SPRITE-SAT Project mission for sprites and TGFs studies *Yukihiro Takahashi 1, Mitsuteru Sato 2, Umran Inan 3, David Smith 4, Sparite-Sat team 1 Tohoku University, Japan 2 Hokkaido University, Japan 3 Stanford University, U.S.A. 4 University of California, Santa Cruz, U.S.A 2008 TLE Workshop, Corte, 26 Jun. 2008

0. Scientific background -- sprites Sprite ↑ QE Model Optical energy of sprites and CMC Takahashi et al (this workshop) CMC Occurrence ratio Occurrence probability of sprites Hu et al., 2002 Hu et al. 2002

Adachi et al., 2003 (GRL) Ohkubo et al., 2005 (GRL) CG suggesting the important roles of: - EMP - intra-cloud (including horizontal) currents in sprites generation/formation processes # of columns is determined not by CMC, but by EMP. Sprites appear in the period of sferics cluster, not at CG. ELF VLF

1. TLEs (Transient Luminous Events) Figs. Horizontal lightning current plays crucial role. [Valdivia et al., 1997] TLEs ・・・ transient optical emission in the stratosphere, mesosphere and lower thermosphere caused by lightning discharges. Nadir Obs. from Space! Issues to be answered -time delay and horizontal displacement from parent lightning - CM deviations from QE-model predictions - what determines number of columns and locations

2. TGFs (Terrestrial Gamma-ray Flashes) Observational results from CGRO satellite and RHESSI satellite. Discovery of terrestrial gamma-ray flash High occurrence rate (1 event/2 day) Associated with lightning discharges? Timing between TGF and lightning is important Smith et al. [2005] TGF and Lightning Runaway Electron Which lightning process generates TGFs? Relationship between TLEs and TGFs? Energy (MeV) Counts / (s MeV) Simultaneous Obs. from a Satellite!

The first university satellite in Japan dedicated to the geoscienceunder collaboration of Science and Eng. faculties History: 2003 Oct. applied for grant-in-aid for 7 kg satellite with 1 camera and 1 (MeV) electron sensor (failed) 2004 Oct. applied for grant-in-aid again 2004 RHESSI results presented in AGU fall meeting 2005 Apr M USD for 4 years adopted (0.24M USD used) modified to 10-kg satellite with 1 camera and 1 TGF counter (TGC) 2006 Apr. modified to 40-kg satellite with 4 cameras, 1 TGC 2006 Dec. chosen as one of the candidates for piggyback launch of JAXA 2007 May chosen as a piggyback satellite 2007 Jun. funded by grant-in-aid (~3.5 M USD/4years) for scientific research in the highest category for individual groups, not by space agency, JAXA. Outline of the SPRITE-SAT project

Total cost: ~3.8 M USD - including fundamental development and ground measurements/facilities - ~1 M USD for satellite manufacturing Launch: Jan Lifetime: 5 years expected Telemetry: UHF(up), S-band at 9600bps (down) Ground facilities: 2.4 m S-band dishes at Sendai (Japan) and Kiruna => 5 MB/day … 5-10 events/day

3. Objects of SPRITE-SAT Achievement of the first synthetic observation of lightning, TLEs and TGFs from a satellite To investigate, Global distribution of TLEs and TGFs Horizontal distribution of sprites and their relation to lightning discharges Relationship between lightning, TLEs and TGFs SPRITE-SAT Science Instruments Imaging of TLEs, detection of Gamma-ray, observation of VLF waves (a)CMOS Camera (LSI): 2 sets (b)CCD Camera (WFC):2 sets (c)Gamma-ray detector (TGC):1 set (d)VLF antenna (VLF):1 set

4. SPRITE-SAT Fig. SPRITE-SAT when in orbit Fig. Top viewFig. Side view Specification Size:480×480×482 mm Mass:41 kg Power:17 W (max) Stabilization:gravity gladient Orbit:SSO (13-01 LT) Altitude:660 km Inclination:98 deg. Period:98 min.

