Transcript Cosmic Rays in the Earth vicinity

Cosmic Rays
in the Earth vicinity
Roberta Sparvoli
University of Rome Tor Vergata and INFN
Prologue
Observing cosmic rays from the Earth’s surface is like
making astronomical observations from the bottom of an
ocean.
Cosmic rays arriving to Earth have survived collisions
with gas atom in interstellar or interplanetary space, and
may have collided in atmosphere (not treated here!).
Moreover, just as important as
collisions, is the influence of
the Earth magnetic field
and that of the Sun.
The Earth Magnetic Field
Originated by electric currents
running inside the Earth core.
To a first approximation it is a
dipolar field:
• Coordinates: 79°N, 70°W and 79°S, 110°E, reversed
with respect to geographic Poles, about 11° inclined
with Earth axis and shifted by 320 km.
The field changes slowly
over the years, producing a
secular
drift
of
the
magnetic Poles.
Magnetic Field Equations
In spherical coordinates:
B (r, l) = M(1+3sin2l)1/2 /r3
Br = -2Msinl/r3
Bl = Mcosl/r3
where M is the magnetic dipole moment
and l the magnetic latitude. M~ 8.1x1025
Gauss cm3 and thus B(RE) ~ 0.31 Gauss.
The field lines have this form:
r = r0 cos2l
The module of the field B along the line has its minimum
for l=0. If l=0, r= r0 and this is the radial distance to the
field line over the equator. Adopting R=r/RE, in Earth-radii,
the field line equation becomes:
R = R0 cos2l
Dipole representation accurate to ~30% at distances  2-3 RE.
A better empirical representation is based on a multipole
expansion (international geomagnetic reference field IGRF
model), with slowly time-dependent coefficients.
To describe the field, also in non-dipole approximation,
usually the McIlwain coordinates (B,L) are used.
A point P in the space is defined by:
L
distance (in RE) of the field line passing for P,
measured on the equatorial plane. A measure of “equatorial
radius”. L analogous to R0 (dipole field);
B
magnetic field intensity in P. A measure of “latitude”.
Influence on cosmic ray fluxes
The interaction between the Earth magnetic field and
cosmic rays was seen by:
• Latitude effect: the CR flux depends on the latitude, is
higher at the poles than at the equator.
Conclusion: CR are mainly charged! They arrive from all
directions and are deflected by the magnetic field. Each
latitude has a cut-off rigidity (p/z) below which no
vertically arriving particles can penetrate.
• East-West effect: the cut-off rigidity depends on the
arrival direction. Positive CRs are more abundant if they
enter from West, negative if from East.
East-West asymmetry (detected): cosmic rays are mainly
positive!
Particle trajectories
Let us consider a particle with Z and p detected at a point x.
We can trace back the particle path to its origin (en electric
charge moving in a static non-homogeneus magnetic field).
We can find:
a) the trajectory originates from Earth’s surface or in atmosphere;
b) the trajectory remains confined in the volume RE < r < ;
c) the trajectory reaches infinity.
Trajectories a) and b) are “forbidden” because no cosmic
rays from far away can reach the Earth along them. The
others are “allowed”. Positive particles with rigidity higher
than the cut-off are allowed.
The effect of the geomagnetic field (static) is to remove
particles from the forbidden trajectories, without deforming
their spectrum.
Trapped particles
No attention was given to the “forbidden” orbits, though
mathematically known, until they were truly discovered
(Van Allen, Explorer I and II, 1958).
The CR counters onboard above 2000 Km
seemed to stop working--> saturation!
It was discovered the existence of two radiation
belts around the Earth, the internal (Inner) full
of protons, the external (Outer) rich of electrons.
The intensity of the radiation was controlled by the magnetic
and not the geographic latitude.
Note that the Sputnik I and II had already flown, but because of the orbit
and the missing telemetry no radiation belts had been seen.
Motion of trapped particles
Combination of 3 periodic motions:
• Gyration: a helix around the field line;
• Bounce: oscillation around the equatorial plane between
almost symmetrical mirror points. Only small oscillations are
possible, the mirror point cannot hit the Earth surface.
Pitch-angle a0: angle between p and B at the equator.
Condition for trapping: |sin a0| R0-5/4 (4 R0-3)-1/4 ;
• Drift: longitudinal. It is due to
dishomogeneity of the field and
variations of the gyroradius.
