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Field‐Aligned Photoelectron Energy Peaks at High Altitude and on the Nightside of Titan
Author(s) -
Cao Y.T.,
Wellbrock A.,
Coates A. J.,
CaroCarretero R.,
Jones G. H.,
Cui J.,
Galand M.,
Dougherty M. K.
Publication year - 2020
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1029/2019je006252
Subject(s) - photoelectric effect , titan (rocket family) , physics , x ray photoelectron spectroscopy , ionosphere , photoionization , zenith , solar zenith angle , electron spectrometer , ionization , atomic physics , magnetic field , electron , computational physics , atmospheric sciences , optics , geophysics , ion , astronomy , nuclear magnetic resonance , quantum mechanics , cathode ray
The ionization of N2by strong solar He II 30.4‐nm photons produces distinctive spectral peaks near 24.1 eV in Titan's upper atmosphere, which have been observed by the Electron Spectrometer (ELS) as part of the Cassini Plasma Spectrometer. The ELS observations reveal that, in addition to the dayside, photoelectron peaks were also detected on the deep nightside where photoionization is switched off, as well as at sufficiently high altitudes where the ambient neutral density is low. These photoelectron peaks are unlikely to be produced locally but instead must be contributed by transport along the magnetic field lines from their dayside source regions. In this study, we present a statistical survey of all photoelectron peaks identified with an automatic finite impulse response algorithm based on the available ELS data accumulated during 56 Titan flybys. The spatial distribution of photoelectron peaks indicates that most photoelectrons detected at an altitude above 4,000 km and a solar zenith angle above 100° are field aligned, which is consistent with the scenario of photoelectron transport along the magnetic field lines. Our analysis also reveals the presence of a photoelectron gap in the deep nightside ionosphere where almost no photoelectrons were detected. It appears to be very difficult for photoelectrons to travel to this region, and such a feature may not be driven by the changes in the orientation between the solar and corotation wakes.