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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, 2002Fig. 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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