Jou nal o Geophysical Resea ch: A mosphe es
Obse a ion o in insically b igh e es ial gamma ay
flashes om he Medi e anean basin
T. Gjes eland1,2, N. Øs gaa d2, S. La iola3, M. M. Miglie a4, E. A none3, M. Ma isaldi2,5,
F. Fuschino5, A. B. Collie 6, F. Fab ó7, and J. Mon anya7
1Depa men o Enginee ing Sciences, Uni e si y o Agde , G ims ad, No way, 2Bi keland Cen e o Space Science,
Depa men o Physics and Technology, Uni e si y o Be gen, Be gen, No way, 3CNR-ISAC, Bologna, I aly, 4CNR-ISAC,
Lecce, I aly, 5INAF-IASF Bologna, Bologna, I aly, 6School o Chemis y and Physics, Uni e si y o KwaZulu-Na al, Du ban,
Sou h A ica, 7Depa men o Elec ical Enginee ing, Poly echnical Uni e si y o Ca alonia, Ba celona, Spain
Abs ac We p esen h ee e es ial gamma ay flashes (TGFs) obse ed o e he Medi e anean basin
by he Reu en Rama y High Ene gy Sola Spec oscope Image (RHESSI) sa elli e. Since he occu ence o
hese e en s in he Medi e anean egion is qui e a e, he cha ac e iza ion o he e en s was op imized
by combining diffe en app oaches in o de o be e define he cloud o o igin. The TGFs on 7 No embe
2004 and 16 Oc obe 2006 came om clouds wi h cloud op highe han 10–12 km whe e o en a s ong
pene a ion in o he s a osphe e is ound. This kind o cloud is usually associa ed wi h hea y p ecipi a ion
and in ense ligh ning ac i i y. Ne e heless, he analysis o he cloud ype based on sa elli e e ie als shows
ha he TGF on 27 May 2004 was p oduced by an unusual shallow con ec ion. This esul appea s o be
suppo ed by he model simula ion o he pa icle dis ibu ion and phase in he uppe oposphe e. The
TGF on 7 No embe 2004 is among he b igh es e e measu ed by RHESSI. The analysis o he ene gy
spec um o his e en is consis en wi h a p oduc ion al i ude ≤12 km, which is in he uppe pa o he
cloud, as ound by he me eo ological analysis o he TGF-p oducing hunde s o m. This e en mus be
unusually b igh a he sou ce in o de o p oduce such a s ong signal in RHESSI. We es ima e ha his TGF
mus con ain ∼3×1018 ini ial pho ons wi h ene gy >1 MeV. This is 1 o de o magni ude b igh e han
ea lie es ima ions o an a e age RHESSI TGF.
1. In oduc ion
Te es ial gamma ay flashes a e bu s s o gamma adia ion o igina ing om hunde clouds and we e fi s
disco e ed by he Comp on Gamma Ray Obse a o y in 1994 [Fishman e al., 1994]. TGFs ha e a ypical du a-
ion o a ew hund ed mic oseconds and con ain pho ons wi h ene gy up o se e al ens o MeV [Smi h e al.,
2005; Ma isaldi e al., 2010]. Due o he high a mosphe ic abso p ion, TGFs we e fi s claimed o be p oduced
a al i udes >30 km in o de o escape o sa elli e al i ude [Fishman e al., 1994]. Howe e , de ailed analysis
o he Reu en Rama y High Ene gy Sola Spec oscopic Image (RHESSI) da a showed ha he cumula i e
TGF ene gy spec um was consis en wi h p oduc ion a ∼15 km al i ude [Dwye and Smi h, 2005]. O he
s udies o gamma a enua ion [Williams e al., 2006] and spec al shape [Ca lson e al., 2007; Øs gaa d e al.,
2008; Gjes eland e al., 2010] ha e confi med his esul , and i is now widely accep ed ha TGFs a e p oduced
ia b emss ahlung om high-ene gy elec ons ha a e accele a ed in elec ical fields inside hunde s o ms
[Dwye e al., 2012]. Mos TGF obse a ions a e ob ained om he opical egion, whe e ligh ning occu ence
is also he highes [Smi h e al., 2010; Fab o e al., 2015]. Howe e , he e a e also examples o TGFs in sub op-
ical egions [G e ens e e e al., 2009; Gjes eland e al., 2012] . In his pape we p esen h ee TGFs om he
Medi e anean basin. Two o he TGFs (27 May 2004 and 16 Oc obe 2006) a e new e en s iden ified by he
sea ch algo i hm p esen ed by Gjes eland e al. [2012] and one o he e en s (7 No embe 2004) was p e i-
ously p esen ed by [Smi h e al., 2007] as an unusual TGF. These TGFs occu ed in he sub opical egion, which
is ou side o he ypical TGF-p oducing egion. Fo each o he h ee e en s we ha e ca e ully analyzed he
me eo ological p ope ies o he TGF-p oducing hunde s o m. Fo he b igh TGF on 7 No embe 2004 we
ha e p e o med a spec al analysis. Fo he TGF on 16 Oc obe 2006 we p esen ligh ning loca ion da a. Fo
all h ee TGFs we ha e es ima ed he ini ial b igh ness o hese TGFs, hei peak cu en momen , and o al
cha ge momen change.
