Ci a ion: Pie a d, V.; Winan , A.;
Bo ek, E.; Pé e s de Bonhome, M. The
Mo he ’s Day Sola S o m o 11 May
2024 and I s E ec on Ea h’s
Radia ion Bel s. Uni e se 2024,10, 391.
h ps://doi.o g/10.3390/uni e se
10100391
Academic Edi o : Ezio Ca oli
Recei ed: 4 Sep embe 2024
Re ised: 27 Sep embe 2024
Accep ed: 5 Oc obe 2024
Published: 10 Oc obe 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
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4.0/).
uni e se
A icle
The Mo he ’s Day Sola S o m o 11 May 2024 and I s E ec on
Ea h’s Radia ion Bel s
Vi iane Pie a d 1,2,*,† , Alexand e Winan 1,2,†, Edi h Bo ek 1and Maximilien Pé e s de Bonhome 1,2
1Sola Wind, Space Physics and Sola -Te es ial Cen e o Excellence, Royal Belgian Ins i u e o Space
Ae onomy (BIRA-IASB), A enue Ci culai e 3, B-1180 B ussels, Belgium;
alexand e.winan @ae onomie.be (A.W.); [email p o ec ed] (E.B.);
[email p o ec ed] (M.P.d.B.)
2Cen e o Space Radia ions (CSR), Ea h and Li e Ins i u e—Clima e Sciences ELI-C, Uni e si é Ca holique
de Lou ain (UCLou ain), B-1348 Lou ain-la-Neu e, Belgium
*Co espondence: i iane.pie a [email p o ec ed]
†These au ho s con ibu ed equally o his wo k.
Abs ac : The mon h o May 2024 was cha ac e ized by sola ene ge ic pa icles e en s di ec ed
owa ds he Ea h, especially he big e en causing a s ong e es ial geomagne ic s o m du ing
he nigh om 10 o 11 May 2024, wi h au o as obse ed e e ywhe e in Eu ope. This was he
s onges s o m o he las 20 yea s wi h a Dis u bed S o m Time index Ds <
−
400 nT. In he
p esen wo k, we show wi h obse a ions o GOES, PROBA-V/EPT and Me OP/MEPED ha his
excep ional e en was associa ed wi h he injec ion o ene ge ic p o ons in he p o on adia ion bel ,
wi h impo an consequences o he Sou h pa o he Sou h A lan ic Anomaly (SAA). In addi ion,
he geomagne ic s o m caused by he sola e up ion has had emendous impac s on he elec on
adia ion bel s. Indeed, we show ha o 0.3 o 1 MeV elec ons, he s o m led o a long las ing
ou bel s con igu a ion which was no obse ed be o e wi h EPT launched in 2013, un il a smalle
geomagne ic s o m ook place a he end o June 2024. Mo eo e , o he i s ime since i s launch,
obse a ions o he EPT show ha ul a- ela i is ic elec ons wi h
E>
2 MeV ha e been injec ed in o
he inne bel down o McIlwain pa ame e L = 2.4, iola ing he impene able ba ie p e iously
es ima ed o be loca ed a L = 2.8.
Keywo ds: geomagne ic s o m; sola ene ge ic pa icles; sola e en ; adia ion bel s; ene ge ic p o ons;
ela i is ic elec ons
1. In oduc ion: The 11 May 2024 E en in he Con ex o P e ious S o ms
Th oughou he mon h o May 2024, se e al ac i e egions o a ed ac oss he sola disc.
One o hem, called NOAA 13664, especially g ew in size and magne ic complexi y, and
ga e ise o nume ous la ge la es, including se e al X-class la es [
1
]. Associa ed wi h he
ac i i y a ound his ac i e egion we e a numbe o mode a e o la ge as Co onal Mass
Ejec ions (CMEs), many o which we e Ea h di ec ed. Ac i e egion NOAA 13664 was
he la ges sunspo g oup in mo e han 10 yea s. I p oduced he s onges sola la e so
a his sola cycle, and was he mos la e p oduc i e egion in mo e han 3 decades. I is
he sou ce o he co onal mass ejec ion ha led o he s onges geomagne ic s o m in wo
decades. On 8 and 9 May, he Sun p oduced no less han 28 M- and X-class la es, 20 o
hem o NOAA 13664. NOAA 13664 now occupies he 4 h spo in e ms o highes M- and
X-class la e p oduc ion since he s a o he GOES measu emen s in 1976. The sunspo
g oup is ob iously he la ges o he sola cycle so a (2400 million hs o a sola hemisphe e,
o 14 imes he o al su ace o he Ea h). Due o he quick succession and a ying
speeds o he CMEs, se e al o hem me ged and in e ac ed as hey a elled h ough he
in e plane a y medium leading o enhanced e ec s in he Ea h’s space en i onmen . These
subsequen complex magne ic s uc u es a e sing he in e plane a y medium inc eased
he geo-e ec i eness o he e en s a Ea h.