4. SPRITE-SAT SPRITE-SAT Science Instruments (Dept. of Geophysics, Tohoku Univ.) Bus System (Dept. of Aerospace Engineering, Tohoku Univ.) CMOS, CCD Cameras H/W logic S/W development Optics development (Dept. of Geophysics, Tohoku Univ.) Gamma-ray Detector (IASA/JAXA, UCSC) VLF-ANT (Stanford Univ.) H/W Development (AD. Co., Ltd.) Development Team

5. SPRITE-SAT Bus System MTM (Mechanical Test Model) of the satellite Fig. MEM without shield panel. Fig. MEM with shield panel.

In-house assembling with supports by small companies… Making of flight model

6. Observation Modes Fig. : Obs. geometry SPRITE MODE TGF MODE - detection of TGFs and imaging of their parent lightning (30/60 fps) - TGF and VLF timing with accuracy of 0.5 ms. At SPRITE and TGF modes, event trigger scheme will be used. imaging of lightning/sprite horizontal distribution (29 fps) Nadir obs. using LSI-1,2 (FOV/35 deg) VLF is also operated TGC, WFC(FOV/140 deg), VLF-ANT

Sprite Obs. Mode Using two CMOS cameras with band- pass filters （ nm, 762nm ）, we will identify the horizontal distribution of sprite columns. TGF Obs. Mode Using TGC and CCD camera with fish-eye lens, we will identify the spatial relationship between TGFs and lightning. Cloud-to-Ground Discharge Scattered Lightning Flashes Sprite 762nm nm Gamma-Ray FOV=30 deg FOV=140 deg

7. Science Instruments

7. Imager (LSI-1, -2) low alt. lightning emission → absorption high alt. sprite emission (100  J) → very weak absorption Fig. : (top) Sprite emission spectrum calculated by numerical simulation [Milikh et al., 1997]. (bottom) Lightning emission derived from optical observation [Christian et al., 1996]. LSI-2 LSI-2 (for sprite detection) : Most strong emission line of sprites severe O 2 absorption 762 nm nm Lightning and Sprite Imager (LSI) LSI-1 LSI-1 (for lightning detection) : Strong emission line of lightning >90% lightning has energy of >20  J

LSI-1 & 2 - CMOS image sensors: “STAR250”, product of Cypress Semiconductor Co. are sued - FOV: ~30° WFC - CCD image sensor: “ES2” by WATEC Co., Ltd. is used. - Equipped with fish eye lens. - FOV: ~140°. HSS. - CCD image sensor: “ES2”, by WATEC Co., Ltd. is used. - FOV: ~40°

7. Imager (LSI-1, -2, WFC) Lightning City light Noise calcuration of differencial trigger if the value exceed the threshold Differential of same pixel Differential of same pixel (C n -C n-1 ) ・・・ > threshold ? (C n -C n-1 > T P ) Integration of (C n -C n-1 ) (F n =F n + (C n -C n-1 )) > threshold ? (F n -F n-1 > T F ) Transient Emission （ Lightning, TLEs ） dark image x4 gain LSB Fig.: sample dark image of CMOS camera 29 frames/sec for LSI 30 or 60 frames/sec for WFC

7.1 Verification of the trigger logic PC CMOS(LSI) Time data Clock counting in FPGA dT = 100 μsec 1PPS (Pulse Per Second) Image header = 16 Byte Time dataCamera parameters Image data PC Imaging of display Lightning simulator Lightning simulator GPS time counting system

7.1 Verification of the trigger logic Fig. : Lightning emission with city light. Lightning flash with city light contamination All the lightning events are triggered

comparison of LSI-1 (lightning) and LSI-2 (sprites) intensities 7.2 Sprite judgment logic by CPU Lightning Sprite Lightning without sprite with sprite differential of two images is used in sprite judgment

8. VLF receiver Fig. SPRITE-SAT with extended boom BeCa boom for gravity gradient stabilization also works as VLF antenna. Analog circuit for the VLF receiver provided by Stanford Univ. Specification freq. range:<25 kHz sampling freq.:100 kHz

