Particle energization by substorm dipolarizations

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

2. 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

3. Example of a dipolarization

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

5. Electron trajectories

6. Energy gain by electrons

7. Energy gain by electrons

8. Electron density enhancements Assuming uniform initial distribution of 5 keV electrons

9. 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

10. Earthward propagating pulse

11. Equatorial electrons: optimizing ti for max energization

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

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

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

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

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

17. 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