Uni e se 2024,10, 391. h ps://doi.o g/10.3390/uni e se10100391 h ps://www.mdpi.com/jou nal/uni e se
Uni e se 2024,10, 391 2 o 18
The cha ac e is ics o he sola wind measu ed by OMNI when i eached 1 AU a e
illus a ed on Figu e 1. The a i al ime o e en a 1 AU wi h ega d o sola wind densi y,
eloci y and p essu e is 10 May 2024 a 17:07 UT. A clea peak o sola wind densi y, bulk
eloci y ( op panel), empe a u e and p essu e (second panel) is obse ed on 11 May 2024.
The bulk eloci y peak ( ha eached almos 1000 km/s) and he empe a u e pe u ba ions
a e longe in ime han he densi y shock. The p essu e combines he densi y and eloci y
e ec s, and de e mines he posi ion o he magne opause. The in e ed nega i e peak o he
Z componen o he In e plane a y Magne ic Field Bz ( hi d panel) showing a sou hwa d
di ec ion implies a s ong answe o he geomagne ic ield. The sola e en indeed ga e
ise o he la ges geomagne ic s o m obse ed in 20 yea s in he nigh o 11 May. This
s o m has been called “Mo he ’s Day” e en , because he da e co esponds o Mo he ’s
Day in many coun ies a ound he wo ld.
Figu e 1. Pa ame e s o he sola wind a 1 AU and geomagne ic indices om OMNI om 1 May o
30 June 2024. Top panel: sola wind densi y (blue) and sola wind speed ( ed). Second panel: sola
wind p essu e (blue) and sola wind empe a u e ( ed). Thi d panel: Sou hwa d componen o he
in e plane a y magne ic ield
Bz
.Bo om panel: Ds index (blue) and Kp index mul iplied by 10 ( ed).
The geomagne ic ac i i y indices o Ba els (Kp) and Dis u bed s o m ime (Ds in
nanoTesla) a e illus a ed in he bo om panel o Figu e 1. The ini ial phase o he s o m
s a ed a he a i al o he sola wind pe u ba ion, i.e., on 10 May 2024 a 17:07 UT. A his
poin in ime, a s ong sudden commencemen is obse ed in he Ds index. Du ing he
main phase, Ds d as ically d opped o
−
412 nT (and a highe esolu ion minimum SYM-H
Uni e se 2024,10, 391 3 o 18
o
−
512 nT no shown he e) on 11 May 2024 a 02:14. I is he la ges geomagne ic s o m
(lowes alue o Ds ) since he No embe 2003 Halloween e en s. I is only in Oc obe -
No embe 2003 and No embe 2004 ha s o ms o compa able s eng h a e ound. The
11 May 2024
s o m is he 7 h day wi h he mos nega i e Ds alue since he measu emen s
s a ed in 1957 (WDC Kyo o—h ps://wdc.kugi.kyo o-u.ac.jp/ (accessed on 2 Sep embe
2024)). I anks jus behind 20 No embe 2003 bu be o e o he majo s o ms such as on 26
May 1967, 31 Ma ch 2001 and 30 Oc obe 2003. Only he amous 13–14 Ma ch 1989 s o m
(
−
589 nT) and 3 days in he la e 1950s ha e a mo e nega i e Ds . As Ds alues a e only
a ailable since 1957, one should no o ge he e we e also o he big s o ms in he pas , such
as he Ca ing on e en o Sep embe 1859, he 4 Feb ua y 1872 s o m, and he 13–15 May
1921 s o m. Fo hese e en s, much s onge indi idual Ds alues o espec i ely
−
949 nT,
−834 nT [2,3] and −907 nT [4] ha e been es ima ed.
The 11 May 2024 s o m pe sis ed h ough he weekend gi ing ise o au o as a low
magne ic la i udes (below 45°) in bo h hemisphe es [
5
]. The space wea he a his ime also
esul ed in s ong adio blackou s and sola ene ge ic pa icle e en s wi h adia ion-bel
enhancemen s h oughou he days ollowing. This big e en had s ong e ec s on he
e es ial adia ion bel s ha will be explained in he p esen wo k.
Sec ion 2p esen s he ins umen s and da a used o measu e he luxes in he adia ion
bel s. Sec ion 3p esen s he injec ion o p o ons du ing he e en , while Sec ion 4shows
he modi ica ions o he elec on luxes. The new esul s a e discussed in Sec ion 5and
conclusions a e p esen ed in he Sec ion 6.
