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ASK observations and modelling of N 2 first positive emission in aurora – II . Cross sections

ASK observations and modelling of N 2 first positive emission in aurora – II . Cross sections Joanna Sullivan (presenter) Mina Ashrafi Betty Lanchester Dirk Lummerzheim Nickolay Ivchenko O. Jokiaho D. Whiter ASK workshop Stockholm, June 2008. 20 18 16 14 12 10. N 2.

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ASK observations and modelling of N 2 first positive emission in aurora – II . Cross sections

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  1. ASK observations and modelling of N2 first positive emission in aurora – II . Cross sections Joanna Sullivan (presenter) Mina Ashrafi Betty Lanchester Dirk Lummerzheim Nickolay Ivchenko O. Jokiaho D. Whiter ASK workshop Stockholm, June 2008

  2. 20 18 16 14 12 10 N2 (5,2) and (4,1) bands potential energy (eV) 10 Excitation 5 8 6 4 2 0 10 5 15 (Ground Level) 10 5 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 0 internuclear distance ( ) N2 1st positive system • N2 First positive system stems from transition of the B electronic state of the molecular nitrogen to the A state. • ASK1 filter @ 673.0 nm records two bands of this system: (5,2) and (4,1). • (5,2) at 670.4 nm observed intensity in normal aurora is ~18.2 kR and (4,1) at 678.8 nm • intensity is ~17.59 kR (Vallance Jones, 1971, Table 4.12) • N2 1st positive observed total band intensity is 882 kR.

  3. Relative scale (5,2) & (4,1) N2 first positive system, 5,200-10,000 ASK1 filter transmission curve (673.0 nm) Band transmittance 0.85 0.67 Synthetic spectra (Jokiaho et al., 2008) was used for determining the ASK1 filter transmission factor (TF): 300 K : TF= 0.76 1000K : TF= 0.66

  4. 1 minute guide to modelling Precipitating Electrons energy spectrum (Maxwellian+mono+powerlaw) Excitation cross section of atmospheric constituents N2, O2 , O, etc… , … other inputs Model Inputs Model Intensity of N2 measured by ASK1 (calibrated and background subtracted) ASK1 filter transmission factor Modelled N2 intensity ( direct + cascading ) Constant determining the contribution from (4,1) and (5,2) bands compare to the total 1st positive band (V. Jones, 1971):

  5. Ne (/m3) radar data ASK1 @ 673.0 Intensity (kR) Event 22 Oct 2006

  6. Energy 1st positive N2 • Nitrogen 1st positive emission intensity measured by ASK includes cascades’ contribution from higher states (B’, C and W) as well as direct excitation of the B state. • In order to take into account population of B state resulting from cascades, modelled population of these higher states were also added to produce output emission intensities.

  7. Cross section for excitation of the N2 B state Cross section for excitation of the N2 B, C, B’ and W states (R. Link, private communication with D. Lumm.) Cross section for excitation of the N2 B, C, B’ and W states (Itikawa, 2005)

  8. Electron impact excitation cross sections of the N2 B state vs. emissions cross sections of the first positive system A more direct solution for modelling the N21P band intensity might be to use measured emission cross sections, so that all cascades are included.

  9. Results • Emission cross section of Stanton and St. John produce the best estimate of the N2 emissions. • Uncertainties associated with the reported cross sections could explain the discrepancies between modelled and measured emissions. • Brunger et al. (2003) estimated the uncertainty of up to 35% for B-state cross sections, 30% for B’ and C and 40% for W state. Stanton and St. John estimated a possible systematic error of 18% for their emissions cross sections.

  10. Summary and conclusion • Maxwellian and mono-energetic flux and energies are inputs to the model, these are calculated and adjusted using radar and N2 emission intensity. • Modelled electron densities agree well with the E-region electron density profiles measured by EISCAT radar and therefore the input flux and energy are accurate estimates of the precipitating electron spectrum. • Excitation cross sections by R. Link and Itikawa (2005) underestimate the N2 first positive intensity by ~30%. • Emissions cross sections of Shemansky and Broadfoot (1971) overestimates the N2 emission intensity by ~40%. • Using emissions cross sections of Stanton and St. John (1969) the modelled emission has relatively good agreement with the observed intensities by ASK. • More accurate and recent cross sections are yet to be found. We will use Stanton and St. John (1969) emissions cross sections for this study.

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