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Advances in Space Research 35 (2005) 458–464

Solar cosmic rays in the near Earth space and the atmosphere

G.A. Bazilevskaya *

Lebedev Physical Institute, Russian Academy of Sciences, 53, Leninsky prospect, Moscow 119991, Russia

Received 4 October 2004; received in revised form 12 November 2004; accepted 15 November 2004

Abstract

Solar energetic particles (SEPs) constitute a distinct population of energetic charged particles, which can be often observed in the

near Earth space. SEP penetration into the Earth�s magnetosphere is a complicated process depending on particle magnetic rigidity

and geomagnetic field structure. Particles in the several MeV energy range can only access to periphery of the magnetosphere and the

polar cap regions, while the GeV particles can arrive at equatorial latitudes. Solar protons with energies higher than 100 MeV may

be observed in the atmosphere above �30 km, and those with energies more than 1 GeV may be recorded even at the sea level. There

are some observational evidences of SEP influence on atmospheric processes. Intruding into the atmosphere, SEPs affect middle

atmosphere odd-nitrogen and ozone chemistry. Since spatial and temporal variations of SEP fluxes in the near Earth space are con-

trolled by solar activity, SEPs may present an important link between solar activity and climate. The paper outlines dynamics of SEP

fluxes in the near Earth space during the last decades. This can be useful for tracing relationship between SEPs and atmospheric

processes.

� 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar energetic particles; Solar activity; Magnetosphere; Atmosphere

1. Introduction

Solar energetic particles (SEPs) have attracted con-

siderable interest since their discovery in the late 1940s

of the 20th century (Forbush, 1946). SEPs cause veryimpressive rapid and sometimes huge enhancement of

extra-terrestrial radiation. This phenomenon is worth

careful monitoring and exploration since it is closely

related to a number of fundamental and practical prob-

lems. It is a rich source of information about non-

thermal processes in the space plasma and a base for

endless discussions about place of SEP acceleration –

whether it is a flare or a shock related to a coronal massejection (e.g. Gosling, 1993; Reames, 1999). The study

of SEPs contributes to understanding of particle acceler-

0273-1177/$30 � 2004 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2004.11.019

* Tel.: +7 095 4854263; fax: +7 095 4086102.

E-mail addresses: bazilevs@fian.fiandns.mipt.ru, gbaz@rambler.ru

(G.A. Bazilevskaya).

ation near the Sun and in the inner heliosphere, in con-

nection to solar flares, coronal mass ejections (CMEs),

solar wind structures. Although influence of SEPs on

spacecraft systems as well as potential danger of SEPs

for spacecraft crews have been always born in mind,the importance of radiation conditions in the near Earth

space has been fully recognized only recently. Now, SEP

monitoring and analysis is included in all the projects on

space weather modeling and prediction (Flueckiger,

2004).

In the last decade, possible links between solar activ-

ity and the Earth�s weather and climate become a hot

point for scientific community. Cosmic rays are the mainsource of ionization in the atmosphere at altitudes below

55–60 km. Since the cosmic ray fluxes are strongly mod-

ulated by solar activity they could be a translator of so-

lar influence on the Earth�s environment. Possible links

of SEPs with weather may be due only to their ability

to ionize the atmospheric constituents. From this point,

ved.

0.01

0.1

1

10

100

1000

23 25 27 29

J, c

m-2

s-1

sr-1

Go E>10 MeV

Go E>30 MeV

Go E>50 MeV

Go E>100 MeV

Bal, E>150 MeV

NM, E>450 MeV

G.A. Bazilevskaya / Advances in Space Research 35 (2005) 458–464 459

there is no difference between galactic and solar cosmic

rays (e.g., Krivolutsky et al., 2002). However, SEPs have

an advantage of being episodical events with a sharp

start. In addition, due to steep energy spectrum SEPs

have very strong latitudinal effect and are strongly ab-

sorbed in the atmosphere. These features facilitatesearch for connection between cosmic rays and atmo-

spheric processes. This paper outlines features of SEPs

that should be accounted for while establishing their

connection with the atmospheric processes.

August, 2002

Fig. 1. Intensity–time profile of the solar proton event of 24 August

2002. Four top curves relate to geostationary GOES spacecraft

(protons with energies above 10, 30, 50 and 100 MeV), rhombs are

the results of balloon measurements, lower curve presents the data of

the Apatity neutron monitor.