2. Ins umen s and Da a
2.1. PROBA-V/EPT
The Ene ge ic Pa icle Telescope (EPT) measu es he luxes o high ene gy pa icles in
he adia ion bel s. This ins umen was de eloped by he Cen e o Space Radia ion (CSR)
a UCLou ain in Belgium, wi h he collabo a ion o he Royal Belgian Ins i u e o Space
Ae onomy and Qine iQ Space (p esen ly Redwi e Space). This ins umen was launched on
7 May 2013 onboa d he ESA (Eu opean Space Agency) sa elli e PROBA-V. The spacec a
was sen o a sun-synch onous pola Low Ea h O bi (LEO) a an al i ude o 820 km, wi h
an inclina ion o 98.73° and a descending node a 10:30 am local ime [
6
]. The concep o
he EPT is based on he Be he-Block o mula gi ing he ela ionship be ween he s opping
powe o a ma e ial and he ene gy o inciden cha ged pa icles [
7
]. The EPT was designed
o eal- ime and con amina ion- ee measu emen s o cha ged pa icle spec a in he space
en i onmen and is able o disc imina e be ween elec ons, p o ons, alpha pa icles and
hea ie ions while pe o ming di ec measu emen s o hei ene gy spec a [
8
]. The EPT
ea u es wo ene gy sec ions. The Low Ene gy Sec ion (LES) only measu es lowe ene gy
elec on luxes, while he High Ene gy Sec ion (HES) measu es elec on, p o on and hea ie
pa icle luxes o highe ene gy. The EPT measu es di e en ial lux o elec ons abo e
500 keV in 6 ene gy channels, and o p o ons abo e 9.5 MeV in 10 ene gy channels.
2.2. SEM-2/MEPED
The Space En i onmen Moni o 2 (SEM-2) [
9
] consis s o a sui e o ins umen s ha
moni o he lux o p o ons and elec ons in nea -Ea h space. One o he wo main ins u-
men s o he SEM-2 sui e is he Medium Ene gy P o on and Elec on De ec o (MEPED)
which ocuses on measu ing luxes o ene ge ic p o ons and elec ons coming di ec ly
om he Sun and apped in he Ea h’s geomagne ic ield, i.e., o ming he adia ion
bel s. The SEM-2 sui e was i s i ed on he NOAA-15 (Na ional Oceanic and A mo-
sphe ic Adminis a ion) POES (Pola O bi ing En i onmen al Sa elli es) in 1998 and hen
on all la e e sions NOAA-16 o -19. In 2006, SEM-2 was also accommoda ed on he ESA
Me Op-A and also on he la e e sions Me Op-B and Me Op-C. All he sa elli es on which
SEM-2 is i ed on ope a e on heliosynch onous LEO a an a e age al i ude o 850 km o
NOAA/POES and 820 km o Me Op.
Uni e se 2024,10, 391 4 o 18
The MEPED ins umen is composed o wo pai s o di ec ional de ec o s. The i s
pai is dedica ed o he measu emen o p o ons wi h ene gies anging om 30 keV o
200 MeV in 5 di e en ial channels and one in eg al channel. The second pai o de ec o s
measu es he luxes o elec ons o ene gies be ween 30 keV o 2500 keV in 3 in eg al
channels. Fo a gi en ype o pa icles, he wo elescopes a e a anged pe pendicula
o one ano he and a e e e ed o as he 0° elescope and he 90° elescope. On Me Op,
he 0° elescope poin s di ec ly o he zeni h and he 90° elescope poin s o he an i am
di ec ion (i.e., opposi e o he eloci y ec o o he spacec a ). The pa icula a angemen
o he MEPED elescopes allows o cha ac e ize bo h apped and p ecipi a ing pa icles.
Indeed, he 90° elescope mos ly measu es pa icles ha a e apped in he geomagne ic
ield whe eas he 0° elescope mainly measu es pa icles in he Bounce Loss Cone (BLC)
and hus p ecipi a ing in o he a mosphe e.
3. The E ec s o he 11 May S o m on he P o on Radia ion Bel
3.1. P o ons Obse ed by GOES a Geos a iona y O bi
The Geos a iona y Ope a ional En i onmen al Sa elli es (GOES), ope a ed by NOAA,
also p o ide obse a ions o sola p o on luxes a he geos a iona y o bi [
10
]. Ene ge ic
p o ons de ec ed by GOES du ing he e en a e illus a ed on Figu e 2. The sola e en is
associa ed o he a i al o sola ene ge ic p o ons: he lux
>
10 MeV (in blue) s a s inc eas-
ing on May 8 a 08h00 and eaches on 10 May a 11h00 he h eshold o 10
cm−2s−1s −1
,
gene ally used o quali y an e up ion as a sola ene ge ic pa icle (SEP) e en . Peak lux
o
>
10 MeV p o ons is i s eached on 10 May a 17h00 (co esponding o he Ds s o m),
quickly ollowed by a peak on 11 May a 09h00, isible also o highe ene gy p o ons, e en
o E>100 MeV o ins ance (in g een).
