The formation of the stormtime ring current is a result of the inward transport and energization of plasma sheet ions. Previous studies have demonstrated that a significant fraction of the total inward plasma sheet transport takes place in the form of bursty bulk flows (BBFs), known theoretically as flux tube entropy-depleted “bubbles.’ However, it remains an open question to what extent bubbles contribute to the buildup of the stormtime ring current. Using the Multiscale Atmosphere Geospace Environment (MAGE) Model, we present a case study of the March 17, 2013 storm, including a quantitative analysis of the contribution of plasma transported by bubbles to the ring current. We show that bubbles are responsible for at least 50\% of the plasma energy enhancement within 6 R$_E$ during this strong geomagnetic storm. The bubbles that penetrate within 6 R$_E$ transport energy primarily in the form of enthalpy flux, followed by Poynting flux and relatively little as bulk kinetic flux. Return flows can transport outwards a significant fraction of the plasma energy being transported by inward flows, and therefore must be considered when quantifying the net contribution of bubbles to the energy buildup. Data-model comparison with proton intensities observed by the Van Allen Probes show that the model accurately reproduces both the bulk and spectral properties of the stormtime ring current. The evolution of the ring current energy spectra throughout the modeled storm is driven by both inward transport of an evolving plasma sheet population and by charge exchange with Earth’s geocorona.

Deep penetration of energetic electrons (10s-100s of keV) to low L-shells (L<4), as an important source of inner belt electrons, is commonly observed during geomagnetically active times. However, such deep penetration is not observed as frequently for similar energy protons, for which underlying mechanisms are not fully understood. To study their differential deep penetration, we conducted a statistical analysis using phase space densities (PSD) of μ=10-50 MeV/G, K=0.14 G^1/2Re electrons and protons from multi-year Van Allen Probes observations. The results suggest systematic differences in electron and proton deep penetration: electron PSD enhancements at low L-shells occur more frequently, deeply, and faster than protons. For μ=10-50 MeV/G electrons, the occurrence rate of deep penetration events (defined as daily-averaged PSD enhanced by at least a factor of 2 within a day at L<4) is ~2-3 events/month. For protons, only ~1 event/month was observed for μ=10 MeV/G, and much fewer events were identified for μ>20 MeV/G. Leveraging dual-Probe configurations, fast electron deep penetrations at L<4 are revealed: ~70% of electron deep penetration events occurred within ~9 hours; ~8%-13% occurred even within 3 hours, with lower-μ electrons penetrating faster than higher-μ electrons. These results suggest non-diffusive radial transport as the main mechanism of electron deep penetrations. In comparison, proton deep penetration happens at a slower pace. Statistics also show that the electron PSD radial gradient is much steeper than protons prior to deep penetration events, which can be responsible for these differential behaviors of electron and proton deep penetrations.

Dipolarization events with inductive, radial electric fields are examined, using Van Allen Probes data between 2013 and 2018. Two cases are studied, followed by statistical analyses. These events were observed between evening and premidnight magnetic local times (MLTs) under moderate geomagnetic activities. Radial electric field variations, azimuthal magnetic field variations, and energetic protons were often observed when horizontal magnetic fields started to decrease in the dip region. Magnetic field lines were stretched with their motion similar to the gradient B/curvature drift velocities of energetic protons. Signs of electric fields changed when horizontal magnetic fields started to increase in the dipolarization front (DF). Electric field variations were correlated with magnetic field ones with ~ 90 deg. phase shift. These observations are mainly interpreted in terms of energetic proton structures drifting toward the probe locations, while being accompanied by standing waves.

A dipolarization of the background magnetic field was observed during a conjunction of the Magnetospheric Multiscale (MMS) spacecraft and Van Allen Probe B on 22 September 2018. The spacecraft were located in the inner magnetosphere at L~6-7 just before midnight magnetic local time (MLT). The separation between MMS and Probe B was ~1 Re. Gradual dipolarization or an increase of the northward component Bz of the background field occurred on a timescale of minutes. Since both MMS and Probe B measured similar gradual increases, the spatial scale was of the order of the separation between these two. On top of that, there were Bz increases, and a decrease in one case, on a timescale of seconds, accompanied by large electric fields with amplitudes > several tens of mV/m. Spatial scale lengths were of the order of the ion inertial length and the ion gyroradius. The inertial term in the momentum equation and the Hall term in the generalized Ohm’s law were sometimes non-negligible. These small-scale variations are discussed in terms of the ballooning/interchange instability (BICI) and kinetic Alfven waves. It is inferred that physics of multiple scales was involved in the dynamics of this dipolarization event.