Academic Edi o : Jianyong Lu
Recei ed: 22 Janua y 2025
Re ised: 17 Feb ua y 2025
Accep ed: 26 Feb ua y 2025
Published: 4 Ma ch 2025
Ci a ion: Pie a d, V.; Ve huls ,
T.G.W.; Che alie , J.-M.; Be geo , N.;
Winan , A. E ec s o he Geomagne ic
Supe s o ms o 10–11 May 2024 and
7–11 Oc obe 2024 on he Ionosphe e
and Plasmasphe e. A mosphe e 2025,
16, 299. h ps://doi.o g/10.3390/
a mos16030299
Copy igh : © 2025 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
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A ibu ion (CC BY) license
(h ps://c ea i ecommons.o g/
licenses/by/4.0/).
A icle
E ec s o he Geomagne ic Supe s o ms o 10–11 May 2024 and
7–11 Oc obe 2024 on he Ionosphe e and Plasmasphe e
Vi iane Pie a d 1,2,* , Tobias G. W. Ve huls 3, Jean-Ma ie Che alie 4, Nicolas Be geo 4
and Alexand e Winan 1,2
1
Sola Te es ial Cen e o Excellence (STCE) and Space Physics, Royal Belgian Ins i u e o Space Ae onomy,
B-1180 B ussels, Belgium; alexand [email p o ec ed]
2Ea h and Li e Ins i u e–Clima e Sciences ELI-C, Uni e si é Ca holique de Lou ain,
B-1348 Lou ain-la-Neu e, Belgium
3Sola Te es ial Cen e o Excellence (STCE), Royal Me eo ological Ins i u e o Belgium,
B-1180 B ussels, Belgium; obias. e [email p o ec ed]
4Sola Te es ial Cen e o Excellence (STCE), Royal Obse a o y o Belgium, B-1180 B ussels, Belgium;
[email p o ec ed] (J.-M.C.); [email p o ec ed] (N.B.)
*Co espondence: i iane.pie a d@ae onomie.be
Abs ac : On 10 May 2024 a 17 h:07 UTC, he simul aneous a i al o se e al sola co onal
mass ejec ions (CMEs) gene a ed he s onges geomagne ic s o m o he las wen y yea s,
wi h a minimum Ds =
−
412 nT, usually e e ed o as he Mo he ’s Day e en . On
10 Oc obe 2024, he second s onges e en o sola cycle 25 appea ed wi h a Ds =
−
335 nT,
p eceded on 8 Oc obe by an e en wi h a Ds =
−
153 nT. In he p esen wo k, wi h
measu emen s o he e ical o al elec on con en and wi h ionosonde obse a ions om
Eu ope, USA, and Sou h Ko ea, we show ha he ioniza ion o he uppe a mosphe e
sho ly inc eased a he a i al o he CME o hese di e en e en s, ollowed by a as
dec ease a all la i udes. The ioniza ion emained e y low o mo e han a ull day. While
he eco e y s a ed a he beginning o he second day a e he onse o bo h e en s in
Oc obe , he sudden eco e y in he middle o he second day on 12 May is much mo e
unusual. The analysis o he obse a ions a di e en la i udes and longi udes shows
ha he causes o he ioniza ion a ia ions du ing he supe s o ms we e mainly due o
s ong pe u ba ions in he ionosphe ic F laye , ampli ied by he plasmasphe e’s in luence
on he e ical o al elec on con en (VTEC). The e osion o he plasmasphe e du ing
hese wo s ong e en s led o a plasmapause loca ed a excep ionally low adial dis ances
smalle han 2 Re (Ea h’s adii) in he pos -midnigh sec o and a o a ing plume in he
a e noon–dusk sec o clea ly isible in he BSPM plasmasphe e model. I ook se e al
days a e he s o ms o eco e no mal ioniza ion a es.
Keywo ds: Mo he ’s Day; geomagne ic s o ms; ionosphe e; plasmasphe e; VTEC
1. In oduc ion: S onges S o ms o Sola Cycle 25
The geomagne ic s o m o 10–11 May 2024, also called he Mo he ’s Day e en , ga ne s
a lo o scien i ic in e es because his e en is he s onges s o m o he las 20 yea s [
1
],
and i caused au o as a unusually low la i udes [
2
–
4
]. Many s udies ha e been published
o a e in p og ess abou his e en . Published wo ks conce ning he ionosphe ic e ec s o
his supe s o m in May 2024 ocus, in gene al, on measu emen s a speci ic local places
like Pe u [
5
], Asia [
6
], Asian–Aus alian and Ame ican sec o s [
7
,
8
], La in Ame ica [
9
], o
Eu ope [
10
]. Nea he equa o , s ong eas wa d ionosphe ic elec ic ields ha e been ob-
se ed a dusk du ing he no hwa d In e plane a y Magne ic Field [
11
]. Plasma oun ains
A mosphe e 2025,16, 299 h ps://doi.o g/10.3390/a mos16030299
A mosphe e 2025,16, 299 2 o 25
du ing he Mo he ’s Day s o m we e unusually s ong ac oss di e en local ime sec o s.
These oun ains we e shown o be sus ained by he combined e ec s o a s ong pene a ion
elec ic ield and me idional wind [12].
A high la i udes, plasma li ing du ing he s o m caused mid-la i ude displacemen s
o ionosphe ic peak heigh by as much as 300 km o e he cou se o 1 h [
13
]. TEC maps
show he in ensi ica ion and sp ead o he 11 May 2024 ex eme au o al e en ac oss he
con inen al US o e a 20 min in e al [
14
]. A ypical spo adic E laye ype was de ec ed o
he i s ime du ing nigh ime in he Sou h Ame ican Magne ic Anomaly [15].
A a lowe al i ude in he mesosphe e and he mosphe e, a he mosphe ic NO adia i e
cooling lux was obse ed [
16
], while s ong empe a u e inc eases we e also de ec ed [
17
],
as well as modi ica ions o a mosphe ic composi ion [
18
]. The s o m was p oduced due o
a highly comp essed magne osphe e, wi h he magne opause pushed below geos a iona y
o bi (6.6 R
E
) con inuously o 6 h [
19
]. The highly comp essed magne osphe e led an
in ense ing cu en a a much close dis ance o he Ea h.
These p e ious s udies open some in e es ing ques ions:
•
Is i possible o know exac ly how deep he ionosphe ic deple ion will be, and how
long i will las ?
•
How does he a mosphe ic esponse o geomagne ic s o ms change depending on he
la i ude and longi ude?
•
How is he a mosphe ic ioniza ion modi ied as a unc ion o he al i ude, including
abo e he ionosphe ic laye s in he plasmasphe e?
In he p esen s udy, we use an inno a i e app oach ha signi ican ly ad ances
he unde s anding o geomagne ic supe s o ms beyond p io wo ks by p o iding
he ollowing:
1.
Including he plasmasphe e ha is di ec ly coupled o he ionosphe e and p o ides
magne osphe ic e ec s in h ee dimensions;
2.
P o iding a compa ison be ween measu emen s o di e en ins umen s (VTEC,
ionosondes, and plasmapause), helping o di e en ia e he e ec s a di e en al i udes;
3.
Analyzing hese measu emen s a di e en places all a ound he wo ld
(Eu ope, No h A ica, Ame ica, and Asia) o di e en ia e he e ec s a di e en
la i udes and longi udes;
4.