RESEARCH ARTICLE
10.1002/2015JD023704
Key Poin s:
• We p esen me eo ological condi ions
o TGF o e he Medi e anean
• The TGFs come om cloud wi h high
and shallow con ec ion
• One o he TGF is es ima ed o be e y
b igh a sou ce
Co espondence o:
T. Gjes eland,
[email p o ec ed]
Ci a ion:
Gjes eland, T., N. Øs gaa d, S. La iola,
M. M. Miglie a, E. A none,
M. Ma isaldi, F. Fuschino, A. B. Collie ,
F. Fab ó, and J. Mon anya (2015),
Obse a ion o in insically b igh
e es ial gamma ay flashes om
he Medi e anean basin, J. Geophys.
Res. A mos.,120, 12,143–12,156,
doi:10.1002/2015JD023704.
Recei ed 22 MAY 2015
Accep ed 11 NOV 2015
Accep ed a icle online 13 NOV 2015
Published online 14 DEC 2015
©2015. The Au ho s.
This is an open access a icle unde he
e ms o he C ea i e Commons
A ibu ion-NonComme cial-NoDe i s
License, which pe mi s use and
dis ibu ion in any medium, p o ided
he o iginal wo k is p ope ly ci ed, he
use is non-comme cial and no
modifica ions o adap a ions a e made.
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,143
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
2. Da a and Obse a ion Me hods
The TGFs s udied in his pape we e measu ed by RHESSI, which is a small NASA explo e designed o s udy
sola fla es. The de ec o will de ec omnidi ec ional gamma fluxes and will he e o e de ec TGFs [Smi h
e al., 2005; G e ens e e e al., 2009; Gjes eland e al., 2012]. The RHESSI ins umen consis s o nine Ge ma-
nium de ec o s inside an aluminum c yos a . The ene gy ange used o TGF de ec ion is 25 keV o 17 MeV
[G e ens e e e al., 2009]. Due o he aluminum c yos a co e ing he RHESSI de ec o s, low-ene gy pho ons
may be abso bed o sca e ed. The measu ed ene gy o he low-ene gy coun s is he e o e unce ain. The
o al effec i e a ea o he RHESSI de ec o is 239 cm2 o a ypical TGF spec um [G e ens e e e al., 2008]. The
RHESSI ela i e ime esolu ion is 1 bina y mic osecond (2−20 s).
The h ee TGFs analyzed in his s udy we e iden ified by he sea ch algo i hm p esen ed by Gjes eland e al.
[2012]. Two o he e en s we e oo dim o mee he equi emen s o he fi s RHESSI ca alog. The las e en
has been p e iously epo ed as an unusually b igh TGF [Smi h e al., 2007].
The con ec ion o he TGF-p oducing hunde s o m is s udied by he Mic oWa e Cloud Classifica ion (MWCC)
me hod [Miglie a e al., 2013]. The compu a ional co e o he MWCC algo i hm exploi s he h ee wa e
apo abso p ion equencies a 183.31 ±1∕±3∕±7GHz on boa d he Ad anced Mic owa e Sounding
Uni -B/Mic owa e Humidi y Sounde flying on he NOAA and Me Op sa elli es [Funa su e al., 2007; Hong
e al., 2005]. Due o he e ical de elopmen o he diffe en cloud ypes, he ypical ex inc ion o adia ion
a 183.3±1∕±3∕±7GHz in clea -sky condi ions is pe u bed as a unc ion o cloud ype and cloud e ical
de elopmen . S a ified hin clouds, o example, usually ha e less impac on he wa e apo channels
peaking a lowe al i udes and o en appea anspa en o comple ely masked by he abso p ion o e laying
a mosphe ic wa e apo . On he con a y, hick s a us clouds o con ec i e cells, due o hei p onounced
e ical de elopmen , significan ly pe u b he adia ion pa h. The e o e, om he analysis o he signal
a ia ions in he channels a 183.3±1∕±3∕±7GHz, which peak a diffe en al i udes in he a mosphe e,
i is possible o de ec he p esence o clouds by assessing hei al i ude and ype, ei he con ec i e o
s a i o m. Recen ly, he compu a ional scheme o he MWCC was imp o ed wi h a p obabili y-based
module o hail de ec ion and is cu en ly in es ing s age. In suppo o he MWCC cloud ype/al i ude
es ima es a obus es (he ea e B igh ness empe a u e diffe ence (BTD)) based on he combina ion o he
Me eosa Second Gene a ion (MSG) Spinning Enhanced Visible and In a ed Image channels a 6.2μm and
10.8μm has been applied o de ec he p esence o he o e shoo ing op o med du ing deep con ec ion
[Schme z e al., 1997]. Finally, he esul s om his me hod we e compa ed wi h he ou pu o he Wea he
Resea ch and Fo ecas ing (WRF) model gi ing an indica ion o he cloud dis ibu ion in he uppe a mosphe e.