Figu e 2. GOES obse a ions o p o on luxes wi h ene gy
>
10 MeV (blue),
>
50 MeV (o ange) and
>100 MeV (g een) a he geos a iona y o bi om 1 May 2024 o 30 June 2024.
On 13 May, a second peak o he
>
10 MeV lux appea s abo e he h eshold. An
inc ease is hen also isible o >50 MeV, bu no o >100 MeV.
No e ha a e a ull sola o a ion o 27 days, he sunspo s ill p esen a he Sun’s
su ace was esponsible o a new sola ene ge ic pa icle e en appea ing on 8 June 2024
oge he o all ene gies. Since he p o ons o igina e om he same ac i e egion, he
a ia ion o he p o on lux in ime is e y simila o ha o 11 May, wi h a second peak
a ew days la e o 10 and 50 MeV. I is wo h men ioning ha he i s maximum luxes
eached a all ene gies a e highe du ing he la e e en . Ne e heless, his second SEP
e en did no c ea e a geomagne ic s o m, which is no so unusual since only 75% o SEP
e en s a e ollowed by Ds s o ms <−50 nT [11].
3.2. P o on Injec ions Obse ed a LEO
Ene ge ic p o ons de ec ed by PROBA-V/EPT and Me Op/MEPED on a low pola
o bi a e illus a ed in Figu es 3and 4. Because hose wo ins umen s a e loca ed on a
low Ea h o bi , hey a e able o measu e he lux o p o ons ac oss he en i e ange o he
Uni e se 2024,10, 391 5 o 18
adia ion bel s, i.e., a di e en alues o he McIlwain [
12
] pa ame e L, as opposed o
GOES which pe o m measu emen s a a ixed loca ion in he ou e bel (L∼6.6).
Figu e 3. P o on di e en ial luxes obse ed by PROBA-V/EPT om 1 May o 30 June 2024 as a
unc ion o he McIlwain pa ame e L ( e ical axis) and ime (ho izon al axis) in he i s 5 EPT
p o on ene gy channels. Fluxes a e a e aged in bins which a e 6 h long in ime and 0.25 in L. F om
op o bo om, he ene gy o each channel inc eases and hey all sha e he same colo ba .
Figu e 4. P o on di e en ial luxes obse ed by MEPED om 1 May o 30 June 2024 as a unc ion o
he McIlwain pa ame e L ( e ical axis) and ime (ho izon al axis) in 5 h p o on ene gy channel o
MEPED. Fluxes a e a e aged in bins which a e 6 h long in ime and 0.25 in L. Top panel: 0° elescope,
Bo om panel: 90° elescope.
In Figu e 3, he i s 5 p o on ene gy channels o EPT a e displayed, wi h inc easing
ene gy om he op panel o he bo om one. P o on luxes a e a e aged in L and ime bins
(L bins: 0.25, ime bins: 6 h). Simila o Figu e 3, Figu e 4p esen s he di e en ial p o on
luxes measu ed by he 0° and 90° elescopes o MEPED in hei highes ene gy channel
(2.5–6.9 MeV). MEPED luxes ha e been a e aged in he same bins in ime and L as o
he EPT.
Uni e se 2024,10, 391 6 o 18
I is impo an o no e ha e en hough EPT and MEPED a e bo h obse ing pa icle
luxes a a simila al i ude, hei obse a ions canno be di ec ly compa ed o one ano he ,
a leas quan i a i ely. The e a e wo main easons o his:
1.
The ene gy anges o he measu ed p o ons a e no he same o he i s channel o
he EPT and he i h channel o MEPED. I is hus expec ed ha he lux o p o ons
obse ed by MEPED is highe han hose obse ed by he EPT, since i measu es
luxes a lowe ene gy.
2.
The pi ch angles o he obse ed pa icles di e be ween he ins umen s. Indeed, he
MEPED 90° elescope can be conside ed o measu e luxes o apped pa icles and he
0° elescope obse es he p ecipi a ing pa icles (a leas a high la i udes and hus high
L alues), while EPT measu es pa icle luxes wi h a pi ch angles be ween 60° and
−60°. Thus, he EPT measu es a combina ion o apped and p ecipi a ing pa icles.
A high L alues, especially a ound L = 6.6, he a ia ions o he p o on lux a a ious
ene gies in ime a e e y consis en wi h he obse a ions om GOES. Obse a ions om
EPT, p o iding he dependence in ene gy o he lux, con i m ha lowe ene gy p o ons
(
∼
10 MeV) a e injec ed i s a high L, apidly ollowed by he highe ene gy p o ons. EPT
ac ually shows ha he e a e al eady no mo e double peaked injec ion o p o ons wi h
ene gies >61 MeV, as con i med by he >100 MeV measu ed by GOES in eg al channels.