Compa ing he Mo he ’s Day e en wi h he second s onges supe s o m o his
sola cycle, he e en o Oc obe 2024, o which he p esen s udy is a pionee o ou
knowledge. This allows us o de e mine how he in ensi y o he geomagne ic s o ms
modi ies he a mosphe ic ioniza ion.
The wo s onges geomagne ic e en s o sola cycle 25 in 2024 s udied he e a e
as ollows:
1. The Mo he ’s Day s o m on 10–11 May 2024.
2. The successi e s o ms on 8–11 Oc obe 2024.
Fo hese e en s, we show he obse a ions o sola wind pa ame e s ha gene a e he
geomagne ic ac i i y, ionosonde measu emen s (up o he maximum o elec on densi y
in he F laye o he ionosphe e), e ical o al elec on con en (VTEC) up o he GNSS
(Global Na iga ion Sa elli e Sys em) o bi al i ude, plasmapause measu emen s by Swa m
sa elli es, and he h ee-dimensional BSPM plasmaspehe e model [
20
] in o de o de e mine
he e olu ion in ime and space o he plasma densi y a ia ions.
The obse a ion me hods a e desc ibed in Sec ion 2, he esul s a e desc ibed in
Sec ion 3 o he 10–11 May 2024 Mo he ’s Day e en and in Sec ion 4 o 8–11 Oc obe
2024 s o ms, while Sec ion 5discusses and summa izes he esul s.
A mosphe e 2025,16, 299 3 o 25
2. Obse a ion Me hods
Fo bo h pe iods o geomagne ic dis u bances, in his sec ion, we desc ibe he models
and he me hods used o analyze he obse a ions om di e en ins umen s ha a e used
in his wo k.
2.1. Sola Wind and Geomagne ic Indices
OMNI is a mul i-sou ce da a po al o he nea -Ea h sola wind’s magne ic ield and
plasma pa ame e s p o ided by NASA. We use i o ob ain he sola wind pa ame e s
measu ed a 1 As onomy Uni (UA) (i.e., he a e age dis ance be ween he Sun and Ea h)
om in si u spacec a and he geomagne ic ac i i y indices.
2.2. Ve ical To al Elec on Con en VTEC
One o he key pa ame e s used o cha ac e ize ionosphe ic condi ions is he e ical
o al elec on con en (VTEC). The TEC ep esen s he o al numbe o ee elec ons in-
eg a ed along he ay pa h be ween a sa elli e o he Global Na iga ion Sa elli e Sys em
(GNSS) loca ed a an al i ude o 19,100 km o 23,300 km ( hus, a adial dis ance om he
cen e o he Ea h o app oxima ely 4 Ea h adii, Re) and he ecei e on he g ound. The
TEC is measu ed in TEC uni s (TECu), whe e 1 TECu = 10
16
elec ons/m
2
. The VTEC is he
e ical p ojec ion o TEC a he Ionosphe ic Pie ce Poin (IPP), which in e sec s abo e he
elec on densi y peak a a 450 km heigh .
The VTEC is di ec ly ela ed o he GNSS signal p opaga ion delay caused by he
ionosphe e. These ionosphe ic condi ions a e con inuously moni o ed by he STCE in
B ussels based on he GNSS obse a ions o he EUREF Pe manen Ne wo k (EPN) [
21
]
and p ocessed using he ROB-IONO so wa e [22].
Real- ime GNSS da a om ~150 s a ions (GPS + Galileo + GLONASS) a e used o
es ima e he VTEC a he IPP e e y 30 s, co e ing a 5 min ime span. This app oach
p o ides ex ensi e spa ial and empo al co e age o moni o ing he ionosphe e s a e. Fo
each 5 min in e al, median VTECs a he IPP o each sa elli e– ecei e pai a e es ima ed
along wi h hei s anda d de ia ion and a e hen in e pola ed o p oduce VTEC and VTEC
a iabili y maps abo e Eu ope. The me hodology o p oducing hese maps and hei
alida ion agains widely used pos -p ocessed Global Ionosphe ic Maps such as IGS and
ESA, wi h mean di e ences o 1.3
±
0.9 and 0.4
±
1.6 TECu, espec i ely, a e de ailed
in [22].
The VTEC ime se ies p o ided in his pape a e ex ac ed om hese maps a
h ee loca ions:
(a)
In he no he n pa o Eu ope (61◦N, 5◦E);
(b)
Abo e B ussels, in mid-la i ude Eu ope (50.5◦N, 4.5◦E);
(c)
In No h A ica (36◦N, 5◦E).
The expec ed VTEC a each loca ion co esponds o he median o he 15 p e ious
ones o he same local ime.
2.3. Ionosonde Obse a ions
Ionosondes employ high equency (HF) adio ansmissions o a ious equencies
o de e mine he elec on densi y p o ile abo e he ins umen . F equencies below he
plasma equency
ω=qNee2
ε0m
(whe e eis he elec on cha ge, mis i s mass, and
ε0
is he
pe mi i i y o ee space) a e e lec ed by he plasma in he ionosphe e. The e o e, he
elec on densi y p o ile
Ne(h)
can be econs uc ed om he e lec ion o a ange o di e en
equencies. The main limi a ion o his echnique is ha i allows obse a ions only up o
he heigh o he g ea es elec on densi y. This al i ude a ies signi ican ly bo h du ing
quie condi ions and as a esul o pa icula dis u bances such as geomagne ic s o ms.
A mosphe e 2025,16, 299 4 o 25
In he p esen s udy, we use da a om h ee ionosondes in Eu ope (JR055, DB049, and
EB040), a di e en la i udes, as well as wo ionosondes in o he longi ude sec o s: IC437 in
Sou h Ko ea and MHJ45 in he USA. Ionog ams we e ob ained om he DIDBase o he
GIRO eposi o y [
23
]. This allows us o compa e he s o m e ec s a di e en la i udes
as well as he e ec s o he local ime a he onse o he e en . The coo dina es o he
obse a o ies a e lis ed in Table 1. Each o hese obse a o ies uses a simila Digisonde
DPS-4D ionosonde and p oduces egula soundings o he ionosphe e a in e als o 5 ( o
he Eu opean obse a o ies) o 7.5 min ( o he Ame ican and Sou h Ko ean obse a o ies).
All ionog ams we e manually inspec ed in o de o de i e he cha ac e is ics o he a ious
laye s (he e, we use he c i ical equencies oF
1
and oF
2
o he wo F egion densi y peaks),
only e aining he da a poin s which could be ob ained eliably acco ding o he s anda d
URSI (In e na ional Union o Radio Science) ules.
Table 1. Ionosondes om which da a a e used.
Name (Coun y) URSI Code La i ude Longi ude
Julius uh (Ge many) JR055 54.60◦N 13.40◦E
Dou bes (Belgium) DB049 50.10◦N 4.60◦E
Roque es (Spain) EB040 40.80◦N 0.50◦E
Mills one Hill (USA) MHJ45 42.60◦N 288.50◦E
I-Cheon (Sou h Ko ea) IC437 37.14◦N 127.54◦E
The excep ional ionosphe ic condi ions du ing he e en s analyzed he e cause some
pa icula di icul ies in he in e p e a ion o he ionog ams. The e a e h ee easons o
occasional gaps in he ime se ies o he c i ical equencies. Fi s , du ing he nigh ime, he
sp eading o he F
2
ace is obse ed (see he le panel o Figu e 1 o an example). This
can ende i impossible o accu a ely de e mine he c i ical equency oF
2
. Du ing he i s
nigh o he 11 May s o m, he e we e also spo adic E laye s obse ed, p oduced by pa icle
impac s (which a e associa ed wi h he isible au o as seen a hose same imes e en a low
la i udes). An example o his is shown in Figu e 1on he igh . In his example, he Es
laye ex ends o abo e 4 MHz, making i impossible o de ec he deple ed F laye abo e.
