Energetic Particle Injection During Short Isolated Bubble as Seen in RCM Simulation and Spacecraft Observations in the Flow Braking Region.

Although energetic particle (EP) injections are commonly thought to be formed by the flow burst intrusion from the magnetotail, important details and quantitative aspects of their transport, acceleration and flow braking need further investigation and understanding. Motivated by frequent observations of short transient EP injections being not associated with substorms, we analyze high‐resolution Rice Convection Model simulations of a short (5‐min long) localized (∼3RE width) density depletion (evacuating 90% of flux tube content) initiated at the tailward simulation boundary (∼18RE) and allowed to evolve within an otherwise typical plasma sheet environment. We note that, driven by betatron‐like acceleration, the peak EP flux at fixed energy dramatically increases in a couple of minutes when the bubble head enters the inner magnetosphere at r < 8–10 RE giving rise to a localized injection of subsequently drifting EP clouds. Here the 50–200 keV electron flux reaches values as high as #105 (cm2 s sr keV)−1, and even higher energies (up to 1 MeV) may briefly appear. Surprisingly, at a later stage of bubble penetration, after termination of bubble jet from the tail, the injection boundary of high energy (HE) particles detaches from the bubble earthward boundary while the latter continues moving inward. Time History of Events and Macroscale Interactions during Substorms multi‐spacecraft mission observations of a short bubble‐like flow burst at the spacecraft cluster located near the flow stopping point, show much similarity with simulation results but also reveal important differences between responses of HE protons and electrons attributed to the finite gyroradius effect. Plain Language Summary: Large sudden increases of energetic particle (EP) fluxes in the outer radiation belts, associated with sporadic intrusions of localized fast plasma jets (plasma bubbles) from the tail plasma sheet, were studied by many authors. Still there were very few simulations of the entire process, from bubble launch till generation of energetic electron cloud drifting around the Earth in the radiation belt, which include the drift physics and self‐consistently and realistically treat the flow braking region. To fill this gap we analyze high‐resolution Rice Convection Model simulation of short (5‐min long) localized (3 RE width) density depletion starting from 18 RE, focusing on the formation of intense EP cloud during flow braking phase of elementary intrusion. Although particle acceleration acts all the way during flow burst propagation, EP energy and fluxes are significantly enhanced during last couple minutes of intrusion in the radially confined (couple RE) region near geostationary orbit, where injected accelerated particles are suddenly deflected azimuthally by the magnetic drift. While observations of Time History of Events and Macroscale Interactions during Substorms multi‐spacecraft mission spacecraft group in the flow braking region confirm close association of energetic electron flux and magnetic variations, different from simulations, proton flux increases started 1–2 min ahead of bubble arrival, which is explained by kinetic interaction with very narrow bubble front. Key Points: Energy flux of energetic particles explosively rises when bubble enter the flow braking regionSelf‐consistent dynamical sharp magnetic field gradients help form the injection boundary and assist in deeper inward penetrationofinjectionSharp appearance of high energy proton burst observed before the electron burst and dipolarization front as kinetic effect of non‐adiabatic protons [ABSTRACT FROM AUTHOR]