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 a

nd jets

Study their dynamical evolutions and spectroscopi

c characteristics

Elucidate the importance of TLEs in the Earth’s en

vironment; a key energy source for the upper atm

osphere at the night-hemisphere?

Exploring the global distributions of airglow and au

rora

5

Blue (370 - 450 nm)Red (530 - 650 nm)

ISUAL sensor packages

ICCD imager

Array Photometer (AP)

Spectrophotometer (SP)

Filter wheel623 – 754 nm762 nm630 nm557.7 nm427.8 nm425 – 890 nm

Six bandpasses150-290nm(FUV)333.5-341.2nm387.1-393.6nm658.9-753.4nm773.6-783.4nm240-400nm(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 -25o to +45o, -45o to +25o Latitude, resp.• >90,000 triggered and recorded events • Event consists of 10-30 msec integration images with: -

0-1 pre-trigger (dark) image, the triggered image, and 4-6 post-trigger images (mostly N2-1P 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, JGRChristian 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

SP

SP

SP

SP

SP

AP

AP

VLFVLF 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 2 3

15

Sprites occur often delayed from original lightning during continuing current

SP

SP

SP

SP

SP

AP

AP

AP

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 streamerTo explain observed emission ratios electric field in streamer has to be greater than

3x local breakdown field

20

Investigation of elvesAtmospheric transmittance at tangent heights 10-90 km

Comparison of observed emission intensity with model and confirmation of model

Kuo et al., JGR, 2007

21

TLE observations in O2 emission at 762 nm

Every emission below 80 km is absorbedLightning signature comes from continuum

N2-1P

N2-1P55 km

We possibly see contribution from lower altitude from the N2-1P (3-1) at 762.68 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

24Harrison, 2005

geophysical processes to link the global atmospheric electrical circuit and the climate system

25

Possible electrification processes in lower atmosphere

Elve

Lightning w/ high Ipeak

Sea surface temperature

Vertical flowPrecipitation

Walker circulation

Hadleycirculation

Surface temperature

Positive correlationPositive correlation suggested by literatures

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-midnightregion. A side-viewing image from ISUAL imager taken through a 630 nm filter. A double-layered nightglow enhancement is evident.

OI emissionOH(9,3) emission

Stitched image

27

FOV

SAT.Exposure Interval : 1.4 sFilter : 630.0 nm

12/21/2006 event：Substorm breakup arc

FORMOSAT-2 Satellite Observations FORMOSAT-2 Satellite Observations of Auroral Substormsof Auroral Substorms

ISUAL / FORMOSAT-2 PSSCNCKU

28

Fig. auroral substorm events observed by the all-sky imager in Alaska.

Ground and satellite observations of substorm onset arcsGround 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 PSSCNCKU

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.