Multi-event Study on the Connection between Subauroral Polarization Streams and Deep Energetic Particle Injections in the Inner Magnetosphere

by Sam Califf
Cooperative Institute for Research in Environmental Sciences and NOAA National Centers for Environmental Information


Energetic electron injections with energies of 100s keV are frequently observed at low L shells (L <4) during moderate to strong geomagnetic storms. For these energies, radial transport through large-scale electric fields is thought to be the dominant energization mechanism, where the solar wind drives a dawn-dusk convection electric field that moves particles radially toward Earth on the nightside and increases their energy by driving the particles into a region of stronger magnetic field. A uniform convection electric field is expected to create a similar effect on protons with similar energies, but observations demonstrate that the protons do not reach the same low L shells as electrons. Subauroral polarization streams (SAPS) are a common magnetosphere-ionosphere coupling phenomena that produce strong radial electric fields in the post-dusk sector, which causes asymmetry in the large-scale electric field in the inner magnetosphere. This paper explores the relationship between SAPS and the differential response of 100s keV electrons and protons using Van Allen Probes observations.


We selected three geomagnetic storms where there were direct observations of SAPS and energetic electron injections to low L shells, and here we focus on the 17 March 2013 storm. Figure 1 shows radial profiles of phase space density for electrons and protons that were derived from Van Allen Probes flux measurements for several consecutive. Conceptually, phase space density is a quantity that is expected to be conserved if particles are transported radially by large-scale electric fields, and 𝜇 corresponds to particle energy. The persistent inward motion of the steep radial gradient for electrons (left panels) is an indication of inward radial transport. The protons (right panels) display a dramatically different response – at lower energies, the protons show inward transport that persisted for only a single orbit, and there was little response for the higher-energy protons.

Figure 1. Phase space density as a function of L shell for electrons (left) and protons (right) on 17 March 2013. The bottom panels show the relationship between the first adiabatic invariant, 𝜇, and particle energy.

The Van Allen Probes orbit crossed through the post-dusk sector during this event, offering an opportunity to directly measure the electric field in the SAPS region (Figure 2). The large peaks in the radial electric field are signatures of SAPS (Figure 2, panels a and e). SAPS are generated by coupling between the magnetosphere and the ionosphere through field-aligned currents that are driven by pressure gradients at the inner edge of the 10s keV ion population (Figure 2, panels d and j). The SAPS electric field was observed across the same L range as the large increases in electron flux (panels e and k) between these two Van Allen Probes orbits, which were separated by 1.5 hr.

Figure 2. SAPS measurements from consecutive duskside passes by RBSPB (left panels) and RBSPA (right panels) on 17 March 2013. (a,g) Radial and azimuthal components of the electric field in the frame co-rotating with Earth, (b,h) plasma density with the plasmapause marked by a red line, (c,i) <50 keV electron fluxes, (d,j) <50 keV proton fluxes, (e,k) 100s keV electron fluxes, and (f,l) 100s keV proton fluxes.

To illustrate the impact of SAPS on the energetic particle dynamics, drift trajectories for electrons and protons were evaluated assuming a uniform dawn-dusk convection electric field (Volland-Stern) and an empirical SAPS model that accounts for the radial electric field. Figure 3 demonstrates that electrons move further inward under the influence of SAPS than they would under a uniform dawn-dusk field alone, and the protons move radially outward. This simple model is consistent with the observations of inward transport for electrons when SAPS are observed and the lack of inward transport for protons.

Figure 3. (Left) electron and (right) proton drift trajectories under the Volland-Stern electric field alone (dashed lines) and under the combined Volland-Stern and SAPS electric fields (solid lines).


This study provides examples of direct SAPS observations occurring at the same time and spatial location as 100s keV electron enhancements, with little response in protons of similar energies. Under a uniform dawn-dusk convection electric field, the electrons and protons should respond similarly, but SAPS modifies the inner magnetospheric electric field and the drift paths of energetic particles by introducing a radial electric field in the post-dusk sector. The differential response between electrons and protons under the influence of SAPS was investigated using a simple SAPS model, which showed that SAPS causes electrons to move radially inward and protons to move radially outward. These results indicate that SAPS play an important role in energetic particle dynamics in the inner magnetosphere.


Califf, S., Zhao, H., Gkioulidou, M., Manweiler, J. W., Mitchell, D. G., and Tian, S. (2022). Multi‐Event Study on the Connection Between Subauroral Polarization Streams and Deep Energetic Particle Injections in the Inner Magnetosphere. Journal of Geophysical Research: Space Physics, 127(2), e2021JA029895.

Biographical Note

Sam Califf is a research scientist at the Cooperative Institute for Research in Environmental Sciences and NOAA National Centers for Environmental Information. His research interests include inner magnetospheric electric fields, energetic particle dynamics, magnetometer calibration and magnetic field modeling.

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