The Ad anced Resea ch Wea he Resea ch and Fo ecas ing (WRF) model e sion 3.1 (see www.w -model.o g)
[Skama ock e al., 2008] is used in he p esen s udy. I is a nume ical wea he p edic ion sys em ha sol es
he ully comp essible, nonhyd os a ic Eule equa ions using a e ain- ollowing hyd os a ic-p essu e e ical
coo dina e. In he p esen simula ions 41 e ical le els a e employed, mo e closely spaced in he plane-
a y bounda y laye . Simula ions a e pe o med on wo domains, wi h a g id spacing o 16 km and 4 km,
ex ending, espec i ely, 108 ×108 and 145 (in eas wes ) ×169 g id poin s (in no h-sou h), one-way nes ed
one in o he o he . The domains a e cen e ed in he a ea whe e con ec ion was obse ed, and he simula ions
las 36 h, co e ing he pe iod whe e TGFs we e epo ed. Eu opean Cen e o Medium-Range Wea he Fo e-
cas s analysis is used as ini ial and bounda y condi ions (upda ed e e y 6 h). The ollowing pa ame e iza ion
schemes we e used: he Thompson e al. [2004] mic ophysics; he Kain [2004] cumulus pa ame e iza ion in he
coa se g id (no pa ame e iza ion is used in he inne g id); he apid adia i e ans e model o long-wa e
adia ion, based on Mlawe e al. [1997]; he Dudhia [1989] scheme o sho wa e adia ion; he Mello Yamada
Janjic, a u bulen kine ic ene gy closu e scheme o he bounda y laye [Janjic, 2002]; and he Noah land
su ace model [Niu e al., 2011].
Fo he TGF on 16 Oc obe 2006 we also ha e ligh ning da a. The Wo ld Wide Ligh ning Loca ion Ne wo k
(WWLLN) is a global ligh ning ne wo k ha measu es ligh ning wi h empo al accu acy ∼30 μs and spa ial
accu acy o <10 km [Rodge e al., 2005; Jacobson e al., 2006]. In 2006, which is he ime o in e es , WWLLN
had a de ec ion efficiency o ∼5 o 6% o all ligh ning s okes and ∼15% o cloud o g ound (CG) s okes
[Rodge e al., 2009]. Fo 2006 WWLLN had a 12.4 % ma ch wi h RHESSI TGFs [Collie e al., 2011; Gjes eland
e al., 2012].
LINET is a 3D ligh ning de ec ion ne wo k in Eu ope de eloped a he Uni e si y o Munich, and s a ed i s
ope a ion 1 May 2006 [Be z e al., 2009]. LINET can de ec bo h in acloud (IC) and CG ligh ning. Fo ligh ning
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,144
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Figu e 1. (a) The ligh cu e and a sca e plo o pho on ene gy and ime o he RHESSI TGFs on 27 May 2004. (b) MWCC a 20:32 UTC. A weak con ec ion (C01) is
o med close o he TGF measu emen s. (c) MSG 10.8μm cloud empe a u e measu ed a 19:45 in g ey scale. BTD is shown in colo . The maps ange is la i ude
25∘ o 50∘and longi ude −5∘ o 40∘. (d) WRF model inne g id ou pu : cloud wa e and cloud ice con en (colo s) and e ical eloci y (con ou s) along he c oss
sec ion whe e he con ec ion is mo e de eloped op, c oss sec ion a 33.75∘N, a 2100 UT. Fo he e ical eloci y, only he alues w=4,8,12 m/s a e shown.
occu ing wi h a baseline less han ∼250 km LINET is capable o dis inguishing be ween CG and IC and i also
gi es IC emission al i udes. LINET also p o ides ligh ing s oke peak cu en es ima es.
CESI - SIRF (Cen o Ele o ecnico Spe imen ale I aliano - Sis ema I aliano Rile amen o Fulmini) is an I alian
ligh ning de ec ion ne wo k based on Vaisala echnology [Cummins e al., 1998] which s a ed i s ope a ion
in 1994 [Io io and Fe a i, 1996] and de ec s CG ligh ning o e he I alian e i o y. I p o ides loca ion, pola i y
ype o ligh ning, and peak cu en es ima es.
