Van Allen Radiation Belt Electron Dynamics During Geomagnetic Storms
Creators: Olifer, Leonid
Publisher: name: University of Alberta Library
Year: 2022

<p>Space radiation is often identified as one of the most prominent dangers of space exploration. It mostly originates at the Sun and streams into the space between planets, creating hazardous radiation conditions at every point of the solar system which is not shielded by an atmosphere. Therefore, it is crucial to characterize this radiation and determine the worst-case radiation levels to aid in the design of radiation-tolerant spacecraft and satellites, mitigate damaging space radiation effects, and protect humans during the exploration of deep space. This thesis focuses on studying the dynamics of the electron radiation trapped in the magnetic field of the Earth, also known as Van Allen radiation belts, during geomagnetic storms that create the most hazardous radiation conditions around our planet.  </p>

<p>Often, the dynamics of the electron radiation belt are asserted to result from a delicate balance between acceleration and loss. In contrast, this thesis shows evidence for some remarkable repeatability in flux dynamics associated with both the loss and acceleration phases of geomagnetic storms. In relation to loss, this thesis presents a study of 69 geomagnetic storms that demonstrate intense and isolated compressions of the magnetopause, characterized by the location of the last closed drift shell (LCDS), to assess the repeatability of loss processes associated with magnetopause shadowing. A superposed epoch analysis of the particle dynamics associated with periods of low LCDS reveals 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 the fast outward radial diffusion of the electrons to the compressed LCDS. </p>

<p>On the other hand, the lower energy electron population 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 (&lt;800 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. A superposed epoch analysis of 70 strong geomagnetic storms shows that the Kennel-Petschek limit impacts the radiation belt electron dynamics in almost every storm of at least strong intensity in the Van Allen Probes era (2012-2019). The study presented in this thesis also reveals how the Kennel-Petschek theory elegantly explains the evolution and hardening of the electron spectrum up to strongly relativistic (up to 2.6~MeV) energies, introducing an energy-dependent maximum flux limit. No current radiation belt models include this theory. As shown here, this is an important process that 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. </p>

<p>Finally, this thesis presents studies of geomagnetic storms with very fast radiation belt acceleration that increases the electron flux of relativistic or strongly relativistic electrons by orders of magnitude over very short periods of time (&lt;1 hr). The multipoint measurements by the Van Allen Probes reveal how the apparent phase space density (PSD) peak, commonly associated with local acceleration by the chorus waves, can instead arise from aliasing monotonic PSD profiles which are rapidly increasing due to acceleration from very fast inwards radial diffusion. In the absence of such multi-satellite conjunctions during fast acceleration events, such peaks might otherwise be interpreted as caused by local acceleration processes. As also shown in this thesis, utilizing an even larger electron radiation belt measurement dataset from 20 Global Positioning System satellites can reveal even faster radiation belt dynamics with greater detail.</p>

<p>Overall, this thesis provides an extensive study of the storm-time radiation belt dynamics, the main results of which not only contribute to the understanding of the dynamics of the near-Earth plasma but also can be used for the improved radiation specification models for designing radiation-hard space infrastructure.</p>