1._CRD Geophysical_AJ0 Concept_NN1 
1.1_CRD ._SENT Origins_NN2 
Images_NN2 of_PRF the_AT0 far_AV0 ultraviolet_AJ0 (_PUL FUV_NP0 )_PUR dayglow_NN1 observed_VVN in_PRP the_AT0 direction_NN1 of_PRF the_AT0 Earth_NN1 's_POS disk_NN1 have_VHB been_VBN made_VVN for_PRP more_AV0 than_CJS 40_CRD years_NN2 ,_PUN for_PRP example_NN1 by_PRP the_AT0 Apollo_NP0 16_CRD Lunar_AJ0 Camera_NN1 (_PUL Carruthers_NP0 &_NP0 Page_NP0 ,_PUN 1976a_CRD ,_PUN 1976b_CRD )_PUR ,_PUN the_AT0 Dynamics_NN0 Explorer_NN1 (_PUL Frank_NP0 and_CJC Craven_NP0 ,_PUN 1988_CRD )_PUR and_CJC the_AT0 POLAR_AJ0 missions_NN2 (_PUL Germany_NP0 et_NP0 al._NP0 ,_PUN 1994_CRD ;_PUN Frank_NP0 et_NP0 al._NP0 ,_PUN 1995_CRD ;_PUN Torr_NP0 et_NP0 al._NP0 ,_PUN 1995_CRD )_PUR ._SENT Often_AV0 these_DT0 observations_NN2 were_VBD used_VVN to_TO0 provide_VVI context_NN1 for_PRP in_PRP situ_NN1 measurements_NN2 without_PRP requirement_NN1 for_PRP quantitative_AJ0 assessment_NN1 ._SENT Despite_PRP the_AT0 application_NN1 of_PRF forward_AJ0 models_NN2 to_TO0 assess_VVI magnitudes_NN2 and_CJC morphologies_NN2 of_PRF the_AT0 imaged_AJ0 emission_NN1 rates_NN2 (_PUL e.g._AV0 ,_PUN Drob_NN1 et_NP0 al._NP0 ,_PUN 1998_CRD ;_PUN Germany_NP0 et_NP0 al._NP0 ,_PUN 1994_CRD ;_PUN Meier_NP0 et_NP0 al._NP0 ,_PUN 1995_CRD )_PUR and_CJC empirical_AJ0 models_NN2 to_TO0 assess_VVI variability_NN1 (_PUL Immel_NP0 et_NP0 al._NP0 ,_PUN 2000_CRD )_PUR ,_PUN the_AT0 extraction_NN1 of_PRF quantitative_AJ0 atmospheric_AJ0 composition_NN1 information_NN1 from_PRP the_AT0 data_NN0 proved_VVD challenging_AJ0 because_CJS the_AT0 FUV_NP0 disk_NN1 images_NN2 depict_VVB column_NN1 emission_NN1 rates_NN2 (_PUL CER_NP0 )_PUR without_PRP revealing_VVG the_AT0 altitudes_NN2 where_AVQ the_AT0 signal_NN1 originates_VVZ ._SENT In_PRP contrast_NN1 ,_PUN observations_NN2 of_PRF the_AT0 ultraviolet_AJ0 emission_NN1 from_PRP Earth_NN1 's_POS limb_NN1 do_VDB contain_VVI quantitative_AJ0 information_NN1 (_PUL e.g._AV0 ,_PUN Meier_NP0 &_NP0 Anderson_NP0 ,_PUN 1983_CRD ;_PUN Meier_NP0 &_NP0 Picone_NP0 ,_PUN 1994_CRD ;_PUN Meier_NP0 et_NP0 al._NP0 ,_PUN 2015_CRD )_PUR because_CJS their_DPS altitude_NN1 variations_NN2 above_PRP the_AT0 horizon_NN1 allow_VVI extraction_NN1 of_PRF composition_NN1 profiles_NN2 ._SENT 
Strickland_NP0 et_NP0 al._NP0 (_PUL 1995_CRD )_PUR overcame_VVD the_AT0 disk_NN1 problem_NN1 ,_PUN for_PRP the_AT0 first_ORD time_NN1 ,_PUN by_PRP demonstrating_VVG a_AT0 functional_AJ0 relationship_NN1 between_PRP the_AT0 ratios_NN2 of_PRF observed_AJ0 far_AJ0 ultraviolet_AJ0 airglow_NN1 emission_NN1 intensities_NN2 and_CJC the_AT0 ratios_NN2 of_PRF the_AT0 column_NN1 densities_NN2 of_PRF O_ZZ0 and_CJC N2_CRD ._SENT The_AT0 latter_DT0 quantity_NN1 is_VBZ designated_VVN herein_AV0 as_PRP ΣO_NP0 /_PUN N2_CRD to_TO0 distinguish_VVI the_AT0 ratio_NN1 of_PRF column_NN1 densities_NN2 from_PRP the_AT0 volume_NN1 density_NN1 