3. Obse a ions
3.1. The 27 May 2004 E en
This TGF occu ed in a well-o ganized con ec ion close o he coas line o Tunis. Figu e 1a shows he ligh cu e
o he TGF measu ed by RHESSI ( op) and a sca e plo o ime and ene gy o each de ec ed pho on (bo om).
This is a weak TGF con aining only 13 coun s, and i has a du a ion o 0.312 ms. The du a ion is calcula ed
by aking ±2𝜎o a Gaussian fi o he ligh cu e. Figu e 1b shows a map wi h RHESSI loca ion a he ime
o he e en , and he con ec i e clouds a e shown in colo . The con ec i e cloud ype is de e mined by he
MWCC me hod. The MWCC measu emen s we e ob ained a 20:34 UTC, which is 54 min a e he TGF measu e-
men . The con ec ion cloud ype is classified as weak con ec ion wi h cloud op heigh (−60∘C) in he middle
oposphe e (a ound 6 km). Al hough he sensi i i y o he high- equency mic owa es is linked o he sca -
e ing by la ge pa icles, he esul s o he MWCC a e suppo ed by he independen BTD es sea ching he
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,145
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
possible in usion in o he s a osphe e due o ex eme hunde s o ms. The BTD es based on da a om
20:30 UT demons a es ha a e y small a ea (169 MSG pixels) shoo s ou o he con ec ion up o he
s a osphe e indica ing ha i was no an in ense hunde s o m. The BTD es based on da a om 19:45 UT
(blue pixels in Figu e 1c) shows simila beha io . To u he in es iga e his case, a simula ion wi h he WRF
model shows ha ela i ely shallow con ec ion de eloped du ing he e en . The model uns show peculia
condi ions since he e is a s ong inhibi ion ex ending along an unusually deep laye (abou 3000 m abo e
he g ound), due o he p esence o e y d y ai o con inen al o igin in he low le els and con ec i e clouds
ex ending abo e om abou 650 g/cm2. Compa ed wi h he o he cases, he cloud ex ension, conside ing
he olume wi hin he cloud con ou lines o 0.5 g/kg, emained confined well below he opopause heigh
(which is a abou 250 g/cm2), and he la ge con en was confined below 400 g/cm2, p obably as a
consequence o he weak e ical mo ion (peak o abou 5 m/s) confined o lowe le els. (Below 300 g/cm2;
Figu e 1d).
In ag eemen wi h his esul , he a mosphe e empe a u e p ofile measu ed a T apani Radiosonde S a ion
18:00 UTC shows a minimum a mosphe ic empe a u e o 217 K be ween 10 and 11 km al i ude. MSG
measu ed an a e age cloud op empe a u e o 220–225 K which co esponds o an al i ude o 10 km
(316 g/cm2).
3.2. The 7 No embe 2004 E en
This TGF occu ed in he open Medi e anean basin. The TGF came om a con ec i e sys em whe e wo deep
co es we e o med. The le pa o Figu e 2a shows he ligh cu e and a sca e plo o he TGF measu ed by
RHESSI. This TGF is among he b igh es TGFs measu ed by RHESSI. The TGF con ains 79 coun s and has a
du a ion o 0.816 ms. Due o he high coun ing a e se e al pho ons a e los in he ead ou elec onics o his
e en . This is called dead ime losses. I we use he me hod p esen ed in Øs gaa d e al. [2012] o de e mine
o al numbe o pho ons we es ima e ha 115 ±15 pho ons hi he RHESSI de ec o s o his TGF.
Figu e 2b shows RHESSI loca ion a he ime o he measu emen s and he con ec i e clouds. The RHESSI
loca ion was be ween wo con ec i e co es. The MWCC esul s a e a 16:15 UTC, 48 min be o e he TGF and he
con ec ion ype classifica ion is C03 indica ing s ong con ec ion in he uppe a mosphe e. Fu he mo e, ew
pixels in he MWCC e ie al a e associa ed wi h hail pa icles o supe cooled wa e o e laying he ice pa icle
laye s [Mulle e al., 1994], which some imes exp ess a adiome ically simila signa u e in passi e mic owa es.
The confi ma ion o he ozen op is issued by he BTD es based on da a om 16:15 UT desc ibing a compac
o e shoo ing op (1843 MSG pixels) o e lying he con ec i e co es ma king he p esence o in usion in o he
s a osphe e consis en wi h MWCC esul s. In his case, he modeled cloud ops each he le el o 200 g/cm2;
hus, hey c oss he opopause, which he model simula es a abou 250 g/cm2. The ins abili y was mode a e
(Con ec i e A ailable Po en ial Ene gy (CAPE) o abou 1200 J/kg), while he inhibi ion was weak, and he
ascending mo ion was ela i ely s ong (abou 9 m/s) e en a 300 g/cm2. The o e shoo ing op is also seen
in MSG a 17:00 UT (Figu e 2c). The a e age cloud op empe a u e was 210–213 K which co esponds o an
al i ude o 12 km (227 g/cm2).
