Science results of the Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on FORMOSAT-2 Alfred Bing-Chih Chen[1]; Cheng-Ling Kuo[2]; Rue-Ron Hsu[3]; Han-Tzong Su[3]; Jyh-Long Chern[5]; H.U. Frey[6]; S.B. Mende[6]; Yukihiro Takahashi[7]; Lou-Chuang Lee[2] [1] Department of Physics, National Cheng Kung University; [2] Space science, Natl. Central Univ., Taiwan; [3] Department of Physics, National Cheng-Kung University; [4] Cheng Kung Univ.; [5] Department of Photonics, National Chiao-Tung University; [6] U.C.Berkeley; [7] Dept. of Geophysics, Tohoku Univ.

2 Outlines Brief introduction to ISUAL mission –FORMOSAT-2 Satellite –Scientific objectives Major scientific results –Global distributions of TLEs –Characteristics of TLEs –Airglow and Aurora Conclusions

3 FORMOSAT-2  Earth remote sensing satellite  Mission orbit: 891 km circular, sun synchronous, 99.1 degree inclination, 14 orbits per day, ~103 min/orbit, passes through Taiwan twice daily  Satellite Lift-off Mass (Wet): 758 kg  Size: Hexagonal, height 2.4 meters, outer radius ~1.6 meters (with solar panels folded)  Payloads: Remote Sensing Instrument (RSI) and Imager of Sprites and Upper Atmospheric Lightning (ISUAL)  Remote sensing resolution: 2 meters for B&W images and 8 meters for color images  Image Swatch ≧ 24km; Limb view angle±45°, capable of capturing 3-D images  Mission Life: 5 Years  Launch date: May 21, 2004 (Taiwan local time)

4 Objectives of the ISUAL experiment  Obtain of the global distribution of sprites, elves and jets  Study their dynamical evolutions and spectroscopic characteristics  Elucidate the importance of TLEs in the Earth’s environment; a key energy source for the upper atmosphere at the night-hemisphere?  Exploring the global distributions of airglow and aurora

5 Blue ( nm) Red ( nm) ISUAL sensor packages ICCD imager Array Photometer (AP) Spectrophotometer (SP) Filter wheel 623 – 754 nm 762 nm 630 nm nm nm 425 – 890 nm Six bandpasses nm(FUV) nm nm nm nm nm(MUV)

6 International Collaboration of ISUAL Taiwan –National Space Organization (NSPO) –National Cheng-Kung University (NCKU) –National Central University (NCU) United States –UC Berkeley –Duke University –Pennsylvania State University –Penn State Lehigh Valley Japan –Tohoku University –Kyoto University –Hokkaido University

7 Simple facts Launch May 20, 2004, science since July 2004 Sun-synchronous TLE limb-observations at ~2300 LT Coverage -25 o to +45 o, -45 o to +25 o Latitude, resp. >90,000 triggered and recorded events Event consists of msec integration images with: pre-trigger (dark) image, the triggered image, and 4-6 post-trigger images (mostly N 2 -1P band); - ~200 msec 6-channel photometer records (FUV  NIR); - 2-channel altitude resolved red/blue recordings Observations of >7,000 elves, >800 sprites, >800 halos, >20 gigantic jets Additional observations of aurora, equatorial anomaly, and gravity waves

8 Zoo of Transient Luminous Events (TLEs) Sprite halo

9 Images of TLE Elve time sequence Gigantic jet Elve “Blue Event” (jet?) Halo Sprite

10 Global distribution and occurrence rate of TLEs Chen et al., 2008, JGR Christian et al., JGR, 2003

11 VLF confirms negative CG Halo initiation by -CG Frey et al., GRL, 2007

12 Where do these negative halos occur? Field of view over Central America * Positive CG or unclear v Negative CG confirmed by VLF/ELF * Estimated as negative CG v Over Water! Frey et al., GRL, 2007

13 FUV emission and ionization by elves Mende et al., JGR 2005 ionization takes place in elves the reduced electric field which characterizes the local electron energy distribution is >200 Td the elves produce an average electron density of 210 electrons cm -3 over a large circular region FUV signature of elve is confirmed

14 A Elves often caused by –CG with Beta-type stepped leader SP AP VLF VLF confirms elve after -CG Frey et al., GRL, 2005 Half of all elves were produced by lightning that shows a three-step signature: 1.An initial brightening in all except the FUV channels (initial breakdown ) 2.a slow brightness decrease for the next 2–5 milliseconds (beta-type stepped leader ) 3.an impulsive increase of signal in all channels (bright return stroke, -CG) 1 23

15 Sprites occur often delayed from original lightning during continuing current SP AP C D VLF VLF confirms sprite after positive CG C Frey et al., GRL, 2005

16 Estimation of electric field Adachi et al., GRL, 2006

17 Electric field transition between the diffuse and streamer regions of sprites Electric field in the diffuse region are 0.5–0.7 Ek, which support the theoretical expectation that their optical emissions could be produced without significant ionization. those in the streamer region are 1–2 Ek which is a few times less than predicted fields in the streamer head. Adachi et al., GRL 2006

18 Electric field in sprites from SP data Residual intensities after background and lightning subtraction Electric field in streamer region is about 2-4 times the local electric breakdown field at sprite streamer altitude Kuo et al., GRL, 2005

19 Analysis of emission ratios Liu et al., GRL, 2006 First analysis of FUV emission of sprite streamer To explain observed emission ratios electric field in streamer has to be greater than 3x local breakdown field

20 Investigation of elves Atmospheric transmittance at tangent heights km Comparison of observed emission intensity with model and confirmation of model Kuo et al., JGR, 2007

21 TLE observations in O 2 emission at 762 nm Every emission below 80 km is absorbed Lightning signature comes from continuum N2-1P 55 km We possibly see contribution from lower altitude from the N 2 -1P (3-1) at nm, (in aurora 64 kR of 882 kR total) 35 km

22 Electron density change from elve Cheng et al., JGR, 2007 Elve created local electron density enhancement of 460 cm -3

23 High altitude sprite current Result: Delayed VLF signal does not come from low altitude lightning, but from current flowing in high altitude sprite Cummer et al., GRL, 2006

24 Harrison, 2005 geophysical processes to link the global atmospheric electrical circuit and the climate system

25 Possible electrification processes in lower atmosphere

26 Side-viewing observations of OI(1D) and OH night airglows by ISUAL FOV In the eclipse, FORMOSAT-2 tracks from south to north along a sun- synchronized orbit. The imager looks across the track toward the pre-midnight region. A side-viewing image from ISUAL imager taken through a 630 nm filter. A double-layered nightglow enhancement is evident. OI emission OH(9,3) emission Stitched image

27 FOV SAT. Exposure Interval : 1.4 s Filter : nm 12/21/2006 event ： Substorm breakup arc FORMOSAT-2 Satellite Observations of Auroral Substorms ISUAL / FORMOSAT-2 PSSC NCKU

28 Fig. auroral substorm events observed by the all-sky imager in Alaska. Ground and satellite observations of substorm onset arcs Fig. ISUAL auroral substorm events observed over the Alaska-Canada border region. 01/18/2007 event ISUAL / FORMOSAT-2 PSSC NCKU ISUAL

29 Conclusions The ISUAL experiment successfully achieves the scientific goals defined for this mission, and reveals a lot of new viewpoints in the atmospheric electricity, ocean- atmosphere-ionosphere coupling, and auroral dynamics. In the second phase of ISUAL mission, year-to-year and seasonal variations among ocean, atmosphere electricity and circulation are going to be explored and they are important to understand the long-term and global climate change. We would like to thank the great support from NSPO and NSC.