ASK observations and modelling of N2 first positive emission in aurora – II . Cross sections

Joanna Sullivan (presenter) Mina Ashrafi

Betty LanchesterDirk LummerzheimNickolay Ivchenko

O. JokiahoD. Whiter

ASK workshop Stockholm, June 2008

2ASK workshop, Stockholm, June 2008 N2 modelling

• 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.

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

20

18 16

14

12

10

8

6

4

2

0

internuclear distance ( )οA

pote

ntia

l ene

rgy

(eV

)

N2

ΠB3

ΣA3

ΣX1(Ground Level)

0

5

0

10

15

5

5

10

10

(5,2) and (4,1) bands

Exc

itatio

n

N2 1st positive system

3ASK workshop, Stockholm, June 2008 N2 modelling

N2 first positive system, 5,200-10,000

(5,2) & (4,1)

Rel

ativ

e sc

ale

0.85

Band transmittance

0.67

ASK1 filter transmission curve

(673.0 nm)

οA

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

4ASK workshop, Stockholm, June 2008 N2 modelling

1 minute guide to modelling1 minute guide to modelling

TFF(VJ)IMIA Intensity of N2

measured by ASK1 (calibrated and

background subtracted)

Modelled N2 intensity ( direct + cascading )

"'

)1,4()2,5(

vvIII

Constant determining the contribution from (4,1) and (5,2) bands compare

to the total 1st positive band (V. Jones, 1971):

ASK1 filter transmission factor

Model

Precipitating Electrons energy spectrum

(Maxwellian+mono+powerlaw)

Excitation cross section of atmospheric constituents

N2, O2 , O, etc…

Model Inputs

, … other inputs

5ASK workshop, Stockholm, June 2008 N2 modelling

Ne (

/m3 )

ra

dar d

ata

AS

K1

@ 6

73.0

Inte

nsity

(kR

)

Event Event 22 Oct 200622 Oct 2006

6ASK workshop, Stockholm, June 2008 N2 modelling

ΠC3

ΣB'3

ΣA3

ΠB3ΔW3

ΣX'

1st positive

N2

Ene

rgy

• 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.

7ASK workshop, Stockholm, June 2008 N2 modelling

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)

8ASK workshop, Stockholm, June 2008 N2 modelling

Electron impact excitation cross sections of the N2 B statevs. emissions cross sections of the first positive system

A more direct solution for modelling the N21P band intensitymight be to use measured emission cross sections, so that all

cascades are included.

9ASK workshop, Stockholm, June 2008 N2 modelling

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% fortheir emissions cross sections.

ResultsResults

10ASK workshop, Stockholm, June 2008 N2 modelling

Summary and conclusionSummary 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.