A mosphe e 2025, 16, x FOR PEER REVIEW 4 o 27
du ing quie condi ions and as a esul o pa icula dis u bances such as geomagne ic
s o ms.
In he p esen s udy, we use da a om h ee ionosondes in Eu ope (JR055, DB049,
and EB040), a diffe en la i udes, as well as wo ionosondes in o he longi ude sec o s:
IC437 in Sou h Ko ea and MHJ45 in he USA. Ionog ams we e ob ained om he DIDBase
o he GIRO eposi o y [23]. This allows us o compa e he s o m effec s a diffe en la i-
udes as well as he effec s o he local ime a he onse o he e en . The coo dina es o
he obse a o ies a e lis ed in Table 1. Each o hese obse a o ies uses a simila Digisonde
DPS-4D ionosonde and p oduces egula soundings o he ionosphe e a in e als o 5 ( o
he Eu opean obse a o ies) o 7.5 min ( o he Ame ican and Sou h Ko ean obse a o-
ies). All ionog ams we e manually inspec ed in o de o de i e he cha ac e is ics o he
a ious laye s (he e, we use he c i ical equencies oF
1
and oF
2
o he wo F egion densi y
peaks), only e aining he da a poin s which could be ob ained eliably acco ding o he
s anda d URSI (In e na ional Union o Radio Science) ules.
Table 1. Ionosondes om which da a a e used.
Name (Coun y) URSI Code La i ude Longi ude
Julius uh (Ge many) JR055 54.60° N 13.40° E
Dou bes (Belgium) DB049 50.10° N 4.60° E
Roque es (Spain) EB040 40.80° N 0.50° E
Mills one Hill (USA) MHJ45 42.60° N 288.50° E
I-Cheon (Sou h Ko ea) IC437 37.14° N 127.54° E
The excep ional ionosphe ic condi ions du ing he e en s analyzed he e cause some
pa icula difficul ies in he in e p e a ion o he ionog ams. The e a e h ee easons o
occasional gaps in he ime se ies o he c i ical equencies. Fi s , du ing he nigh ime,
he sp eading o he F
2
ace is obse ed (see he le panel o Figu e 1 o an example).
This can ende i impossible o accu a ely de e mine he c i ical equency oF
2
. Du ing
he i s nigh o he 11 May s o m, he e we e also spo adic E laye s obse ed, p oduced
by pa icle impac s (which a e associa ed wi h he isible au o as seen a hose same imes
e en a low la i udes). An example o his is shown in Figu e 1 on he igh . In his exam-
ple, he Es laye ex ends o abo e 4 MHz, making i impossible o de ec he deple ed F
laye abo e.
Figu e 1. Examples o sp ead-F condi ion ((le ), eco ded by IC437 on 11 May 2024 a 20:15:00 UTC)
and pa icle-induced Es laye (( igh ), eco ded by DB049 on 11 May a 00:50:02 UTC). No e ha he
heigh ( e ical axis) anges om 0 o 800 km in bo h cases, bu he equencies (ho izon al axis)
ange om 1 o 12 MHz in he le panel and om 1 o 9 MHz in he igh panel.
Figu e 1. Examples o sp ead-F condi ion ((le ), eco ded by IC437 on 11 May 2024 a 20:15:00 UTC)
and pa icle-induced Es laye (( igh ), eco ded by DB049 on 11 May a 00:50:02 UTC). No e ha
he heigh ( e ical axis) anges om 0 o 800 km in bo h cases, bu he equencies (ho izon al axis)
ange om 1 o 12 MHz in he le panel and om 1 o 9 MHz in he igh panel.
Finally, he e a e some gaps in he da a ela ed o he abso p ion o he ionosonde signals
in he D egion o he ionosphe e due o an inc eased X- ay lux o pola and au o al abso p ion
A mosphe e 2025,16, 299 5 o 25
( he la e mos ly a ec s da a om he Mills one Hill obse a o y). On 11 May 2024, he e
was an X5.89 la e peaking a 01:23 UTC (Uni e sal Time Coo dina ed) and an X1.54 la e
peaking a 11:44 UTC. In addi ion, du ing he pe iod o in e es , he e we e some M-class
la es as well. Fo mos o 11 May, e en he minima o he X- ay lux emained abo e
he le el o an M1 la e. Depending on he signal- o-noise a io usually ob ained by he
di e en ionosondes, and he local ime a each du ing he la es, his abso p ion in he
lowe ionosphe e esul s in gaps in he obse a ion o he F laye .
2.4. Plasmasphe e Model and Da a
The Belgian SWIFF plasmasphe e model (BSPM) is a 3D kine ic semi-empi ical model
o he plasmasphe e [
24
], coupled o he ionosphe e [
25
]. I has ecen ly been imp o ed
o ake in o accoun he mos ecen ly iden i ied ele an physical p ocesses [
20
]. I can
be un o any da e o ob ain o e e y hou he numbe densi y and empe a u e o he
elec ons and p o ons inside and ou side he plasmasphe e, as well as he posi ion o he
plasmapause, as a unc ion o he geomagne ic ac i i y d i en by he Kp index. The model
uses he kine ic app oach o he pa icle densi ies and he mechanism o quasi-in e change
ins abili y o he o ma ion o he plasmapause. The densi y in he plasma ough egion
has ecen ly been imp o ed using obse a ions o Van Allen P obes [
26
]. The esul s o
he plasmasphe e model ha e been compa ed o NASA’s IMAGE mission (2000–2005)
global EUV images o he plasmasphe e [
27
], Clus e [
28
] and Themis [
29
], among o he s,
allowing he alida ion o he plasmasphe e e osion and dynamical e olu ion. The mos
ecen e sion has been made a ailable on he PITHIA [
30
] pla o m: h ps://esc.pi hia.eu/
(accessed on 5 No embe 2024).
Mo eo e , he magne ic and plasma obse a ions o he low-Ea h o bi ing Swa m
sa elli es allow us o de i e he midnigh plasmapause [
31
]. Launched on 22 No embe 2013
in o a nea -pola low-Ea h o bi (LEO), Swa m is a cons ella ion o h ee iden ical sa elli es
ope a ed by he Eu opean Space Agency (ESA) wi h he pu pose o mapping Ea h’s
magne ic ield [
32
]. The ini ial al i ude o he sa elli e pai Swa m A and Swa m C was abou
490 km in Ap il 2014, and i was abou 510 km o Swa m B, and bo h o bi al i udes do
slowly decay wi h ime. Based on in si u elec on densi y and empe a u e [
33
], GPS-de i ed
TEC, and au o al ield-aligned cu en obse a ions a he Swa m sa elli es, new Swa m
p oduc s ha e ecen ly been de eloped o cha ac e ize plasmapause- ela ed bounda ies in
he opside ionosphe e. This p oduc also includes a plasmapause index, a p oxy o he
midnigh plasmapause posi ion. This index is de i ed om Swa m obse a ions simila ly
o he ones desc ibed in [34].
