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Investigations of lightning-induced sudden

brightening in the OH airglow layer observed by

ISUAL onboard FORMOSAT-II Satellite

1.Physics Department, Penn State Lehigh Valley, USA

2.Department of Physics, National Cheng Kung University, Tainan, Taiwan.

3.Institute of Space Science, National Central University, Taiwan

4.Physics Department, National Central University, Taiwan.

TLE workshop, 23-27 June 2008, University of Corsica, Corte, France

T.-Y. Huang1, C. Y. Chiang2, C. L. Kuo3, J. B. Nee4, A. B. Chen2, H. T. Su2, and R. R. Hsu2

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The ISUAL January 2007 Campaign

Elves and Sprites are the two types of Transient Luminous Events (TLEs) in the MLT region.

Since these phenomena are now understood to be lightning-induced, they will be referred to as Lightning-Induced Transient Emissions (LITEs).

ISUAL’s TLE observations have sometimes shown an enhancement below or in the OH airglow layer when there is lightning activity.

The first reported sighting of lightning induced sudden brightness in the airglow layer was in 1992 (Boeck et al.) from a space shuttle.

Similar lightning-induced sudden brightening was also observed from the Columbia space shuttle during the MEIDEX sprite campaign (Israelevich et al., 2004). They named the enhancement as Transient Airglow Enhancement (TAE)

The filters used in these studies are broadband filters.

Objectives

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The mechanism of elves is that the EMPs produced by lightning accelerate the electrons (Inan et al., 1996), leading to optical emissions of gas species.

It is generally believed that the species is N2.

Huang et al. (2007) proposed that OH species could play a role in LITEs occurred in the airglow layer.

Due to too much overlap between OH and N2 emissions in the 623-750 nm range, ISUAL’s broadband filter is unable to discern the respective contribution from OH and N2.

We conducted a 9-day (5-8th and 17-21st) campaign in January 2007 with some observations made exclusively by the 630 nm filter (filter 3) for the investigation.

The ISUAL January 2007 Campaign

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ISUAL CCD Imager

ISUAL CCD Imager Filters

Filter # (nm) TLE/Airglow Observations Airglow Peak Altitude

1 623-750 N2 1PG & OH bands OH bands at 87 km

2 762 O2 atmospheric band O2 at 95 km

3 630 OH(9,3) band & OI red line OH(9,3) band at 87 km & O(1D) at 250 km

4 557.7 557.7 nm OI green line Green line at 96 km

5 427.8 N2+ (0,1) band O2 Herzberg band at 90 km

6 Broad band General purpose Non-specific

Sprite mode was used: 29 ms of exposure time and 1 ms dead time

Six images in each set after it is triggered

Field of view in each image frame: 1000 km (horizontal) x 250 km (vertical)

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ISUAL CCD Imager Filter 3 response function

Bandwidth ~ 7 nm

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The Occurrence of LITEs in or Below the OH Airglow Layer

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3

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Features noted From the ISUAL Observations

The airglow enhancement observed by the narrowband filter was significant.

The enhancement observed by either the broadband or narrowband filter has never been found above the OH airglow layer.

Observations show that LITEs occur much more frequently in the OH airglow layer than below the airglow layer.

A case study on one event (20070109R21) shows that there was a significant intensity enhancement (~80%) when there was lightning and a somewhat substantial post-lightning intensity enhancement (~ 25%) in the airglow layer after lightning has ceased.

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20070109R21, Lat=2.5S

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20070117R17, Lat=11.3 S

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Species that emit light near 630 nm

N21P : N21P(10,7) & N21P(11,8), weak emissions; ~6 us

OH(9,3) band; ~ a few ms

N2+(Meinel) band: quenched in the 85-90 km region (Vallance

Jones, 1974)

OI red line: quenched below 150 km (Baggaley, 1976)

O2+ : requires threshold electron energy > 30 eV (Borst and

Zipf, 1970)

Species

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From Hampton et al, GRL, 1996

The Spectra of N2 1P and Sprites

This figure shows that the intensity of N2 1P band is very small at 630 nm.

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The simulated N2 1P Intensity near 630 nm

We use the Frank-Condon factors for the N21P lines and the filter response function to estimate the intensity of N21P that could be observed by filter 3.

The calculated values show that to be 0.24% of N2 1P band, within the bandpass of filter 3.

Courtesy of C. –L. Kuo

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The Simulated OH (9,3) Band Intensity near 630 nm

We estimated the OH(9,3) line emission intensity within the bandpass of 630-nm filter to be 42.3% of OH(9,3) band.

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Possible mechanisms for the induced OH nightglow enhancements

N2*

OH

OH*energy transfer

de-excitation excitation

de-excitation

N2

N2*

OH

Induced

OH Nightglow Emissions

OH* OH

OH

OH

H

HO2

OO3

minor speciesin OH chemistry

e–

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Summary

A 9-day worth of data collected in January 2007 has been analyzed to help delineate the causes for the lightning-induced enhancements often observed in the OH airglow layer.

Observations show that LITEs occur more in the airglow layer than below.

The analysis of the data shows that there was a significant intensity enhancement when there was lightning and a somewhat substantial post-lightning intensity enhancement in the airglow layer after lightning.

Three mechanisms were proposed to explain the LITEs in the OH airglow layer.

A kinetic model is in preparation to validate/test the proposed mechanisms.

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Derivation of vertical profile from the CCD images

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Figure taken from Gurevich and Zybin, Physics Today, Vol.58, No. 5, 2005

Three Major Ways For Electrons To Lose Energy:

Ways For Electrons To Lose Energy

1. to excitation of major species N2 vibrational levels.

2. to optical emission

3. to ionization

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2

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As the figure shows, electrons with energy less than 10 eV are more likely to lose energy via excitation of N2 vibrational levels. This thermal energy should not be confused with the optical energy or other types of energy.

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20070106R7, Lat=3 S

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20070117R8, Lat=5.9 S

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20070117R18, Lat=40 N

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20070119R24, Lat=16.7S

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20070121R8, Lat=0

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20070108R5, Lat=8.5 N