2. Solar energetic particles and technique of observation

Solar energetic particles, which are also called solar

cosmic rays, constitute a distinct population of energetic

charged particles, which can be sporadically observed in

the near Earth space as a rapid enhancement of charged

particle intensity against the background of galactic cos-

mic rays. An event lasts from hours to days depending on

particle energy, the intensity decay being usually much

longer than its growth. The bulk of SEPs are protons,so the episodes of SEP occurrence are sometimes called

‘‘the solar proton events’’. However, SEPs contain also

about <5% of ions (fully or partially stripped nuclei)

and electrons. The lower energy limit of solar protons

is adopted to be around several MeV. The upper limit

usually does not exceed several tens of GeV, although

a problem of possible upper limit is under discussion

(Karpov et al., 2003).Intensity–time profiles of SEP events are various

and energy dependent. The simplest events occurring

in quiet conditions have a diffusion-type shape with ra-

pid enhancement and rather long decay. However, such

factors as multiple or long-lasting particle ejection,

additional acceleration in the interplanetary space,

and modulation of SEP fluxes by solar wind structures

lead to the diversity of profiles observed. An exampleof a SEP event is shown in Fig. 1 where characteristic

times of duration in different energy ranges are well

seen. It should be noted that SEP events are fairly of-

ten observed in series being associated with the same

solar active region during its life on the disk. In this

case the enhanced fluxes of charged particles may have

a complicated time history and last during several

weeks.Fig. 1 introduces also the instruments for the SEP

observation. Low-energy SEPs (E < 100 MeV) can only

be measured by spacecraft borne devices. Now, the en-

ergy range of onboard spectrometers is up to �900

MeV, but there are some problems with high energy

channels due to side particle penetrating (Smart and

Shea, 1999). The SEP monitoring from the space is per-

formed beginning from 1986 onboard the GOES space-craft series (http://spidr.ngdc.noaa.gov). The GOES

satellites are on the geosynchronous orbit at the height

of 6.6Re, and protons with energies above several

MeV can freely achieve the detector, i.e. their intensity

is equal to that outside the magnetosphere. The homo-geneous data on SEPs with E > 10 MeV are available

since 1964 and can be found in the Catalogues (Akini-

yan et al., 1983; Bazilevskaya et al., 1986, 1990; Dodson

et al., 1975; Sladkova et al., 1998). Shea and Smart

(1990) extended the series of SEP events with >10

MeV protons back to 1955 using the polar ionospheric

absorption records.

The SEPs in the energy interval of 100–500 MeV aresuitable for observation on balloons. Such observations

are being performed since 1958 by Lebedev Physical

Institute (LPI) at high and mid-latitudes (Bazilevskaya

et al., 2003). Protons with E 6 500 MeV penetrate in

the atmosphere down to �15 km, and can be directly

measured at higher altitudes. Solar protons with

E > 500 MeV lose their energy in interactions with

the air nuclei producing secondary particles. High en-ergy particles initiate nuclear-electromagnetic cascades

in the atmosphere. Secondary nucleons, mostly neu-

trons, reach the ground level and are recorded by neu-

tron monitors. Such events are called ground level

enhancements (GLEs). The majority of particles re-

corded during GLEs by the ground-based installations

(mostly neutron monitors, but also ionization cham-

bers and muon telescopes) are related to the SEPs withenergy above 1 GeV (several GeV for ionization

chambers).

3. The SEP energy spectrum and event size distribution

The energy spectrum of SEPs is rather steep and

time-dependent. Since the SEP intensity–time profileshave maximum, it is convenient to constitute the spec-

trum of the peak intensity for each energy interval (note

1970 -2003

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+00 1.E+02 1.E+04 1.E+06

J(>10 MeV), cm-2s-1sr-1

Eve

nts

/ in

tens

ity in

terv

al

observation

appr. g= 1.34

Fig. 3. Size distribution of SEP events over peak intensity of protons

with energy above 10 MeV.

460 G.A. Bazilevskaya / Advances in Space Research 35 (2005) 458–464

that protons of different energies reach usually their

peak intensity not at the same time). In the case of the

instant particle ejection from the Sun and diffusive prop-

agation in the interplanetary space, such a spectrum is

similar to the source spectrum. Energies of SEPs cover

more than four orders of magnitude and intensities,more than eight orders of magnitude. The examples of

the integral energy spectra of SEPs are depicted in

Fig. 2. In spite of wide range of SEP intensities, the spec-

tra demonstrate roughly similar slopes. In the power-law

presentation J(>E) � E�c spectral index c is usually

between 1 and 4. It should be noted that c may depend

on energy and more sophisticated spectral form are used

by the specialists with the aim to get insight into theacceleration mechanism and conditions. Fig. 2 demon-