3. Mo he ’s Day S o m: 10–11 May 2024
3.1. Obse ed Sola Wind and Geomagne ic Indices
An excep ionally s ong geomagne ic s o m occu ed du ing he nigh om 10 o
11 May 2024, wi h au o as obse ed a ound he wo ld [
2
–
4
]. This was he s onges s o m
o he las wen y yea s wi h a minimum Dis u bance S o m Time index o Ds <
−
400 nT
(see bo om panel o Figu e 3). Due o he a ying speeds and quick succession o di e en
co onal mass ejec ions (CMEs) coming om he ac i e egion NOAA 13664, 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 [
1
]. These CMEs we e associa ed wi h
he X2.2 la e on 9 May and h ee X1 la es on 8 May.
The cha ac e is ics o he sola wind when i eached 1 AU, ob ained om OMNI, a e
shown in Figu e 2. The bulk eloci y o he sola pa icles eached mo e han 1000 km/s (see
Figu e 2, 2nd panel), and peaks o high empe a u e > 10
6
K we e obse ed (panel 4). The
high-densi y peak (panel 1) a he a i al o he ho plasma shows he appa i ion o a shock.
A mosphe e 2025,16, 299 6 o 25
The p essu e (panel 3) combines he densi y and eloci y e ec s and de e mines he posi ion
o he magne opause. The downwa d peak o he Z componen o he In e plane a y
Magne ic Field Bz (panel 5) shows a sou hwa d di ec ion ha can explain he s ong answe
o he e es ial magne ic ield [35].
A mosphe e 2025, 16, x FOR PEER REVIEW 6 o 27
(see Figu e 2, 2nd panel), and peaks o high empe a u e > 10
6
K we e obse ed (panel 4).
The high-densi y peak (panel 1) a he a i al o he ho plasma shows he appa i ion o a
shock. The p essu e (panel 3) combines he densi y and eloci y effec s and de e mines
he posi ion o he magne opause. The downwa d peak o he Z componen o he In e -
plane a y Magne ic Field Bz (panel 5) shows a sou hwa d di ec ion ha can explain he
s ong answe o he e es ial magne ic ield [35].
Figu e 2. Sola wind densi y n ( op), bulk eloci y (2nd panel), p essu e P (3 d panel), p o on
empe a u e T (4 h panel), and he sou hwa d componen o he In e plane a y Magne ic Field Bz
(bo om panel) om 1 o 24 May 2024, obse ed by OMNI a 1 AU.
The a i al ime o enhanced sola plasma a 1 AU wi h ega d o densi y, eloci y,
and p essu e is 10 May 2024 a 17:07 UT. I di ec ly caused he sudden commencemen o
he s ong geomagne ic s o m (see Figu e 3), ollowed by he main phase, when he Ds
index dec eased down o a minimum alue o −412 nT (see panel 3) in he nigh o 11
May, and he plane a y geomagne ic ac i i y index Kp inc eased o he maximum alue
o 9 (panel 1). The geomagne ic ac i i y indices Kp, Ap (in nano esla) ha eached 400
nT (see panel 2), and Ds (also in nT) a e illus a ed in Figu e 3, oge he wi h he F10.7
sola adio lux a 10.7 cm (panel 4) and he in ensi y o he Lyman alpha line in he sola
spec um (panel 5) ha a e daily sola ac i i y indices.
The bo om panel o Figu e 3 illus a es he Ds index om 1 Janua y 2000 o 31 Jan-
ua y 2025. The wo analyzed e en s a e indica ed by he ed e ical lines. This panel
clea ly shows ha he las e en wi h Ds < −400 nT appea ed in No embe 2003 (known
as he Halloween Day s o m [1]) and ha he in ensi y o he geomagne ic s o m o Oc o-
be 2024 was no seen since No embe 2004.
Figu e 2. Sola wind densi y n ( op), bulk eloci y (2nd panel), p essu e P (3 d panel), p o on
empe a u e T (4 h panel), and he sou hwa d componen o he In e plane a y Magne ic Field Bz
(bo om panel) om 1 o 24 May 2024, obse ed by OMNI a 1 AU.
The a i al ime o enhanced sola plasma a 1 AU wi h ega d o densi y, eloci y,
and p essu e is 10 May 2024 a 17:07 UT. I di ec ly caused he sudden commencemen o
he s ong geomagne ic s o m (see Figu e 3), ollowed by he main phase, when he Ds
index dec eased down o a minimum alue o
−
412 nT (see panel 3) in he nigh o 11 May,
and he plane a y geomagne ic ac i i y index Kp inc eased o he maximum alue o 9
(panel 1). The geomagne ic ac i i y indices Kp, Ap (in nano esla) ha eached 400 nT
(see panel 2), and Ds (also in nT) a e illus a ed in Figu e 3, oge he wi h he F10.7sola
adio lux a 10.7 cm (panel 4) and he in ensi y o he Lyman alpha line in he sola spec um
(panel 5) ha a e daily sola ac i i y indices.
The bo om panel o Figu e 3illus a es he Ds index om 1 Janua y 2000 o 31 Janua y
2025. The wo analyzed e en s a e indica ed by he ed e ical lines. This panel clea ly
shows ha he las e en wi h Ds <
−
400 nT appea ed in No embe 2003 (known as he
Halloween Day s o m [
1
]) and ha he in ensi y o he geomagne ic s o m o Oc obe 2024
was no seen since No embe 2004.
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Figu e 3. Geomagne ic ac i i y indices o Ba els Kp ( op panel), Ap in nT (2nd panel), Dis u bed
S o m Time Ds in nT (3 d panel), daily sola ac i i y indices F10.7, sola adio lux a 10.7 cm (4 h
panel), and in ensi y o he Lyman alpha line in he sola spec um (5 h panel) om 1 o 24 May
2024. The (bo om panel) illus a es Ds om 1 Janua y 2000 o 31 Janua y 2025. The wo analyzed
e en s a e indica ed by he ed e ical lines.
3.2. Ionosphe ic Ve ical To al Elec on Con en
Figu e 4 shows he ime se ies o he VTEC (in ed), he expec ed VTEC, i.e., he 15
p e ious days median (in g ey, wi h s anda d a ia ion), and he 5 min VTEC a iabili y
(in blue) in he no h o Eu ope, middle o Eu ope (B ussels) and mo e sou hwa d (no h
o A ica). The h ee nea - eal- ime VTEC ime se ies showed abno mal a ia ions du ing
he nigh o 10 o 11 May 2024, ollowed by a long deple ion in VTEC un il 12 May 2024.
This is due o he s ong CME impac de ec ed du ing he a e noon o 10 May. By com-
pa ing wi h Figu e 3, hese VTEC luc ua ions (high a ia ions du ing he s o m and long
deple ion a e ) canno be explained by a ia ions in F10.7 du ing hese days.
Figu e 3. Geomagne ic ac i i y indices o Ba els Kp ( op panel), Ap in nT (2nd panel), Dis u bed
S o m Time Ds in nT (3 d panel), daily sola ac i i y indices F10.7, sola adio lux a 10.7 cm
(4 h panel), and in ensi y o he Lyman alpha line in he sola spec um (5 h panel) om 1 o 24 May
2024. The (bo om panel) illus a es Ds om 1 Janua y 2000 o 31 Janua y 2025. The wo analyzed
e en s a e indica ed by he ed e ical lines.
