Introduction
0
50
100
150
200
250
300
1978 1983 1988 1993 1998
year
s
u
n
s
p
o
t
n
u
m
b
e
r
1365.0
1365.5
1366.0
1366.5
1367.0
1367.5
T
o
t
a
l
S
o
l
a
r
I
r
r
a
d
i
a
n
c
e
(
W
/
m
2
)
R TSI
Figure 1.1. Confrontation between Total Solar Irradiance variations (pink line) and sunspot number
series (orange line)
Recently we have demonstrated that the influence of the 11 year solar cycle is recorded in
climate indices measured in Ionian Sea cores dated with high precision and covering the last
millennia (Cini Castagnoli et al., 2002a). Solar activity variations over time scales longer than
the 11 year cycle also exist (see e.g. Cini Castagnoli et al., 1998a and b, 2002b).
There is also the possibility of the existence of feedback mechanisms increasing the effects of
the small solar constant variations. Different studies to understand the amplifying
mechanisms of the direct effect of the TSI variation are in progress.
An interesting mechanism was proposed by Svensmark and co-authors: during the solar cycle
22 Earth’s cloud cover underwent a modulation more closely in phase with the galactic
cosmic ray flux than other solar activity parameters. Further was found that Earth’s
temperature follows more closely decade variations in galactic cosmic ray flux and solar cycle
length than other solar cycle parameters. The main conclusion is that the average state of the
Heliosphere affects Earth’s climate (Svensmark, 1998). In a later paper they observed a
dependence on the height of clouds: only low (< 3.2 km) clouds seems to agree with GCR
flux variations, both in their total cover and top temperature. The observed ~2% absolute
4
Introduction
change in low cloud cover over a solar cycle would give a change in net low cloud forcing of
~1.4 W/m
2
(Marsh and Svensmark, 2000, Figure 1.2).
Figure 1.2. Global average of monthly cloud anomalies for (a) high (<440 hPa), (b) middle (440-680 hPa),
and (c) low (>680 hPa) cloud cover (blue). To compute the monthly cloud anomalies the annual cycle is
removed by subtracting the climatic monthly average (July 1983–June 1994) from each month on an
equal area grid before averaging over the globe. The global average of the annual cycle over this period
for high, middle, and low IR detected clouds is 13.5%, 19.9%, and 28.7%, respectively. The cosmic rays
(red) represent neutron counts observed at Huancayo (cutoff rigidity 12.91 GeV) and normalized to
October 1965.
Note that the link between GCR flux and cloud cover is still a matter of debate: Kernthaler et
al. (1999) and Kristjànsson and Kristiansen (2000) challenged Svensmark’s declaration,
pointing out that the interpretation of results is rendered difficult by the short time series of
5
Introduction
ISCPP and ERBE data used by Svensmark and by uncertainties concerning instrument
calibrations and changes of satellites.
On the other hand, new interesting results support Svensmark’s theory: Tinsley et al. (2000,
2001) estimated the influence of solar wind on the global electric circuit and inferred the
effects on cloud microphysics, temperature, and dynamics in the troposphere with models of
electroscavenging in clouds with broad droplet size distributions and weak electrification;
Eickorn et al. (2002) found the first observational evidences of cosmic ray-induced aerosol
formation in the upper troposphere.
Detailed observations in a widespread spectrum of electromagnetic waves and of cosmic-ray
particles are available only for the last decades by ground-based and spaceborn instruments.
The knowledge of the GCR flux grows with the progresses in the instruments for their
detection. Muon chambers data are available since the end of the 30s, Neutron Monitors since
the 50s, the first measurements of entire spectra from balloons came in the early 60s and then
satellites and spacecrafts permit continuous measurements of the interplanetary medium at 1
AU and have extended our knowledge to the limits of the Heliosphere (Voyager, Pioneer) and
out of the ecliptical plane (Ulysses). All these measurements show the short-scale variations
of the GCR flux, with the well established periodicities from days to decades.
On longer time scales, we know that the sunspot number time series, available since 1700,
shows an amplitude modulation with prolonged solar quiet periods with a periodicity of about
100 years (Gleissberg cycle (Gleissberg, 1967), see Figure 1.3). Since the effects associated
with the solar activity variations are important it seems worthwhile to investigate proxy data
to draw information over different time scales in the past, when direct observations were not
undertaken.
6
Introduction
0
20
40
60
80
100
120
140
160
180
200
1700 1750 1800 1850 1900 1950 2000
year
S
u
n
s
p
o
t
N
u
m
b
e
r
R
Maunder
Minimum
Dalton
Minimum
Modern
Minimum
Figure 1.3. The Sunspot number R series. Here are marked the prolonged solar minima.
The most important proxies of the past solar activity are the radionuclides produced by
nuclear interactions of CR in the Earth’s atmosphere, in meteorites and in planetary surfaces.
In fact the production rate of these cosmogenic radioisotopes depends on the CR flux and
energy spectrum of these, in turn, vary in the inner heliosphere dominantly by solar
modulation. Therefore, the study of cosmogenic isotopes in terrestrial archives and in
meteorites, which fell in different times, gives information on the solar activity in the past.