strates the most noted SEP events which have had most

energetic protons (more than several GeV) and the high-

est intensity of low-energy protons. The more is the peak

intensity at a given energy the less is the SEP event

occurrence rate. The size distribution of events which

is well fitted by a power-law with an index g = �1.34

is shown in Fig. 3. It is generally accepted that numberof SEP events is calculated in the logarithmically equal

intervals (e.g., Miroshnichenko et al., 2001). After that

the number of SEP events is divided by the width of

the interval. To obtain the number of events in a given

interval it is necessary to multiply the ordinate meaning

by the width of the interval. The most powerful events

are very rare but they are of utmost interest from both

fundamental and application points of view. From1970 to 2003, only seven SEP events with the peak inten-

sity of J(> 10 MeV) > 104 cm�2 s�1 sr�1 were observed.

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+00 1.E+02 1.E+04 1.E+06

Energy, MeV

Jmax

(>E

), c

m-2

s-1sr

-1

23 Feb. 1956

4 Aug. 1972

29 Sept.1989

19 Oct.1989

23 Mar. 1991

28 Oct.2003

Fig. 2. Integral energy spectra for a number of most powerful SEP

events. Data for events 1972–1991 are taken from the Catalogues

(Bazilevskaya et al., 1986; Sladkova et al., 1998), the spectrum for 23

February 1956 is from (Miroshnichenko, 2003), the spectrum of 28

October 2003 is preliminary.

4. SEP penetration through the Earth�s magnetosphere

and atmosphere

The bulk of SEPs are declined by the geomagnetic

field and do not reach the middle and low latitudes.

Charged particle movement in the geomagnetic field de-

pends on its magnetic rigidity R = Pc/Ze, where P is aparticle momentum, c is light velocity, Z is a particle

charge, and e is an electron charge. For protons, R is

connected to energy as R ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE2 þ 1876E

p, where R is

in GV and E, in GeV. For each site on the globe, there

is a geomagnetic cutoff such as particles with magnetic

rigidity below this cutoff are prohibited from penetrating

from outside the magnetosphere. Fig. 4 (Shea and

Smart, 2004) shows the contours of constant cutoff

rigidities at 1 GV intervals. The outer contour refers

to R = 1 GV (E = 433 MeV for protons). Going back

to Fig. 2 we see that only the most energetic ‘‘tail’’ of

Fig. 4. Contours of constant cutoff rigidities at 1 GV intervals (Shea

and Smart, 2004). The outer counter is for R = 1 GV, the inner one is

for 17 GV.

Apatity, Murmansk region, 27 Oct. - 3 Nov. 2003

0

5

10

15

20

25

30

35

40

100 1000 10000 100000

Count rate of a Geiger counter, min-1

Alti

tude

, km

75.5

185.6

118.4

290.9

469.5

E, MeV

48.2

Fig. 5. Cosmic ray fluxes in the atmosphere as measured by a Geiger

counter during quiet time (left curve) and during solar proton events.

Data plotted by various symbols relate to different balloon flights. The

right-hand axis gives energy of protons penetrating down to altitude,

indicated on the left-hand axis.

0

20

40

60

80

100

120

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

Ion production rate, cm-3s-1

Alti

tude

, km

Solar particles,14 July,2000Galactic cosmicrays, 1964Galactic cosmicrays 1959

Fig. 6. Ion production rate in the polar atmosphere during SEP event

of 14 July 2000 (Quack et al., 2001) and due to galactic cosmic rays in

the minimum (1964) and maximum (1959) phases of the 11-year solar

cycle (Neher, 1971).

G.A. Bazilevskaya / Advances in Space Research 35 (2005) 458–464 461

SEPs would reach latitudes equatorward of 50�. Only

about ten GLEs have had such energetic particles in

the course of the ground-based observations.

The geomagnetic field acts also as an angular ana-

lyzer. A particle with certain rigidity above the cutoff

can arrive at a given site only from the definite direc-tion outside the magnetosphere. The asymptotic direc-

tions of particle arrival depend not only of particle

rigidity but also on the time of observation and magne-

tosphere conditions. In case of anisotropic particle

fluxes stations with equal geomagnetic cutoffs record

different SEP fluxes. For example, at the time of max-

imum SEP fluxes during a giant event of 29 September

1989, the Oulu and Thule neutron monitors accepted 1GV protons from asymptotic directions close in lati-

tude but �120� distant in longitude. As a result the so-

lar proton flux recorded at Thule was about twice

larger as that at Oulu.

