1 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, June 2008, University of Corsica, Corte, France T.-Y. Huang 1, C. Y. Chiang 2, C. L. Kuo 3, J. B. Nee 4, A. B. Chen 2, H. T. Su 2, and R. R. Hsu 2

2 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

3  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 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

4 ISUAL CCD Imager ISUAL CCD Imager Filters Filter # (nm) TLE/Airglow ObservationsAirglow Peak Altitude N 2 1PG & OH bandsOH bands at 87 km 2762O 2 atmospheric bandO2 at 95 km 3630OH(9,3) band & OI red lineOH(9,3) band at 87 km & O( 1 D) at 250 km nm OI green lineGreen line at 96 km N 2 + (0,1) bandO2 Herzberg band at 90 km 6Broad bandGeneral purposeNon-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)

5 ISUAL CCD Imager Filter 3 response function Bandwidth ~ 7 nm

6 The Occurrence of LITEs in or Below the OH Airglow Layer

7 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 ( R21) 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.

R21, Lat=2.5S

R17, Lat=11.3 S

10 Species that emit light near 630 nm  N 2 1P : N 2 1P(10,7) & N21P(11,8), weak emissions;  ~6 us  OH(9,3) band;  ~ a few ms  N 2 +( Meinel) band: quenched in the km region (Vallance Jones, 1974)  OI red line: quenched below 150 km (Baggaley, 1976)  O 2 + : requires threshold electron energy > 30 eV (Borst and Zipf, 1970) Species

11 From Hampton et al, GRL, 1996 The Spectra of N 2 1P and Sprites  This figure shows that the intensity of N 2 1P band is very small at 630 nm.

12 The simulated N 2 1P Intensity near 630 nm  We use the Frank-Condon factors for the N 2 1P lines and the filter response function to estimate the intensity of N 2 1P that could be observed by filter 3.  The calculated values show that to be 0.24% of N 2 1P band, within the bandpass of filter 3. Courtesy of C. –L. Kuo

13 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.

14 Possible mechanisms for the induced OH nightglow enhancements N2*N2*  OH OH * energy transfer de-excitation excitation collision de-excitation N2N2 N2*N2* OH Induced OH Nightglow Emissions OH * OH   H HO 2 O O3O3 minor species in OH chemistry e–e–

15 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.

16 Derivation of vertical profile from the CCD images

17 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 N 2 vibrational levels. 2. to optical emission 3. to ionization As the figure shows, electrons with energy less than 10 eV are more likely to lose energy via excitation of N 2 vibrational levels. This thermal energy should not be confused with the optical energy or other types of energy.

R7, Lat=3 S

R8, Lat=5.9 S

R18, Lat=40 N

R24, Lat=16.7S

R8, Lat=0

R5, Lat=8.5 N