Quantifying loss of current sheet scattered electrons during the substorm growth phase

• Main Author: Beever, Zachary
• Other Authors: Semeter, Joshua
• Language: en_US
• Published: 2021
• Subjects: Plasma physics; Aurora; Magnetosphere; Nonadiabatic; Precipitation; Simulation; Space
• Online Access: https://hdl.handle.net/2144/42586

Particles trapped in the magnetosphere are naturally accelerated by the exchange of electromagnetic and kinetic energy, resulting in relativistic plasma populations. Through a number of processes, these particles can be scattered into the atmosphere and lost to interactions. Such precipitating particles can affect radio communications, ozone chemistry, and thermal structures. For these reasons, it is important to characterize loss mechanisms and quantify precipitation rates. This thesis examines one particular loss mechanism known as current sheet scattering (CSS). If interactions are negligible, charged particles in a magnetic field have approximately conserved quantities that characterize their motion provided the background field changes sufficiently slowly over space and time. The first of these ‘adiabatic invariants,’ the magnetic moment, is related to the particle’s mirror point along its bounce trajectory—the location at which the particle reverses direction in its journey from weaker to stronger B. In the equatorial region of the near-Earth magnetotail, where the radius of field line curvature of the magnetic field can become comparable to the gyroradius of ≈ 100 keV electrons, the homogeneity conditions needed for conservation of the magnetic moment of this population are broken. Upon passing through this location, known as the current sheet, these particles experience a chaotic change in their magnetic moment, and thus an alteration of their mirror point. This is the phenomenon of CSS. If the resulting mirror point lies within the atmosphere, the particle will most likely be lost through interactions. CSS is often investigated for highly relativistic electrons. However, recent observations suggest that this mechanism may account for a significant proportion of precipitating electrons between 100 and 300 keV during the substorm growth phase, a common space weather event wherein magnetic field lines in the near-Earth magnetotail become highly stretched. In this thesis, we test the efficacy of CSS as a loss mechanism for < 300 keV electrons by developing a relativistic charged particle tracer capable of solving complex trajectories in realistic magnetospheric magnetic field models. We then find distributional characteristics through Monte Carlo methods, comparing simulated ratios of loss- to total-flux with observations of the same quantities for a single substorm event. These observations are obtained by comparison of in situ measurements made by THEMIS (Time History of Events and Macroscale Interactions during Substorms) with ionospheric energy flux remotely sensed by PFISR (Poker Flat Incoherent Scatter Radar). Given an input distribution from THEMIS satellite measurements, we find agreement between observed and simulated loss- to total-flux ratios within an order of magnitude, with closer agreement for electrons between 100 and 300 keV. This implies CSS can explain a significant proportion of observed precipitation for the event studied and demonstrates its role as a prominent radiation belt loss mechanism. In particular, these findings suggest that the measured loss flux of < 300 keV electrons during such events can be immediately related to the geometry of the near-Earth magnetotail. This is further supported by a parametric study of initially field aligned distributions spawned at various nightside locations, showing a low-energy peak in the loss- to total-flux ratio at the boundary between the outermost radiation belt and the magnetotail. Measurements of particle orientation taken from THEMIS are low resolution, and agreement between simulated and observed loss- to total-flux ratios can be increased by assuming a more field aligned distribution for electrons below 100 keV. This suggests the presence of other physical processes besides CSS that may preferentially structure the pitch angle distributions of low energy electrons to be field aligned. Additional analysis is needed to identify these possible mechanisms. In summary, findings from this work support the role of CSS as an important contributor to < 300 keV electron loss during the substorm growth phase. Though there is an underestimation of loss for < 100 keV electrons, it is known that the empirical magnetic field models employed overestimate the radius of curvature in the current sheet. Furthermore, the dawn-dusk electric field has been neglected, though it has the possibility to produce field aligned electrons through current sheet acceleration. The inclusion of these effects in future studies may further improve agreement between simulation and observations.