SPRITE-SAT will provide important practical information to following missions, such as, TARANIS, ASIM and GLIMS/ISS - Brightness and spectrum of lightning flashes /sprites measured with nadir looking cameras - Detection frequency of sprite /TGF events using same time of instruments - will contribute to improve the triggering method/criteria

10. Summary SPRITE-SAT equipped with 3 cameras, VLF receiver and TGF detector, will be launched in January Manufacturing of flight model of BUS and SI is almost completed. Now final assembling of satellite. The delivery to JAXA will be in November. SPRITE-SAT must be a good precursor mission for TARANIS, ASIM and GLIMS, providing important practical information. Collaborative proposal with ground-based / space observations are very welcome.

Abstract SPRITE-SAT is now being developed in-house by the Tohoku University team, which will be launched in summer, The total weight of the satellite would be less than 50 kg, including 4-5 kg science mission payload. There are two scientific objectives in this micro- satellite mission: (1) to identify the generation mechanisms of sprites by investigating their horizontal structures, (2) to identify the generation mechanisms of TGFs by investigating their source location and relationship to lightning discharges. Lightning and Sprite Imager-1 and -2 (LSI-1 and 2) are CMOS cameras with 512 x 512 pixels and the pixel size of 25  m, which pointed at nadir to take images of the horizontal structures of lightning and sprites. In order to detect lightning emissions, we equip LSI-1 with a broadband filter between 740 and 830 nm. We also equip LSI-2 with a rather narrow band- pass filter centered at 762 nm. The optics and the detector array altogether yield an effective field of view (FOV) of 35 deg, giving the pixel resolution of less than 660 m from the altitude of 660 km. Wide Field CCD imager (WFC) is a CCD camera with 659 x 494 pixels and the pixel size of 7.4 um, which takes images of lightning discharges inducing TGFs. WFC is also pointed at nadir and is equipped fish-eye lens (FOV is larger than180 deg). The outputs of all cameras are digitized by 10 bit A/D conversion. One instrumental case contains LSIs and WFC and the total weights is 630 g. In order to detect TGFs, terrestrial gamma-ray counter (TGC) which consists of CsI scintillator is installed at the satellite. TGC can detect gamma-rays in the energy range from 30keV to a few MeV. This satellite also equips a VLF antenna which receives VLF radiations from lightning discharges. At the presentation, we will show the specifications of the instruments and the status of the satellite development more in detail.

7. Imager (LSI-1, -2, WFC) LSI-1 LSI-2 WFC LSI-1 、 2 WFC Mass ： 630 g Power ： 1.94 W CMOS 、 512x512 pix 440x440pix 1/38 s1/38 s, 10bit output <660 m 1.23 /dayFOV=24.8 x 24.8 deg Spatial Res. : <660 m Detection Rate: 1.23 /day 20μJ = 3LSB (SN=12)20μJ = 3LSB (SN=12) CCD 、 659x494 pix 1/60 s1/60 s, 10bit output FOV > 140 degFOV > 140 deg 20μJ = 230LSB (SN=81)20μJ = 230LSB (SN=81)

7.1 Verification of the trigger logic 10km Possible to detect <3 LSB emission Fig. : Typical lightning image Possible to detect >90% lightning events All the lightning events are triggered Lightning emission without city lights

7. Imager (LSI-1, -2, WFC) SRAM (8M) DSP(TMS320VC 5510) ROM data compression Control of SI FPGA (Virtex-2) ROM 1st trigger (lightning detection) 2nd trigger (TLEs detection) Temporal data save memory SHU Science Instruments Satellite BUS data CMD

7. Imager (LSI-1, -2, WFC)FOV 28.7×28.7 pix. Spatial Res. 0.6 km to detect columniform sprites Frame Rate29 fpsto reduce background noise Pixel Size 512×512max value (variable) Detection Rate > 1 event/day global survey FOV 134×180 deg Frame Rate30 or 60 fpsto reduce background noise Pixel Size659×494detect >90% lightning events LSI-1, 2 WFC (Wide Field CCD Camera) Specification