ratio_NN1 ,_PUN O_ZZ0 /_PUN N2_CRD ._SENT Strickland_NP0 et_NP0 al._NP0 found_VVD minimal_AJ0 ambiguity_NN1 in_PRP the_AT0 relationship_NN1 when_AVQ the_AT0 vertical_AJ0 column_NN1 densities_NN2 are_VBB calculated_VVN above_PRP an_AT0 altitude_NN1 ,_PUN z17_AJ0 ,_PUN corresponding_VVG to_PRP an_AT0 N2_AJ0 column_NN1 (_PUL from_PRP infinity_NN1 down_AVP to_PRP z17_CRD )_PUR of_PRF 1017_CRD cm−2_CRD (_PUL =_NP0 NN2_CRD in_PRP Equation_NN1 1_CRD ;_PUN NO_NN1 is_VBZ the_AT0 O_ZZ0 column_NN1 density_NN1 above_PRP z17_CRD )_PUR ._SENT Thus_AV0 ,_PUN ΣO_NP0 /_PUN N2_NP0 is_VBZ defined_VVN as_PRP :_PUN equation_NN1 
At_PRP NN2_CRD =_NP0 1017_CRD cm−2_CRD ,_PUN Strickland_NP0 et_NP0 al._NP0 found_VVD little_DT0 dependence_NN1 of_PRF the_AT0 relationship_NN1 between_PRP ΣO_NP0 /_PUN N2_NP0 and_CJC the_AT0 135.6_CRD nm_NN1 /_PUN LBH_NP0 emission_NN1 ratio_NN1 on_PRP the_AT0 details_NN2 of_PRF the_AT0 model_NN1 atmosphere_NN1 ,_PUN although_CJS they_PNP did_VDD find_VVI a_AT0 dependence_NN1 on_PRP solar_AJ0 zenith_NN1 angle_NN1 and_CJC viewing_NN1 angle_NN1 from_PRP nadir_NN1 (_PUL Evans_NP0 et_NP0 al._NP0 ,_PUN 1995_CRD )_PUR ._SENT The_AT0 main_AJ0 point_NN1 of_PRF their_DPS discovery_NN1 is_VBZ that_CJT the_AT0 column_NN1 density_NN1 ratio_NN1 can_VM0 be_VBI found_VVN from_PRP the_AT0 ratio_NN1 of_PRF disk_NN1 radiances_NN2 without_PRP knowing_VVG how_AVQ O_ZZ0 and_CJC N2_CRD are_VBB distributed_VVN throughout_PRP the_AT0 atmosphere_NN1 ._SENT 
Further_AV0 ,_PUN they_PNP were_VBD able_AJ0 to_TO0 quantify_VVI the_AT0 solar_AJ0 extreme_AJ0 ultraviolet_NN1 (_PUL EUV_NP0 )_PUR energy_NN1 flux_NN1 required_VVN to_TO0 produce_VVI the_AT0 observed_AJ0 airglow_NN1 by_PRP using_VVG the_AT0 absolute_AJ0 magnitude_NN1 of_PRF either_DT0 the_AT0 O_ZZ0 or_CJC the_AT0 N2_CRD CERs_NN2 ._SENT Strickland_NP0 et_NP0 al._NP0 designated_VVD this_DT0 solar_AJ0 variable_NN1 as_PRP Qeuv_NP0 the_AT0 integral_AJ0 over_PRP wavelength_NN1 of_PRF the_AT0 model_NN1 solar_AJ0 spectral_AJ0 irradiance_NN1 from_PRP 1_CRD to_PRP 45_CRD nm_NN2 ,_PUN in_PRP units_NN2 of_PRF W_NP0 m−2_NP0 ,_PUN a_AT0 quantity_NN1 directly_AV0 comparable_AJ0 to_PRP independent_AJ0 measurements_NN2 of_PRF solar_AJ0 EUV_NP0 spectral_AJ0 irradiance_NN1 (_PUL e.g._AV0 ,_PUN Woods_NP0 et_NP0 al._NP0 ,_PUN 2008_CRD )_PUR ._SENT This_DT0 allows_VVZ for_PRP determination_NN1 of_PRF the_AT0 internal_AJ0 consistency_NN1 of_PRF the_AT0 FUV_NP0 airglow_NN1 observations_NN2 and_CJC extracted_VVD geophysical_AJ0 quantities_NN2 ._SENT 