A lowe ene gies (
∼
2 MeV), MEPED obse a ions a e simila o hose o EPT in he
i s channel. None heless, he di e en ene gy anges o EPT show he di e en beha iou s
depending on he ene gy. The i s peak lux o lowe ene gy p o ons is much la ge han
abo e 10 MeV. Mo eo e , he inc eased p o on luxes emain longe (1 mo e day) han a
highe ene gies.
Du ing he e en o 11 May, p o ons we e injec ed deep in o he adia ion bel . P o on
lux sha ply ose, eaching L
∼
2.5 in all ene gy channels displayed in Figu e 3, and lux
inc eases we e also obse ed up o channel 8 (182–205 MeV).
Fo p o ons be ween 9.5 MeV and 13 MeV, obse a ions o EPT show ha p o ons
we e injec ed in he inne bel . This is he i s ime since he launch o EPT ha di ec
injec ion o p o on in he p o on apped bel is obse ed [
11
]. In channel 1 o EPT, he
injec ion o p o ons in he inne bel appea s be ween L = 1.9 and 2.1 and hese p o ons
o m an addi ional inne bel emaining apped du ing mo e han one mon h, wi h a lux
sligh ly dec easing wi h ime. This is also con i med by he obse a ions om he MEPED
90° elescope. Indeed, channel 5 o MEPED (shown in Figu e 4) was p o en o be ee o
elec on con amina ion in [
13
] by simula ions wi h GEANT-4. E en hough hey canno be
di ec ly compa ed o one ano he , he i s EPT channel and he i h MEPED channel a e
complemen a y, and clea ly show ha low ene gy p o ons (i.e., 2.5 MeV o 13 MeV) we e
injec ed in he inne bel .
In channels 2 o 4, s a ing on 13 May, p o ons a e also obse ed a highe L (be ween
L = 2.5 and L = 4), o ming an addi ional empo a y bel which emains only o se e al
days. Those obse a ions may ac ually come om con amina ion o hese p o on channels
by elec ons. Indeed, hose addi ional bel s a e appea ing a he same ime and a he same
L shells as he elec on luxes appea a e he s ong geomagne ic s o m associa ed wi h
his e en .
3.3. E ec s in he Sou h A lan ic Anomaly
Figu e 5shows he map o 9.5–13 MeV p o ons obse ed by he EPT i s channel a
di e en cha ac e is ic phases o he e en desc ibed abo e. A high la i udes (co espond-
ing also o high L), luxes a e obse ed only in panels (b,d) co esponding o he pe iods o
p o on injec ions in May and June.
Uni e se 2024,10, 391 7 o 18
Figu e 5. P o on luxes obse ed in he i s EPT p o on channel a e aged in longi ude and la i ude
bins du ing ou di e en pe iods co e ing mos o he pe iod conside ed in p e ious igu es. The
a e aging bins he e ha e a wid h o 10° in longi ude and 5° in la i ude. Each panel co esponds
o a di e en pe iod: (a) quie condi ions om 1 May o 9 May, (b) s o m ime and beginning o
he eco e y om 10 May o 20 May, (c) eco e y pe iod om 21 May o 31 May, (d) second p o on
injec ion om 1 June o 15 June.
Figu e 5also clea ly illus a es he Sou h A lan ic Anomaly (SAA), a ea whe e he
luxes a e always e y high abo e Sou h Ame ica and Sou h A lan ic. The SAA is a egion
whe e inne adia ion bel pa icles can mi o a lowe al i udes, inc easing he local
pa icle lux. This is due o he ac ha he Ea h’s magne ic ield is pa icula ly low in he
SAA, caused by he il o app oxima ely 11° o he Ea h’s magne ic axis wi h espec o
he Ea h’s o a ion axis and he loca ion o he magne ic idealized dipole some 400 km
away om he Ea h’s cen e .
EPT obse a ions indica e ha p o ons a his ene gy a e injec ed in he SAA co -
esponding o he inne adia ion bel a low al i udes. One can see ha i is only he
sou he nmos pa o he SAA ha is illed a e his e en in he channel 9.5–13 MeV.
Such injec ions we e ne e di ec ly obse ed by EPT in he pas , e en i hey could ex-
plain he di e en cha ac e is ics obse ed be ween he sou h and no h pa o he SAA
no ed in [
11
,
14
]. This allows us o exclude he e ec s o elec on con amina ion in his
ene gy ange, since he egions illed by he elec ons a e loca ed a di e en L alues (see
Sec ion 4).
