An Enhanced "Flux-Corrected Transport"-Based Plasmasphere Refilling Model

TL;DR

Geomagnetic storms erode the plasmasphere, and plasmasphere refilling is governed by multi-ion, two-stream transport along flux tubes. The authors extend a flux-c corrected transport framework by solving the electron energy equation, allowing spatially and temporally varying $T_e$ and updating the ambipolar field via $E_{\parallel} = -\frac{1}{e n_e} \frac{\partial}{\partial s} (n_e k T_e)$, thereby producing a self-consistent pressure gradient that alters ion transport. They demonstrate the emergence of two-stage refilling, with enhanced early-time contributions from heavier ions (notably O$^+$) and a temperature-driven coupling between H$^+$ and He$^+$ through the ambipolar field, across L-shells $L=3$ and $L=4$. The results validate the approach, quantify ion-specific roles, and set the stage for 3D extensions and coupling to global ionosphere–thermosphere models for improved storm-time recovery predictions.

Abstract

A previously developed multi-ion, two-stream Flux-Corrected Transport (FCT) hydrodynamic model for plasmasphere refilling has been extended to incorporate self-consistent electron temperature evolution. The past assumption of a constant temperature along the modeled flux tube has been replaced by solving the electron energy equation, permitting spatially and temporally varying temperature. This improvement provides a more physically complete representation of the pressure and ambipolar electric-field gradients that influence ion transport. The extended model allows us to investigate two-stage refilling behavior established by prior observations and simulations. The model continues to reproduce the expected dominance of H+, enhanced early-time O+ contributions, and the coupling between H+ and He+ through the ambipolar electric field during the transition between stages. Sensitivity experiments with modified initial ion concentrations, including cases representing seasonal effects, highlight the distinct roles of each ion species in shaping the refilling trajectory. Comparisons across L-shells 3 and 4 further confirm the robustness of the model framework for future extension to three-dimensional geometries. Overall, by incorporating more realistic temperature variations, this enhanced model strengthens the physical understanding for interpreting complex multi-ion transport processes during plasmasphere recovery following geomagnetic storms.

Figures (5)

• Figure 1: Initial concentrations for the southern hemisphere (dashed) and northern hemisphere (solid) streams assumed for a depleted plasmasphere. The maximum concentrations for each ion and hemisphere's stream are set manually, which are symmetric about the equator for this standard case. The latitudes outside a stream's designated hemisphere are set to 0.2 cm$^{-3}$ and intermediate latitudes are solved analytically.
• Figure 2: Electron temperature across latitude at 10-minute intervals after a constant initial temperature of 3560K.
• Figure 3: (Top) Concentration of H$^{+}$ as a function of latitude and time. (Middle) Equatorial concentration of H$^{+}$, He$^{+}$, and O$^{+}$ ions as functions of time. (Bottom) The fractions of He$^{+}$ and O$^{+}$ out of the total ion concentration at the equator as functions of time.
• Figure 4: Equatorial electron temperature for the H$^{+}$ (top), He$^{+}$ (middle), and O$^{+}$ (bottom) simulations depicted in Fig. 4. The simulations for each ion correspond to those in Fig. 4 where the initial value of the ion was the Standard, decreased by a factor of 0.5 in both the northern and southern hemispheres (Symmetric), or decreased by a factor of 0.5 in only the southern hemisphere (Asymmetric).
• Figure 5: Total equatorial ion concentration (sum of H$^{+}$, He$^{+}$, and O$^{+}$) and equatorial temperature over time for L=3 and L=4.