Evidence for significantly greater N₂ Lyman‐Birge‐Hopfield emission efficiencies in proton versus electron aurora based on analysis of coincident DMSP SSUSI and SSJ/5 data

Abstract

The launch of the Defense Meteorological Satellite Program (DMSP) satellite F16 in 2003 provided the first opportunity to analyze extensive sets of high‐quality coincident auroral particle and FUV data obtained by the onboard sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor Auroral Particle Sensor (SSJ/5). Features of interest are Ly α (121.6 nm), Lyman‐Birge‐Hopfield short (LBHS, the SSUSI 140–150 nm channel), and Lyman‐Birge‐Hopfield long (LBHL, 165–180 nm). We report on comparisons of column emission rates (CERs) by deriving simulated SSUSI values using SSJ/5 electron and ion (treated as proton) spectra. Field‐line tracing is performed to determine the locations of coincidences. CERs are obtained by integrating the products of particle spectra and monoenergetic emission yields. A technique is reported for deriving these yields from nonmonoenergetic CERs obtained by our particle transport model. SSJ/5 ion spectra are extrapolated above 30 keV using a statistical representation based on Polar Orbiting Environmental Satellites particle data. Key quantities of interest are ratios of SSUSI to SSJ/5‐based CERs (S‐S ratios) and corresponding ratios of proton‐produced to total emission (unity for Ly α and from 0 to 1 for LBHS and LBHL). SSJ/5‐based CERs are used to derive the latter ratios. Median ratio values are determined in order to reduce the error budget to primarily calibration and model errors. The median LBH S‐S ratios increase by a factor of ∼2.5 from electron to proton aurora and support significantly higher proton LBH emission efficiencies (3 times the electron efficiencies) assuming reported calibration uncertainties. This calls for significant increases in proton and/or H‐atom LBH cross sections. In turn, FUV auroral remote‐sensing algorithms must explicitly address both electron and proton aurora.

THE HIGH-RESOLUTION EXTREME-ULTRAVIOLET SPECTRUM OF N 2 BY ELECTRON IMPACT

“…The excitation of the N 2 Rydberg-valence states responsible for these emissions occurs in the short wavelength portion of the EUV between 800 and 1000 Å. This excitation, emission, and associated dissociation and ionization play a role in establishing the physical composition of the N 2 -bearing atmospheres of Earth (Meier et al 2005;Meier & Picone 1994;Knight et al 2008;Strickland et al 1999Strickland et al , 2004aStrickland et al , 2004b, Titan (Ajello et al 2007(Ajello et al , 2011bStevens et al 1994Stevens et al , 2011Stevens 2001;Broadfoot et al 1981;Fulchignoni et al 2005), Triton (Broadfoot et al 1989;Summers & Strobel 1991), and Pluto (Zhu et al 2014;Stern et al 2008;Bagenal et al 1997), and in interstellar clouds and extrasolar protoplanetary disks (Maret et al 2006;Pascucci et al 2009;Li et al 2013).…”

Laboratory studies of UV emissions from proton impact on N2: The Lyman-Birge-Hopfield band system for aurora analysis

“…Knight et al [2008] reported unexpectedly high efficiencies of LBH emission for proton precipitation, higher than predicted by current models. The earliest measurements of LBH excitation by H + impact on N 2 were obtained by energy loss spectroscopy (from 20 to 120 keV by Schowengerdt and Park [1970] and from 0.6 to 4 keV by Moore [1972]).…”

“…[4] The 2003 launch of the Defense Meteorological Satellite Program (DMSP) F16 satellite, which contains, among its suite of sensors, Special Sensor Auroral Particle Sensor (SSJ/5) and Special Sensor Ultraviolet Spectrographic Imager (SSUSI) [e.g., Paxton et al, 1992aPaxton et al, , 1992b made it possible, for the first time, to undertake extensive statistical studies of auroral output at far ultraviolet (FUV) wavelengths as a function of auroral input of energetic electrons and ions [Knight et al, 2008; J. T. Correira et al, A downward revision of a recently reported proton auroral LBH emission efficiency, submitted to Journal of Geophysical Research, 2010]. Independently, FUV can also be used to infer the solar energy input [e.g., Strickland et al, 2007] and also determine characteristics of auroral particles that deposit energy in the thermosphere [e.g., Strickland et al, 1993;Germany et al, 1994aGermany et al, , 1994bLummerzheim et al, 1991].…”

“…It is known that energetic protons and electrons do not interact in the same way within the atmosphere Strickland et al, 1993;Galand et al, 1997;Galand and Richmond, 2001;Galand and Lummerzheim, 2004;Knight et al, 2008], one difference being higher LBH emission efficiency for proton aurora. While current usage of cross sections leads to ∼50% higher efficiency, Knight et al found even greater values with some downward revision recommended by Correira et al (submitted manuscript, 2010) (also see section 4).…”

“…[7] LBH emissions result from collisions of N 2 with energetic electrons, combined neutral H and protons, as well as from secondary electrons produced by ionizing events [Knight et al, 2008]. Knight et al [2008] reported unexpectedly high efficiencies of LBH emission for proton precipitation, higher than predicted by current models.…”