On the Similar and Repeatable Dynamics of Relativistic and Seed Electrons in the Terrestrial Van Allen Belt

Abstract:

Radiation belt electron flux dynamics are often asserted to result from a delicate balance between acceleration and loss. In contrast, here we show evidence for some remarkable repeatability in flux dynamics associated with both the loss and acceleration phases of geomagnetic storms. In relation to loss, we focus on 69 storms which demonstrate intense and isolated compressions of the last closed drift shell (LCDS) to assess the repeatability of loss processes associated with magnetopause shadowing. We perform a statistical superposed epoch analysis of the particle dynamics associated with minimum LCDS which reach L* < 5.8, revealing a clear and repeatable organization of the loss as a function of both L* and energy for electrons with energies above 600 keV. The high repeatability reveals almost the same fraction of pre-existing particles is rapidly lost in every event. This can be explained by fast outward radial diffusion of the electrons to the compressed LCDS. This suggests that the effectiveness and the domain of action of this LCDS-related loss are amenable to parameterization in a relatively simple analytical loss model. On the other hand, the lower energy electron population (<200 keV) does not experience loss over the course of the storm but is instead accelerated to levels two to three orders of magnitude above the pre-storm level within a period of hours at the beginning of the storm. Significantly, the maximum flux reached in every event for this lower energy population (< ~ 200 keV) is identical in almost every storm! Remarkably, the maximum flux reaches the theoretically derived limit exactly as predicted by the Kennel and Petschek (1966) theory developed more than 50 years ago. By performing superposed epoch analysis, we show that the Kennel-Petschek limit impacts the radiation belt electron dynamics in almost every geomagnetic storm event in the Van Allen Probes era. No current radiation belt models include the non-linear saturation of the Kennel-Petschek theory. As we show here, this is an important process which has a strong and sometimes controlling and limiting effect on electron flux. Its impacts appear to be essential for understanding and accurately predicting the absolute limits of the most extreme electron space radiation.