Positive particles drift westward,
negative eastward.
Fluxes of trapped particles
Origin:high energy CR interactions
in atmosphere, producing neutrons
and then protons and electrons.
Also Solar Wind and influences of
the ionosphere.
Inner Radiation Belt: protons with E up to hundreds MeV.
Mean life time: years. It extends to 1.5 RE.
Outer Radiation Belt: electrons with E of a few MeV.
Mean life time: days. It extends to 4.5 RE.
Death: distortions in the magnetic field (also due to solar
activity) bring particles to jump to different field lines
which go down to dense atmosphere. Collisions.
Also collisions among themselves.
South Atlantic Anomaly
Above South America, about 200 - 300 kilometers off the
coast of Brazil, and extending over much of South America,
the nearby portion of the Van Allen Belt forms what is called
the South Atlantic Anomaly.
This is an area of enhanced radiation caused
by the offset and tilt of the geomagnetic axis
with respect to the Earth's rotation axis,
which brings part of the radiation belt to
lower altitudes.
The inner edge of the proton belt dips below the line
drawn at 500 km altitude.
Albedo particles
Albedo particles are produced by cosmic ray interactions in
atmosphere (40 km). They are rebound to space by the
Earth magnetic field and have energies below the cut-off.
According to pitch-angle, we can have:
1. Only one bounce: albedo
2. More than one bounce: quasi-trapped
3. Trapped
with almost equal fluxes (Grigorov, 1977).
Differences between albedo and trapped:
- the origin traces back into atmosphere or ground level;
- shorter flight time (from source to sink).
- energy up to GeV.
The magnetosphere
The outer regions of the Earth field are sensitive to the
magnetic field carried by the Solar Wind, important for
distant orbits. 10 RE (on the Sun side) is considered the
boundary of the magnetosphere.
The Sun
compresses one
side of the
magnetosphere
and stretches the
other. A very
complex region is
formed when the
two fields meet
each other.
The influence of the Sun
The Sun energy, originated by fusion reactions inside,
radiates in all directions, maintaining a steady level (quiet
Sun).
The photosphere, visible surface of the Sun, has a
temperature of T=6000 ºK, but the overlying corona has a
T exceeding 106 ºK.
At these temperatures, part of the
ionized gas of the solar ambient has
speed enough to escape the solar
gravitational attraction.
Solar Wind
Evidences for the Solar Wind
The existence of the Solar Wind was firmly established only
in 1960, by summing up several evidences:
• Sunspots: observed to have a 27 days period
(the Sun rotation), but modulated over intervals
of 11 years.
• Magnetic storms: disturbances
in Earth electrical power systems
and telecommunications, often
accompanied by auroras in polar
regions.
They
were
also
correlated with the sunspot
number.
•Antimodulation CR:
neutron monitors at
ground found an anticorrelation between the
particle fluxes and the
sunspot number.
• Comets: a mixture of frozen water, frozen gases and dust. Near
the Sun they become visible because the gases and ice melt and
form a "tail" pointing away from the Sun.
The Solar Wind and the interplanetary
magnetic field force the ionized gas to
stream behind the comet. Following
the movements of the tails it was
possible to infer speed and number of
particles in the Solar Wind.
Characteristics of the Solar Wind
• Composed of protons and electrons (also He and heavier
elements), neutral;
• The gas is highly ionized;
• The stream holds a magnetic field;
• Because of the Sun rotation, the stream
is emitted like from a ‘garden-hose’;
• At 50-100 AU it is thought
that the Solar Wind terminates
abruptly in a “shock”, a
complex boundary between
the
interplanetary
and
interstellar regions.
Quiet and Active Sun
The Solar Wind is a manifestation
of the quiet Sun.
Solar Cosmic Rays (Solar Energetic
Particles) are instead a short-lived
manifestation of the active Sun,
and are associated to energetic
solar events.
The two groups of particles are distinguished by their energy:
Solar Wind protons:
Energy ~ KeV
Solar CR protons:
Energy ~ MeV
Origin of SEP events
• Solar Flares: until the 90ies thought to be
responsible of the most intense SEPs and
geomagnetic storms. The Solar Flare is an
explosive
release
of
energy
(both
electromagnetic and charged particles)
within a relatively small (but greater than
Earth-sized) region of the solar atmosphere.