3.2. Ionosphe ic Ve ical To al Elec on Con en
Figu e 4shows he ime se ies o he VTEC (in ed), he expec ed VTEC, i.e., he
15 p e ious days median (in g ey, wi h s anda d a ia ion), and he 5 min VTEC a iabili y
(in blue) in he no h o Eu ope, middle o Eu ope (B ussels) and mo e sou hwa d (no h o
A ica). The h ee nea - eal- ime VTEC ime se ies showed abno mal a ia ions du ing he
nigh o 10 o 11 May 2024, ollowed by a long deple ion in VTEC un il 12 May 2024. This is
due o he s ong CME impac de ec ed du ing he a e noon o 10 May. By compa ing wi h
Figu e 3, hese VTEC luc ua ions (high a ia ions du ing he s o m and long deple ion
a e ) canno be explained by a ia ions in F10.7 du ing hese days.
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Figu e 4. VTEC ime se ies a 3 loca ions in Eu ope om 9 o 14 May 2024. The (le panel) shows
he ime e olu ion o he e ical o al elec on con en (VTEC) (in ed) a h ee loca ions illus a ed
in he ( igh panel) on a map o Eu ope: (a) in he no he n pa o Eu ope (61° N, 5° E), (b) abo e
B ussels (50.5° N, 4.5° E), and (c) in No h A ica (36° N, 5° E).
Consequen ly, he geomagne ic s o m had a s ong impac on he ionosphe ic elec-
on con en , wi h sudden inc eases and dec eases o 20 TECu om 18:00 UTC 10 May o
12:00 UTC 11 May in he no h o Eu ope. A mid-la i ude, he VTEC ends o dec ease
s a ing a 18:50 UTC, wi h a minimum o −10 TECu, wi h espec o he expec ed quie
ionosphe e, and wi h a sudden peak o an inc ease o 8 TECu (wi h espec o he quie
ionosphe e) a 22:20 UTC. A he same ime, a a low la i ude, he VTEC s a s o dec ease
wi h espec o he quie ionosphe e. Fo bo h mid- and low la i udes, he VTEC a iabili y
also emains high, un il midnigh on 11 h May. These VTEC a ia ions and apid a ia-
bili y a hese h ee la i udes a e due o he injec ion o he pa icles associa ed wi h he
s o m and leading o he occu ence o au o as a unusually low la i udes.
The maximum a iabili y in he VTEC is obse ed a 20:20 UTC (see Figu e 5).
Figu e 4. VTEC ime se ies a 3 loca ions in Eu ope om 9 o 14 May 2024. The (le panel) shows he
ime e olu ion o he e ical o al elec on con en (VTEC) (in ed) a h ee loca ions illus a ed in he
( igh panel) on a map o Eu ope: (a) in he no he n pa o Eu ope (61
◦
N, 5
◦
E), (b) abo e B ussels
(50.5◦N, 4.5◦E), and (c) in No h A ica (36◦N, 5◦E).
Consequen ly, he geomagne ic s o m had a s ong impac on he ionosphe ic elec on
con en , wi h sudden inc eases and dec eases o 20 TECu om 18:00 UTC 10 May o
12:00 UTC 11 May in he no h o Eu ope. A mid-la i ude, he VTEC ends o dec ease
s a ing a 18:50 UTC, wi h a minimum o
−
10 TECu, wi h espec o he expec ed quie
ionosphe e, and wi h a sudden peak o an inc ease o 8 TECu (wi h espec o he quie
ionosphe e) a 22:20 UTC. A he same ime, a a low la i ude, he VTEC s a s o dec ease
wi h espec o he quie ionosphe e. Fo bo h mid- and low la i udes, he VTEC a iabili y
also emains high, un il midnigh on 11 h May. These VTEC a ia ions and apid a iabili y
a hese h ee la i udes a e due o he injec ion o he pa icles associa ed wi h he s o m
and leading o he occu ence o au o as a unusually low la i udes.
The maximum a iabili y in he VTEC is obse ed a 20:20 UTC (see Figu e 5).
In Figu e 5, i can be obse ed ha an inc ease in TEC wi h espec o he quie
ionosphe e occu s a high and low la i udes, while his is nega i e a mid-la i ude. O e
Scandina ia, he VTEC a iabili y eached a maximum o 5 TECu o a 5 min in e al
a 20:20 UTC. This could be linked wi h he heigh alues in he a e o TEC change
index (ROTI) obse ed in [
2
], suppo ing he luc ua ions, implying dis u bances in
GNSS applica ions.
The o he e ec o he s o m is he deple ion o he VTEC down o 30 TECu a low
la i udes un il 12 May a ound 05:00 UTC. As men ioned in [
36
,
37
], he pe sis ing deple ion
in TEC a ew days a e he onse is due o he con ac ion o he plasmasphe e, implying
dis u bances in he he mosphe ic ci cula ion and a chemical loss o ioniza ion in he
ionosphe e. This con ac ion o he plasmasphe e is cohe en wi h he obse a ion in
Figu e 9 Sec ion 3.4 o he p esen pape .
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Figu e 5. VTEC maps, diffe ences wi h expec ed beha iou and a iabili y o e Eu ope on 11 May
2024 a 20:20 UTC. (Top): VTEC maps es ima ed in eal- ime. The do s ep esen he VTEC da a
used o he in e pola ion. (Bo om le ): diffe ences be ween VTEC maps and he expec ed VTEC
(median o e he pas 15 days). (Bo om igh ): he VTEC a iabili y e lec ing he ionosphe ic s a e
a ia ions du ing he 5 min ime span o he in e pola ion.
In Figu e 5, i can be obse ed ha an inc ease in TEC wi h espec o he quie iono-
sphe e occu s a high and low la i udes, while his is nega i e a mid-la i ude. O e Scan-
dina ia, he VTEC a iabili y eached a maximum o 5 TECu o a 5 min in e al a 20:20
UTC. This could be linked wi h he heigh alues in he a e o TEC change index (ROTI)
obse ed in [2], suppo ing he luc ua ions, implying dis u bances in GNSS applica ions.
Figu e 5. VTEC maps, di e ences wi h expec ed beha iou and a iabili y o e Eu ope on 11 May
2024 a 20:20 UTC. (Top): VTEC maps es ima ed in eal- ime. The do s ep esen he VTEC da a used
o he in e pola ion. (Bo om le ): di e ences be ween VTEC maps and he expec ed VTEC (median
o e he pas 15 days). (Bo om igh ): he VTEC a iabili y e lec ing he ionosphe ic s a e a ia ions
du ing he 5 min ime span o he in e pola ion.
3.3. Ionosonde Obse a ions
Figu e 6shows he peak elec on densi ies o he F
1
(in blue) and F
2
(in ed) laye s
o he h ee Eu opean ionosondes o a h ee-day pe iod co e ing 10 o 12 May 2024. The
A mosphe e 2025,16, 299 16 o 25
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Figu e 11. VTEC ime se ies a he same 3 loca ions as in Figu e 4 om 7 o 13 Oc obe 2024 included.
(a) in he no he n pa o Eu ope (61° N, 5° E), (b) abo e B ussels (50.5° N, 4.5° E), and (c) in No h
A ica (36° N, 5° E).