The most useful cosmogenic isotopes in terrestrial archives (independently dated) are
14
C in
tree rings and
10
Be in ice cores and sediments. The concentration of the cosmogenic isotopes
in these archives is also affected by terrestrial influences such as climatic and geomagnetic
field variations (Stuiver and Braziunas, 1993; Lal, 1996). On the contrary, the cosmogenic
isotopes in meteorites are produced by CR in the interplanetary space; therefore the
production rate of these isotopes does not depend on terrestrial effects.
The century scale modulation has been shown in the cosmogenic
44
Ti (T
1/2
= 59.2 yr) activity
measured in meteorites which fell in the last two centuries (Bonino et al., 1995; 1999) and
in the time series of
14
C in tree rings (e.g. Damon and Sonett, 1991). On this scale during
7
Introduction
prolonged solar quiet periods, like the Gleissberg minima, the cosmogenic radionuclide
concentrations were higher than during the short lasting recent decadal minima.
We observed that the
44
Ti variations from century minima and maxima are about four time
higher than calculated on the basis of the GCR flux measured in the last decades and
extrapolated in the past simply on the basis of the sunspot number (Bonino et al., 1995;
1999).
The contents of this Ph.D. thesis are: a brief explanation of the interaction of the GCR with
meteoroids in the interplanetary space and the description of the model for the calculation of
production rates of cosmogenic radionuclides I proposed (chapter 2); the experimental
equipment in the underground laboratory of Monte dei Cappuccini where measurements were
performed, and in particular the description of the new γ spectrometer GEM 150 I set up
during my Ph.D. courses (chapter 3); some results of measurements of cosmogenic
radionuclides in meteorites (chapter 4); the study of the short-lived cosmogenic radionuclides
in meteorites and the confrontation with my model (chapter 5); the reconstruction of the GCR
flux in the past back to 1700 and a reconstruction of
10
Be in ice cores (chapter 6); the
calculation of the
44
Ti activity in meteorites using both my semiempyrical model and a
physical model by Michel and Neumann (1998) and the confrontation with the experimental
data (chapter 7); some conclusions.
8
2 Cosmogenic radionuclides in meteorites
The modulation of GCR as a function of position, energy and time in the heliosphere is a
complex combination of different mechanisms (e.g. Jokipii, 1991; Potgieter, 1993). Models of
the inward transport of GCR have been successful. However, because of the complexity of the
modulation dynamics they do not allow to determine unambiguously the GCR fluxes for any
solar activity cycle.
Cosmic-ray particles interact with meteoroids in the interplanetary space, thereby producing a
large variety of radioactive and stable nuclides: the radionuclides concentration reveals the
exposure history during a time interval of about three half-lives before the fall; the stable
isotopes concentration is the integration of the production rate during the entire time of
exposure.
Due to gravitational effects and to collisions with other meteoroids some fragments can be
subtracted from their primitive orbit (which stands between 2 and 4 AU from the Sun) and
can intersect Earth’s orbit, eventually falling to the ground and then becoming meteorites.
During the fall a part of the original mass is destroyed by the ablation due to the friction with
the atmosphere.
Once fallen the meteorites are shielded from the cosmic ray flux from the atmosphere, so the
production of cosmogenic radionuclides ceases and the activity of them is only influenced
from the permanence of the meteoroid in the interplanetary space and constitutes a proxy of
the GCR flux during the exposure time. For this reason, moreover, only FALLS (and not
FINDS) can be used for the measurement of the cosmogenic radionuclides.
9
Cosmogenic radionuclides in meteorites
2.1 Cosmogenic radionuclides production rates in meteorites
The interpretation of the measured concentrations of cosmogenic isotopes in terms of the
history of CR irradiation requires a precise and accurate modelling of the production rates in
the interplanetary space. These depend:
1. on the type of the nuclear interacting particles;
2. on their flux and spectral distribution;
3. on the chemical composition and size of the meteoroid;
4. on the shielding depth of the considered sample.
2.1.1 Type of interacting particles
1. Solar wind
The solar wind is a continuous flux of ionised and rarefied particles (a plasma) which
starts from the solar corona’s base and scatters in the whole solar system. The solar wind
is accelerated by solar radiative pressure and reaches supersonic speeds (~ 450 km/s at 1
AU). Solar wind proton and electron densities at 1 AU are ~7 cm
-3
, temperatures are ~105
K.
The solar wind brings frozen magnetic field lines through the Heliosphere. Due to the
solar rotation they form a spiral structure known as Parker spiral (Figure 2.1)
Figure 2.1 The solar wind Parker spiral
10
Cosmogenic radionuclides in meteorites
2. Solar Cosmic Rays and solar energetic particles
The Sun’s particle emission has two components: a continuous one (the solar wind) and a
strongly variable one, bound to sporadical and violent events with great loss of energy
both as radiation and particles.