• Coronal Mass Ejections (CMEs):
violent eruptions of coronal mass,
known to be the very responsible of
particle acceleration. Often, not always,
associated to a flare. The fast CME
explosion in the slow Solar Wind
produces a shock wave which
accelerates particles.
What else arrives to Earth?
Anomalous Cosmic Rays (ACRs):
represent a sample of the local interstellar medium. They
have not experienced such violent processes as GCRs,
and indeed they have a lower speed and energy.
ACRs include He, O, Ne and other elements with high
FIP.
They are a tool for studying the movement of energetic
particles within the solar system, for learning the general
properties of the heliosphere, and for studying the nature
of interstellar material itself.
Mechanism of ACRs
While interstellar plasma is kept outside the heliosphere by an
interplanetary magnetic field, the interstellar neutral gas flows
through the solar system like an interstellar wind. When closer to
the Sun, its atoms undergo the loss of one electron in photoionization or by charge exchange.
Once these particles are charged, the Sun's magnetic field picks
them up and carries them outward to the solar wind termination
shock. They are called pickup ions during this part of their trip.
The ions repeatedly collide with
the termination shock, gaining
energy in the process. This
continues until they escape from
the shock and diffuse toward the
inner heliosphere. Those that are
accelerated are then known as
Anomalous Cosmic Rays.
Trapping of ACRs
The high M/Z of singly ionized AC nuclei enables them to
penetrate deeply into the magnetosphere. AC nuclei
travelling near a low altitude mirror point easily encounter
sufficient grammage to be stripped of remaining orbital
electrons. After stripping, the particle gyroradius is
reduced by a factor 1/Z and the ion can become stablytrapped.
The SAMPEX spacecraft has
provided the first detailed look at
trapped ACRs, which form a
specific radiation belt. This
radiation belt includes significant
abundances of O, N and Ne, but
very little C or other elements.
The L-shell distribution of the
observed trapped ACRs is
sharply peaked at L-shell~ 2.
Summary of Cosmic Rays
Plus sub-cutoff particles
Trapped
Quasi-trapped
Albedo
Cosmic ray missions in space
• Balloon (ISOMAX, MASS,CAPRICE..)
- choice of the geographic location
for the physics
- background calculations
• Space Station (MIR-SilEye, ISS-AMS, ISS-EUSO...)
- safety of humans onboard
- choice of the orbit for the physics
• Satellite (NINA, PAMELA, EGRET,
AGILE, GLAST….)
- choice of the orbit for the physics
- background calculations
Possible orbits
A real case: experiments NINA
Scientific interest:
Study of the nuclear and isotopic component of cosmic rays:
H - Fe --> 10--200 MeV/n (full containment)
--> 1 GeV/n (out of containment)
Choice of the orbit: POLAR
so to be able to encounter different
families of cosmic rays.
The detector
a silicon wafer 6x6 cm2 ,
380 mm thick with 16
strips, 3.6 mm wide in
X -Y views.
32 wafers arranged in 16
planes, 1.4 cm apart. In
total almost 12 mm of
silicon.
Lateral and Bottom AC
for Full Containment
NINA-2 mission
Satellite MITA:
PERIOD
ALTITUDE
INCLINATION
MASS
~ 100 min.
~ 400 km
98.7 deg.
2500 kg
Launch: 14 July 2000
Space - Base Plesetsk
End of mission: 13th April 1999.
NINA mission
Satellite RESURS-01 n.4:
PERIOD
ALTITUDE
INCLINATION
MASS
~ 100 min.
~ 840 km
98.7 deg.
2500 kg
Launch: 10 July 1998
Space - Base Baikonur
End of mission:
13th April 1999.
Orbit analysis
Polar regions:
GCRs
ACRs
SCRs
Mid-latitudes:
Trapped
Quasi-trapped
Albedo
Results in Polar Regions
• Galactic Cosmic Rays
Performed in solar quiet periods,
at high L-shells.
Results in Polar Regions
• SEP events
Performed in
active Sun periods,
at high L-shells.
Results at mid-latitude
• Particles trapped in SAA
• Albedo Particles
Conclusions
Cosmic rays travel much and are distorted before reaching
the Earth. The Earth magnetic field, the Sun and the
atmosphere influence the cosmic ray flux.
The knowledge of the radiation environment related to a
space mission is necessary for:
Strategy of the mission
Data analysis and interpretation
Possible discoveries?