Taranis衛星によるMeV帯域の地球ガンマ線フラッシュ観測
和田有希 (東京大学/ 理化学研究所: wada@juno.phys.s.u-tokyo.ac.jp)
Philippe Laurent (CEA Saclay / Laboratoire APC, France), 中澤知洋 (名古屋大学), ほかTaranis XGREチーム

1. MeV帯域における大気中での電子加速

3. XGRE検出器によるTGF観測

地球ガンマ線フラッシュ (Terrestrial Gamma-ray Flash: TGF)
・雷放電に同期した、雷雲から宇宙に向かうフラッシュ
1994年にCGRO/BATSEで発見 (Fishman+1994)
・数十マイクロ秒から数ミリ秒の継続時間 (Foley+2014)
・スペクトルは40 MeV以上まで伸びる冪関数型 (Smith+2005)
・1017-18個の電子をMeV帯域まで加速・増幅 (Mailyan+2016)
REPORTS-> 制動放射による光子や電子が衛星に到達
Lightcurves (4 events)
Spectrum (all events)

・XGRE: X / Gamma / Relativistic Electron Detector
・日本から東京大学と名古屋大学が開発に参加
センサーユニット

検出ユニットSmith+2005

・3台のセンサーユニットで構成
・1台のセンサーは4台の検出器ユニットで構成
・検出ユニットは3層のシンチレータで構成- 8.7 mm厚のLaBr3結晶- 5 mmのプラスチックシンチレータ2枚

time forthe brightest RHESSI detected so far. (Right) Summed
energy spectrum of all the RHESSITGFs,
RHESSIで観測されたTGFのライトカーブの例(左)と積算エネルギースペクトル(右)versus time for the brightest, shown with the expected instrumental response (solid curve) to isotropic
rom upper left). RHESSI TGF thin-target bremsstrahlung from 35 MeV electrons (dashed line).

検出範囲：光子 0.03–10 MeV / 電子 1–10 MeV
デッドタイム：< 350 ns
6
最大カウントレート：>10 particles/s
相対 / 絶対時刻精度：1 μs / 1 ms
有効面積：800 cm2@1 MeV / 300 cm2@1 MeV (Sarria+2017)
検出数 (予測)：202 TGFs/yr (Sarria+2017)
Sarria+2017(1センサーユニット・LaBr3)-1

207Bi 0.570 MeV

電波受信機
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18 FEBRUARY 2005

207Bi 1.064 MeV
40K 1.462 MeV

1020

LaBr3の内在BGD
0.5

11.5
Energy (MeV)

2

2.5

3

4. Taranis衛星によるTGF観測のサイエンス・電波 (長波: <1 MHz / 短波: 0.1-35 MHz)との同時観測-> すべてのTGFにおいて対応する雷放電を観測可能
・100 keV未満のスペクトル -> 吸収による発生高度推定
・LaBr3による高速光子計測 -> パイルアップを回避
・ガンマ線衛星と直交する軌道 -> 2機での同時観測に期待・2018年にISSに搭載されたAtmosphere-SpaceInteraction Monitor (ASIM) との協調観測にも期待・256 ms毎のスペクトル、128 μsビンのライトカーブは常に取得, イベントを検出したら光子イベントの1087 エネルギーと時刻を記録-> MeV帯域のshort GRBやフレア観測？

可視光カメラ・測光器- IMM
磁場プローブ- SI
イオンプローブ

103

102. Taranis衛星による全球観測- IME-BF/HF

207Bi照射スペクトル104-1

tively. The electron accelerator responsible foratmospheric cross sections rise below 100 keV,TGFs may thus work to higher energies thanand assuming that all the photons come fromany other natural accelerator in Earth_s atmoa single altitude, we can constrain that altisphere or magnetosphere. If the acceleration istude to be 925 km by noting that the lowest
theby a DC electric field, it requires a potential
RHESSI energy point shown (about 60 keV)uladrop of at least 30 MV (higher when taking
does not have an extra e-folding of absorpthe
frictional energy losses into account). This istion compared to the points at several huncomparable to predicted potentials between
dred keV. Monte Carlo simulations of electron
cloudtops and the ionosphere (25).
and gamma-ray propagation in the atmofor
To estimate the global average rate of TGFs,
sphere will produce stronger altitude confour
we need RHESSI_s footprint for TGF detection.
straints and address other scenarios that might
panAlthough RHESSI has a line of sight to thefit the data, such as a hard electron spectrum
thehorizon at 2700 km, only an extremely bright
below 25 km combined with a softer spectrum
maflash at that distance would be detected aboveat higher altitudes. Future orbiting detectors(Dwyer that2004)
background. Assuming
TGFs are isotropic
sensitive to 海抜高度における大気中での電子に生じる抗力
lower energies (È10 keV) would
sugemitters and equally luminous, and considergive even better constraints, due to the muching the dynamic range of our events, we estihigher cross sections for atmospheric absorppons onmate an effective footprint of radius È1000 km.
tion at lower energies.
Then the observed TGF rate of 86 events in
The TGFs that make up the composite
m is・フランス国立宇宙科学センターの小型衛星(150kg級)l by
183 days corresponds to È50 events per day
spectrum may have different spectra (18). Alsummed over the latitudes RHESSI covers.
though each TGF has too few photons for aThe・2020年上半期にフランス領ギアナよりVEGAで打ち上げ
cted
Upward beaming of the photons would reduce
good spectral fit, the mean photon energy
・高度700へ投入予定
the radius of detectability. One
model predicted
for each TGF kmの太陽同期軌道
can be compared (Fig. 3). The (軌道時刻22:30)
ung
not,a È100-km beam (21), in which case the truesmooth curve is a larger set of artificial TGFs,
・国立環境物理化学研究所がミッションコントロール
global rate could be two orders of magnitudeeach one having the mean spectrum in Fig. 2
ates・CEA,LATMOS,exhigher. Even 5000 TGFs per day is only 0.1%
and the APC,
same numberof counts as IRAPなどが開発に参加。
one ranodelof the global lightning rate, which the spacedomly selected real TGF. The width of this・世界初の放射線・電波・可視光の軌道上同時観測！
conbased Optical Transient Detector recently meacurve indicates that some, but not all, of theomsured to be 44 T 5 per second (26). We cannot
spectral variation we see is due to the small
・6つ観測機器s atrule out an even larger population of TGFs
number of photons per TGF. Nemiroff et al.
forbelow RHESSI_s detection threshold.
(18) and Feng et al. (24) found that BATSE
XGRE
xis,
The average number of relativistic elecTGF spectra evolved from hard to soft withinross X線・ガンマ線・電子検出器
trons in each flash can be estimated. Assuming
each flash, and that the bursts were somewhat23).
monoenergetic 35-MeV electrons (Fig. 2), anlonger at low energies. We see both effects in- IDEE
200 keV電子検出器
average photon energy of 2.5 MeV, thick-target
the RHESSI flashes, but at a low level; moreatebremsstrahlung, isotropic emission, a distancedetailed analyses will be forthcoming.

検出ユニットの断面図 (Sarria+2017)

Counts s MeV・どのようにして大量の電子を加速・増幅させるか？
-rials
陽電子が電場中を逆流して雪崩増幅のサイクルにIndividual photons greater than 10 MeV apsurrounding the detectors. The BATSE
pear in 60 of the 86 TGFs.
Summingdata
down to 25 keV were consistent
with a
フィードバック？
(相対論的フィードバックモデル:Dwyer
2003)all theTGFs, we find 47 photon events that exceed
power law when corrected for the instrumen-tal放電路に生じる絶縁破壊を遥かに超える強電場中で
one detector_s threshold of È18 MeV and 9response (18). These results suggest thatphoton events that, because they deposited enerTGFs occur relatively high in the atmosphere
熱電子もすべて加速？
(リーダー種電子モデル: Celestin+2011)
gy in more than one detector, we can be certain
and are probably not from the same source
were above 20 MeV. The expected values ofthat produces gamma rays seen on the ground・TGFを発生させる強電場領域はどのような雷放電
these numbers at normal background rates(4, 5). Assuming that the intrinsic TGF specで生成されうるか？
would be 16.5 and 2.8 photon events, respectrum does not rise suddenly just where the(c) CNES

Fishman et al., Science, 1994
Smith et al., Science, 2005
Foley et al., JGR Space Phys., 2014
Mailyan et al., JGR Space Phys., 2016
Dwyer, Geophys. Res. Lett., 2003

Celestin & Pasko, JGR Space Phys., 2011
Dwyer, Geophys. Res. Lett., 2004
Sarria et al., Geosci. Inst. Methods
Data Sys., 2017

第2回MeVガンマ線天文学研究会@東京大学本郷キャンパス 2019年9月26日-27日