3.4. Neu on Moni o s Obse a ions
Two main p ocesses esponsible o he p esence o apped p o ons a low L alues
ha e been iden i ied: he Cosmic Ray Albedo Neu on Decay (CRAND) and he adial
di usion o injec ed sola p o ons [
15
,
16
]. While CRAND mainly con ibu es o he p o ons
lux peaking a L
∼
1.5 o highe ene gy, inwa d di usion o injec ed sola p o ons is he
p incipal sou ce o he main p o on bel a lowe ene gies [
17
]. In his la e wo k, ocusing
on p o on obse a ion be ween 1998 and 2006, sola p o on apping we e shown o mainly
ake place a L = 2.3, whe eas du ing he e en o 11 May 2024, sola p o ons we e di ec ly
injec ed in he seconda y p o ons bel a L = 2, co esponding o he sou he nmos pa o
he SAA.
In case o sola ene ge ic pa icle e en like on 11 May and 8 June 2024, p o ons wi h
enough ene gy and igidi y can each he uppe a mosphe e and in e ac wi h neu al
a oms o p oduce neu ons. Ea h-bound ene ge ic neu ons a e measu ed by g ound
neu on moni o s (NM). Backsca e ed neu ons en e ing he magne osphe e a e a possible
Uni e se 2024,10, 391 8 o 18
sou ce o geomagne ic apped p o ons and elec ons h ough Be a-decay (Sola Neu on
Decay), i is he e o e in e es ing o analyze obse a ions o NM as well.
The e en o 11 May has been measu ed by many NM, e en a la i udes as low as 49°,
as illus a ed in Figu e 6. The cha ac e is ics o he s a ions whe e he neu on moni o s
ha e measu ed a e summa ized in Table 1.
Figu e 6. Obse a ions o neu on moni o s a di e en s a ions speci ied in Table 1, loca ed a all
la i udes like Dou bes (Belgium, 50° la ) and SOPO (Sou h Pole la i ude
−
90°). The pe u ba ion
du ing he nigh o 10 o 11 May 2024 is well isible. The neu on dec ease du ing he s o m (Fo bush
dec ease) is immedia ely ollowed by a G ound Le el Enhancemen .
Table 1. Cha ac e is ics o he g ound-based neu on moni o s a ions whose da a a e shown in
Figu e 6: Full name, sho name, al i ude, Cu o Rigidi y Rc, la i ude and longi ude.
Full Name Sho Name Al (m) Rc (GV) Coo dina es (La , Longi ude) in Deg ee
Lomnicky LMKS 2634 3.84 49.2000, 20.2200
Dou bes DRBS 225 3.18 50.0971, 4.59003
Kiel KIEL 54 2.36 54.3399, 10.1199
Oulu OULU 15 0.81 65.0544, 25.4681
Apa i y APTY 181 0.65 67.5704, 33.3935
Sou h Pole SOPO 2820 0.1 −90.000, 0.00000
Neu on moni o s show a clea dis u bance du ing he geomagne ic s o m. Figu e 6
shows a d op o neu ons, called a “Fo bush dec ease” ha s a ed in he e ening o 10 May.
I is he esul o he a i al o he ene ge ic pa icles o he CME a he Ea h. Du ing he
s o m, mo e sola pa icles we e injec ed in he a mosphe e, bu less Galac ic cosmic ays,
because he sola pa icles c ea e a s onge magne ic shield agains he cosmic ay pa icles
o igina ing om ou side ou sola sys em. Tha is why he numbe o de ec ed neu ons
dec eases du ing he s o m. I is qui e unusual ha his e ec is obse ed e en a low
la i udes (such as in he Dou bes s a ion in Belgium).
The small bump in he neu on coun , which happens jus a e he dec ease, igh
du ing he geomagne ic s o m o 10–11 May, is a G ound Le el Enhancemen (GLE). I is
only he second GLE so a du ing his sola cycle and numbe 74 since he measu emen s
s a ed back in he 1940s (Oulu, h ps://gle.oulu. i (accessed on 2 Sep embe 2024)). I is
Uni e se 2024,10, 391 9 o 18
associa ed wi h he a i al o p o on luxes wi h ene gy highe han 100 MeV (see Figu e 2).
This GLE 74 is no pa icula ly s ong, bu obse ed due o he Fo bush dec ease. A e
he e en , he le el o neu ons emains lowe han be o e he s o m e en a e one mon h.
No e ha no hing special was isible on 8 June 2024 in he neu on moni o measu emen s
a he a i al o he second SEP e en .
4. E ec s on he Elec on Radia ion Bel s
4.1. Obse a ions o New Elec on Bel s by EPT
Sola P o on Albedo Neu on Decay (SPAND) can also p oduce elec ons in he slo
egion du ing GLEs, bu only a L
>
2 because he lowe ene gy o SEP’s compa ed o
cosmic ays limi s hei access o smalle L’s [
15
]. Fo lowe L, CRAND e ec makes a small
bu con inuous con ibu ion o he elec ons in he inne bel and slo egion.