Subsequent_AJ0 studies_NN2 arising_VVG from_PRP the_AT0 pioneering_AJ0 work_NN1 of_PRF Strickland_NP0 et_NP0 al._NP0 (_PUL 1995_CRD )_PUR leave_VVB no_AT0 doubt_NN1 about_PRP the_AT0 geophysical_AJ0 importance_NN1 of_PRF ΣO_NP0 /_PUN N2_NP0 and_CJC the_AT0 value_NN1 of_PRF its_DPS close_AJ0 relationship_NN1 to_PRP O_ZZ0 and_CJC N2_NP0 FUV_NP0 emission_NN1 rates_NN2 ._SENT Embodied_VVN in_PRP Equation_NN1 1_CRD is_VBZ the_AT0 prescription_NN1 for_PRP comparison_NN1 of_PRF observed_AJ0 ΣO_NN1 /_PUN N2_CRD to_PRP atmospheric_AJ0 model_NN1 predictions_NN2 :_PUN simply_AV0 integrate_VVB the_AT0 model_NN1 N2_CRD density_NN1 vertically_AV0 downward_AJ0 until_CJS the_AT0 altitude_NN1 (_PUL z17_CRD )_PUR is_VBZ found_VVN where_CJS the_AT0 column_NN1 density_NN1 is_VBZ 1017_CRD cm−2_CRD ._SENT Next_ORD ,_PUN the_AT0 O_ZZ0 column_NN1 density_NN1 is_VBZ computed_VVN above_PRP that_DT0 altitude_NN1 and_CJC divided_VVN by_PRP 1017_CRD to_TO0 obtain_VVI the_AT0 model_NN1 ΣO_NN1 /_PUN N2_CRD for_PRP comparison_NN1 with_PRP observations_NN2 ._SENT This_DT0 definition_NN1 of_PRF altitude_NN1 is_VBZ only_AV0 needed_VVN for_PRP comparison_NN1 with_PRP models_NN2 ._SENT It_PNP does_VDZ not_XX0 violate_VVI the_AT0 Strickland_NP0 et_NP0 al._NP0 finding_VVG that_CJT the_AT0 ΣO_NN1 /_PUN N2_NP0 retrieved_VVD from_PRP observed_VVD OI_ITJ 135.6_CRD /_PUN N2_NP0 LBH_NP0 has_VHZ little_DT0 dependence_NN1 on_PRP the_AT0 altitudinal_AJ0 distribution_NN1 of_PRF composition_NN1 ._SENT 
For_PRP an_AT0 atmosphere_NN1 in_PRP pure_AJ0 diffusive_AJ0 equilibrium_NN1 ,_PUN ΣO_NP0 /_PUN N2_NP0 is_VBZ independent_AJ0 of_PRF temperature_NN1 ._SENT A_AT0 simple_AJ0 example_NN1 of_PRF this_DT0 independence_NN1 is_VBZ an_AT0 isothermal_AJ0 atmosphere_NN1 :_PUN temperature_NN1 cancels_VVZ in_PRP the_AT0 ratio_NN1 of_PRF O_ZZ0 to_PRP N2_CRD scale_NN1 heights_NN2 ._SENT The_AT0 independence_NN1 is_VBZ also_AV0 true_AJ0 for_PRP the_AT0 Bates-Walker_NP0 (_PUL Walker_NP0 ,_PUN 1965_CRD )_PUR and_CJC Jacchia_NP0 (_PUL 1977_CRD )_PUR diffusive_AJ0 equilibrium_NN1 models_NN2 ._SENT But_CJC in_PRP realistic_AJ0 atmospheres_NN2 ,_PUN column_NN1 density_NN1 ratio_NN1 and_CJC temperature_NN1 can_VM0 be_VBI related_VVN ._SENT For_PRP example_NN1 ,_PUN high_AJ0 latitude_NN1 joule_NN1 heating_NN1 causes_VVZ upwelling_AJ0 that_CJT results_NN2 in_PRP increased_AJ0 molecular_AJ0 concentrations_NN2 relative_AJ0 to_PRP atomic_AJ0 oxygen_NN1 ._SENT This_DT0 lower_AJC ΣO_NN1 /_PUN N2_CRD ,_PUN hotter_AJC air_NN1 is_VBZ redistributed_VVN globally_AV0 by_PRP equatorward_AJ0 winds_NN2 ,_PUN especially_AV0 at_PRP night_NN1 ,_PUN that_DT0 can_VM0 result_VVI in_PRP dramatic_AJ0 changes_NN2 in_PRP the_AT0 column_NN1 density_NN1 ratio_NN1 (_PUL Meier_NP0 et_NP0 al._NP0 ,_PUN 2005_CRD )_PUR ._SENT I_PNP have_VHB even_AV0 noticed_VVN high_AJ0 latitude_NN1 decreases_VVZ in_PRP GUVI_AJ0 observations_NN2 of_PRF ΣO_NP0 /_PUN N2_CRD for_PRP very_AV0 weak_AJ0 geomagnetic_AJ0 activity_NN1 increases_VVZ in_PRP Ap_NP0 from_PRP 2_CRD to_PRP 4._CRD As_CJS well_AV0 ,_PUN Crowley_NP0 et_NP0 al._NP0 (_PUL 2008_CRD )_PUR have_VHB related_VVN 7-_AJ0 and_CJC 9-days_AJ0 oscillations_NN2 in_PRP the_AT0 solar_AJ0 wind_NN1 to_PRP modulation_NN1 of_PRF ΣO_NP0 /_PUN N2_CRD by_PRP vertical_AJ0 winds_NN2 at_PRP high_AJ0 latitudes_NN2 and_CJC by_PRP inference_NN1 ,_PUN thermal_AJ0 expansion_NN1 at_PRP low_AJ0 latitudes_NN2 ._SENT 