Figu e 12 shows he s a e o he ionosphe e du ing his maximal VTEC diffe ence
obse ed in he sou he n loca ion a a low la i ude. As we can see, he VTEC obse ed is
mo e han 30 TECu highe han he expec ed alues. This is no seen in he ionosonde
elec on densi y obse ed by he ionosonde in Spain (see Figu e 13, EB040), whe e no sig-
ni ican inc ease is obse ed. This diffe ence be ween VTEC and elec on densi y om
ionosonde can be in e p e ed as an inc ease in he elec on densi y abo e he F
2
laye up
o he plasmapause. This densi y peak is ollowed by a sha p dec ease, leading o a alue
lowe han he a e aged densi y o he quie imes (g ey line) du ing he nigh and he ull
nex day and nigh . This a e -s o m deple ion s ands ~1.5 days (un il he 12 h 08:00 UTC)
as seen o he Mo he ’s Day s o m, wi h a minimum o −55 TECu in he low la i ude on
11 May a 13:50 UTC.
Figu e 11. VTEC ime se ies a he same 3 loca ions as in Figu e 4 om 7 o 13 Oc obe 2024 included.
(a) in he no he n pa o Eu ope (61
◦
N, 5
◦
E), (b) abo e B ussels (50.5
◦
N, 4.5
◦
E), and (c) in No h
A ica (36◦N, 5◦E).
Figu e 12 shows he s a e o he ionosphe e du ing his maximal VTEC di e ence
obse ed in he sou he n loca ion a a low la i ude. As we can see, he VTEC obse ed
is mo e han 30 TECu highe han he expec ed alues. This is no seen in he ionosonde
elec on densi y obse ed by he ionosonde in Spain (see Figu e 13, EB040), whe e no
signi ican inc ease is obse ed. This di e ence be ween VTEC and elec on densi y om
ionosonde can be in e p e ed as an inc ease in he elec on densi y abo e he F
2
laye up
o he plasmapause. This densi y peak is ollowed by a sha p dec ease, leading o a alue
lowe han he a e aged densi y o he quie imes (g ey line) du ing he nigh and he ull
nex day and nigh . This a e -s o m deple ion s ands ~1.5 days (un il he 12 h 08:00 UTC)
as seen o he Mo he ’s Day s o m, wi h a minimum o
−
55 TECu in he low la i ude on
11 May a 13:50 UTC.
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Figu e 12. VTEC maps, diffe ences wi h expec ed beha iou and a iabili y o e Eu ope on 10 Oc-
obe 2024 a 21:10 UTC. (Top): VTEC maps es ima ed in eal- ime. The do s ep esen he VTEC
da a used o he in e pola ion. (Bo om le ): diffe ences be ween VTEC maps and he expec ed
VTEC (median o e he pas 15 days). (Bo om igh ): he VTEC a iabili y e lec ing he ionosphe ic
s a e a ia ions du ing he 5 min ime span o he in e pola ion.
Figu e 12. VTEC maps, di e ences wi h expec ed beha iou and a iabili y o e Eu ope on 10
Oc obe 2024 a 21:10 UTC. (Top): VTEC maps es ima ed in eal- ime. The do s ep esen he VTEC
da a used o he in e pola ion. (Bo om le ): di e ences be ween VTEC maps and he expec ed
VTEC (median o e he pas 15 days). (Bo om igh ): he VTEC a iabili y e lec ing he ionosphe ic
s a e a ia ions du ing he 5 min ime span o he in e pola ion.
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Figu e 13. Peak elec on densi ies o F
2
( ed) and F
1
(blue) o he JR055 ( op), DB049 (middle), and
EB040 (bo om) ionosondes om 10 o 13 Oc obe 2024 0:00 UTC. No e ha he maximum densi y
( e ical axis) he e is 2.5 × 10
6
(ins ead o 1.5 × 10
6
as in Figu es 6 and 7).
4.3. Ionosonde Obse a ions
Figu e 13 shows he equi alen obse a ions as seen in Figu e 6 bu o he s o m o
11 Oc obe 2024. The clima ological condi ions in Oc obe a e diffe en om hose du ing
he May e en . The oF
1
peak is no as p onounced in Oc obe as i is in May. On he o he
hand, he oF
2
is sligh ly highe du ing he Oc obe e en han du ing he May s o m.
No e ha he e a e again some gaps in he ime se ies, in pa icula du ing he nigh
o he s o m a he highes la i ude obse a o y (JR055). These a e p ima ily due o he
p esence o pa icle-induced spo adic laye s p ecluding obse ing he F egion o he ion-
osphe e. Blanke ing o he obse a ions by egula Es laye s is no impo an o his e en
Figu e 13. Peak elec on densi ies o F
2
( ed) and F
1
(blue) o he JR055 ( op), DB049 (middle), and
EB040 (bo om) ionosondes om 10 o 13 Oc obe 2024 0:00 UTC. No e ha he maximum densi y
( e ical axis) he e is 2.5 ×106(ins ead o 1.5 ×106as in Figu es 6and 7).
4.3. Ionosonde Obse a ions
Figu e 13 shows he equi alen obse a ions as seen in Figu e 6bu o he s o m o
11 Oc obe 2024. The clima ological condi ions in Oc obe a e di e en om hose du ing
he May e en . The oF
1
peak is no as p onounced in Oc obe as i is in May. On he o he
hand, he oF2is sligh ly highe du ing he Oc obe e en han du ing he May s o m.
No e ha he e a e again some gaps in he ime se ies, in pa icula du ing he nigh
o he s o m a he highes la i ude obse a o y (JR055). These a e p ima ily due o he
p esence o pa icle-induced spo adic laye s p ecluding obse ing he F egion o he
ionosphe e. Blanke ing o he obse a ions by egula Es laye s is no impo an o his
A mosphe e 2025,16, 299 19 o 25
e en as such laye s a e mo e a ely p esen a hese obse a o ies in Oc obe as compa ed
o May.
A he onse o he s o m, a s eep d op in he F
2
laye densi y can again be obse ed,
indica ing he sudden deple ion o he ionosphe ic plasma. The main phase and eco e y,
on he o he hand, look di e en o his s o m. The cases o G-condi ion being obse ed
a e limi ed, in pa icula in he sou he nmos obse a o y, and he backg ound condi ions
we e es o ed al eady in he mo ning o 12 Oc obe .
Figu e 14 shows he c i ical equencies obse ed by he Ame ican and Ko ean ionoson-
des. Once again, i should be no ed ha he F
1
peak is no as clea ly obse ed as du ing
he May e en , and ha he oF
2
alues a e sligh ly highe . A hese obse a o ies, no
G-condi ion was obse ed. This is due o he local ime o he onse o he s o m. As can be
seen om Figu e 13, he pe iod o he G-condi ion obse ed by he Eu opean ionosondes
was ela i ely sho , compa ed o wha was seen du ing he May e en , om a ound 06:00
o a ound 12:00 UTC a JR055. This coincides wi h a pe iod when no F
1
laye is de ec ed a
MHJ45 and IC437, so he e can be no G-condi ion.
A mosphe e 2025, 16, x FOR PEER REVIEW 20 o 27
as such laye s a e mo e a ely p esen a hese obse a o ies in Oc obe as compa ed o
May.
A he onse o he s o m, a s eep d op in he F
2
laye densi y can again be obse ed,
indica ing he sudden deple ion o he ionosphe ic plasma. The main phase and eco e y,
on he o he hand, look diffe en o his s o m. The cases o G-condi ion being obse ed
a e limi ed, in pa icula in he sou he nmos obse a o y, and he backg ound condi ions
we e es o ed al eady in he mo ning o 12 Oc obe .