Figu e 7shows he empo al e olu ion o he EPT elec on di e en ial luxes a e aged
on 6 h and 0.25 L bins om 1 May o 30 June 2024. Each panel co esponds o an ene gy
channel o he EPT, inc easing in ene gy as panels a e loca ed lowe on he igu e.
Figu e 7. Elec on di e en ial luxes obse ed om 1 May o 30 June 2024 as a unc ion o he
McIlwain pa ame e L ( e ical axis) and ime (ho izon al axis) in he 6 EPT elec on ene gy channels.
Fluxes a e a e aged in bins which a e 6 h long in ime and 0.25 in L. F om op o bo om, he ene gy
o he channels inc eases and hey all sha e he same colo ba .
P e ious s udies ha e shown ha geomagne ic s o ms ha e a s ong impac on he
adia ion bel s, especially he ou e elec on bel (e.g., [
18
]). Fo mos s o ms, he pe u ba-
ions a e ela i ely sho li ed and ollow he same pa e n, as also obse ed du ing his
supe s o m. A low al i udes co esponding o he EPT o bi , he main phase o he s o m
(i.e., when he Ds dec eases) causes a sudden d op in he elec on lux a high L alues
called a d opou e en [
19
]. D oupou e en s a e apid e osions o he ou e edge down o
lowe L alues. The minimum L alues eached by he d opou depends on he in ensi y o
Uni e se 2024,10, 391 16 o 18
wi h di e en wa e-pa icle in e ac ions ha depend on he ene gy, he posi ion o he
pa icles and he geomagne ic condi ions.
6. Conclusions
The mo he ’s day sola s o m o 11 May led o s ong injec ion o ene ge ic pa icles in
he adia ion bel s, oge he wi h a geomagne ic s o m o excep ional magni ude. Following
hose e en s, he pa icle popula ions o he adia ion bel s we e g ea ly dis u bed p o iding
us wi h some a e obse a ions o he dynamics o he bel s unde such ex eme o cing.
Du ing he pe iod o obse a ion p esen ed in his wo k (May and June 2024), di e en
ins umen s obse ed wo sepa a e ene ge ic p o on e en s: The i s e en occu ed on
11 May 2024 and injec ed p o ons in he apped p o on bel , o he i s ime since he
launch o EPT, while he second ook place on he 8 o June, which pu s hem 27 days
apa co esponding o a o a ion o he Sun on i sel . The empo al e olu ion o he lux a
L = 6.6
is simila o he wo e en s, wi h a double peak o bo h
>
10 MeV and
>
50 MeV
p o ons while a highe ene gies only one peak is obse ed. Those wo cha ac e is ics
indica e ha he wo p o on injec ions ha we e obse ed du ing he pe iod o in e es
o igina e om he same sola ac i e egion. The disc epancies be ween he wo e en s
can be associa ed o he s ong di e ence on he geo-e ec i eness o he wo e en s (see
Figu e 1). Indeed, he i s e en was associa ed wi h he s onges geomagne ic s o m
obse ed in wo decades, while du ing he second SEP e en no s o m was obse ed. The
much deepe pene a ion o p o ons in he apped bel du ing he 11 May e en can be
explained by he la ge sudden commencemen (SC) o he geomagne ic s o m leading o
he apid comp ession o he magne osphe e which in u n can cause a pulse in he elec ic
ield, accele a ing and injec ing pa icles o lowe L alues [
32
]. The inne adia ion bel
is known o be mo e s able in ime han he ou e bel , wi h a s ong esponse o sola
cycle [11].
The geomagne ic s o m o 11 May caused ex eme elec on lux a ia ion in bo h he
inne and ou e bel , bu also led o he o ma ion o complex bel s uc u es se e al weeks
a e he s o m. One impo an ea u e shown in Figu e 9is ha he injec ion o elec ons
ollowing he s o m ini ially akes place a lowe L o low ene gy. Obse a ions om
MEPED
>
300 keV elec ons also con i ms ha injec ion a hose low ene gies akes place
a low L due o he ex emely low Ds alue. Rapidly a e he injec ions, he elec on
p o iles a e modi ied by losses appea ing a e y speci ic L ha a e di e en as a unc ion
o ene gy (see o ins ance Figu es 7and 9). This sugges s ha he accele a ion mechanisms
o elec ons be ween 300 keV and se e al MeV, and e en mo e he losses a e ex emely
dependen on ene gy.
Those g adual losses lead o he o ma ion o mul iple bel s depending on he ene gy.
Fo 300 keV o 800 keV elec ons, wo dis inc bel s a e o med abo e he main slo and wo
in he inne bel s. In he highes ene gy ange o he EPT, ul a- ela i is ic elec ons a e
di ec ly injec ed in he inne bel , iola ing he impene able ba ie obse ed by he Van
Allen P obes [21], o a leas dec easing i s limi s o L = 2.4.