In_PRP summary_NN1 ,_PUN it_PNP is_VBZ now_AV0 established_VVN that_CJT observed_AJ0 "_PUQ two-color_AJ0 "_PUQ disk_NN1 images_NN2 of_PRF FUV_NP0 dayglow_NN1 emissions_NN2 (_PUL i.e._AV0 ,_PUN measured_VVN at_PRP oxygen_NN1 and_CJC nitrogen_NN1 wavelengths_NN2 )_PUR are_VBB readily_AV0 convertible_AJ0 into_PRP a_AT0 geophysically_AV0 meaningful_AJ0 quantity_NN1 ,_PUN ΣO_NP0 /_PUN N2_CRD which_DTQ ,_PUN in_PRP turn_NN1 ,_PUN can_VM0 be_VBI analyzed_VVN with_PRP simulations_NN2 by_PRP atmospheric_AJ0 models_NN2 ._SENT Research_VVB incorporating_VVG and_CJC comparing_VVG observed_VVN and_CJC modeled_VVD ΣO_NP0 /_PUN N2_NP0 has_VHZ established_VVN the_AT0 dramatic_AJ0 responsivity_NN1 of_PRF this_DT0 ratio_NN1 to_PRP thermospheric_AJ0 dynamical_AJ0 processes_NN2 that_CJT change_VVB the_AT0 abundance_NN1 of_PRF O_ZZ0 relative_AJ0 to_PRP N2_CRD (_PUL e.g._AV0 ,_PUN Crowley_NP0 et_NP0 al._NP0 ,_PUN 2008_CRD ;_PUN Meier_NP0 et_NP0 al._NP0 ,_PUN 2005_CRD ;_PUN Zhang_NP0 et_NP0 al._NP0 ,_PUN 2004_CRD )_PUR ._SENT A_AT0 connection_NN1 between_PRP ΣO_NP0 /_PUN N2_CRD and_CJC both_DT0 F-region_AJ0 peak_NN1 electron_NN1 density_NN1 and_CJC total_AJ0 electron_NN1 content_NN1 is_VBZ also_AV0 apparent_AJ0 (_PUL e.g._AV0 ,_PUN Lean_NP0 ,_PUN Meier_NP0 ,_PUN et_NP0 al._NP0 ,_PUN 2011_CRD ;_PUN Strickland_NP0 et_NP0 al._NP0 ,_PUN 2001_CRD )_PUR ,_PUN surmised_VVD to_TO0 be_VBI the_AT0 result_NN1 of_PRF ionospheric_AJ0 F-region_NN1 photochemistry_NN1 that_CJT relates_VVZ the_AT0 volume_NN1 densities_NN2 of_PRF electrons_NN2 to_PRP O_ZZ0 /_PUN N2_CRD in_PRP photochemical_AJ0 equilibrium_NN1 ._SENT 
So_AV0 far_AV0 ,_PUN this_DT0 paper_NN1 has_VHZ categorically_AV0 referred_VVN to_PRP the_AT0 N2_AJ0 emission_NN1 rate_NN1 in_PRP the_AT0 denominator_NN1 of_PRF the_AT0 Strickland_NP0 et_NP0 al._NN1 algorithm_NN1 as_PRP "_PUQ LBH_NP0 ._SENT "_PUQ Specifically_AV0 ,_PUN it_PNP means_VVZ emission_NN1 from_PRP the_AT0 a1Πg_AJ0 electronic_AJ0 state_NN1 of_PRF the_AT0 molecule_NN1 to_PRP the_AT0 X_ZZ0 1Σg+_NN0 ground_NN1 state_NN1 ._SENT Details_NN2 of_PRF the_AT0 LBH_NP0 spectrum_NN1 used_VVN by_PRP Strickland_NP0 et_NP0 al._NP0 (_PUL 1995_CRD )_PUR and_CJC in_PRP this_DT0 paper_NN1 are_VBB taken_VVN from_PRP Conway_NP0 (_PUL 1982_CRD )_PUR ._SENT Conway_NP0 's_POS spectral_AJ0 synthesis_NN1 includes_VVZ bands_NN2 ranging_VVG from_PRP 127.3_CRD to_PRP 360_CRD nm_NN2 ._SENT So_AV0 ,_PUN it_PNP is_VBZ important_AJ0 to_TO0 define_VVI what_DTQ is_VBZ meant_VVN by_PRP "_PUQ LBH_NP0 ._SENT "_PUQ The_AT0 definition_NN1 depends_VVZ on_PRP the_AT0 type_NN1 of_PRF measurement_NN1 ;_PUN so_AV0 a_AT0 unique_AJ0 Strickland_NP0 et_NP0 al._NN1 algorithm_NN1 must_VM0 be_VBI derived_VVN for_PRP each_DT0 instrument_NN1 ._SENT For_PRP the_AT0 ICON_NN1 FUV_NN1 instrument_NN1 ,_PUN the_AT0 LBH_NP0 channel_NN1 covers_VVZ the_AT0 wavelength_NN1 range_NN1 ,_PUN 152_CRD –_PUN 162_CRD nm_NN2 ,_PUN with_PRP peak_NN1 responsivity_NN1 at_PRP 158_CRD nm_NN2 (_PUL Mende_NP0 et_NP0 al._NP0 ,_PUN 2017_CRD )_PUR ._SENT Convolution_NN1 of_PRF the_AT0 instrument_NN1 responsivity_NN1 with_PRP the_AT0 LBH_NP0 emission_NN1 spectrum_NN1 results_NN2 in_PRP a_AT0 measurement_NN1 of_PRF 6.8_CRD %_NN0 of_PRF the_AT0 total_AJ0 band_NN1 emission_NN1 ._SENT FUV_NP0 remote_AJ0 sensing_NN1 missions_NN2 should_VM0 always_AV0 report_VVI the_AT0 fraction_NN1 of_PRF the_AT0 total_AJ0 LBH_NN1 band_NN1 that_DT0 is_VBZ observed_AJ0 ._SENT 
1.2_CRD ._SENT Conceptual_AJ0 Confusion_NN1 
Much_DT0 of_PRF the_AT0 current_AJ0 confusion_NN1 about_PRP the_AT0 thermospheric_AJ0 column_NN1 O_ZZ0 /_PUN N2_CRD ratio_NN1 and_CJC its_DPS geophysical_AJ0 interpretation_NN1 concerns_VVZ the_AT0 reference_NN1 altitude_NN1 ,_PUN z17_CRD ,_PUN for_PRP which_DTQ the_AT0 N2_AJ0 column_NN1 reaches_VVZ 1017_CRD cm−2_CRD ._SENT Some_DT0 researchers_NN2 have_VHB attempted_VVN to_TO0 assign_VVI geophysical_AJ0 significance_NN1 to_PRP this_DT0 altitude_NN1 ,_PUN by_PRP postulating_VVG that_CJT it_PNP is_VBZ a_AT0 fundamental_AJ0 variable_NN1 (_PUL Yu_NP0 et_NP0 al._NP0 ,_PUN 2020_CRD ;_PUN Zhang_NP0 &_NP0 Paxton_NP0 ,_PUN 2011_CRD ,_PUN 2012_CRD )_PUR ._SENT Strickland_NP0 et_NP0 al._NP0 (_PUL 2012_CRD )_PUR objected_VVD to_PRP this_DT0 interpretation_NN1 ,_PUN asserting_VVG that_CJT "_PUQ the_AT0 proper_AJ0 and_CJC necessary_AJ0 way_NN1 to_TO0 understand_VVI O_ZZ0 concentration_NN1 changes_NN2 from_PRP satellite_NN1 observations_NN2 of_PRF OI_ITJ 135.6_CRD nm_NN2 and_CJC N2_CRD LBH_NP0 dayglow_NN1 from_PRP the_AT0 Earth_NN1 's_POS disk_NN1 is_VBZ not_XX0 in_PRP terms_NN2 of_PRF altitude_NN1 but_CJC in_PRP terms_NN2 of_PRF column_NN1 densities_NN2 ,_PUN including_PRP total_AJ0 column_NN1 density_NN1 ._SENT "_PUQ 
The_AT0 present_AJ0 study_NN1 aims_VVZ to_TO0 resolve_VVI this_DT0 argument_NN1 by_PRP demonstrating_VVG with_PRP explicit_AJ0 examples_NN2 in_PRP Sections_NN2 2_CRD and_CJC 3_CRD ,_PUN that_CJT ΣO_NP0 /_PUN N2_NP0 must_VM0 follow_VVI the_AT0 Strickland_NP0 et_NP0 al._NN1 interpretation_NN1 in_PRP terms_NN2 of_PRF thermospheric_AJ0 column_NN1 density_NN1 (_PUL or_CJC equivalently_AV0 optical_AJ0 depth_NN1 )_PUR ._SENT Strickland_NP0 et_NP0 al._NP0 found_VVD that_CJT the_AT0 relationship_NN1 between_PRP ΣO_NP0 /_PUN N2_NP0 and_CJC the_AT0 135.6_CRD nm_NN1 /_PUN LBH_NP0 ratio_NN1 becomes_VVZ less_AV0 accurate_AJ0 for_PRP column_NN1 densities_NN2 less_AV0 than_CJS or_CJC greater_AJC than_CJS NN2_CRD =_NP0 1017_CRD cm−2_CRD ._SENT The_AT0 way_NN1 to_TO0 understand_VVI this_DT0 is_VBZ to_TO0 recognize_VVI that_CJT the_AT0 emission_NN1 rates_NN2 are_VBB sensitive_AJ0 to_PRP the_AT0 atmospheric_AJ0 