The geomagnetic field is subject to the evolution lead-

ing to changes in cutoff rigidities which are important

for analysis of the long-term cosmic ray variations (Shea

and Smart, 2004). However, rapid changes in cutoffrigidity in response to interplanetary disturbances are

much more important for SEP penetration into the mag-

netosphere. According to Galper and Koldashov (2001)

a significant decrease in the cutoff rigidity at midlati-

tudes and 400 km height occurred during several hours

in SEP event of 31 October 1991. Direct observations

onboard the spacecraft during SEP events showed

changes of geomagnetic cutoff locations by more than5� in less than 1 day (Leske et al., 2001).

SEPs with energy above geophysical cutoff are al-

lowed to enter the atmosphere where they lose energy

in interactions with the air nuclei. Solar protons with

E < 500 MeV lose their energy mainly through ioniza-

tion. They are just absorbed in the atmosphere, their

path length being dependent on energy: a 100-MeV pro-

ton achieves altitude of �32 km, while a 500-MeV pro-ton �14.5 km. The charge particle fluxes in the

atmosphere of polar latitudes are presented in Fig. 5.

In the absence of SEPs, the secondary cosmic ray flux

versus altitude has a form of so called transition curve

with a maximum at about 20–25 km. During solar or

magnetospheric particle intrusion, there is a growth of

particle fluxes toward the atmospheric boundary. On

the right-hand scale of Fig. 5 the energy of protons pe-netrating down to given altitude is indicated. Higher en-

ergy SEPs produce nuclear and electromagnetic

cascades, i.e. there is not only absorption but also some

multiplication of particles. Therefore, SEP intrusion

into the atmosphere of middle latitudes leads to a

growth of the transition curve at altitudes higher �10

km.

Ion production rate depends on the particle intensityand the air density. Because of the falling energy spec-

trum of the SEPs, the larger fluxes are observed at higher

altitudes, but the air density decreases with the altitude

increasing. The maximum ion production rate during

SEP events is observed in the polar latitudes at the alti-

tudes of 20–80 km (depending on SEP energy spectra)

and amounts �103–104 cm�3 s�1 (Quack et al., 2001)

while the galactic cosmic rays yield the maximum ion

production �30 cm�3 s�1 at �12 km (Neher, 1971) as

is shown in Fig. 6.It should be mentioned that short-lasting enhance-

ments in the ionization rate in the upper atmosphere

(above 50–70 km) are also caused by solar X-ray bursts.

An additional source of ionization is precipitation of

magnetospheric electrons (e.g. Callis, 2001). Relativistic

electrons create the first maximum of ion production at

70–80 km (hundreds of ions cm�3 s�1), and the secondary

462 G.A. Bazilevskaya / Advances in Space Research 35 (2005) 458–464

one (much lower) at �30 km due to bremsstrahlung

X-rays (Resnell et al., 2000).

5. SEP events and galactic cosmic rays

Bearing in mind that both galactic and solar cosmic

rays ionize air atoms it is relevant to consider SEP

behavior in the course of an 11-year solar cycle together

with behavior of galactic cosmic rays, which are a back-

ground for SEP events. Fig. 7 demonstrates the rate of

SEP event occurrence and galactic cosmic ray intensity

for two energy thresholds alongside with time history

of sunspot number. Monthly averaged data on SEPswith E > 100 MeV and galactic cosmic ray intensity

beginning from 1958 were obtained in the LPI balloon

observations (Bazilevskaya et al., 2005). Before 1958,

the galactic cosmic ray intensity for E > 1.5 GeV was

reconstructed using the data of ionization chambers

(Okhlopkov and Stozhkov, 2005). Data on solar pro-

tons with E > 1 GeV were deduced from events recorded

by neutron monitors (GLEs) using specific yield func-tions (Lockwood et al., 1974; Debrunner, private com-

munication). These values should be considered as

estimations as they were obtained under assumption of

Fig. 7. Upper panel: monthly averaged peak intensity of solar protons

with E > 100 MeV (vertical bars) and intensity of galactic cosmic ray

with E > 100 MeV (grey curve). Middle panel: peak intensities of solar

protons with E > 1 GeV derived from GLEs observations (vertical

bars) and intensity of galactic cosmic ray with E > 1.5 GeV (grey

curve). Lower panel: 7-month running averages of sunspot number

values.

the SEP flux isotropy outside the Earth�smagnetosphere.

