On the variability of seed, relativistic and ultra-relativistic electron population in the outer Van Allen radiation belt
Authors: Κατσαβριάς Χρήστος 
Published: September 2021

After more than half a century, from the unintentional discovery of the radiation belts by the team of J. Van Allen , the dynamic processes occurring in them (particle acceleration, loss, amplification, etc.) are not entirely understood.Outer belt electrons have been observed to reach ultra–relativistic energies well above 10 MeV (i.e., γ > 20). Taking into account that the typical energy of electrons in the terrestrial ionosphere is less than 1 eV and the typical energy of solar wind electrons is about 10 eV, it is obvious that how these electrons come to be energized is a major theme of space research, with both astrophysical and societal significance. In this dynamic area, the sources, losses, acceleration mechanisms and processes of transport of energetic particles remain primary issues in magnetospheric physics. Modern observation instruments and long term measurements (over a solar cycle) have significantly revised our perception of how electrons with initial energies of only a few eV can reach energies of several MeVs. From the somewhat simplistic view of relativistic electrons being just the plain, recurrent result of magnetic storms, we have come to the realization of the much more complex ways the flux of relativistic electrons is influenced by different plasma waves, which in turn are driven in variable ways by different interplanetary disturbances.Here, we – statistically – investigate the response of the outer Van Allen belt electrons to various types of solar wind (Interplanetary Coronal Mass Ejections–ICMEs, Stream Interaction Regions–SIRs and High Speed Solar Wind Streams–HSSWSs) and internal magnetospheric forcing (geospace magnetic storms of different intensities and to intense magnetospheric substorms). To that end, multi–point observations of electron Phase Space Density (PSD), as well as, various wave activity (ULF–Pc5 and whistler chorus waves) will be presented, during, more than 70 events, spanning the maximum and descending phase of Solar cycle 24 (2012–2018).In detail, after a brief introduction including the necessary physical background and the presentation of the missions and instruments used in this dissertation, we start by presenting a number of procedures concerning pre–processing methods for radiation belt particle data. These methods include: background determination, instrument saturation, signal-to-noise analysis, despiking and/or cross-species contamination. Furthermore, a method for cross-calibration of particle detectors is presented and, in detail, the cross-calibration of the THEMIS/SST instrument using the latest generation RBSP/MagEIS spectrometer.Chapter 4, presents a statistical analysis of 71 geospace disturbances and investigates the differences between events that resulted in relativistic electron enhancement and depletion. These events cover the time period from September 2012 to April 2018 (i.e. the maximum and the declining phase of the solar cycle 24). PSD calculations for 3 values of the first adiabatic invariant are used: 100 MeV/G corresponding to seed electrons (0.1–0.6 MeV), 900 MeV/G corresponding to relativistic electrons (0.7–2.4 MeV) and 4200 MeV/G corresponding to ultra–relativistic electrons (1.9–6.5 MeV), as well as simultaneous observations of Pc5 (2–7 mHz) and chorus waves.Chapter 5, presents a statistical analysis concerning the differences between ICME and SIR driven disturbances as well as their result in the various electron population (seed, relativistic and ultra–relativistic) of the outer radiation belt. The methodology and the the data used, are the same as in the previous chapter. Here we compare 32 events that were caused by SIR without shock features, 12 events caused by SIR with shock features and 14 events that caused by ICME with shock features. These events, as in the previous chapter, cover the time period from September 2012 to April 2018.Chapter 6, revisits the differences between events that resulted in relativistic electron enhancement and depletion by presenting detail analysis of two single and isolated events: the March 17, 2013 event which resulted in relativistic electron enhancement and the September 12, 2014 event which resulted in relativistic electron depletion. Finally, chapter 7, investigates, in detail, the mechanisms which affect acceleration and loss of relativistic and ultra–relativistic electrons during non–storm events, i.e. events during which the conditions in the interplanetary field did not allow magnetic reconnection. The first part of this chapter includes an analysis of the period from mid–April to mid–May 2017, a period with relatively weak geospace disturbances (SYMHmin ≈-50nT) that led to a long–lasting enhancement of relativistic and ultra–relativistic electrons (up to ≈10 MeV) similar to that of St. Patrick’s event in 2015 (SYM-Hmin ≈- 235nT). The second part of the chapter includes an analysis of the April 14, 2013 event, a period with consistently positive Dst values, which resulted in relativistic electron loss.We can summarize the conclusions of this dissertation in three categories:1. Episodes that resulted in acceleration/loss of relativistic electrons A common feature, in all the episodes examined, is the combined effect of magnetopause shadowing and outward radial diffusion driven by Pc5 waves. This combination seems to be the dominant loss mechanism and in addition, it is present in both enhancement/depletion event groups. This indicates that it’s the combination of acceleration mechanisms at "decides" the result of each geospace disturbance, regardless of the driver.2. Events driven by ICME and SIRThe fast solar wind streams that follow the SIR are more effective in accelerating electrons in relativistic energies, especially at higher L–shells. The acceleration is basically caused by the high solar wind speed values as well as the prolonged Pc5 and substorm activity. On the other hand, ICMEs are more effective in causing losses of relativistic electrons, especially in higher L–shells and during the sheath area. These losses are related to to combined effect of magnetopause shadowing and outward radial diffusion due to the combination of the maximum Pc5 power and the maximum compression of magnetopause.3. Episodes without geomagnetic stormResults during non–storm events show that the mechanisms responsible for acceleration and loss of the relativistic electron population of the outer belt are mainly internal. Even during a relatively "closed magnetosphere", these mechanisms are capable of producing the same (or similar) result to that of an intense geomagnetic storm.