Au ho Con ibu ions: Concep ualiza ion, V.P. and A.W.; me hodology, V.P.; so wa e, A.W.; ali-
da ion, V.P., A.W. and E.B.; o mal analysis, A.W.; in es iga ion, V.P. and A.W.; esou ces, V.P.; da a
cu a ion, A.W.; w i ing—o iginal d a p epa a ion, V.P.; w i ing— e iew and edi ing, A.W., E.B. and
M.P.d.B.; isualiza ion, V.P., A.W. and M.P.d.B.; supe ision, V.P.; p ojec adminis a ion, V.P.; unding
acquisi ion, V.P. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding: The p ojec 21GRD02 BIOSPHERE has ecei ed unding om he Eu opean Pa ne ship
on Me ology, co- inanced by he Eu opean Union’s Ho izon Eu ope Resea ch and Inno a ion
P og amme and by he Pa icipa ing S a es, and om Ho izon 2020 PITHIA-NRF p ojec wi h G an
Ag eemen 101007599.
Uni e se 2024,10, 391 17 o 18
Da a A ailabili y S a emen : The da a p esen ed in his s udy a e a ailable in a eposi o y a DOI
e e ence numbe 10.5281/zenodo.13626553. These da a we e de i ed om he ollowing esou ces
a ailable in he public domain: PROBA-V/EPT da a a e publicly a ailable on he Space Si ua-
ional Awa eness websi e o ESA h ps://swe.ssa.esa.in /space- adia ion (accessed on 2 Sep embe
2024). POES da a a e a ailable on h ps://sa da .ngdc.noaa.go /sem/poes/da a (accessed on 2
Sep embe 2024). GOES on h ps://www.swpc.noaa.go /p oduc s/goes-p o on- lux (accessed on
14 Augus 2024) and OMNI on h ps://omniweb.gs c.nasa.go /h ml/ow_da a.h ml (accessed on 1
Augus 2024).
Acknowledgmen s: The p ojec 21GRD02 BIOSPHERE has ecei ed unding om he Eu opean
Pa ne ship on Me ology, co- inanced by he Eu opean Union’s Ho izon Eu ope Resea ch and
Inno a ion P og amme and by he Pa icipa ing S a es. The au ho s acknowledge he Ho izon 2020
PITHIA-NRF p ojec wi h G an Ag eemen 101007599. All au ho s a e g a e ul o he PROBA-
V/EPT eams, a B.USOC and ESA/Redu o hei in ol emen in he da a acquisi ion p ocess,
o he membe s o CSR o da a alida ion, and o Belgian Science Policy-Space Resea ch and
Applica ions (BELSPO) o suppo in da a exploi a ion. The esul s p esen ed in his documen ely
on da a p o ided by he Communi y Coo dina ed Modeling Cen e a Godda d Space Fligh Cen e
h ough hei in eg a ed Space Wea he Analysis (iSWA) sys em’s HAPI se e (h ps://iswa.gs c.
nasa.go /IswaSys emWebApp/hapi (accessed on 2 Sep embe 2024)). The CCMC is a mul i-agency
pa ne ship be ween NASA, AFMC, AFOSR, AFRL, AFWA, NOAA, NSF and ONR. These da a
we e accessed ia he Uni e si y o Colo ado’s Space Wea he Technology, Resea ch, and Educa ion
Cen e ’s (h ps://colo ado.edu/spacewea he (accessed on 2 Sep embe 2024)) Space Wea he Da a
Po al (h ps://lasp.colo ado.edu/space-wea he -po al). We acknowledge he NMDB da abase
(www.nmdb.eu) ounded unde he Eu opean Union’s FP7 p og amme (con ac no. 213 007), and he
PIs o indi idual neu on moni o s, including a : Oulu (Sodankyla Geophysical Obse a o y o he
Uni e si y o Oulu, Finland), Thule (Uni e si y o Delawa e Depa men o Physics and As onomy
and he Ba ol Resea ch Ins i u e, USA), Sou h Pole (Uni e si y o Wisconsin, Ri e Falls, USA),
Te e Adelie (Obse a oi e de Pa is and he F ench Pola Ins i u e IPEV, F ance), Apa i y (Russia),
Lomnicky (Slo akia), Kiel (Ge many) and Dou bes (Belgium).
Con lic s o In e es : The au ho s decla e no con lic s o in e es .
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Disclaime /Publishe ’s No e: The s a emen s, opinions and da a con ained in all publica ions a e solely hose o he indi idual
au ho (s) and con ibu o (s) and no o MDPI and/o he edi o (s). MDPI and/o he edi o (s) disclaim esponsibili y o any inju y o
people o p ope y esul ing om any ideas, me hods, ins uc ions o p oduc s e e ed o in he con en .