SEP events happen more often during high solar

activity, although the so called Gnevyshev gaps, i.e.

quiet periods free of SEPs can sometimes be observed

around solar maximum (Storini et al., 2003). For in-stance, in Fig. 7, a salient Gnevyshev gap is seen in

1999. Intensity of galactic cosmic ray changes in the

opposite phase with solar activity. It is seen in Fig. 7 that

at E > 100 MeV SEP peak fluxes exceed the galactic cos-

mic ray background often enough. At lower energies

such cases are more frequent. According to GOES mea-

surements the >10 MeV proton flux was 1.5 times larger

than a quiet day background during �50% of days in2000–2002 and this flux was five times larger than a

quiet background during �30% of days in 2001. Inten-

sity of relativistic solar protons as recorded by neutron

monitors (middle panel of Fig. 7) rarely exceeds a galac-

tic cosmic ray background. The great SEP events of

1940–1950s before the neutron monitor epoch (Duggal,

1979) manifested much higher intensity than the recent

GLEs. The study of SEP events in the past as they wereretained by the nitrate deposition in the Greenland ice

core (McCracken et al., 2001) found five episodes

around 1610, 1710, 1790, 1870, and 1950 when the rate

of powerful SEP events was several times higher than

that in the present era. Since the periodicity in enhanced

SEP production agrees with �80–90 year Gleissberg

cycle we can expect an increase in SEP activity in the

near future.The number of powerful SEP events is surprisingly

constant from one 11-year cycle to another even though

the lengths of cycles are different. In the 20–22 solar

cycles the numbers of GLEs recorded were consequently

12, 12, and 15 (Shea and Smart, 2001) while the corre-

sponding numbers of SEP events recorded by balloons

(E > 100 MeV) were 23, 23, and 22 (Bazilevskaya

et al., 2003). In the current, not yet complete, solar cyclewe have 13 GLEs and 25 balloon events. In the 21 and

22 solar cycles numbers of SEP events with peak inten-

sity more than 1 cm�2 s�1 sr�1 at E > 10 MeV were 141

and 137 (Bazilevskaya et al., 1986; Sladkova et al.,

1998).

It should be noted that SEP events often occur dur-

ing Forbush decreases, i.e. against a diminished galac-

tic cosmic ray background. For example, the recentGLE of 29 October 2003 caused a 7.5% peak in the

count rate of the Apatity neutron monitor. Several

hours earlier this neutron monitor recorded a Forbush

effect with amplitude of �30% and a recovery phase

being about 10 days. The energy spectrum of Forbush

decreases is very hard in comparison with that of SEPs.

Therefore, the enhancements in charged particle fluxes

at high latitudes and altitudes may be observed simul-taneously with decreased particle fluxes at lower alti-

tudes and latitudes.

G.A. Bazilevskaya / Advances in Space Research 35 (2005) 458–464 463

6. Summary

Cosmic ray fluxes are strongly modulated by solar

activity and so is the ionization produced by cosmic

rays in the atmosphere. The ionization certainly plays

a significant role in the atmospheric processes. Numer-ous observations corroborate the relationship between

the cosmic ray flux temporal modulation and the

changes in the dynamical, electrical and chemical states

of the atmosphere both for long-term and day-to-day

variations. Due to their special temporal and energy

features SEPs are rather often used for tracing links be-

tween cosmic rays and weather (e.g., Jackman et al.,

2001; Krivolutsky et al., 2001, 2003; Morozova andPudovkin, 2002; Quack et al., 2001; Veretenenko and

Thejll, 2004). Summary of SEP characteristics impor-

tant to their possible influence on weather and climate

is the following.

Solar energetic particles intrude into the Earth�s mag-

netosphere and atmosphere more than 100 times during

an 11-year solar cycle. SEP behavior in the magneto-

sphere and the atmosphere depend on SEP energy,height and latitude of observation site: most events do

not penetrate below the altitude of 50 km and latitude

lower than �60�. Only 12–15 events per solar cycle

can be recorded at the ground level.

Variations in galactic cosmic ray background (For-

bush decreases) and other concurrent phenomena (fast

changes in geomagnetic cutoff rigidity, solar X-ray

bursts, energetic electron precipitation) should be takeninto account while estimating SEP impact on the

atmosphere.

The interpretation of the found correlations between

SEPs and weather is still far from fulfillment. Hard work

is needed to reach understanding of the underlying phys-

ical processes and feedbacks.

Acknowledgements

This work is partly supported by the Russian Foun-

dation for Basic Research, Grants 02-02-16262, 04-02-

31007, and 04-02-17380.

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