Figu e 14 shows he c i ical equencies obse ed by he Ame ican and Ko ean iono-
sondes. Once again, i should be no ed ha he F
1
peak is no as clea ly obse ed as du ing
he May e en , and ha he oF
2
alues a e sligh ly highe . A hese obse a o ies, no G-
condi ion was obse ed. This is due o he local ime o he onse o he s o m. As can be
seen om Figu e 13, he pe iod o he G-condi ion obse ed by he Eu opean ionosondes
was ela i ely sho , compa ed o wha was seen du ing he May e en , om a ound 06:00
o a ound 12:00 UTC a JR055. This coincides wi h a pe iod when no F
1
laye is de ec ed
a MHJ45 and IC437, so he e can be no G-condi ion.
Figu e 14. Time se ies o oF
1
(blue) and oF
2
( ed) obse ed by he MHJ45 and IC437 ionosondes
du ing he 11 Oc obe s o m. No ice ha he e ical axes a e again ex ended o 2.5 × 10
6
.
Some deple ion o he F
2
laye du ing he i s day a e he s o m can s ill be seen a
bo h obse a o ies hough. In bo h cases, he oF
2
a he end o he pe iod has no ye e-
co e ed en i ely o he alues seen be o e he s o m. In addi ion, he p esence o la ge-
scale a elling ionosphe ic dis u bances can be seen. Especially du ing 11 Oc obe a
IC437, he e a e pe iodic oscilla ions e iden in oF
2
.
Figu e 14. Time se ies o oF
1
(blue) and oF
2
( ed) obse ed by he MHJ45 and IC437 ionosondes
du ing he 11 Oc obe s o m. No ice ha he e ical axes a e again ex ended o 2.5 ×106.
Some deple ion o he F
2
laye du ing he i s day a e he s o m can s ill be seen
a bo h obse a o ies hough. In bo h cases, he oF
2
a he end o he pe iod has no
ye eco e ed en i ely o he alues seen be o e he s o m. In addi ion, he p esence o
la ge-scale a elling ionosphe ic dis u bances can be seen. Especially du ing 11 Oc obe a
IC437, he e a e pe iodic oscilla ions e iden in oF2.
A mosphe e 2025,16, 299 20 o 25
4.4. Plasmasphe e
The BSPM shows ha on 10 Oc obe 2024 12:00 UT, be o e he supe s o m, he
plasmasphe e is al eady aniso opic in he geomagne ic equa o ial plane (see Figu e 15,
op le panel) due o he p e ious s o m o 8 Oc obe . On 10 Oc obe 22:00 UT, du ing he
s o m, a double plume ha ex ends up o 6 Re appea s in he a e noon MLT sec o (see
Figu e 15, bo om le panel). Like o all s o ms, he plumes o a e wi h he Ea h [
45
]. The
plasmasphe e is e oded and a plasmapause close o he Ea h a ound 2.4 Re is ound a e
midnigh a MLT~3.
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4.4. Plasmasphe e
The BSPM shows ha on 10 Oc obe 2024 12:00 UT, be o e he supe s o m, he plas-
masphe e is al eady aniso opic in he geomagne ic equa o ial plane (see Figu e 15, op
le panel) due o he p e ious s o m o 8 Oc obe . On 10 Oc obe 22:00 UT, du ing he
s o m, a double plume ha ex ends up o 6 Re appea s in he a e noon MLT sec o (see
Figu e 15, bo om le panel). Like o all s o ms, he plumes o a e wi h he Ea h [45]. The
plasmasphe e is e oded and a plasmapause close o he Ea h a ound 2.4 Re is ound a e
midnigh a MLT~3.
Figu e 15. Elec on densi y ob ained wi h he BSPM coupled wi h he ionosphe e on 10 Oc obe
2024 a 12:00 UTC (be o e he supe s o m, ( op panels)) and 10 Oc obe 2024 a 22:00 UTC (du ing
he supe s o m, (bo om panels)). The ed dashed line on he obse ed Kp op panels indica es he
wo illus a ed imes.
A mosphe e 2025,16, 299 21 o 25
A lowe posi ion o he plasmapause a 2 Re is e en obse ed by Swa m a midnigh
MLT (see Figu e 16), a leas on 11 Oc obe , while no measu emen s a e accessible on
10 Oc obe 2024. These excep ional obse a ions o Kp > 8+ allow us o imp o e he BSPM,
by inc easing he ex apola ed in ensi y o he elec ic ield model o such high Kp alues,
since he empi ical E5D model [
46
] had been alida ed only o lowe alues o Kp due o
insu icien s ong e en s.
A mosphe e 2025, 16, x FOR PEER REVIEW 22 o 27
Figu e 15. Elec on densi y ob ained wi h he BSPM coupled wi h he ionosphe e on 10 Oc obe
2024 a 12:00 UTC (be o e he supe s o m, ( op panels)) and 10 Oc obe 2024 a 22:00 UTC (du ing
he supe s o m, (bo om panels)). The ed dashed line on he obse ed Kp op panels indica es he
wo illus a ed imes.
A lowe posi ion o he plasmapause a 2 Re is e en obse ed by Swa m a midnigh
MLT (see Figu e 16), a leas on 11 Oc obe , while no measu emen s a e accessible on 10
Oc obe 2024. These excep ional obse a ions o Kp > 8+ allow us o imp o e he BSPM,
by inc easing he ex apola ed in ensi y o he elec ic ield model o such high Kp alues,
since he empi ical E5D model [46] had been alida ed only o lowe alues o Kp due o
insufficien s ong e en s.
Figu e 16. Midnigh plasmapause p oxy de i ed om he magne ic and plasma obse a ion o he
low-Ea h o bi ing Swa m sa elli es (like in Figu e 9) om 9 o 12 Oc obe 2024.
5. Discussion and Conclusions
In he p esen wo k, we show ha a s udy o he ionosphe ic obse a ions as a unc-
ion o he la i ude and he Magne ic Local Time using VTEC, ionosondes, and plasmas-
phe ic obse a ions is c ucial o unde s anding he causes o he densi y a ia ions asso-
cia ed wi h he magne ic s o ms. We ha e analyzed he elec on densi y measu ed by ion-
osondes in he F
1
and F
2
laye s, he VTEC measu ed om he g ound o GNSS sa elli es,
and he posi ion o he midnigh plasmapause measu ed by Swa m du ing he wo bigges
geomagne ic s o ms o he p esen sola cycle 25, in May and Oc obe 2024. The obse a-
ions a diffe en la i udes and longi udes allow us o de e mine he spa ial and empo al
effec s and o s udy he ac ion o elec on densi y due o he ionosphe ic peak and o he
plasmasphe ic s o m esponse.
The obse a ions du ing he wo analyzed s o ms show he ollowing:
Figu e 16. Midnigh plasmapause p oxy de i ed om he magne ic and plasma obse a ion o he
low-Ea h o bi ing Swa m sa elli es (like in Figu e 9) om 9 o 12 Oc obe 2024.