3.3. The 16 Oc obe 2006 E en
The TGF discussed in his sec ion was gene a ed by a ypical mesoscale con ec i e sys em in he Medi e -
anean basin. The MWCC shows ha he mos ac i e con ec i e co es had wo con ibu ions (Figu e 3b). The
smalles was loca ed in he wes e n pa close o he G eek coas line while he bigges , which p oduced he
TGF, de eloped nea he wes coas o Sicily. Bo h sys ems a e cha ac e ized by deep con ec ion in he uppe
a mosphe e wi h he op al i ude highe han 8–10 km. Fu he mo e, a da k a ea co esponding o deep con-
ec ion shows he p esence o high sca e ing by ice ypically associa ed wi h g aupel pa icles. The BTD es
based on da a om 05:00 UT co obo a es he MWCC esul s showing a dense dis ibu ion o cold pa icles
pene a ing in o he s a osphe e and o ming a wide o e shoo ing op. Figu e 3b shows he RHESSI loca ion
and he con ec i e clouds. The MWCC measu emen s a e a 04:54 UTC, which is 2.5 h be o e he TGF.
Fo his case, he WRF model simula ed unusually deep con ec i e clouds. The cloud ops (defined as in
sec ion 3.1) eached 150 g/cm2; hus, hey clea ly c oss he opopause, which in he p esen case can be
es ima ed a abou 250 g/cm2. The ins abili y o his case was e y s ong (simula ed Con ec i e A ailable
Po en ial Ene gy (CAPE) o 2500 J/kg), while he inhibi ion was negligible. I is ele an ha in his case an
in ense e ical mo ion affec s he uppe oposphe e, wi h e ical eloci y s ill la ge han 12 m/s a abou
250 g/cm2. The BTD es based on da a om 07:15 UT is shown in Figu e 3c. The a e age MSG cloud op
empe a u e was 205–210 K which co esponds o an al i ude ≥12 km (≤234 g/cm2).
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,146
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Figu e 2. (a) The ligh cu e and a sca e plo o pho on ene gy and ime o he RHESSI TGFs on 7 No embe 2004. (b) MWCC a 16:15 UT. Two deep con ec i e
co es (C03) a e de ec ed by he MWCC me hod co espondingly o he o e shoo ing op e ie ed by MSG. (c) MSG 10.8 μm cloud empe a u e measu ed a
17:00 in g ey scale. BTD is shown in colo . The maps ange is la i ude 25∘ o 50∘and longi ude −5∘ o 40∘. (d) WRF model inne g id ou pu : cloud wa e and
cloud ice con en (colo s) and e ical eloci y (con ou s) along he c oss sec ion whe e he con ec ion is mo e de eloped: middle, c oss sec ion a 18.55∘E, a
1500 UT. Fo he e ical eloci y, only he alues w=4,8,12 m/s a e shown.
The TGF was p oduced a he ma u e s age o he s o m a e a p og essi e inc ease in size o mo e han 7 h.
A he momen o he TGF he s o m eached an a ea o ∼23⋅103km2. Al hough om ha momen he s o m
size kep s able o abou 2 h, he con ec i e pa <−55∘C s a ed o inc ease in size o he nex 2 h. A he
ime o he TGF he con ec i e co es ep esen ed 34 % o he s o m. A ha momen he s o m had he ypical
condi ions o ansien luminous e en (TLE) p oduc ion [Soula e al., 2009].
Figu e 3a shows he ligh cu e and a sca e plo o he TGF measu ed by RHESSI. He e we ha e assumed ha
he WWLLN loca ion was he sou ce o he TGF. We ha e hen co ec ed o he ligh a el ime om sou ce
o RHESSI and also added he RHESSI offse o 1.8 ms [G e ens e e e al., 2009; Gjes eland e al., 2012]. This TGF
con ains wo gamma pulses sepa a ed by 2.4 ms. The e ical lines show he imes o ligh ning measu ed by
he loca ion ne wo ks. These imes a e also p esen ed in Table 1.
WWLLN de ec ed wo s okes ela ed o his TGF. The fi s WWLLN s oke occu ed 0.6 ms be o e he fi s pulse
o he TGF, and he second s oke occu ed simul aneous wi h he second TGF pulse. Bo h WWLLN ligh ning
had he same loca ion 73 km eas o he RHESSI nadi poin .
LINET classified he s oke as a nega i e cloud o g ound (−CG) wi h peak cu en o −81.6 kA. The loca ion is
<5km away om he WWLLN s oke, and he ime be ween he second WWLLN ligh ning and he LINET ligh -
ning is 48 μs. The ligh ning was measu ed ou side o he LINET ne wo k, and i has an es ima ed unce ain y
o 2.5 km (H. Be z, pe sonal communica ion, 2015).
