Dartmouth Events

Abstract: During the mid-twentieth century, the first earth-orbiting satellites unexpectedly discovered a two-zone structure of MeV charged particles trapped in the Earth’s magnetic field with the inner zone dominated by protons while electrons dominate the outer zone. The flux of these energetic particles, particularly the electrons, fluctuates dramatically during geomagnetic activity and the mechanisms driving this phenomenon are not well understood. However, global simulations provide a valuable tool to study the dynamics of these energetic particles in a controlled manner by using a combination of satellite measurements, the Lyon-Fedder-Mobarry (LFM) 3D magnetohydrodynamic (MHD) model, rbelt3d test-particle code and radial diffusion model. The first of the two mechanism studied was a gradual transport of electrons over several orbits via radial diffusion. The LFM MHD code was run for the 1 June 2013 solstice storm as well as the 17 March 2015 equinoctial storm. Both events were characterized by a Coronal Mass Ejection (CME)-triggered shock arrival at Earth and resulted in strong geomagnetic storms. However, the 1 June 2013 storm resulted in a loss of radiation belt electrons while the 17 March 2015 storm resulted in strong enhancement of MeV electron flux. The second mechanism studied was a prompt acceleration of electrons in drift resonance with the electric field impulse due to CME-shock compression of the magnetopause. Two such events occurred on 17 March 2015 and 6 September 2017, where electrons accelerate by ~1 MeV on a drift period timescale as the impulse propagates from the dayside to nightside around the flanks of the magnetosphere. Ringing ULF oscillations follow the bipolar electric field impulse and produce energy independent localized radial oscillations in the electron flux measured by the REPT instrument on the Van Allen Probes. The longitudinally localized acceleration produces energy dependent drift echoes at >1 MeV energy. MHD test-particle simulations reproduced drift echo signatures observed in the REPT measurements, including energy and pitch angle dependence of the drift echoes. The flux enhancement was short-lived for both events due to inward motion of the magnetopause following compression. However stronger events, such as the March 1991 and the Halloween 2003 storms, were observed by earlier spacecraft to produce transport deeper into the magnetosphere and a long-lived enhancement.