Download Pertti Mäkelä The Catholic University of America

Pertti Mäkelä
The Catholic University of America, Washington, DC, USA
and
NASA Goddard Space Flight Center, Greenbelt, MD, USA
SCOSTEP/ISWI
International School on Space Science
November 7-17, 2016, Sangli, India
Energy measured in units of eV: 1 GeV (109 eV), 1 TeV (1012 eV), 1 PeV (1015 eV), 1 EeV
(1018 eV)
Motion of charge particles affected by magnetic fields:
Rigidity (momentum over charge)
[V]
CR particles propagate with relativistic speeds: (E is the kinetic energy, E0 the restmass energy (proton=938.28 MeV), Z the atomic number and A the mass number of the
particle)
• 1912 Victor Hess measured change in ionization by
ascending in a balloon up to an altitude of 5.3 km on 7
April 1912 during a solar eclipse: (1) The radiation
increased with height, ruling out its terrestrial origin; (2)
Flight during a solar eclipse ruled out the Sun as a
source of the “Höhenstrahlung.”
• 1925 Robert Millikan invented the term "cosmic rays."
• 1927 Jacob Clay detects a latitudinal effect: the radiation
closer the equator lower than at higher (magnetic)
latitudes (CRs are charged particles).
• 1930 Carl Størmer (Stoermer) calculated particle
trajectories in the geomagnetic field -> latitudinal effect
was due to shielding by the geomagnetic field.
• 1936 Georg Pfotzer discovered a maximum in CR
intensity at an altitude of about 15 km due to the
interaction between CRs and the atmosphere.
• 1937 Scott Forbush observed a world-wide decrease in
cosmic rays during a strong magnetic storm (Forbush
decrease) suggesting that solar activity affects CRs.
Continuous highly isotropic (only faint sidereal anisotropy reported) flux of
charged particles (mostly protons).
Consists of two components: primary (from source) and secondary (from
interactions of primary particles during propagation) particles.
Particle kinetic energies range from MeVs to 1020 eV (1020 eV ≈ 16 J; the kinetic
energy of a tennis ball moving at the speed of 85 km/h)
Energy density ≈ 1 eV/cm3 (comparable to that of the visible star light ≈ 0.3
eV/cm3 , the galactic magnetic fields ≈ 0.25 eV/cm3 , the microwave
background ≈ 0.25 eV/cm3)
Power needed to sustain a constant flux of particles ≈ 1034 J/s (1041 erg/s)
Originate most likely from both the galactic (GCRs) and extragalactic (Ultra
High Energy Cosmic Rays; UHECRs) sources
E-3.0
E-3.3
E-2.7
Power-law spectrum: J ≈ E-γ
Solar modulation at the lowest energies
Changes in spectral slope occur at
the knee (3-4 PeV), the 2nd knee (400
PeV), and the ankle (3 EeV).
C O
Fe
Abundance of C, O and Fe
Reduced abundance of H and He
Overabundance of Li (Lithium), Be (beryllium), B (boron)
Excess of elements just below Fe
Zweibel, (2013)
Charged particles trajectories are deflected the galactic magnetic fields,
so GCRs cannot be tracked back to their sources.
The bulk of GCRs is assumed to be accelerated in strong shock fronts of
supernova remnants (SNRs).
Accelerated through diffusive shock acceleration (DSA).
An efficiency of ∼ 10% in particle acceleration at the SNR shocks is
required for produce GCRs.
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Neutral pion
The W44 supernova remnant is
nestled within and interacting with the
molecular cloud that formed its parent
star. Fermi's LAT detects GeV gamma
rays (magenta) produced when the
gas is bombarded by cosmic rays,
primarily protons. Radio observations
(yellow) from the Karl G. Jansky Very
Large Array near Socorro, N.M., and
infrared (red) data from NASA's
Spitzer Space Telescope reveal
filamentary structures in the remnant's
shell. Blue shows X-ray emission
mapped by the Germany-led ROSAT
mission. Credit: NASA/DOE/Fermi LAT
Collaboration, NRAO/AUI, JPLCaltech, ROSAT
This multiwavelength composite shows the supernova remnant IC 443, also
known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in
magenta, optical wavelengths as yellow, and infrared data from NASA's Widefield Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns),
cyan (4.6 microns), green (12 microns) and red (22 microns). Cyan loops indicate
where the remnant is interacting with a dense cloud of interstellar gas. Credit:
NASA/DOE/Fermi LAT Collaboration, NOAO/AURA/NSF, JPL-Caltech/UCLA
CRs diffuse through the turbulent interstellar magnetic fields in the
interstellar medium (ISM) in the Galaxy (galactic disc + halo).
CRs are also deflected multiple times by the randomly oriented magnetic
fields (B ∼ 3 µG).
Suffer energy losses and produce spallation secondary particles in
interactions with matter and radiation fields.
CRs also escape from the Galaxy in a way that is energy dependent at
high energy.
The spectrum and composition of CRs measured at Earth is different
from the spectrum and composition of CRs at their source.
The pattern of relative abundances of secondary and primary CRs tell
about the propagation history of the primary CRs, which can be used to
estimate the contribution from propagation to the spectrum at Earth.
CRs interact with interstellar matter (ISM) and produce secondary particles.
The isotopes 6Li, 9Be, and 10B are created mainly by CR spallation in the ISM.
Some secondary particles produced in CR interactions are unstable and
decay. These secondary particles are called cosmic ray clocks.
The ratio of secondaries to primaries CR useful to estimate the amount of
traversed interstellar matter.
One of the most sensitive quantities is the ratio of boron to carbon,
because boron is purely secondary and its main progenitors, carbon and
oxygen, are primaries.