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,147
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Figu e 3. (a) The ligh cu e and a sca e plo o pho on ene gy and ime o he RHESSI TGF on 16 Oc obe 2006. All RHESSI coun s a e ime shi ed back o
he WWLLN sou ce. The e ical lines indica e he ime o he s e ic measu emen s. WWLLN e en s a e ma ked in ed, he LINET is g een, and CESI is pu ple.
(b) MWCC a 04:54 UT. A compac deep con ec ion wi h a well-defined o e shoo ing op. The con ec ion de elops wi h a ypical s uc u e om weak con ec ion
o deep con ec ion showing ice pa icles on op classified as hails ones. (c) MSG 10.8 μm cloud empe a u e measu ed a 07:15 in g ey scale. BTD is shown in
colo . The maps ange is la i ude 25∘ o 50∘and longi ude −5∘ o 40∘. (d) WRF model ou pu c oss sec ion a 35.70∘N, a 06:00 UT. Fo he e ical eloci y,
only he alues w=4,8,12 m/s a e shown.
LINETs algo i hm o dis inguish be ween CG and IC wo ks well when he dis ance be ween he ligh ning and
he closes s a ion is less han 125 km and he baseline be ween wo s a ions a e less han 250 km [Be z
e al., 2009]. The TGF occu ed ∼700 km om he wo closes senso s, and he baseline be ween he wo
senso s a e ∼1000 km. When he LINET ne wo k s a ed in 2006, he pola i y o ligh ning de ec ed a ou side
he ne wo k could be e e sed. In 2007 his unce ain y was emo ed. The ne wo k can he e o e no ule ou
he possibili y ha his flash is in ac o posi i e pola i y and no nega i e (H. Be z, p i a e communica ion,
2015).
The ligh ning loca ion ne wo k ope a ed by CESI classified he ligh ning as a cloud o g ound (CG) s oke o
−83.8kA. The ime o he s oke is ∼0.4 ms a e he second pulse o he TGF, and i was loca ed 128 km
Table 1. Ligh ning De ec ion Rela ed o he TGF on 16 Oc obe a
Ligh ning Ne wo k La i ude (∘N) Longi ude (∘E) Time (s)bType
WWLLN (fi s s oke) 35.3997 17.3051 17.069439 NA
WWLLN (second s oke) 35.4021 17.3307 17.072720 NA
LINET 35.4133 17.3443 17.072768 −CG 81.6 kA
CESI 36.0525 16.3592 17.073123 −CG 83.8 kA
aThe sou ce ime o he wo TGF pulses a e 17.07028 and 17.07268. NA is o no applicable.
bTime is seconds since 16 Oc obe 2006 07:21:00.
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,148
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Figu e 4. The map shows he RHESSI loca ion o he TGF on 16 Oc obe 2004. The blue do s a e ligh ning measu ed
by WWLLN ±30 min a he ime o he TGFs, while he g een do s a e WWLLN wi hin ±30 s. The unce ain y ellipses o
WWLLN and CESI is also shown. LINET unce ain y is 2.5 km.
no hwes o he WWLLN and LINET ligh ning. The s e ic was loca ed using wo s a ions, being a ou o he
ne wo k domain e i o y. The e o e, i has a s e ched unce ain y ellipse wi h majo semiaxis o 99 km as
shown in Figu e 4. I he CESI sou ce in ac was om he same loca ion as he WWLLN s e ics, i mus occu
ea lie due o he ligh p opaga ion ime. I we use he speed o ligh as he eloci y o he s e ic p opaga ing
om he sou ce o he ecei e s and mo e CESI o he end o he e o ellipse (99 km), he CESI s oke mus
occu 0.33 ms ea lie . This is he ime used in Figu e 3.
Figu e 4 shows a map o RHESSI subsa elli e posi ion and he ligh ning loca ions. Bo h WWLLN ligh ning
s okes we e loca ed 73 km eas o he RHESSI nadi poin . The WWLLN 10 km unce ain y is also shown. The
LINET ligh ning, which was loca ed <5km om WWLLN is shown in o ange.
The blue do s in Figu e 4 a e WWLLN ligh ning wi hin ±30 min o he TGF, and he g een do s a e WWLLN
ligh ning wi hin ±30 s.
4. Discussion
4.1. The 27 May 2004 E en
This TGF occu ed du ing sp ing. The con ec i e cloud measu ed 54 min a e he TGF is C01. This indica es
con ec ion limi ed al i ude. The opopause measu ed a T apani s a ion o his day was be ween 10 and
11 km al i ude. This TGF did no come om a s ong con ec i e cloud. The MSG cloud op empe a u e
measu ed a 20:30 UT indica es ha he op o he cloud is loca ed a 10 km al i ude (316 g/cm2).
4.2. The 7 No embe 2004 E en
The me eo ological obse a ions show high con ec ion up o 8–10 km (414–316 g/cm2) al i ude and a cloud
op a 12 km al i ude (227 g/cm2) 48 min be o e he ime o he TGF. The cloud op empe a u e measu ed a
17:00 UT indica es ha he op o he cloud was loca ed a 12 km al i ude (227 g/cm2).
This TGF was so b igh ha i is possible o do a spec al analysis o he RHESSI TGF measu emen . The TGF
also occu ed be o e he degene a ion o he RHESSI de ec o s [G e ens e e e al., 2009] such ha he RHESSI
de ec o esponse ma ix can be used o compa e simula ed spec um wi h he measu emen s.
Figu e 5 shows he RHESSI ene gy spec um wi h unce ain ies in black. F om ou Mon e Ca lo model
desc ibed in Øs gaa d e al. [2008] we simula e TGFs p oduced a 6, 8, 10, 12, and 14 km al i ude. We use a
ypical Rela i is ic unaway elec on a alanche (RREA) spec um, dN/dE∝1∕E×exp(E∕7.5MeV). We also
assume ha RHESSI is wi hin he TGF emission cone, which we assume o be 30∘[Gjes eland e al., 2011].
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,149
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Figu e 5. RHESSI TGF on 7 No embe 2004 ene gy spec um wi h unce ain ies is shown in black. The cu es a e he
esul om ou MC simula ion, wi h 6, 8, 10, 12, and 14 km p oduc ion al i ude, olded h ough he RHESSI de ec ion
esponse ma ix and no malized o he RHESSI measu emen s. The p alue on he figu e is he p obabili y ha he
simula ed cu es fi he obse a ions. The p alue in pa en heses is calcula ed only using pho ons wi h ene gy
>140 keV. A p oduc ion al i ude o 8 km gi es he bes fi o he measu emen s.
We ha e used he Pea sons chi-squa e es o compa e ou simula ed spec a o he RHESSI measu emen s.
The null hypo hesis is ha he simula ed spec a and he measu emen s a e equal. Fo each simula ed
spec um we calcula e he p alue as shown in Figu e 5. The p alues shown in pa en heses a e calcula ed by
using coun s wi h ene gy >140 keV (no including he fi s bin in Figu e 5). The eason o also discussing he
p alue wi hou he 100–140 keV bin is ha he aluminum c yos a ha houses he de ec o s may influence
he low-ene gy coun s. We canno ejec he null hypo hesis o simula ed TGFs wi h p oduc ion al i ude
6–14 km (536–227 g/cm2) a a significan le el o 0.05. I we include he 100–140 keV bin, only 8 km
(409 g/cm2) p oduc ion al i ude has a p alue ≥0.05.
F om he me eo ological analysis o he hunde cloud 8–10 km (409–307 g/cm2) co esponds o he op o
he s ong con ec ion in he cloud. A TGF p oduc ion al i ude a his al i ude is no ejec ed by he spec al
analysis. The cloud op measu ed by MSG a 17:00 UT was 12 km (227 g/cm2). I he TGF was p oduced a his
al i ude, i is also no ejec ed by he spec al analysis.
4.3. The 16 Oc obe 2006 E en
The me eo ological obse a ions show high con ec ion up o 10 km (313 g/cm2) al i ude and a cloud op
o ≥12 km (≤227 g/cm2) 2.5 h be o e he ime o he TGF. The cloud op empe a u e measu ed a 7:15 UT
indica es ha he op o he cloud is loca ed a 12 km al i ude (234 g/cm2). Howe e , he WRF simula ion
shows ha he cloud had owe s ha eached up o 150 g/cm2.
Fo his TGF we ha e ligh ning loca ion measu emen s om se e al ne wo ks. As shown in sec ion 3.3, he
second pulse o he TGF was simul aneous wi h ligh ning measu ed by h ee ligh ning loca ion ne wo ks.
All measu emen s a e simul aneous o wi hin 100 μs. Howe e , he absolu e iming o he RHESSI clock is
unde discussion. The RHESSI clock has been sugges ed o ha e 1–2 ms andom unce ain y [G e ens e e
e al., 2009]. The ime be ween he wo pulses o his TGF is 2.4 ms. The ime be ween he wo WWLLN s e ics
is 3.3 ms. I is he e o e no possible ha bo h WWLLN s e ics a e caused by he wo ela i is ic unaway
a alanches ha p oduced he wo pulses o he TGF. I is likely o assume ha he second TGF pulse caused
he second WWLLN s e ic and also possibly he LINET and CESI s e ic. Howe e , he fi s WWLLN s e ic was no
ela ed o a gamma pulse ha was measu able a sa elli e al i ude.
