple magnetotail model with few adjustable parameters, controlling magnetotail thickness and transition from dipole to tail-like fields (Kabin et al. JGR 2011, 2017) We view dipolarization as a tailward retreat of this transition area We also consider electron energization by an earthward propagating EM pulse

Example of a dipolarization

Particle trajectories We trace electrons in the calculated dipolarization fields using Guiding Center equations. For equatorial electrons GC simplifies dramatically

Electron trajectories

Energy gain by electrons

Energy gain by electrons

Electron density enhancements Assuming uniform initial distribution of 5 keV electrons

Parameters used: E0=87.5 mV/m, V0=125 km/s, Model of the EM pulse Similar to Li et al 1998, Sarris et al, 2002, Zaharia et al, 2000, Gabrielse et al., 2016, etc, but 3D Parameters used: E0=87.5 mV/m, V0=125 km/s, b=1.5 RE, h=0.7 RE, =0.2 rad

Earthward propagating pulse

Equatorial electrons: optimizing ti for max energization

Equatorial electrons: radial transport and energization Dependence on the initial position

Equatorial electrons: radial transport and energization Dependence on the initial energy

Non-equatorial electrons (pitch angle dependence) Co-longitude of the mirror points as a function of the equatorial pitch angle

Non-Equatorial electrons: radial transport and energization Dependence on the initial equatorial pitch angle, 10 keV initial energy

Non-Equatorial electrons: pitch angle changes Dependence of the final pitch angle on the initial one

Conclusions We considered electron energization by two different processes: a tailward retreat of the near-earth transition region, and by an earthward propagating Electromagnetic impulse In both cases we observed substantial electron energization factors of 10-25 Energization factors are the largest for equatorial electrons and decrease for other initial pitch angles Azimuthal electric field drives electron pitch angles towards 90