column_NN1 where_AVQ the_AT0 solar_AJ0 EUV_NP0 radiation_NN1 is_VBZ deposited_VVN ._SENT At_PRP column_NN1 base_NN1 less_AV0 than_CJS NN2_CRD =_NP0 1017_CRD cm−2_CRD ,_PUN the_AT0 lower_AJC accuracy_NN1 is_VBZ likely_AJ0 a_AT0 consequence_NN1 of_PRF not_XX0 including_VVG in_PRP the_AT0 algorithm_NN1 ,_PUN the_AT0 production_NN1 of_PRF airglow_NN1 by_PRP solar_AJ0 EUV_NP0 photons_NN2 that_CJT have_VHB passed_VVN through_AVP to_PRP greater_AJC column_NN1 depths_NN2 ._SENT Similarly_AV0 ,_PUN for_PRP a_AT0 column_NN1 density_NN1 base_NN1 in_PRP excess_NN1 of_PRF NN2_CRD =_NP0 1017_CRD cm−2_CRD ,_PUN there_EX0 is_VBZ little_DT0 or_CJC no_AT0 airglow_AJ0 production_NN1 ,_PUN so_AV0 the_AT0 numerator_NN1 and_CJC denominator_NN1 of_PRF Equation_NN1 1_CRD needlessly_AV0 include_VVB large_AJ0 non-contributing_AJ0 values_NN2 that_CJT reduce_VVB the_AT0 accuracy_NN1 of_PRF the_AT0 relationship_NN1 between_PRP the_AT0 column_NN1 density_NN1 ratio_NN1 and_CJC the_AT0 emission_NN1 rate_NN1 ratio_NN1 ._SENT An_AT0 N2_AJ0 column_NN1 density_NN1 base_NN1 of_PRF 1017_CRD cm−2_CRD defines_VVZ a_AT0 region_NN1 that_CJT contains_VVZ most_DT0 of_PRF the_AT0 photon_NN1 production_NN1 and_CJC therefore_AV0 results_NN2 in_PRP the_AT0 optimal_AJ0 algorithmic_AJ0 relationship_NN1 for_PRP sunlight_NN1 incident_NN1 on_PRP a_AT0 terrestrial_AJ0 atmosphere_NN1 ._SENT 
1.3_CRD ._SENT Other_AJ0 Complexities_NN2 
As_CJS aforementioned_AJ0 ,_PUN the_AT0 simple_AJ0 relationship_NN1 between_PRP column_NN1 density_NN1 ratio_NN1 and_CJC emission_NN1 rate_NN1 ratio_NN1 does_VDZ depend_VVI on_PRP the_AT0 solar_AJ0 zenith_NN1 angle_NN1 and_CJC the_AT0 viewing_NN1 angle_NN1 from_PRP nadir_NN1 ._SENT Consequently_AV0 ,_PUN when_CJS viewing_VVG the_AT0 disk_NN1 away_AV0 from_PRP nadir_NN1 ,_PUN the_AT0 azimuth_NN1 angle_NN1 relative_AJ0 to_PRP the_AT0 solar_AJ0 azimuth_NN1 angle_NN1 also_AV0 becomes_VVZ an_AT0 essential_AJ0 parameter_NN1 —_PUN viewing_VVG toward_PRP the_AT0 sun_NN1 leads_VVZ to_PRP a_AT0 different_AJ0 result_NN1 than_CJS viewing_VVG away_AV0 from_PRP the_AT0 sun_NN1 ._SENT Figure_NN1 1_CRD illustrates_VVZ the_AT0 geometric_AJ0 concepts_NN2 ._SENT The_AT0 azimuth_NN1 of_PRF the_AT0 instrument_NN1 line_NN1 of_PRF sight_NN1 (_PUL LOS_NP0 )_PUR is_VBZ of_PRF minimal_AJ0 importance_NN1 near_PRP the_AT0 subsolar_AJ0 location_NN1 but_CJC becomes_VVZ critically_AV0 important_AJ0 for_PRP observations_NN2 approaching_VVG twilight_NN1 ._SENT 
