Cosmic Raysin 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 fieldand that of theSun.
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: Br = -2Msinl/r3 B (r, l) = M(1+3sin2l)1/2/r3Bl = 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, thefield 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 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. • 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: 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 It was discovered the existence of two radiation belts around the Earth, the internal (Inner) full of protons, the external (Outer) rich of electrons. • 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! 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 • Sunspots: observed to have a 27 days period (the Sun rotation), but modulated over intervals of 11 years. • The existence of the Solar Wind was firmly established only in 1960, by summing up several evidences: • 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.
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. Antimodulation CR: neutron monitors at ground found an anti-correlation between the particle fluxes and the sunspot number. 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 two groups of particles are distinguished by their energy: Solar Wind protons: Energy ~ KeV Solar CR protons: Energy ~ MeV • 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.
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 photo-ionization 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 ofsingly ionized AC nuclei enables them to penetrate deeply into the magnetosphere. AC nuclei travelling near a low altitude mirror point easily encounter sufficientgrammage to be stripped of remaining orbital electrons. After stripping, the particle gyroradiusis reduced by a factor 1/Zand the ion can become stably-trapped. 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 otherelements. The L-shell distribution of the observed trapped ACRsis sharply peaked at L-shell~ 2.
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
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 mission Satellite RESURS-01 n.4: PERIOD ~ 100 min. ALTITUDE ~ 840 km INCLINATION 98.7 deg. MASS 2500 kg NINA-2 mission Satellite MITA: PERIOD ~ 100 min. ALTITUDE ~ 400 km INCLINATION 98.7 deg. MASS 2500 kg Launch: 10 July 1998 Space - Base Baikonur End of mission: 13th April 1999. Launch: 14 July 2000 Space - Base Plesetsk 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
Strategy of the mission Data analysis and interpretation Possible discoveries? 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: