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Evolution of Pitch AngleDistributed Megaelectron Volt Electrons During Each Phase of the Geomagnetic Storm
Author  Pandya, Megha; Bhaskara, Veenadhari; Ebihara, Yusuke; Kanekal, Shrikanth; Baker, Daniel; 
Keywords  electron flux; inner magnetosphere; Pitch angle distribution; Radiation belts; Van Allen Probes 
Abstract  Using Relativistic Electron Proton Telescope measurements onboard Van Allen Probes, the evolution of electron pitch angle distributions (PADs) during the different phases of magnetic storms is studied. Electron fluxes are sorted in terms of storm phase, urn:xwiley:jgra:media:jgra55457:jgra55457math0001 value, energy, and magnetic local time (MLT) sectors for 55 magnetic storms from October 2012 through May 2017. To understand the potential mechanisms for the evolution of electron PADs, we fit PADs to a sinusoidal function urn:xwiley:jgra:media:jgra55457:jgra55457math0002, where urn:xwiley:jgra:media:jgra55457:jgra55457math0003 is the equatorial pitch angle and n is a real number. The major inferences from our study are (i) at L urn:xwiley:jgra:media:jgra55457:jgra55457math00045, the prestorm electron PADs are nearly isotropic (n urn:xwiley:jgra:media:jgra55457:jgra55457math00050), which evolves differently in different MLT sectors during the main phase subsequently recovering back to nearly isotropic distribution type during the storm recovery phase; (ii) for urn:xwiley:jgra:media:jgra55457:jgra55457math0006 urn:xwiley:jgra:media:jgra55457:jgra55457math0007 3.4 MeV, the main phase electron PADs become more pancake like on the dayside with high n values (>3), while it becomes more flattop to butterfly like on the nightside, (iii) at L = 5, magnetic field strength during the storm main phase enhances during the daytime and decreases during the nighttime. (iv) Conversely, at L urn:xwiley:jgra:media:jgra55457:jgra55457math00083, the electron PADs neither respond significantly to the different phase of the magnetic storm nor reflect any MLT dependence. (v) Main phase, electron fluxes with urn:xwiley:jgra:media:jgra55457:jgra55457math0009 <4.2 MeV shows a persistent 90\textdegree maximum PAD with n ranging between 0 and 2, while for urn:xwiley:jgra:media:jgra55457:jgra55457math0010 urn:xwiley:jgra:media:jgra55457:jgra55457math0011 4.2 MeV the distribution appears flattop and butterfly like. Our study shows that the relativistic electron PADs depend upon the geomagnetic storm phase and possible underlying mechanisms are discussed in this paper. 
Year of Publication  2019 
Journal  Journal of Geophysical Research: Space Physics 
Volume  125 
Number of Pages  
Section  
Date Published  12/2019 
ISBN  
URL  https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019JA027086 
DOI  10.1029/2019JA027086 