Later_AV0 investigations_NN2 have_VHB revealed_VVN additional_AJ0 effects_NN2 that_CJT influence_VVB the_AT0 dependence_NN1 of_PRF the_AT0 column_NN1 density_NN1 ratio_NN1 on_PRP the_AT0 emission_NN1 rate_NN1 ratio_NN1 ._SENT For_PRP example_NN1 ,_PUN variations_NN2 in_PRP the_AT0 spectral_AJ0 distribution_NN1 of_PRF the_AT0 solar_AJ0 EUV_NP0 irradiance_NN1 can_VM0 affect_VVI the_AT0 relationship_NN1 ,_PUN both_AV0 over_PRP a_AT0 solar_AJ0 cycle_NN1 and_CJC during_PRP solar_AJ0 flares_NN2 ._SENT This_DT0 is_VBZ a_AT0 consequence_NN1 of_PRF wavelength_NN1 dependence_NN1 of_PRF the_AT0 atmospheric_AJ0 absorption_NN1 cross_NN1 sections_NN2 that_CJT effectively_AV0 move_VVB the_AT0 energy_NN1 deposition_NN1 to_PRP different_AJ0 column_NN1 depths_NN2 (_PUL Strickland_NP0 et_NP0 al._NP0 ,_PUN 2007_CRD )_PUR ._SENT And_CJC ,_PUN as_CJS this_DT0 paper_NN1 describes_VVZ ,_PUN there_EX0 is_VBZ actually_AV0 a_AT0 slight_AJ0 dependence_NN1 on_PRP atmospheric_AJ0 composition_NN1 (_PUL less_AV0 than_CJS ~_NN1 2_CRD %_NN0 )_PUR of_PRF the_AT0 relationship_NN1 of_PRF the_AT0 135.6_CRD /_PUN LBH_NP0 ratio_NN1 to_PRP ΣO_NP0 /_PUN N2_CRD when_CJS evaluated_VVN over_PRP a_AT0 larger_AJC range_NN1 of_PRF atmospheres_NN2 than_CJS used_VVN by_PRP Strickland_NP0 et_NP0 al._NP0 (_PUL 1995_CRD )_PUR ._SENT As_CJS explained_VVN later_AV0 ,_PUN this_DT0 is_VBZ undoubtedly_AV0 due_AJ0 to_PRP the_AT0 inclusion_NN1 of_PRF physical_AJ0 effects_NN2 traceable_AJ0 to_PRP O2_NN1 or_CJC to_PRP instrumental_AJ0 configuration_NN1 ._SENT 
In_PRP the_AT0 most_AV0 general_AJ0 case_NN1 ,_PUN accurate_AJ0 retrieval_NN1 of_PRF ΣO_NP0 /_PUN N2_CRD from_PRP measurements_NN2 of_PRF the_AT0 135.6_CRD /_PUN LBH_NP0 ratio_NN1 therefore_AV0 requires_VVZ knowledge_NN1 of_PRF 1_CRD )_PUR the_AT0 solar_AJ0 zenith_NN1 angle_NN1 ,_PUN 2_CRD )_PUR the_AT0 view_NN1 angle_NN1 from_PRP nadir_NN1 and_CJC 3_CRD )_PUR its_DPS azimuth_NN1 relative_AJ0 to_PRP the_AT0 sun_NN1 's_POS azimuth_NN1 (_PUL Figure_NN1 1_CRD )_PUR ,_PUN 4_CRD )_PUR the_AT0 solar_AJ0 EUV_NP0 spectral_AJ0 irradiance_NN1 ,_PUN and_CJC 5_CRD )_PUR atmospheric_AJ0 concentrations_NN2 ._SENT The_AT0 solar_AJ0 spectrum_NN1 and_CJC the_AT0 minimal_AJ0 atmospheric_AJ0 effects_NN2 are_VBB readily_AV0 accommodated_VVN in_PRP the_AT0 algorithm_NN1 by_PRP including_VVG a_AT0 priori_NN2 information_NN1 from_PRP empirical_AJ0 models_NN2 that_CJT are_VBB dependent_AJ0 on_PRP solar_AJ0 activity_NN1 ._SENT For_PRP nadir_NN1 viewing_NN1 ,_PUN there_EX0 is_VBZ no_AT0 azimuthal_AJ0 effect_NN1 ;_PUN even_AV0 up_AVP to_PRP 20_CRD °_NN0 from_PRP nadir_NN1 the_AT0 error_NN1 from_PRP using_VVG a_AT0 fixed_AJ0 azimuth_NN1 is_VBZ less_AV0 than_CJS a_AT0 few_DT0 per_PRP cent_NN1 ._SENT Nonetheless_AV0 ,_PUN an_AT0 accurate_AJ0 algorithm_NN1 suitable_AJ0 for_PRP any_DT0 atmospheric_AJ0 observation_NN1 must_VM0 include_VVI all_DT0 five_CRD parameters_NN2 ._SENT Alternatively_AV0 ,_PUN this_DT0 paper_NN1 proposes_VVZ a_AT0 generalized_AJ0 approach_NN1 for_PRP the_AT0 extraction_NN1 of_PRF ΣO_NP0 /_PUN N2_CRD from_PRP radiance_NN1 ratios_NN2 ._SENT 