5. Discussion and Conclusions
In he p esen wo k, we show ha a s udy o he ionosphe ic obse a ions as a unc ion
o he la i ude and he Magne ic Local Time using VTEC, ionosondes, and plasmasphe ic
obse a ions is c ucial o unde s anding he causes o he densi y a ia ions associa ed
wi h he magne ic s o ms. We ha e analyzed he elec on densi y measu ed by ionoson-
des in he F
1
and F
2
laye s, he VTEC measu ed om he g ound o GNSS sa elli es, and
he posi ion o he midnigh plasmapause measu ed by Swa m du ing he wo bigges
geomagne ic s o ms o he p esen sola cycle 25, in May and Oc obe 2024. The obse a-
ions a di e en la i udes and longi udes allow us o de e mine he spa ial and empo al
e ec s and o s udy he ac ion o elec on densi y due o he ionosphe ic peak and o he
plasmasphe ic s o m esponse.
The obse a ions du ing he wo analyzed s o ms show he ollowing:
1.
While he ioniza ion inc eases du ing he main phase o he s o ms, he densi y o
elec ons dec eases o a leas one day a e he s o ms.
2.
The VTEC deple ion is no only due o a dec ease in he ioniza ion in he F
2
laye
bu also o a close plasmapause, as shown using Swa m plasmapause obse a ions.
This con i ms ha sha p elec on densi y deple ion is associa ed wi h plasmasphe e
A mosphe e 2025,16, 299 22 o 25
e osion [
24
]. This was also obse ed in he s udies o p e ious geomagne ic s o ms.
Fo ins ance, i was ound ha he plasmasphe e can lose 40% o mo e o i s o al mass
du ing massi e e osions [
47
]. The ela i e con ibu ion o he plasmasphe e o he
nigh ime (i.e., locally) o al elec on con en (TEC) can easily go beyond 80% du ing
se e ely dis u bed pe iods [
48
]. The plasmasphe e is o en o e looked despi e he
di ec in e ac ion be ween he ionosphe e–plasmasphe e sys em.
3.
The F
2
laye e ills e y suddenly a e he Mo he ’s Day e en , which is e y unusual.
Measu emen s using di e en ins umen s (ionosondes, GNSS) a di e en la i udes
and longi udes on di e en con inen s indica e ha he sudden e illing occu s a he
same ime in he Eu opean and Ame ican sec o s, despi e he local ime di e ence.
The e illing occu ed ea lie a he lowe la i udes.
4.
The compa ison o wo supe s o ms wi h di e en in ensi ies allowed us o de e mine
how di e en mechanisms can ake place depending on he e en s. Indeed, clea
di e ences could be obse ed in he esponse o he ionosphe ic laye s o bo h s o ms.
The s o ms we e simila in s eng hs and in he local ime o he onse . Howe e ,
he backg ound condi ions o he ionosphe e in May and Oc obe a e e y di e en ,
a leas in he lowe ionosphe e. The F
1
peak is mo e p onounced in May han in
Oc obe , bu he F
2
peak is mo e compac in Oc obe , wi h a peak densi y somewha
highe han in May. These di e ences in he s uc u e o he ionosphe ic laye s lead
o he e ec s o he s o m being isible o a longe ime o he May e en , wi h he
F2laye only becoming isible again du ing he second day a e he s o m.
5.
G-condi ion (i.e., when he F
2
laye is no de ec ed because he densi y is highe in he
F
1
laye han in F
2
) is obse ed o he en i e y o 11 May, while i is almos absen
in Oc obe 2024. G-condi ion is ound o always be mo e se e e and longe las ing
a highe la i udes. This is la gely due o he di e en clima ological backg ound
condi ions, because he s o ms happened in di e en seasons. This is consis en
wi h he obse a ions made in [
38
], whe e s o ms om Ma ch and Ap il 2023 we e
discussed. Such seasonal di e ences a e smalle a highe al i udes.
6.
The spec acula loss o F
2
laye ioniza ion obse ed du ing bo h s o ms can be due
o an inc ease in he ecombina ion a es, associa ed wi h a highe empe a u e and
densi y caused by he injec ion o pa icles, in combina ion wi h he ou low o ioniza-
ion. The analysis o Swa m da a du ing he May e en [
10
] shows an equa o wa d
displacemen o he mid-la i ude ionosphe ic ough, con i ming he impo ance o
high-al i ude in luence.
E en hough om a geomagne ic pe spec i e, he s o m le el was simila o bo h
e en s discussed he e, and bo h e en s s a ed a abou he same uni e sal ime, he
eac ions o he ionosphe e and plasmasphe e show some ma ked di e ences. The main
eason o his is he di e en condi ion o he ionosphe e and plasmasphe e a he ime
o he s o m onse . These di e ences in u n a e due bo h o he di e en clima ological
backg ound, i.e., he season du ing which he e en happened, and o he pe sis ing e ec s
om p io , less se e e dis u bances. This illus a es one o he main issues wi h o ecas ing
he geosphe e e ec s o a s o m, and he impo ance o analyzing e ec s in de ail o each
s o m: se e e s o ms a e uncommon and a ely occu in compa able backg ound condi ions.
The e o e, collec ing a da abase wi h obse a ions co e ing all possible condi ions o allow
he alida ion o he models and o ecas ing sys ems is s ill an ongoing wo k in p og ess.
Au ho Con ibu ions: Concep ualiza ion, V.P.; me hodology, V.P. and T.G.W.V.; so wa e, all; alida-
ion, V.P. and T.G.W.V.; o mal analysis, all; in es iga ion, all; esou ces, V.P.; da a cu a ion, T.G.W.V.,
A.W., J.-M.C. and N.B.; w i ing—o iginal d a p epa a ion, V.P.; w i ing— e iew and edi ing, all;
isualiza ion, all; 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 .
A mosphe e 2025,16, 299 23 o 25
Funding: The p ojec 21GRD02 BIOSPHERE has ecei ed unding om he Eu opean Pa ne ship
on Me ology, co- inanced om 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 hank he Ho izon 2020 esea ch and
inno a ion p og amme unding om he Eu opean Union o he PITHIA-NRF p ojec (Plasmas-
phe e Ionosphe e The mosphe e In eg a ed Resea ch En i onmen and Access se ices) wi h G an
Ag eemen 101007599.
Ins i u ional Re iew Boa d S a emen : No applicable.
Da a A ailabili y S a emen : Omni da a we e e ie ed om h ps://omniweb.gs c.nasa.go /h ml/
ow_da a.h ml, Ds da a om h ps://wdc.kugi.kyo o-u.ac.jp/ (accessed on 3 Janua y 2025). The BSPM is
accessible on h ps://esc.pi hia.eu/ (accessed on 3 Janua y 2025). VTEC is a ailable on STCE.be. Swa m
plasmapause p oxy is ound a h ps://swe.ssa.esa.in /el e-plasma- ede a ed (accessed on 3 Janua y
2025). A de ailed desc ip ion o all hese new SWARM p oduc s as well as he de ailed desc ip ion
o he p ocessing algo i hm can be ound a h ps://ea h.esa.in /eoga eway/ac i i ies/plasmapause-
ela ed-bounda ies-in- he- opside-ionosphe e-as-de i ed- om-swa m-measu emen s (PRISM docs)
(accessed on 3 Janua y 2025). The algo i hm o Swa m de ec ion, and he de i a ion o plasmapause
index is published also in Heilig and Lüh (2018). This publica ion uses da a om he ionosphe ic
obse a o y in Roque es, Spain, owned and ope a ed by he Fundació Obse a o i de l’Eb e, and
om he Julius uh Ionosonde, which is owned by he Leibniz Ins i u e o A mosphe ic Physics
Kuehlungsbo n. All ionog ams used a e a ailable om he GIRO eposi o y (gi o.uml.edu, accessed
on 3 Janua y 2025).
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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