The shape of this ratio is highly sensitive to propagation coefficients. Due
to their similar charge, the B/C ratio is the less affected by systematics or
solar modulation.
The ratio of boron and carbon fluxes (B/C) tells the escape time %
i.e. the time CRs spend in the Galaxy before escaping.
&
,
()*% &
The matter (grammage) traversed by CRs, '
,where ( is the
mean gas density in the Galaxy (disc plus halo), μ is the mean mass of the
gas, * is the speed of particles.
For particles with energy per nucleon of 10 GeV/n the measured B/C
corresponds to X ∼ 10 gcm−2.
The high rigidity behavior of
the B/C ratio is compatible
with a power law grammage
+,
'
with
, = 0.3–0.6
The knee in the all-particle energy spectrum due to a break in the spectra
for the light elements.
Breaks start sequentially with the light elements. The maximum energy of
CRs produced in SNRs,
-. ∝ , CRs become progressively heavier
above the proton cutoff energy 3x1015 eV. Results in an increase of the
mean mass of CRs in this energy region.
Finite lifetime of a shock front (∼ 105 a) limits the maximum energy
attainable for particles with charge Z
The existing data show that above 1017 eV (2nd knee) CRs become lighter
Possible explanations for CRs at the knee and up to the ankle:
propagation effects in the Galaxy (Ptuskin et al. 1993) and re-acceleration
at shocks in a Galactic wind (Völk & Zirakashvili 2004)
Berezhko & Völk 2007
The ankle probably indicate the transition from GCRs to extra-galactic CRs.
Extra-galactic CRs presumably produced in active galactic nuclei (Aloisio et al.
2007)
Above particles have gyroradii greater than the size of the Milky Way and they
cannot be confined within the Galaxy.
2nd GCR component predicts a composition at and
above the 2nd knee (helium or a mixture of helium and
2nd knee CNO nuclei)
Ankle
CRs from Wolf-Rayet
stars give better match
than those from
Galactic wind
termination shocks
Thoudam et al. 2016
The suppression feature
probably originates from the
interaction of primaries with the
CMB:
Greisen (1966) and also Zatsepin
and Kuz’min (1966) suggested
that protons with energy around
and above 6 × 1019 eV can
produce pions in their collisions
with the CMB photons.
GZK effect may help to identify the origin of UHECRs.
The distribution of extragalactic matter within the GZK horizon is
inhomogeneous, and this inhomogeneity may induce an anisotropy in the
distribution of the arrival directions.
69 Auger events with energy above
5.5×1019 eV (filled circles) with the
expected CR density derived from
an Active Galatic Nuclei model
42 TA events with energy above
5.7×1019 eV (white dots) and the flux
distribution from a model based on
2MRS galaxies.
Harari 2014
~11 year sunspot cycle
~22 year solar magnetic cycle
GCR intensity and solar activity
are anti-correlated
Gradient and curvature drifts
cause charge-sign dependent
modulation and a 22-year cycle in
CR intensity profiles.
Positive
polarity
Negative
polarity
The solar magnetic field reverses
polarity every
∼ 11 years so that GCRs will drift
towards Earth from different
heliospheric directions.
CR transport in the heliosphere:
Diffusion
Convection with solar wind
Particle drifts
Adiabatic energy changes
Heber & Potgieter 2006
Forbush decrease (FD) caused by solar wind
disturbance passing Earth, mainly co-rotating
interaction regions (CIRs), interplanetary shocks
or interplanetary coronal mass ejections
(ICMEs).
GCR intensity may decrease up to 20% in a few
hours followed by a slow recovery phase lasting
around a week.
The FD amplitude depends on the magnetic field
strength and the level of fluctuations together
with the length of time the Earth stays inside the
disturbance
The recovery phase depends on the magnetic
field strength and size of the disturbance.
Usoskin et al. 2008
Record high CR intensities in 2009
GCR with Z>2 below 500 MeV/n contribute
less than 5% to effective dose
Exposure to cosmic radiation that has been shown
to cause cognitive impairments (Parihar et al. 2016).
Slaba & Blattnig 2014
Direct measurements of the primary CRs
possible up to 1014 eV by stratospheric balloons
or space experiments.
PAMELA (Payload for Antimatter Matter
Exploration and Light-nuclei Astrophysics) on
the Resurs DK1 satellite launched on June 15th
2006.
The Alpha Magnetic Spectrometer (AMS-02) on
the International Space Station (ISS)
CR spectrum is steeply falling spectrum,
therefore the low flux of particles demands
indirect methods: air showers generated by the
interaction of CRs in the atmosphere.
indirect measurements:
1. particles at ground,
like electrons, muons
and hadrons
2. Cerenkov light
(1014-1016 eV)
3. Fluorescence light
from nitrogen (1017 eV)
4. Radio signals
Haungs 2015
Hybrid detection technique: a surface detector (SD) and of a nitrogen
fluorescence detector (FD).
The Pierre Auger Observatory (1400 m above sea level):
Hexagonal grid of 1600 water Cherenkov tanks (SD) with a separation of
1500 m; total area of 3000 km2 .
The area of SD overlooked by 4 fluorescence detectors at the edges of the
SD array.
Telescope Array (TA): 500 scintillator detectors on a 1.2 km square grid;
total area 700 km2 . The SD area overlooked by 3 telescope stations.
Measures the number of CR
particles impacting Earth
Have been used since the
1950s
Geomagnetic cutoff rigidity
sets a lower limit for
measured energies
Integrates over energy, i.e. no
direct measurement of
energy spectrum
Asymptotic direction of
viewing limits
Two site with data and lists of other useful links
Neutron monitor network: http://nmdb.eu
Cosmic rays database https://lpsc.in2p3.fr/cosmic-rays-db