Section_NN1 2_CRD discusses_VVZ the_AT0 basic_AJ0 principles_NN2 that_CJT relate_VVB the_AT0 observed_AJ0 airglow_NN1 ratios_NN2 to_PRP thermosphere_NN1 ΣO_NN1 /_PUN N2_NN1 and_CJC Section_NN1 3_CRD establishes_VVZ the_AT0 irrelevance_NN1 of_PRF the_AT0 reference_NN1 altitude_NN1 ._SENT Section_NN1 4_CRD assesses_VVZ effects_NN2 of_PRF neglecting_VVG the_AT0 various_AJ0 parameters_NN2 in_PRP a_AT0 table_NN1 lookup_NN1 algorithm_NN1 ._SENT Section_NN1 5_CRD describes_VVZ the_AT0 new_AJ0 approach_NN1 for_PRP retrieving_VVG column_NN1 O_ZZ0 /_PUN N2_CRD from_PRP OI_ITJ 135.6_CRD nm_NN2 to_PRP N2_CRD LBH_NP0 emissions_NN2 ,_PUN a_AT0 methodology_NN1 that_CJT abolishes_VVZ the_AT0 need_NN1 for_PRP a_AT0 five-parameter_AJ0 table_NN1 ._SENT 
2._CRD Why_AVQ is_VBZ the_AT0 Airglow_AJ0 Intensity_NN1 Ratio_NN1 Proportional_AJ0 to_PRP the_AT0 Column_NN1 O_ZZ0 /_PUN N2_CRD Ratio_NN1 ?_SENT 
The_AT0 production_NN1 of_PRF FUV_NP0 emission_NN1 is_VBZ initiated_VVN by_PRP solar_AJ0 EUV_NP0 radiation_NN1 incident_NN1 on_PRP and_CJC absorbed_VVN by_PRP the_AT0 atmosphere_NN1 ._SENT Following_AJ0 absorption_NN1 by_PRP atmosphere_NN1 gases_NN2 ,_PUN photons_NN2 with_PRP wavelengths_NN2 shorter_AJC than_CJS about_PRP 45_CRD nm_NN2 have_VHB enough_DT0 energy_NN1 to_TO0 photoionize_VVI O_ZZ0 and_CJC N2_NP0 ,_PUN releasing_VVG photoelectrons_NN2 with_PRP energy_NN1 sufficient_AJ0 to_TO0 excite_VVI the_AT0 FUV_NP0 dayglow_NN1 (_PUL 45_CRD nm_NN1 corresponds_VVZ to_PRP 15.4_CRD eV_NN1 photoionization_NN1 energy_NN1 of_PRF N2_CRD plus_PRP 12_CRD eV_NN1 photoelectron_NN1 energy_NN1 )_PUR ._SENT Such_DT0 photoelectrons_NN2 can_VM0 then_AV0 inelastically_AV0 collide_VVB with_PRP O_ZZ0 and_CJC N2_NP0 ,_PUN exciting_VVG them_PNP to_PRP the_AT0 5S_CRD and_CJC a1Πg_AJ0 electronic_AJ0 states_NN2 ,_PUN respectively_AV0 ._SENT The_AT0 relaxation_NN1 of_PRF excited_AJ0 species_NN0 to_PRP their_DPS ground_NN1 states_NN2 produces_VVZ OI_ITJ 135.6_CRD nm_NN2 and_CJC N2_CRD LBH_NP0 photons_NN2 ._SENT Using_VVG altitude_NN1 for_PRP the_AT0 benefit_NN1 of_PRF visualization_NN1 ,_PUN the_AT0 profiles_NN2 of_PRF the_AT0 volume_NN1 emission_NN1 rates_NN2 of_PRF these_DT0 species_NN0 peak_NN1 between_PRP 150_CRD and_CJC 200_CRD km_NN0 ,_PUN depending_VVG on_PRP solar_AJ0 activity_NN1 ,_PUN atmospheric_AJ0 conditions_NN2 and_CJC solar_AJ0 zenith_NN1 angle_NN1 ._SENT The_AT0 topside_NN1 profiles_NN2 (_PUL at_PRP altitudes_NN2 above_PRP the_AT0 emission_NN1 peak_NN1 )_PUR mostly_AV0 follow_VVB the_AT0 O_ZZ0 and_CJC N2_CRD scale_NN1 heights_NN2 with_PRP some_DT0 modification_NN1 due_AJ0 to_PRP the_AT0 photoelectron_NN1 flux_NN1 profile_NN1 ._SENT The_AT0 bottom_AJ0 sides_NN2 of_PRF the_AT0 emitting_AJ0 layers_NN2 (_PUL at_PRP altitudes_NN2 below_PRP the_AT0 emission_NN1 peak_NN1 )_PUR decrease_VVB rapidly_AV0 with_PRP decreasing_AJ0 altitude_NN1 due_AJ0 to_PRP extinction_NN1 of_PRF the_AT0 solar_AJ0 EUV_NP0 radiation_NN1 ._SENT Meier_NP0 (_PUL 1991_CRD )_PUR ,_PUN and_CJC Strickland_NP0 et_NP0 al._NP0 (_PUL 1999_CRD )_PUR provide_VVB examples_NN2 of_PRF such_DT0 profiles_NN2 ._SENT 