These events are the cause of interplanetary shock waves that travel trough the solar
system with unusual values of speed, density, temperature and magnetic fields.
The solar energetic events can be divided into two main categories: solar flares and
Coronal Mass Ejections (CMEs).
Solar flares are well-known phenomena caused by the formation of magnetic loops in the
solar cromosphere, up to the lower corona. Charged particles are accelerated emitting
radio burts (synchrotron radiation), X-rays from electron Bremsstrahlung and γ-rays from
proton nuclear interactions with the nuclei of the cromosphere. The cromosphere gets
enough energy to explode and cromospheric materials can evaporate in the interplanetary
space (the whole energy of a flare is greater than the superficial heat of the photosphere).
The number of solar flares depends on the number and complexity of the sunspots and
varies with the solar cycle.
CMEs were discovered only in the last decade, even if they are greater and much more
intense events. Big magnetic structures (sometimes greater than the solar diameter) grow
like enormous “bubbles” and break, ejecting tons of material out of the corona. The CMEs
are the major source of particles reaching the Earth.
During solar maxima on an average occur tens of little flares and one-two CMEs each
day, while at solar minima the number of this events is an order of magnitude lower.
Figure 2.2 shows the solar proton flux: energetic protons greater fluxes correlate with
solar maxima periods measured by sunspots.
11
Cosmogenic radionuclides in meteorites
0
50
100
150
200
250
1975 1980 1985 1990 1995 2000 2005
year
s
u
n
s
p
o
t
n
u
m
b
e
r
0
500
1000
1500
2000
2500
3000
p
r
o
t
o
n
f
l
u
x
(
p
a
r
t
i
c
l
e
s
s
-
1
s
r
-
1
c
m
-
2
)
Figure 2.2. Solar proton flux (> 10 MeV) as a function of time (blue line). Sunspot number R is shown for
comparison (orange line)
3. Galactic Cosmic rays
Galactic cosmic rays come from outside the solar system but generally from within our
Milky Way galaxy. GCR are atomic nuclei from which all of the surrounding electrons
have been stripped away during their high-speed passage through the galaxy. They have
probably been accelerated within the last few million years, and have travelled many
times across the galaxy, trapped by the galactic magnetic field. GCR have been
accelerated to nearly the speed of light, probably by supernova remnants.
GCR composition varies with energy, but in the region of maximum flux (10
8
eV ÷ 10
10
eV) GCR are composed by 87% H, 12% He, 1% heavier nuclei (Z≥6). (e.g. Lal, 1988).
4. GCR secondary particles
High energy primary particles can produce a large variety of secondary particles
interacting with the target elements in the meteoroid (see chapter 2.1.3).
12
Cosmogenic radionuclides in meteorites
2.1.2 GCR Flux variations
GCR measurements are available from the beginning of the 1960s; hereafter I consider only
primary protons flux, because their cross section for the production of cosmogenic
radionuclides is greater than α particles’ one (Michel and Neumann, 1998) and they constitute
almost 90% of the total flux. This and other assumptions will be discussed in chapter 2.2.5.
GCR flux J
G
(particles s
-1
sr
-1
m
-2
MeV
-1
) at 1 AU is referred to the top of terrestrial
atmosphere and can be measured by balloons (e.g. Boezio et al., 1999, Seo et al., 1991),
satellites or spacecrafts like the Space Shuttle (Alcaraz et al., 2000), Voyager 1(Webber e
Yushak, 1983) and 2 or Pioneer 10 (McDonald et al.,1992).
Figure 2.3 shows different J
G
spectra measured in 1965 (Ormes and Webber 1968), 1968
(Hsieh et al., 1971), 1980 (Kroeger, 1986), 1989 (Webber et al., 1991) in different phases of
the solar cycle. Note that J
G
has a maximum for energies of the order of 1 GeV and presents a
modulation with the solar cycle, being higher during solar minima and lower during maxima.
Above 20 GeV the flux is constant without any modulation. The same figure shows also some
comparison of the fitted J
G
spectra (using Eq. 2.1, see chapter 2.2.2) with the experimental
values, and giving values of the modulation parameter M = 390, 600, 820, 1080 MeV,
respectively. We can observe a very good agreement between measured and calculated J
G
(see
chapter 2.2.2).
0.001
0.01
0.1
1
10
100
10 100 1000 10000 100000
Energy (MeV)
J
G
(
p
r
o
t
o
n
s
m
-
2
s
-
1
s
r
-
1
M
e
V
-
1
)
1965 data 1968 data 1980 data 1989 data
MeV
820 MeV
1080 MeV
0 MeV
LIS
M
390 MeV
600
Figure 2.3. Differential cosmic-ray spectra obtained from Eq. 2.1 for different values of the solar
modulation parameter M = 390, 600, 820, 1080 MeV corresponding to the measurements performed with
balloons or spacecrafts during 1965, 1968, 1980 and 1989 respectively
13
Ricevi informazioni sui nostri servizi, sulle offerte e non perdere news e consigli su università e lavoro.