I he measu emen s by WWLLN and LINET, which we e simul aneous wi h he second TGF pulse, we e
caused by he RREA ha also p oduced he TGF, his cu en mus be o posi i e pola i y, accele a ing elec-
ons upwa d. LINET classified his flash as being o nega i e pola i y. Howe e , LINET s a ed in 2006 and a
Oc obe 2006 he ne wo k was no ully calib a ed. We can he e o e no exclude he possibili y ha his
s oke was, in ac , o posi i e pola i y (H. Be z, pe sonal communica ion). Also, in p e ious s udy he pola i y o
he cu en p oduced by RREA has been misclassified by o he ne wo ks ha au oma ically classi y ligh ning.
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,150
Jou nal o Geophysical Resea ch: A mosphe es 10.1002/2015JD023704
Table 2. The Table Shows he Resul s F om MWCC and MSGa
TGF Time UT MWCC MSG Cloud Top Tempe a u e
27 May 2004 19:40 C01 220–225 K
8km 10km
535 g/cm2316 g/cm2
(measu ed 20:32 UT) (measu ed 19:45 UT)
7 No 2004 17:03 C03 210–213 K
10 km 12 km
307 g/cm2227 g/cm2
(measu ed 16:15 UT) (measu ed 17:00)
16 Oc 2006 07:21 C03 205–210 K
10 km >12 km
313 g/cm2<234 g/cm2
(measu ed 04:54 UT) (measu ed 07:15 UT)
aThe ime o he measu emen is also gi en. The MSG cloud op empe a u e is he
a e aged alue o he o e shoo ing pixels (shown in colo in Figu es 1c, 2c, and 3c). The
al i ude es ima ion is calcula ed using a lapse a e o 6.5 K/km.
In a s udy by Cumme e al. [2013] s e ics om Na ional Ligh ning De ec ion Ne wo k (NLDN) we e analyzed
and hey ound ha NLDN may ge w ong pola i y. They also ound ha NLDN in some cases classified an IC
as a CG ligh ning. This happens especially o complex s okes such as he ones in ol ing TGFs.
4.4. TGF P oduc ion Al i ude
The obse a ion o he con ec ion and cloud op al i ude o he hunde s o ms a e summa ized in Table 2.
Since he fi s analysis o he cumula i e spec um o RHESSI by Dwye and Smi h [2005], mos s udies ha e
sugges ed ha TGFs o igina e below he opospause and a e p oduced by elec ical fields inside he hunde -
cloud. Williams e al. [2006] es ima ed gamma a enua ion and ound ha i is possible o TGF o be p oduced
in he uppe oposphe e and make i o sa elli e al i ude. Se e al s udies ha e analyzed he TGF ene gy spec-
um, and hey all conclude ha i is consis en wi h a p oduc ion al i ude ≤15 km [Dwye and Smi h, 2005;
Ca lson e al., 2007; Øs gaa d e al., 2008; Gjes eland e al., 2010].
De ailed s udies o he adio emission om TGFs ha e also shown ha TGFs may be p oduced down o
∼10 km al i ude. Fo wo TGFs S anley e al. [2006] used ionosphe ic eflec ion o es ima ed he sou ce
al i ude o he TGF- ela ed s e ics o be 13.6 km and 11.5 km. In a simila s udy by Shao e al. [2010] hey ound
he TGF- ela ed s e ic o igin o be be ween 10.5 and 14.1 km.
Cumme e al. [2014] epo ed wo TGF measu ed by Fe mi. These TGFs we e es ima ed o be p oduced a
11.8±0.4km and 11.9±0.94 km. This places he sou ce egion in he in e io o he hunde s o m be ween
he wo main cha ge laye s and implies an in insic TGF b igh ness o app oxima ely 1018 elec ons.
Lu e al. [2010] epo ed on a RHESSI TGF which occu ed be ween he nega i e and posi i e cha ge laye s,
which we e loca ed a 8.5 and 13 km al i ude. In a ime analysis o a TGF o e Ma acaibo lake by Øs gaa d e al.
[2013] he TGF was ound o occu close o he uppe posi i e cha ge laye du ing he leade p opaga ion o
an IC ligh ning.
The mos compelling heo ies o TGF p oduc ion a e he ela i is ic eedback discha ge model [Dwye , 2012]
and cold RREA om ligh ning leade s as sugges ed by Ca lson e al. [2009] and Celes in and Pasko [2011, 2012].
In bo h hese heo ies he TGF mus be p oduced be ween he midle el nega i e and he main posi i e cha ge
laye o he hunde cloud.
Since he ime be ween he MWCC con ec ion measu emen s and he TGFs a e 48 min, 54 min, and 2.5 h wi h
espec o he TGF ime, we canno ule ou ha he TGF, in ac , a e p oduced a highe al i ude. Howe e , since
p e ious s udies shows ha TGFs a e p oduced inside hunde clouds, we use he MSG cloud op es ima e as
an uppe limi o TGF p oduc ion.
GJESTELAND ET AL. TGFS FROM THE MEDITERRANEAN 12,151
